Projetos Eletronicos

Electronics Projects Vol. 22

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EFY is a reputed information house, specialising in electronics and information technology magazines. It also publishes directories and books on several topics. Its current publications are: (A) CONSTRUCTION PROJECTS 1. Electronics Projects, Vol. 1: A compilation of selected construction projects and circuit ideas published in Electronics For You magazines during 1979 and 1980. 2. Electronics Projects, Vol. 2 to 19 (English version): Yearly compilations (1981 to 1998) of interesting and useful construction projects and circuit ideas published in Electronics For You. 3. Electronics Projects, Vol. 20, 21 and 22 (with CD): Yearly compilations (1999 to 2001). 4. Electronics Projects, Vol. 16 (fgUnh laLdj.k): Yearly compilations (1995) of interesting and useful construction projects and circuit ideas published in Electronics For You. (B) OTHER BOOKS 1. Learn to Use Microprocessors (with floppy): By K. Padmanabhan and S. Ananthi (fourth enlarged edition). An EFY publication with floppy disk. Extremely useful for the study of 8-bit processors at minimum expense. 2. ABC of Amateur Radio and Citizen Band: Authored by Rajesh Verma, VU2RVM, it deals exhaustively with the subject—giving a lot of practical information, besides theory. 3. Batteries: By D.Venkatasubbiah. Describes the ins and outs of almost all types of batteries used in electronic appliances. (C) DIRECTORIES 1. EFY Annual Guide: Includes Directory of Indian manufacturing and distributing units, Buyers’ Guide and Index of Brand Names, plus lots of other useful information. 2. ‘i.t.’ Directory: First comprehensive directory on IT industry covering hardware, software, telecom, dotcom and training institues. 3. Technical Educational Directory: Includes course-wise and state/city-wise listings of technical educational institutes in India, besides the alphabetical main directory offering all the relevant information about them.

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ELECTRONICS PROJECTS VOL. 22

EFY Enterprises Pvt Ltd D-87/1 Okhla Industrial Area, Phase-1 New Delhi 110020

© EFY Enterprises Pvt Ltd. First Published in this Edition, December 2006

All rights reserved. No part of this book may be reproduced in any form without the written permission of the publishers. ISBN 81-88152-17-X

Published by Ramesh Chopra for EFY Enterprises Pvt Ltd, D-87/1, Okhla Industrial Area, Phase-1, New Delhi 110020. Typeset at EFY Enterprises Pvt Ltd and Printed at Nutech Photolithographers, B-38, Okhla Industrial Area, Phase-1, New Delhi 110020

FOREWORD This volume of Electronics Projects is the twenty second in the series published by EFY Enterprises Pvt Ltd. It is a compilation of 21 construction projects and 66 circuit ideas published in ‘Electronics For You’ magazine during 2001. We are also including a CD with this volume, which not only contains the datasheets of major components used in construction projects but also the software source code and related files pertaining to various projects. This will enable a reader to copy these files directly to his PC and compile/run the program as necessary, without having to prepare them again using the keyboard. In addition, the CD carries useful software, tutorials and other goodies (refer ‘contents’ in CD). In keeping with the past trend, all relevant modifications, corrections and additions sent by the readers and authors have been incorporated in the articles. Queries from readers along with the replies from authors/ EFY have also been published towards the end of relevant articles. It is a sincere endeavour on our part to make each project as error-free and comprehensive as possible. However, EFY cannot resume any responsibility if readers are unable to make a circuit successfully, for whatever reason. This collection of a large number of tested circuit ideas and construction projects in a handy volume would provide all classes of electronics enthusiasts—be they students, teachers, hobbyists or professionals—with a valuable source of electronic circuits, which can be fabricated using readily-available and reasonably-priced components. These circuits could either be used independently or in combination with other circuits, described in this and other volumes. We are sure that this volume, like its predecessors, will generate tremendous interest among its readers.

CONTENTS

Section A: Construction Projects



1.

Build Your Own Pentium III PC.................................................................................. 3



2.

Automatic Room Light Controller............................................................................... 17



3.

Intelligent Water Level Controller............................................................................... 21



4.

A Unique Liquid Level Indicator................................................................................. 25



5.

Interface Your Printer with 8085 Microprocessor....................................................... 28



6.

Morse Processor........................................................................................................... 33



7.

Access-Control System................................................................................................ 42



8.

Telephone Line-Interfaced Generic Switching System............................................... 46



9.

Programmable Melody Generator............................................................................... 55



10.

Auto Control for 3-Phase Motors................................................................................ 66



11.

Telephone Remote Control.......................................................................................... 72



12.

Microcontroller-Based School Timer.......................................................................... 75



13.

Digital Capacitance-cum-Frequency Meter................................................................. 80



14.

Fluid-Level Controller with Indicator......................................................................... 84



15.

MGMA—A Mighty Gadget with Multiple Applications............................................ 87



16.

Traffic and Street Light Controller.............................................................................. 91



17.

Lead-Acid Battery Charger with Active Power Control.............................................. 98



18.

Amplitude Measurement of Sub-Microsecond Pulses................................................ 101



19.

Automatic Submersible Pump Controller.................................................................... 104



20.

Transistor Curve Tracer............................................................................................... 107



21.

Tripping Sequence Recorder-cum-Indicator................................................................ 113



Section B: Circuit Ideas



1.

Electronic Starter for Single-Phase Motors................................................................. 119



2.

Modem ‘On/Off’ Indicator.......................................................................................... 120



3.

Touch-Select Audio Source......................................................................................... 121



4.

Precision Attenuator with Digital Control................................................................... 121



5.

Precision Amplifier with Digital Control..................................................................... 122



6.

Random Number Generator Based Game................................................................... 123



7.

9-Line Telephone Sharer.............................................................................................. 124



8.

Electronic Card Lock System...................................................................................... 126



9.

Pulsed Operation of a CW Laser Diode....................................................................... 127



10.

Generation of 1-Sec. Pulses Spaced 5-Sec. Apart....................................................... 128



11.

High-/Low-Voltage Cutout with Timer........................................................................ 129



12.

Automatic Heat Detector............................................................................................. 130



13.

Musical ‘Touch’ Bell.................................................................................................... 131



14.

Non-Contact Liquid-Level Controller......................................................................... 131



15.

High-Power Bicycle Horn........................................................................................... 133



16.

AC Mains Phase-Sequence Indicator.......................................................................... 133



17.

Luxurious Toilet/Bathroom Facility............................................................................ 135



18.

EEPROM W27C512 (Winbond) Eraser...................................................................... 136



19.

Intelligent Electronic Lock.......................................................................................... 137



20.

Stable 455KHz BFO for SSB Reception..................................................................... 139



21.

Auto Shut-off for Cassette Players and Amplifiers...................................................... 139



22.

House Security System................................................................................................ 141



23.

Simple Water-Level Indicator-cum-Alarm.................................................................. 142



24.

Precision Inductance and Capacitance Meter.............................................................. 142



25.

Under-/Over-Voltage Beep for Manual Stabiliser....................................................... 144



26.

Ultra-Sensitive Solidstate Clap Switch........................................................................ 145



27.

15-Step Digital Power Supply..................................................................................... 145



28.

Microphone for Computer........................................................................................... 147



29.

Versatile Zener Diode Tester........................................................................................ 147



30.

DTMF Proximity Detector.......................................................................................... 149



31.

Stepper Motor Control................................................................................................. 149



32.

Low-Cost Intercom...................................................................................................... 150



33.

High-Power Car Battery Eliminator............................................................................ 151



34.

Automatic Plant Irrigator............................................................................................. 152



35.

Simple Telephone Ring Tone Generator...................................................................... 152



36.

Dual-Input High-Fidelity Audio Mixer....................................................................... 153



37.

Unipolar/Bipolar Triangular and Bipolar Square Wave Generator.............................. 154



38.

Anti-Theft Security for Car Audios............................................................................. 155



39.

PC-Based Dial Clock-cum-Electronic Roulette.......................................................... 156



40.

Long-Range Cordless Burglar Alarm.......................................................................... 157



41.

Water-Level Controller................................................................................................ 158



42.

Invisible Broken Wire Detector................................................................................... 160



43.

PC-Based Multi-Mode Light Chaser........................................................................... 161



44.

Fuse Status Indicators for Power-Supplies.................................................................. 163



45.

A Hierarchical Priority Encoder.................................................................................. 164



46.

Digital Mains Voltage Indicator................................................................................... 165



47.

Electronic Dice............................................................................................................ 166



48.

Light-Operated Organ.................................................................................................. 168



49.

Stereo Tape Head Preamplifier for PC Sound Card..................................................... 168



50.

Heart Beat Monitor...................................................................................................... 169



51.

Digital Fan Regulator.................................................................................................. 170



52.

Running Lights and Running Holes............................................................................ 171



53.

A Simple Transistor Tester........................................................................................... 172



54.

12V, 3A Power Supply................................................................................................. 172



55.

Speller Effect Sign Display.......................................................................................... 173



56.

Darkroom Timer.......................................................................................................... 174



57.

Active Shortwave Antenna.......................................................................................... 174



58.

Long-Range Target Shooter......................................................................................... 175



59.

Power Supply for Walkie-Talkies................................................................................ 176



60.

High-Performance Interruption Detector..................................................................... 177



61.

Digital Relay Tester for RAX and MAX..................................................................... 178



62.

Fastest Finger First Indicator....................................................................................... 179



63.

Decorative Signboard.................................................................................................. 180



64.

Condenser Mic Audio Amplifier.................................................................................. 181



65.

Smoke Alarm............................................................................................................... 182



66.

Overload Protector with Reset Button......................................................................... 183

SECTION A: CONSTRUCTION PROJECTS Page = 1

Build Your Own Pentium III PC K.c. Bhasin and neeraj Kundra

T

he procedure presented here would enable you to assemble your own multimedia personal computer. It is assumed that you have a fundamental knowledge of how a PC functions and some basics of electronics. By way of tools you only need Philips-head and flat-blade screwdrivers. A simple multimeter is the only test equipment that you would ever require during assembly, for AC and DC voltage measurement. All the parts needed to assemble this multimedia PC with processor speed of 700 MHz are listed under Parts List. The cost of parts may vary from dealer to dealer and also with time. It is suggested to source these items from authorised dealers who would meet their warranty obligations. We have also mentioned the brand names of the parts that we used during assembly of the basic unit. It is, however, not necessary to use identical makes, except, of course, the main processor and the motherboard, based on identical chipset mentioned later in this article.

there must be some slack after these are installed and connected.) This will improve the cooling and reduce the chances of electromagnetic interfer-

ence between them. • The motherboard contains sensitive components, which can be easily damaged by static electricity. Therefore the motherboard should remain in its original antistatic envelope until it is required for installation. When it is taken out from the envelope, it should be immediately placed on a suitable grounded conductive surface. The motherboard itself should be held from edges and the person taking it

Precautions Before starting the actual assembly of the PC system, the following precautions would help you to avoid any mishap during the assembly process: • While the motherboard has to be fitted at a fixed place inside the PC cabinet, the locations of add-on cards (as and when used) and the drives (hard disk drive, floppy disk drive, and CD-ROM drive) within the drives’ bay of the cabinet can be changed within certain limits. But it is better to place them far away from each other. (Of course, the length of the cable provided for interconnections to the motherboard or add-on cards has to be taken into account, as

Fig. 1: Block diagram of motherboard employing 810E chipset ELECTRONICS PROJECTS Vol. 22

3

Key Features of Motherboard Using Intel 810/810E Chipset

Processor • Full support for the Intel Pentium III and Celeron processors using PGA370 socket. • Supports 66MHz and 100MHz bus speed including all PGA370. • Supports 133MHz bus speed (810E chipset version only). VRM 8.2 (Voltage Regulator Modules) On-board • Flexible motherboard design with on-board VRM 8.2, easy to upgrade with future processors. System Memory • A total of two 168-pin DIMM sockets (3.3V SDRAM types). • Memory size up to 512MB. • Supports SDRAM at 66/100 (PC100) MHz. • Supports symmetrical and asymmetrical DRAM addressing. • Banks of different DRAM types and depths can be mixed. System BIOS • 4-Mbit Intel Firmware hub (with security feature). • PnP, APM, ATAPI, and Windows 95/98. • Full support of ACPI & DMI. • Auto-detects and supports LBA hard disks with capacities over 8.4 GB. • Easily upgradable by end-user. On-board I/O • Supports two PCI-enhanced IDEs PIO mode 3, mode 4, and ultra DMA 33/66 channels (optional ultra DMA 66 cable). Twin headers for four IDE devices including IDE HDDs and CDROMs. • One ECP/EPP parallel port (via a header). • Two 16550A UART parallel port (via a header). • One floppy port. Supports two FDDs of 360KB, 720KB, 1.2MB, 1.44MB, or 2.88MB (via a header). • Four USB ports (via a header, optional). • PS/2 mouse port (via a header, optional). • AT keyboard port (factory option for PS/2 type). • Infrared (IrDA) support. Plug-and-play • Supports plug-and-play specification 1.1. • Plug-and-play for DOS, Windows 3.X, Windows 95, as well as Windows 98. • Fully steerable PCI interrupts. On-board VGA • Hardware motion compensation for S/W MPEG2 decode (DVD). • 3-D hyper pipelined architecture. • Full 2-D hardware acceleration. • 3-D graphics visual enhancements. • Dynamic display memory (DDM) or optional 4MB display cache (810DC100 or 810E chipset version only). • Resolution up to 1,600x1,200. • Win 95 vxd, Win 98/NT5 mini-port drivers support. • VGA port (via a header). On-board AC97 Sound • Integrated AC97 controller with standard AC97 CODEC. • Direct Sound and Sound Blaster compatible. • Full-duplex 16-bit record and playback. • PnP and APM 1.2 support. • Win 95, 98, and NT drivers ready. • Line-in, line-out, mic-in and MIDI/game port. Power Management • Supports SMM, APM and ACPI. • Break switch for instant suspend/resume on system operations. • Energy star ‘Green PC’-compliant. • WAKE-ON-LAN (WOL) header support. • External modem ring-in wake-up support. Expansion Slots • One audio modem riser (AMR). • Four PCI bus master slots (ver 2.1 compliant).

out should wear an antistatic wrist strap that is properly grounded. In the absence of a proper wrist strap, you may make one on your own using a peeled off multi-strand copper cable and ground it properly. Similar handling precautions are also required

4

ELECTRONICS PROJECTS Vol. 22

for DIMMS and cards. • If you are using a motherboard different from the one mentioned in the parts list, modify the guidelines mentioned here as per the directions given in the user’s manual (which is supplied with the motherboard you may be using), since there

would be some differences between any two makes of the motherboard. • Start the assembly only after going through this article at least once. Only when you feel at ease, start the assembly of your machine as per the guidelines included in this article and the applicable user’s manuals. • Never try to insert a card in PC slots or try to plug/unplug a connector with power supply to the PC ‘on’. • Ensure that the mains 3-pin socket or the socket on your stabiliser/UPS that you would be using for connection to the SMPS of the computer and/or the monitor is correctly wired with ‘live’ line on your right hand side. To find out which line is live (phase) and which one is neutral, use your multimeter in 250V AC or higher range. The live line will show full voltage w.r.t. neutral pin and nearly the same voltage w.r.t. the ground pin, while the neutral pin (w.r.t. ground pin) would/ should show very little voltage (less than 10V AC). Else, the mains wiring has a problem that needs to be set right. • Don’t drop any screw or other conducting material on your PC’s motherboard as that might cause shorting of pins/tracks and consequent damage when you switch it ‘on’. • Make sure that you have a large, flat surface area to work on. That will reduce the chances of small screws etc falling and getting lost. • While screwing components on to the chassis, do not use excessive force as that may damage the screws or their grooves/holes.

Pentium III technology Some points to be noted about the Pentium III processor being used here are: • Intel’s Pentium III processors support various clock speeds from 450MHz to 933 MHz. The one meant for desktop version goes up to 1.13 GHz. (We are using here a 700MHz version.) • Integrates P6 dynamic execution architecture and a dual independent bus (DIB) architecture. • Has a multi transaction system bus. • Incorporates Intel’s MMX media enhancement technology. • Supports Internet streaming single-instruction multiple data (SIMD) extensions. • Compared to Pentium II, it has 70 new instructions, enabling advanced 3-D

imaging, streaming audio and video, and speech recognition. • Has a 32k (16k for instructions and another 16k for data) as primary (level 1) non-blocking cache for rapid access to most heavily used data. In addition, it has 512k unified, non-blocking (level 2) cache or 256k advanced transfer cache integrated on die, which runs at the core frequency of the processor with very low memory access time.

The motherboard

Fig. 2: PC Partner motherboard layout diagram

Table I JP1, JP2—System Bus Frequency

JP1

JP2

CPU Clock Speed

1

Open

1

Open

133MHz (100MHz CPU run at 133MHz Front Side Bus)

1

Open

1

1-2

100MHz (66MHz CPU run at 100MHz Front Side Bus)

1

Close*

1

1-2*

Auto*

JP15 - BIOS (Firm Ware Hub)

Boot Block Protect

JP4 - CMOS Clear

JP15

Function

JP4

1

Close*

Unlocked*

1

1-2*

Normal

1

Open

Locked

1

2-3

CMOS Clear

JP34 - On Board Crystal PCI Sound (Optional)

JP34

Function

1

1-2* PCI Sound Enable*

1

2-3

PCI Sound Disable

Function

JP29 - Keyboard Power On Select

JP29

1-2* Powered by +5V*

1

2-3

JP35, JP36 - On Board AC97 Codec Sound

JP35

JP36

Function

1

1-2* 1

2-3* (S)# AC97 Sound Enable*

1

2-3

1-2 (P)#

1

Function

1

AC97 Sound Disable

Powered by +5V Standby (Allows Keyboard Power On) * Default settings # P= Primary AMR, S = Secondary AMR

While the processor is the most important part of the motherboard, the motherboard itself is the most important part of the computer system. Together with the chipset, it forms the brain of your computer. The modern motherboards do away with the large number of controller chips and cards that were used in the older XT and AT versions, such as clock generator, bus controller, timer/counter, monitor/ printer adopter, FDD and HDD controllers, multi-I/O or super IDE controller card, and DMA controller. All the functions performed by these controllers/cards (and others) are now performed by just two or three chips and that too at much higher speed. The motherboard based on Intel’s 810/810E chipset (being used in the present system) combines the advantage of a multimedia (full-screen, fullmotion video with realistic graphics) and enhanced Internet performance at a budget price. With this motherboard, one does not need separate sound, video, or graphics enhancement cards. A block diagram of a motherboard employing 810E chipset is shown in Fig. 1. Key features. The main features of the PC Partner motherboard used in this project are shown in the accompanying box. A layout diagram showing the relative position of the jumpers, connectors, major components, PCI slots, and DIMM and CPU sockets is shown in Fig. 2. Jumper settings. Positions of various jumpers within the motherboard are shown in Fig. 3. The jumper settings for enabling various functions are shown in Table I. Default settings are shown with an asterisk mark. (Note. Leave all these jumpers in their default setting positions for the present project. The processor speed setting is to be done through CMOS setup as indicated later.) ELECTRONICS PROJECTS Vol. 22

5

Fig. 3: Jumper positions within motherboard

manual with 3-year limited warranty. Similarly, ensure that the 64MB SDRAM DIMM bears the label (such as PC100) to indicate that it is compatible with 100MHz system bus speed. Checking cabinet and its accessories. The AT mini tower PC cabinet measures approx.180mm (width) x 330mm (height) x 360mm (depth). The drive bays comprise two 133.35mm (5.25-inch) exposed, one 89mm (3.5-inch)

exposed, and two 89mm (3.5-inch) internal bays. It has 200W SMPS of VESTA make pre-installed (+5V @16A, +12V @6A, -5V @0.5A, and –12V @0.5A). LEDs with 2-pin SIP connectors are provided for power ‘on’ (green and white twisted wires), HDD (orange and white twisted wires) activity indication, and to reset push switch (blue and white twisted wires), which are required to be connected to the appropriate pin pairs (Berg type) on the motherboard. (Please refer Fig. 2 to spot the corresponding connectors near JP34/JP4, but for the

Fig. 4: Power on/off switch wiring

Hardware installation and checkout Verifying components. First, carry out a physical check of all the items as per the parts list to ensure that there are no apparent deficiencies and no signs of any physical damage, and the parts are correct as indicated by the labels on the items/packages. For example, the Pentium processor pack should comprise Pentium III processor labeled 700MHz/100MHz system bus, fan/heat-sink assembly, and installation

Fig. 6: DIMM installation

A

(a)

(b)

A

(c)

(d)

Fig. 5: Installation of Pentium III processor in PGA 370 socket

6

ELECTRONICS PROJECTS Vol. 22

time being, leave them alone.) An 8-ohm, 0.5W speaker (with black and red twisted wires and 4-pin connector), to go into corresponding 4-pin speaker connector on motherboard, also forms part of the cabinet. Checking SMPS. The control console on the cabinet also has a DPDT push-button switch to switch on the mains (230V AC) to SMPS of the computer and a parallel-wired 3-pin AC socket on SMPS for connecting AC power to the monitor used with the PC. At this stage, slide the shielded connectors of the four power supply wires of the SMPS into the corresponding connectors on the DPDT switch as per the diagram provided on the SMPS case (top side). The same is reproduced in Fig. 4. The white and black wires have a return path via blue and brown wires, respectively, when the power supply switch is flipped ‘on’. Connect the 3-pin power cord provided with the cabinet to the socket at the back of SMPS and plug 3-pin plug into the

female power connectors with Item Description Make projection in the AT cabinet with SMPS, power cord, middle. If these power switch, reset switch, speaker, are held such LEDs, complete with connectors and installation hardware packet. IMIL, Chen- that all black nai wires are adjaMotherboard with Intel’s 810 cent to each othchipset PC Partner, USA along with er, this forms a user’s manual, CD (containing 12-pin AT power drivers for onboard devices) and supply connecheaders for motherboard connectors. * (refer check-list) PC Partner tor with orange Pentium PIII-700 Processor Intel wire (carrying 64MB (PC 100)SDRAM (168-pin DIMM) Alpha power good sigHDD (hard disk drive) Seagate nal) emanating FDD (floppy disk drive) 3.5” Sony from pin 1. CD-ROM drive 52X with audio cable Samsung Keyboard Logitech The voltMouse(3-button) Logitech ages on various Colour Monitor 14” LG pins of this joint USB connector bracket with 2 headers 12-pin connector *list of connectors/brackets forming part of motherboard. with their colour Header (connectors with cables) for HDD (40-pin twin) - one codes are shown Header for FDD (34-pin twin) - one in Table II. Header for PS/2 mouse - one Port bracket set with headers for: Check the cor(a) VGA (15-pin ‘D’ connector ending into 16-pin FRC and rectness of these parallel port (25-pin ‘D’ ending into 26-pin FRC) - one voltages within (b) Com1 and Com2 (two 9-pin ‘D’ ending into 10-pin FRC) - two the range as (c) Onboard AC97 sound codec (line-in, line-out, mic-in and given in Table MIDI/game port ending into 26-pin FRC) - one II. Then switch off the power socket of the mains supply or the UPS, supply and take out the 3-pin plug as appropriate. from the mains socket. If the AT power Switch on the SMPS. The fan blower connector voltages are correct, you inside the SMPS should start running, can safely assume that voltages in all indicating availability of +12V supply to other power connectors [4-pin Molex, the fan. Now verify all DC outputs of the carrying +12V (yellow wire) followed SMPS as follows. by two black wires (ground) and +5V There are two distinct 6-pin Molex

Table IV

Parts List

Table II At Power Connector Pin Voltages Pin Voltage Range Wire Pin Voltage Range Colour 1 *P. G. 4.5V (min) Orange 7 Ground - 2 +5V +5%/-4% Red 8 Ground - 3 +12V +5%/-4% Yellow 9 -5V +10%/-8% 4 -12V +10%/-9% Blue 10 +5V +5%/-4% 5 Ground - Black 11 +5V +5%/-4% 6 Ground - Black 12 +5V +5%/-4% *P. G. = Power good signal which is +5V (delayed, 100ms – 500ms).

Table III VGA–VGA Out Connector CN34* Pin Signal Name Pin Signal Name 1 Red signal 9 NC 2 Green signal 10 GND 3 Blue signal 11 NC 4 NC 12 Display data channel data 5 GND 13 Horizontal sync 6 GND 14 Vertical sync 7 GND 15 Display data channel clock 8 GND *This connector is for the VGA display port. Connect a VGA or higher resolution display monitor to it.

Wire Colour Black Black White Red Red Red

Parallel-Port Connector CN6 Pin Signal Name

Pin

Signal Name

1 2 3 4 5 6 7 8 9 10 11 12 13

14 15 16 17 18 19 20 21 22 23 24 25 26

AFD Error INIT SLCTIN GND GND GND GND GND GND GND GND GND

Strobe- Data bit 0 Data bit 1 Data bit 2 Data bit 3 Data bit 4 Data bit 5 Data bit 6 Data bit 7 ACK Busy PE SLCT

Table V

COM1/COM2–Serial Connectors CN4*, CN5* Pin Signal Name Pin Signal Name 1 DCD 6 DSR 2 SIN 7 RTS 3 SOUT 8 CTS 4 DTR 9 RI 5 GND 10 NC *These connectors are for the serial port bracket. Both connectors have the same pinouts.

(red wire)] meant for various drives are also correct. Motherboard fitment. The chassis on which motherboard is to be mounted can be easily removed from the PC cabinet. Unscrew it and gently slide it out from the main casing. Lay it flatly on the antistatic workbench (properly grounded conductive surface). Mark the side facing the keyboard connecter cutout on the chassis. All motherboards have standard mounting holes. The hardware supplied comprises plastic and metallic motherboard retaining fasteners/screw-holders. Metal-type screw-holders are better as these have better strength and also these ground the motherboard to the chassis. You may use four metallic screw-holders for the four corner holes in the motherboard, while the plastic fasteners may be used for the middle holes of the motherboard. Before attempting fitment of the motherboard, align it on the chassis such that the keyboard connector on the motherboard is towards the side marked earlier for this purpose. Now ELECTRONICS PROJECTS Vol. 22

7

Table VI

Table IX

Audio & Game Port Pin Header CN341*

Floppy Connector Pin Definitions (JP26) Pin Function Pin Function

Pin

Signal Name

Pin

Signal Name

Pin

Signal Name

Pin

Signal Name

1 2 3 4 5 6 7

VCC VCC SWC SWA XTC XTA MSOUT

8 9 10 11 12 13 14

GND XTD GND SWB XTB MSIN SWD

15 16 17 18 19 20 21

NC VCC Line-out Line-out GND GND MIC-in

22 23 24 25 26

MIC-in NC GND Line-in Line-in

*This header is for the audio port bracket. It connects audio ports-stereo line-out, stereo line-in and microphone—and a game port (for a joystick or MIDI device) to your system.

fit all the screwholders/fasteners, CN7: USB Port as discussed above, Pin Assignment on the chassis, oppo1 VCC site the holes on the 2 GND motherboard, using 3 USBP1Philips screws pro4 USBP0+ vided in the hard5 USBP1+ 6 USBP0ware packet. Align 7 GND the motherboard 8 VCC above the fasteners and push it down, so that the self-retaining heads of plastic fasteners pop out from the respective holes. For the metallic screw-holders, use Philips screws to secure the motherboard to the chassis firmly without using excessive force. Pentium processor mounting (refer Fig. 5). The processor is to be fitted into the PGA370 (pin grid array with 370-pin recesses) socket, which is a ZIF (zero insertion force) socket. Take out the processor and its heat sink fitted with cooling fan and heat sink retainer clip ‘D’. Now proceed as follows: 1. Lift handle ‘A’ to its vertical position [refer Fig. 5(a)]. 2. Align the processor pins with the socket holes and insert the processor into its socket [refer Fig. 5(b)]. 3. With the processor in its socket, lower handle ‘A’ and bring it to its closed (horizontal) position [refer Fig. 5(c)]. 4. Orient the heat sink (with fan on top) such that the depression on one side of the heat sink matches the corresponding projection on PGA370 socket, and place it (along with fan) over the processor [refer Fig. 5(c)]. 5. On the PGA370 socket, there are two small projections on opposite sides, in which the heat sink clip has to be inserted. While it is fairly easy to insert one side, it is rather tricky to insert the left-out side as it needs to be pulled down with considerable force to engage it into the projection. You may use the flat

Table VIII

Table VII

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ELECTRONICS PROJECTS Vol. 22

IDE Connector Pin Definitions (J18, J19) Pin Function Pin Function 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

Reset IDE Host data 7 Host data 6 Host data 5 Host data 4 Host data 3 Host data 2 Host data 1 Host data 0 GND DRQ3 I/O Write- I/O Read- IOCHRDY DACK3- IRQ14 Addr 1 Addr 0 Chip select 0 Activity

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

GND Host data 8 Host data 9 Host data 10 Host data 11 Host data 12 Host data 13 Host data 14 Host data 15 Key GND GND GND BALE GND IOCS16GND Addr 2 Chip select 1GND

screwdriver tip to do this, but be careful that screwdriver does not slip and damage the tracks on the motherboard [refer Fig. 5(d)]. 6. Connect the 3-pin fan connector to the corresponding connector CN17 marked ‘CPU Fan’ on the motherboard. DIMM installation (Fig. 6). There are two 168-pin SDRAM DIMM sockets on the motherboard with socket 1 marked ‘1’ and socket 2 left unmarked. The two sockets can together accept 512MB SDRAM (i.e. up to 256 MB each). We propose to install a single 64MB DIMM, which is quite adequate for current type of applications. It can be inserted into any of the two sockets and the same will be automatically suitably configured during setup. Remove the DIMM from its antistatic envelope, holding it by its edges. Proceed as follows: 1. Using fingertips, push the retainer clips on either side of the DIMM socket slightly away from the socket. 2. Position the DIMM to be installed above the socket, aligning the two small

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

GND GND Key GND GND GND GND GND GND GND GND GND GND GND GND GND GND

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

FDHDIN Reserved FDEDIN IndexMorot enable Drive select BDrive select AMotor enable DIRSTEPWrite dataWrite gateTrack 00Write protectRead dataSide 1 selectDiskette

Table X PS/2 Mouse Connector* Pin

Description

Pin

Description

1 Mouse data 2 NC 3 Ground 4 +5V 5 Mouse clock 6 NC *This connector is for the optional PS/2 mouse port bracket.

notches at the bottom edge of DIMM with the corresponding keys in the socket. 3. Push the DIMM vertically down, inserting its bottom edge into the socket. 4. Once seated properly, push DIMM down from the top edge until the retainer clips snap into place and the DIMM is firmly held into its position. Cable set installation. While the motherboard chassis is still not replaced into the case, you could install one of the ends of all the cables originating from the motherboard. The installation of cables, which originated from SMPS and the control panel of the case (LEDs, reset switch, and the speaker), would be completed after the motherboard chassis is screwed back into the PC case. The cables to be connected to the FRC-type male connectors/headers on the motherboard are listed below, and the pin assignments are shown in the referred tables. On the motherboard, normally, only start pin 1 is indicated. In an FRC connector, all odd number pins are in one row while even number pins are in the opposite row; pin 2 is opposite pin 1, pin 4 is opposite pin 3, and so on. Pin 1 on the mating FRC female connector can be identified by an arrow mark over it. Ribbon cable wire going into pin 1 is of red (sometimes blue) colour. Some of the FRC connector pairs have a notch

to floppy drive (DS1 in Fig.7). Let us configure the HDD as primary master and CD-ROM drive as primary slave using a single cable emanating from CN1 (IDE-1 header) on the motherboard (refer Fig. 8). Fig. 7: Floppy drive cable for connecting up to two FDDs (We could alternatively configure CDROM drive as secondary master and connect it directly to CN2 (IDE-2 connector) in motherboard, using another 40-pin cable/connector.) The jumper on HDD should be used to short pins 7 and 8 on the jumper block at the rear of HDD (refer Fig. 9). Similarly, there is a jumper block at the rear of CD-ROM drive with the pairs of pins marked as CS (cable select), SL (slave), and MA (master). Ensure that jumper is used in the middle to select the slave mode for CD-ROM. The cable connection arrangement for HDD and CD-ROM is Fig. 8: Connection of HDD and CD-ROM drive shown in Fig. 8. using IDE-1 header Before installation of drives, note down pin-1 orientation/position of the and the corresponding projection, which 34-/40-pin interface cable connectors on serves as a key so that they can go only the drives. the correct way. The cables used for the The CD-ROM drive may be installed drives have an additional connector in in the topmost position for 13.33cm the middle (for slave in case of HDD and (5.25-inch) drive, after pushing out the drive B in case of FDD, which will be explastic piece (used for protection) coverplained later). Using the tips given here, ing the cutout in this drive’s bay. Align you can install the motherboard end of it from the front side of the case to enthe following cables: sure that it is flush with the cabinet’s • 16-pin VGA connector CN34 (refer external surface. Using four Philips Table III). screws (6-32 UNC) secure it in proper • 26-pin parallel-port connector CN6 horizontal position. The screws should (refer Table IV). not be allowed to go more than 3.5 mm • 10-pin serial/com ports 1 and 2, CN4 into the threaded holes. and CN5 (refer Table V). Suitable cutout also exists in the drive • 26-pin sound cable connector CN31 bay for installing the 8.9cm (3.5-inch) (refer Table VI). floppy drive. Before fitting, ensure that • 8-pin USB connector CN7 (refer drive door in the front opens downward Table VII). (hinged towards top). For installing floppy • 40-pin IDE-1 connector for HDD/CDdrive follow the same procedure as used ROM drive CN1 (refer Table VIII). for fixing CD-ROM drive. • 34-pin FDD connector CN3 (refer Table IX). • 6-pin PS/2 mouse connector CN8 (refer Table X). • Installation of drives in drive’s bay. Before proceeding with the physical installation of CD-ROM drive, hard disk drive, and floppy drive in the drive’s bay, you have to plan their configuration. We propose to use only one floppy drive. This drive will be configured as floppy drive ‘A’. The 34-pin floppy drive cable end with twisted wires, emanating from CN3 on the motherboard, needs to be connected

The HDD can now be installed at the lowest closed (without any cutout in front) position in the drive bay. Secure it like the other drives using four Philips screws. • Completing the hardware installation. After having completed the installation of drives and the cable set of the motherboard, install back the assembled motherboard chassis (complete with its cable/connector set) into the PC cabinet and then complete the cabling as follows. You may start with AT power supply connectors. By now you are familiar with two 6-pin Molex connectors from SMPS used for powering the motherboard (refer paragraph under heading ‘Checking SMPS’ in Part I). Take connecter with orange wire (PG signal) first and align it over pin 1 of PW1 connector on motherboard. Projections on Molex connector of SMPS would engage into corresponding holes in PW1 connector. Once you have engaged the connector in this fashion, make it vertical and then simply slide it down. It will snap into its position. (Be careful not to bend the pins and ensure that you have not engaged the wrong pins.) Similarly, insert the other 6-pin Molex connector in the adjacent pins of AT power connector. On installation, all black coloured wires will be adjacent to each other. Some of the connectors originating from the motherboard (e.g. COM1, COM2, and VGA connectors) can be secured into the cutouts provided on the case below the SMPS. Thus secure the ‘D’ connectors for COM1, COM2, and VGA into the respective cutouts using Philips screws. This saves the precious space inside the PC case and gives it an ethical look. For accommodating the panel/bracket for 25-pin ‘D’ connector of parallel port and PS/2 mouse as well as audio panel/bracket, remove two of the cutouts from the rear of the case by just forcing them out with hands, and secure these brackets in the vacant positions using Philips screws. Now you may terminate the connectors originating from control panel on the cabinet at the motherboard. Connect the loudspeaker connector to CN14, power-on LED connector to CN12, HDD LED connector to CN13, and reset switch connector to CN11. (Correct orientation can be ensured by matching the pin connected to coloured ELECTRONICS PROJECTS Vol. 22

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(not white) wire to go into pin 1 of the connectors in motherboard.) Now connect the 40-pin middle connector (in the ribbon cable) originating

Fig. 9: Back-panel connector details of HDD and CD-ROM drives

from CN1 in the motherboard to CD-ROM drive and its end connector to HDD, ensuring that pin 1 of connector pairs correctly match. (Projection/slot in the middle of connectors will help you in proper orientation of the connectors, unless you try to force it in with wrong orientation.) Follow it up by connecting the 34-pin floppy drive end-connector (at the end of twisted cable) to the interface connector of floppy drive. This header originates from CN3 on the motherboard. The 4-pin Molex-type power supply connectors now remain to be connected to the drives. Ensure that rounded shoulder on the female connectors mate correctly with the corresponding male power connectors on CD-ROM drive and HDD. In all cases you will observe that yellow wire (+12V) pin faces the PC case cover. For FDD, use the 4-pin mini power supply connector. This connector, if inserted properly, will lock itself into position. To take out this connector, you should press the retaining lever with your fingertip. Connect one of the 4-pin connectors—CN24 or CN33 or CN32—to analogue audio output connector on CD-ROM drive,

Screenshots CMOS setup menus



Table XI Pin Assignment Internal Audio Connector Internal Audio Connector CN25 : AUX-IN Pin Assignment 1 AUX-L 2 GND 3 GND 4 AUX-R CN24 : CD-IN Pin Assignment 1 CD-L 2 GND 3 GND 4 CD-R CN33 : CD-IN Pin Assignment 1 CD-R 2 GND 3 CD-L 4 GND CN32 : CD-IN Pin Assignment 1 GND 2 CD-L 3 GND 4 CD-R

after correctly matching the ground pin ‘G’ marked over the analogue audio connector on CD-ROM drive (refer Fig. 10) and those of CN24 or CN33 or CN32 as given in Table XI. If you have followed all the tips religiously, your hardware assembly is complete on closing the cover of the cabinet using four to six Philips screws. But before you do that, have a look again to ensure that no loose wires are hanging around. After closing the cover, you may connect the keyboard cable to the keyboard connector, mouse cable to COM1 connector, and amplified speakers’ banana-type stereo jack into the line-out plug on the audio bracket. Now that hardware assembly part of the basic unit is over, installation of other cards, such as LAN card (for networking), internal modem card (for Internet access), and TV tuner card, into the PCI slots, using the software drivers supplied with them, can be attempted subsequently.

Creating a startup disk



Continued

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ELECTRONICS PROJECTS Vol. 22

Eventually you will be using Windows operating system (say, Windows 98), and for that you should be having Microsoft Windows 98 installation CD. Use some other PC having Windows 98 operating system to create a ‘startup disk’. The idea is to have all important files, including system files, Fdisk.exe, and Format.com files, in hand, so that you may proceed

with hardware partitioning and formatting of hard disk once you switch on your newly assembled PC for the first time. To make a startup disk, get a new formatted 8.9cm (3.5-inch) floppy. On the working computer, click ‘start button’, select settings, double click on icon ‘add/ remove programs’, select ‘startup disk’, insert formatted floppy in floppy drive, and click over the ‘create disk’ button seen on monitor’s screen. The program would prompt you for insertion of original Windows 98 CD in CD-ROM drive. Insert the same and click on ‘OK’ button. Even if you do not have the original CD, but have all programs in Win98 directory in ‘C:’ drive, you can give the proper path and the appropriate programs will be copied to the startup floppy disk.





CMOS setup Switch on the newly assembled PC. It performs power-on-self-test (POST). During POST you will find ‘Num Lock’, ‘Caps Lock’, and ‘Scroll Lock’ LEDs flashing. A single short beep during POST indicates that motherboard is ‘OK’. Certain messages will keep appearing on the screen of your monitor, including “Press Del to enter CMOS setup”. When this message appears, press ‘Del’ key to enter setup. The CMOS Setup Utility screen appears on monitor screen (refer screenshot 1). There are seven items on the left, which can be selected using arrow keys on your keyboard. On the right, it shows certain options that are quite obvious and can be interactively executed when required. Select the first item on the left, “Standard CMOS Features”, and press enter to see its screen (refer screenshot 2). Use arrow keys to move between the items and ‘Page Up’ or ‘Page Down’ key to edit or select the options. You may correct the date, including year and century, and the time to their current values.



Continued

You would notice from screenshot 2 that during power up, the BIOS has identified the primary master (Seagate’s 10GB hard disk ST310211A), 52X Samsung’s CD-ROM Drive SC-15, floppy drives, video, and RAM address range (including its breakdown). This latest Award BIOS 1984-2000 does not contain ‘Auto Detect Hard Disk’ as a separate utility in the CMOS setup options. To select any other screen/setup utility option, press ‘Esc’, select the next item from setup utility menu, and press ‘Enter’. The next screenshot (screen shot 3) pertains to ‘Advanced BIOS Features’. Here you may edit and change the first,

second, and third boot devices to read CD-ROM, HDD-0, and floppy, respectively. This will enable you to boot/run the computer from CD-ROM (if you have a Windows installation), CD, HDD (after formatting and transferring the system files), or floppy drive (using the startup floppy created earlier), in that priority. Press ‘Esc’ to come back to the opening screen. For the time being, skip utilities/screens 4 through 7 with their default values. Select the last “Frequency/Voltage Control” menu item. Edit ‘CPU clock/spread spectrum’ item to read ‘100MHz/On’. Thereafter press ‘Esc’ and select ‘Save and Exit Setup’ or F10 key, and then ‘Y’ and ‘Enter’ for saving the edited BIOS selections. ELECTRONICS PROJECTS Vol. 22

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HDD partitioning and formatting

Assuming that you have Windows 98 installation CD in CD-ROM drive, the PC will boot from the CD and start the Windows 98 setup program. Press function key ‘F3’ to come out of the setup program and come to the prompt ‘D:\Win98>’. Type ‘Fdisk’ and press ‘Enter’ for starting with the partitioning of HDD. (Note. We could have used the ‘start up’ floppy in Drive ‘A’ instead of inserting Windows CD in CD-ROM drive and come to ‘A:\>’ prompt for running the ‘Fdisk’ program from ‘A’ drive, if desired.) On pressing ‘Enter’ key, the following FDISK main menu appears: Current fixed disk drive: 1 Choose one of the following: 1. Create DOS partition or logical DOS drive 2. Set active partition 3. Delete partition or logical DOS drive 4. Display partition information Enter choice: [ ] Press Esc to exit FDISK Enter choice 1 above and press ‘Enter’ key. The next menu on page 2 appears as follows: 1. Create primary DOS partition? 2. Create extended DOS partition? 3. Create logical DOS partition? Type ‘1’ and press ‘Enter’ key. The program verifies integrity of the disk and then displays. Do you wish to use max. size for a primary DOS partition and make it active. Y/N? Type ‘N’ and press ‘Enter’. (Because, we propose to create two DOS partitions of equal size.) Once again the program verifies integrity of the disk and prompts you to enter/specify partition in megabytes or percentage of disk space. Type 50% and press ‘Enter’. The program complies. Now press ‘Esc’ key to return to the main FDISK menu. Now enter choice 2. (The primary DOS partition created earlier becomes active.) The program will ask you to enter the number of partitions. As it is currently ‘1’ on ‘C’ drive, therefore type ‘1’ and press ‘Enter’. Again press ‘Esc’. (Do not press ‘Esc’ key more than once, else it will come out of FDISK.) Again you are led to main FDISK menu. Enter choice 1. You will come to menu on page 2. Now enter choice 2 to create extended DOS partition. The program

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ELECTRONICS PROJECTS Vol. 22







will again verify the integrity of the disk and show availability of 50% of the disk space for extended DOS partition. Type 50% for extended DOS partition and press ‘Enter’. Again press ‘Esc’ (only once). The program will ask you to specify the disk space for logical drive. Simply press ‘Enter’ and then press ‘Esc’ to come back to the main FDISK menu. Choose option 4 to display the information. After looking at the partition information that it has been correctly done, press ‘Esc’ to come out. Press keys CTRL+ALT+DEL or RESET button for settings to take effect. The PC will boot from CD-ROM drive as per settings done in the CMOS setup. On booting you will again come to the setup part of Windows 98 program. Hence to come out of it, press

F3. Now your drives are designated as under: C: First partition on hard disk D: Extended partition on hard disk E: CD-ROM drive

Now you will be able to access CDROM drive by typing ‘E:’. After the prompt ‘E:\>’, type ‘Format C:/S/U/V’ and press ‘Enter’. (Here ‘C:’ refers to drive to be formatted, ‘S’ to system (transfer of system files to ‘C’ drive during formatting), ‘U’ to unconditional, and ‘V’ to verification.) After formatting ‘C’ drive, you will come back to the prompt ‘E:\Win98>’. Type ‘setup’ and press ‘Enter’ to install Windows 98 on ‘C’ drive. As the program is interactive, keep answering the questions logically. Choose ‘typical’ while selecting the Windows ver-

sion. Various messages like ‘enter computer name’, workgroup, etc keep appearing, which you may reply suitably. Against ‘date/time zone’ selection, choose India. Computer will show the Agreement format that you are bound to accept. Hence click on the appropriate button. Before proceeding with the Windows installation, the program prompts you for entering the key number of Windows 98 product, which accompanies each original copy. You must type the key number accurately. It will then copy the Windows 98 files to ‘C’ drive in Win98 directory. This will obviate use of Windows CD for creating a startup file, whenever required. To format drive ‘D’, double click on My Computer icon, click the right button on drive D:, choose ‘Format’, and in ‘Format D:’ menu box, choose full and click on ‘Start’ button. After completion of the formatting of ‘D’ drive, it is accessible for read/write operations. This completes partitioning and formatting of the hard disk.

Loading motherboard drivers • On-board VGA display driver. When the PC is running, insert the motherboard driver CD that came with the motherboard (PCPartner driver’s CD, in our case) into CD-ROM drive. Select drive ‘E’, select ‘Intel Chipset Products’, 810, VGA , Win9X, and Graphics, in that order, and double click on its ‘Setup.exe’ icon and follow the instructions on screen. After finishing, shut down the PC as per Windows shutdown procedure and restart to allow the drivers to take effect. • On-board AC97 Codec sound driver. Click on ‘Start’ button, select settings, select control panel, double click on ‘System’ icon, click on ‘Device Manager’, go to ‘Other Devices’, double click on ‘PCI multimedia’, select ‘PCI Audio’, click on ‘Remove button’ (since compatible software drivers have not yet been installed to avoid conflicts), and then click on ‘refresh’ button. Go back to control panel and, click on ‘Add new H/W’. A wizard guides you through rest of the process, and in due course, a message “Found new hardware – PCI multimedia audio, display, sound video” appears. The program asks if you have disk (drivers). Click the ‘Browse’ button, select E:, ‘Intel Chipset Products’, 810 , AC97 Sound, CS4299, Win98, in that

order, and run ‘Setup’. During the setup, when the program prompts you for selection of device, choose ‘Crystal Audio Codec’ and click ‘OK’. Again during the course of driver installation for Crystal Audio Codec, the program will prompt you for location of Windows 98 files, which you may browse and point towards C:\Win98 directory or towards Windows CD as E:\Win98 and click ‘OK’ button. After finishing, you may verify, via ‘Device Manager’ (refer preceding para) by clicking on ‘Sound, Video and Game controller’ icon, that ‘Crystal Audio Codec’ as also ‘Crystal Audio Codec with Game Device’ appear under it. (A sound icon will concurrently appear on the bottom line of your desktop.) • Intel Firmware Hub configuration. In ‘Device Manager’ under ‘Other Devices’, an ‘Unknown Device’ would still appear. This concerns ‘Intel’s Firmware Hub’. To correct this problem, again go to 810 subdirectory on the CD, double click on ‘INF_install’, and then on ‘Setup.exe’ within that subdirectory. A message “Found New Device – Intel Firmware Hub” appears on the screen. This device will be automatically configured when you follow the instructions appearing on the screen properly. To confirm that there are no unknown devices now, open ‘Device Manager’ and check all the items under ‘Other Devices’. With installation of drivers for onboard devices, hardware and software configuration of your multimedia PC is complete. Other secondary functions such as power management functions— APM (advanced power management) or ACPI (advanced configuration and power management interface)—can be incorporated later through CMOS ‘Power Management Setup’ facility. Similarly, you can install Ethernet card for LAN and modem card for the Internet, fax, and e-mail accessibility via telecom lines. A brief information on these additional functions is given

below. • APM. APM caters to the PC to enter an energy-saving standby mode. BIOS enables APM by default. It can be initiated in the following ways: 1. By specifying time-out period in BIOS setup program. 2. By connecting a hardware suspend/ resume switch to CN10 on the motherboard. 3. From ‘Suspend’ menu item in Windows. • ACPI. ACPI provides direct control to the operating system over the power management as well as plug-‘n’-play functions. Features include: 1. Power management control of individual devices, add-on cards, video display, and HDD. 2. Methods for achieving less than 30W operation in ‘Power-on Suspend Sleeping State’ and less than 5W in ‘Suspend to Disk Sleeping State’. 3. A soft-off feature to power off the PC. 4. Support for multiple wake-up events for the PC to resume normal operation. 5. Support for front-panel power and ‘sleep’ mode switch. • Ethernet card for LAN. Ethernet cards capable of running at 10Mbps to 100Mbps, of different makes such as Intel, Real Tek, Mercury and Dax, as Ethernet PCI adapter are available in the market. Each card comes with a bracket, driver ELECTRONICS PROJECTS Vol. 22

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diskette, and user manual. The bracket would have an LED and RJ-45 jack. This jack is used for running a twistedpair unshielded cable (max. length 100 metres) between the card and the hub/ concentrator (10Base-T or 100Base-Tx) to which other computer’s LAN cards are similarly connected. Once the cable is connected to the hub, the LED on Ethernet card would light up. Before installing, remove a cutout opposite the PCI slot to make space for the bracket of Ethernet card. When you install the card, the

power to the PC should be ‘off’. When you switch ‘on’ the computer, it automatically detects its presence and ‘New Hardware Wizard’ appears on the screen to guide you through the installation process. It asks for location of the drivers. The driver’s floppy can be inserted in ‘A’ drive and path can be indicated. You can then proceed further, as per instructions appearing on the screen, to complete its installation. • Modem. 56kbps PnP (plug-‘n’-play compatible) and Windows 95/98 compat-

ible internal modem cards are available from different manufacturers for installation in any of the PCI slots. The modem card will have a telephone line jack for connection of telephone line from wall socket, a parallel phone jack for connecting a telephone set, and Mic and speaker jacks for external mic and speakers for use with voicemail and speakerphone facilities, respectively. For installing the drivers, the procedure would be similar to that used for installation of the Ethernet card. ❏

Readers’ comments: Q1. The authors have shown irresponsibility by planning to install a Pentium III processor on a PGA 370 socket meant for a Celeron or lower processor. Adarsh Soodan Through e-mail Q2. The article is really interesting and useful. Please clarify the following technical terms: 1. PS/2 mouse connector 2. Energy Star, Green PC 3. Audio modem riser (AMR) R. Sreerekha Hareendran Kollam, Kerala Q3. I request the authors to clear the following doubts. 1. Is there any single and reliable dealer in Chennai, Bangalore or Kerala from where I can procure all the components. 2. Is the PC available in kit form? 3. Instead of a 35.5 cm (14-inch) colour monitor, can I use a 43.2 cm (17-inch) colour monitor with this PC, without making any alterations. Further, is there any 43.2 cm LCD, colour monitor available for this PC. In that case what are all the alterations required to be made? A. Venugopalan Unny Palakkad Q4. Please clarify: 1. What is the difference between a boot disk and a start-up disk? 2. How can I increase the HDD capacity to 20 GB? Further, how can I partition HDD into four sections (logical drives) and CD-ROM drive as the fifth drive? 3. Define primary master/slave and secondary master/slave. 4. How can I configure HDD as secondary master and CD-ROM drive as secondary slave? 5. Provide a few tips for attaching a CD-writer and also a DVD drive to the system. T. K. Hareendran, Kadakkal

Q5. I have successfully assembled the PC as per the given procedure using a 128MB RAM instead of a 64MB RAM. Please answer the following regarding this project: 1. How should I proceed to partition my hard disk into four logical drives? 2. The booting speed of my PC is lower than that of my colleague’s PC that uses 500MHz Celeron processor and 64MB RAM. Why so? 3. What is the difference between AMI BIOS and AWARD BIOS? Narla Sankar Through e-mail Q6. Following the guidelines in the article, I have successfully assembled my PC using altogether a different processor (500MHz AMD K6-2) and a different motherboard (Tomato with SIS 530 Chipset) with Award BIOS. All is well except that during the first switching, it flashes “CMOS checksum error” and “CMOS battery failed”. The former message “Checksum error” does not appear on restarting the PC. Is this problem due to wrong orientation of BIOS chip? Vinod D. Buchia Gandhidham Authors, K.C. Bhasin and Neeraj Kundra, state: A1. We have not only planned but also installed the Intel’s Pentium III processor in PGA 370 socket, and the system is up and running superbly at EFY ever since. In fact, Fig. 5 showing its installation in PGA 370 socket is from Intel Pentium III processor installation notes which accompany the Intel Pentium III processor. So the remarks made by the reader are totally unwarranted. A2. 1. PS/2-compatible keyboard and mouse connector are miniature 6-pin DIN connectors unlike the PCAT 5-pin keyboard and 9-/25-pin (comport) connector for the mouse. The pin signals are: 1.

Data; 2. N/C (not connected); 3. Ground; 4. Vcc; 5. Clock; and 6. N/C. 2. Label Energy Star is awarded by Environmental Protection Agency (EPA), USA, for products which meet its specifications. It was introduced in 1992. Green PC is Energy Star program developed by EPA for minimising unnecessary energy consumption and release of harmful chemicals during production, especially chlorofluorocarbons (CFCs) that cause depletion of ozone layer. 3. The AMR (audio modem riser) card is a new modular specification that integrates the audio/modem functions on the motherboard by assigning the analogue I/O functions to a riser card. Integration of the audio/modem function enhances system capabilities while reducing costs. The AMR interface is based on an AC-link that is compliant with Intel audio codec ’97 version 2.1 specification. It supports data, fax, and voice modes. The pin details of its 46-pin edge connector are given in Table I. The general features of the card include: – Transmission protocols supported (ITU-T V.90 and K56flex, V.34, V.32is, V.22bis, V.21, Bell 212A, and Bell 103) – Maximum download speed of 56,000 bps – Virtual COM Port throughput – 460.8 kbps – Call progress monitor – On-/off-hook control – DTMF detection and generation – Distinctive ring for data, fax, and voice – Call ID support (optional) We will try to publish troubleshooting procedures for the PC, in EFY, very soon. A3. 1. We have not carried out a market survey of the cities/states mentioned by you and as such we cannot provide you any related information.

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ELECTRONICS PROJECTS Vol. 22

2. For complete kit or parts, you may contact EFY associates, M/s IT Solutions (India) Pvt Ltd for quotations at [email protected]. 3. One can connect any VGA/SVGA compatible colour monitor or LCD monitor to this PC. For example, LG Electronics' 38.4 cm LCD monitor 500 LC (Windows 95 plug-n-play compatible) can be directly connected. A4. 1. A bootable disk is one that contains the basic system files, namely, the two hidden files (IO.SYS and MSDOS. SYS) and ‘command.com’ file. These files provide I/O resources for application programs as well as an environment to execute programs and interact with the operating system. You can make a floppy bootable by just adding the system files. To do this, use Windows Explorer, select 3½ floppy, and right-click on ‘+’ sign on its left. Now from the pull-down menu, select ‘format’. Select ‘copy system files only’ and click on its start button to copy the above-mentioned files to make the floppy bootable. If you desire to view hidden files in the disk, click ‘view’ on the menu bar of Windows Explorer and select ‘view’ in the ‘folders’ option. Then under ‘hidden files’, select ‘show all files’ and click ‘apply’. You can now see all files including hidden files on the disk. A start-up disk on the other hand has, in addition to the above-mentioned

system files, a few other utilities such as Chkdsk.exe, Fdisk.exe, and Scandisk. exe to help you optimise and maintain the system. 2. To increase the HDD’s capacity to 20 GB, you can simply replace the existing HDD with either a higher-capacity (20GB) HDD or use an additional HDD in one of the two IDE channels (primary or secondary, connected to IDE1 and IDE2 connectors, respectively, on the motherboard) with each channel capable of supporting two devices (anyone as master and the other as slave). The names ‘master’ and ‘slave’ have no sanctity in real terms. For configuring a drive as master and the other as slave, the position of jumpers at the rear on each HDD device should conform to the settings for jumper block in Fig. 2 above. The setting at serial No. 3 is not applicable to the newer PCs that are all compatible with ATA (advanced technology attachment) packet interface. The fourth setting (cable select) is meant for computers that use CSEL option for master and slave device by selecting or deselecting pin 28 of the interface bus, by jumpering pins 5 and 6. One can use the ‘IDE HDD auto detect’ option from the main menu for auto-detection of HDD parameters. In most cases, the BIOS will auto-detect the HDD(s). If it does not, select the ‘User’ option to manually enter the drive’s parameters by referring Table I to its documentation AMR Connector Pin Definitions (AMR) that gives the values for Pin Signal Pin Signal cylinders, heads, and so number number on. The mode should be B1 Audio mute# A1 Audio_PWRDN set to LBA (logical block B2 GND A2 Mono_phone addressing). B3 Mono_out_/PC_beep A3 The procedure for B4 A4 creating one active B5 A5 B6 Primary_DN# A6 GND (bootable) primary parB7 -12V A7 +5V dual/+5V SB tition (drive C:) and one B8 GND A8 USB_OC# extended DOS partition B9 +12V A9 GND (drive D:) has already B10 GND A10 USB+ been covered in the B11 +5VD A11 USB (Key) (Key) article. Assume that we (Key) (Key) want a primary partiB12 GND A12 GND tion of 50 per cent and B13 A13 S/P-DIF_IN three extended DOS B14 A14 GND B15 +3.3VD A15 +3.3V dual/3.3V SB partitions of equal size B16 GND A16 GND of the leftover disk space B17 AC97_SDATA_OUT A17 AC97_SYNC for drives D:, E:, and B18 AC97_RESET# A18 GND F:, respectively. (Note. B19 AC97_SDATA_IN3 A19 AC97_SDATA_IN1 Drive letter ‘G:’ will be B20 GND A20 GND B21 AC97_SDATA_IN2 A21 AC97_SDATA_IN0 automatically assigned B22 GND A22 GND to CDROM drive in the B23 AC97_MSTRCLK+RST A23 AC97_BITCLK process.) In effect, we

will be creating three logical partitions in the extended DOS partition of 50 per cent drive space. Thus in the first line seen on page 49 of February issue, type 33% for the leftover disk space instead of pressing ‘Enter’. You will observe from the screen that drive D: has been created. Press ‘Esc’ to return to the menu on page 1 and repeat the procedure for creating the next drive, with the exception of typing 50% of the leftover drive space. For creating the last drive, follow the procedure given in the starting paragraph on page 49 (February) in toto. You would thus create all the required drives of required capacities. Note that if you install two HDDs in place of a single HDD, then drive letter ‘D:’ is automatically kept reserved by the DOS for the primary drive in the next HDD, and in that case your four drives on the first HDD will be named C:, E:, F:, and G:. The last drive name after partitioning of the next HDD will be assigned to the CD-ROM drive. In the case of two physical HDDs, the page 1 of menu in ‘Fdisk’ will show an additional choice ‘5. Change current fixed disk drive’. 3. The two devices connected to IDE1 connector (refer the motherboard in Fig. 2 of the article) are termed ‘primary’ and those associated with IDE2 connector ‘secondary’. Master and slave will be configured as per the shorting link’s positions. It is a normal practice to configure the drive at extreme connector as master and the one connected to middle connector as slave. (You can also configure them vice-versa.) 4. The shorting link in jumper block of CD-ROM drive (Fig. 9) will need to short pins marked ‘SL’. For HDD, follow the guidance given in the preceding paragraph. Both the devices should be connected to the cable terminating on IDE2 connector. 5. Attaching a CD-R (CD-recorder/ burner/r-drive). There are various versions. Some can be attached externally while the others can be installed like a CD-ROM itself inside the PC. Some will be IDE-compatible and can be configured using IDE cable referred above. Some versions will be SCSI compatible (for higher speed), but for that you will need an extra SCSI adapter card in one of the vacant PCI slots. The latest ones are USB (universal serial bus) port-compatible. (The USB port is available on your Pentium motherboard.) Some external recorders can be connected via the printer port. ELECTRONICS PROJECTS Vol. 22

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The newer models can write as well as read, and as such they can also function as CD-ROM drives. Some of the recording software accompanying these writers would also have a verification feature called ‘pre-mastering’ for comparing the recorded data against source file. Necessary documentation including software drivers for installation, configuration, and utilities will accompany the CD-R. Going through this literature, you can buy the CD-R that suits you the best. Attaching a DVD to the system. The minimum system requirements for installing a DVD in the same place where CD-ROM goes include Pentium with bus speed of 133 MHz (supported by 810E chipset) or better (200 MHz is required for maintaining 30 frames/second rate), Windows 95 or later version (Windows 98 has in-built DVD support), 16MB/32MB RAM, graphics resolution of 800x600 pixels with 16-bit colour, and sound blastercompatible card. You also need to install an MPEGII decoder card in a spare PCI slot for decompression of video and audio. (Software-based MPEG decoders are comparatively much slower.) While a CD can store around 650MB/700MB, a single-sided DVD can store 4.7 to 9 GB and a double-sided DVD up to 18 GB. (A double-sided DVD needs to be flipped to read the second side as existing drives can read from a single side only.) The DVD kit would comprise ATAPI/EIDE-compatible controller (SCSIcompatible are also available in the market), an MPEGII decoder card, cables, and software including sample titles. Considering an ATAPI/EIDE-compatible controller, configure DVD drive as primary slave by placing the shorting link across SL pins on its jumper block at the rear in the same fashion as described above. Read your DVD installation manual carefully, since there are various ways to cable together the DVD drive, MPEG card, existing sound card, and existing CD-ROM drive (if any). You should follow the documentation for audio and video connections. However, here is a typical approach: Connect the internal audio cable from the rear of the DVD drive to the first audio-in connector on the MPEG card. If you find an internal audio cable from your existing CD-ROM drive fastened to your sound card, disconnect that cable from the sound card and plug it into the MPEG card’s secondary audio-in connector. Next connect an audio cable from the audio-out connector on the MPEG card to

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ELECTRONICS PROJECTS Vol. 22

1. 2. 3. 4.

Fig. 2: Settings for jumper block

the sound card’s internal audio-in connector. Remove the monitor connector from your graphics card and plug it into the appropriate connector on the rear of your MPEG card. Find the VGA loop-back cable included in your DVD kit. Attach its one end to your graphics card and the other to the input connector on your MPEG card. If you want to use a TV monitor with your DVD drive, link the video-out connector on your MPEG board to the monitor’s video-in connector. Now proceed to install the drivers and DVD software. Power up your PC. Windows will detect the MPEG board and ask you for a driver. Insert the driver floppy from your DVD kit and click ‘OK’. You may need to restart your PC before proceeding. A5. 1. The first question has already been answered above. 2. The slow booting operation in PC may be attributable to some other reason rather than the processor. The booting process in the PC involves a number of steps. Some of the probable reasons and the suggested remedies are given below: • The power good (PG) signal may not be building fast to its specified value (4.5Vmin). For that, you need to check your PCs’ SMPS. The specified power rating of the power supply to support Intel’s 810 chipset is 145 watts for a typical configuration. • In the CMOS setup utility (refer screenshot 4), edit the field against ‘System BIOS Cacheable’ and ‘Video BIOS Cacheable’ to read ‘Enabled’ in place of ‘Disabled’. • Ensure that 3.3V SDRAM sticks (DIMMs) used by you are 100MHzcompatible. • Ensure firm connections from motherboard to all peripherals. Improper connections can result in BIOS taking time in identifying them/their settings during

POST operations. As time-out margins are fairly long, a lot of time could get wasted that way. • Reload ‘command.com’ file to make sure that it is not corrupt. Also check for presence/removal of any virus using Norton or other anti-virus program. We suggest you to refer the BIOS FAQs posted by Pheonix at ‘www. pheonix.com/platform/awardbios.html’ before coming to any conclusion. 3. AMI (American Megatrends Inc.) and AWARD (AWARD Softwares, which merged with Pheonix Technologies Ltd in Sept.1998) are the two renowned BIOS suppliers. There are a few other suppliers also in the field (e.g. Microid Research Inc., Chips & Technologies, etc). At the user’s level, one does not directly interface with the BIOS, except via the CMOS setup options that are more or less similar, irrespective of the BIOS supplier. BIOS is chipset-/hardware-specific. Hence it will generally differ from one motherboard manufacturer to another. Even the same manufacturer may upgrade his BIOS by some hardware modifications to make it compatible with the new hardware. Only options can be changed via the CMOS set-up utility. A6. The BIOS chip orientation is correct, else you would not even see the initial setup screen or any message. Battery-backed CMOS chip orientation is also correct if your system time is changing correctly, since battery-backed CMOS chip is used for retaining the system set-up data as well as the RTC (real-time clock) data. It seems that either your batterybacked CMOS chip is faulty or some of its pins are not making proper contact with the base and hence its integrity is not getting verified during POST. Check if this CMOS chip has popped up from its base or any of its pins are bent and rectify the same. Else, try changing this IC. There could also be a track discontinuity problem on your motherboard. Check proper setting of jumpers, especially the CPU core voltage selection. For AMD K6-2, this voltage setting is 2.2V. This can be achieved by shorting pins 7-8 of JP5, while leaving all others open, in your Tomato motherboard. Shorting is achieved by moving the link to 1-2 and opening is achieved by moving the link over 2-3. This information may be included in the user manual of your motherboard. Else, you may get it from‘http://www.zida.com/js_t530b.htm’.

Automatic Room Light Controller rejo g. parekkattu

U

Parts List Semiconductors: IC1, IC2, IC3 - NE555, timer IC4 - 74LS192, up/down decade counter IC5 - 74LS85, 4-bit magnitude comparator) IC6 - 7447, BCD to 7-segment decoder/driver IC7 - MCT2E, opto-coupler IC8 - 7805, +5V regulator IC9(N1-N4) - 74LS00, quad 2-input NAND gate IC10(N5-N10) - 74LS14, hex schmitt inverter gate T1, T2 - BC548, npn transistor T3 - SL100, npn transistor D1-D3 - IN4001, rectifier diode IRLED1, IRLED2 - Infrared LED Resistors (all ¼-watt, ±5% carbon, unless stated otherwise): R1 - 3.3-kilo-ohm R2 - 10-kilo-ohm R3 - 100-ohm R4, R5, R21 - 1.2-kilo-ohm R6, R7, R12 - 33-kilo-ohm R8, R9 - 180-kilo-ohm R10, R11 - 1-kilo-ohm R13-R19 - 470-ohm R20 - 100-kilo-ohm VR1 - 10-kilo-ohm preset Capacitors: C1 - 0.001µF, ceramic disk C2, C3, C4 - 0.01µF, ceramic disk C5, C6 - 4.7µF, 16V electrolytic C7, C8 - 10µF, 16V electrolytic C9 - 1µF, 16V electrolytic Miscellaneous: M1, M2 - IR sensor modules DS1 - LT542 (common anode display) RL1 - 12V, 200 ohm, 2 C/O. LDR1 - LDR (Dark resistance > 120 kilo-ohm) L1 - 230V, 100W electric bulb - 12V power supply - Printed circuit board - IC sockets

sually, when we enter our room BCD output of the counter, at any time, in darkness, we find it difficult represents the number of persons inside to locate the wall-mounted switchthe room. The output of the up/down board to switch ‘on’ the light. For a counter is decoded by 7-segment decoder/ stranger, it is tougher still as he has no driver and displayed on 7-sement display. knowledge of the correct switch to be Simultaneously, the output of counter is turned on. Here is a reliable circuit that compared by 4-bit magnitude comparatakes over the task of switching ‘on’ and tor. switching ‘off’ of the light(s) automatically The output of comparator remains when somebody enters or leaves the room high as long as BCD output of counter is during darkness. This circuit has the folgreater than zero. A logic gate is used to lowing salient features: initiate energisation of a relay to switch • It turns on the room light whenever ‘on’ the light when comparator output is a person enters the room, provided that high and it is dark outside. the room light is insufficient. If more than one person enters the room, say, one after The circuit the other, the light remains ‘on’. • The light turns ‘off’ only when the The detailed section-wise description of room is vacant, or, in other words, when the circuit shown in Fig. 2 is as follows: all the persons who entered the room IR transmitter. The IR transmitter have left. circuit consists of an astable multivibra• A 7-segment display shows the tor built around NE555 timer IC1. The number of persons currently inside the output of IC1 at pin 3 is a rectangular room. waveform of around 36kHz frequency. • The circuit is resistant to noise and This output is used to drive two IR LEDs, errors since the detection is based on inwhich transmit modulated IR light at frared light beams. 36kHz frequency. Modulating frequency • The circuit uses commonly available of 36 kHz is used because the IR receiver components and is easy to build and test. modules used in this circuit respond to IR The functional block diagram of the signals modulated at 36kHz frequency. circuit is shown in Fig.1. It comprises The multivibrator frequency can be cor36kHz IR transmitter, two IR detector rectly adjusted with the help of preset VR1 modules, two monostable multivibrators, (10 kilo-ohm). Resistor R3 is a current up/down-counter, 4-bit magnitude comlimiting resistor that keeps the IR LEDs, parator, 7-segment decoder display, light current within the required range. sensor, and relay driver. IR detector modules. The IR deTwo pairs of IR transceivers are emtector modules used in the circuit are ployed in order to detect whether the person is entering or leaving the room. When a person enters the room, IR detector 1 gets triggered, followed by triggering of IR detector 2. Conversely, when a person leaves the room, IR detector 2 gets triggered, followed by triggering of IR detector 1. A priority detector circuit determines which of the two detectors is triggered first and then activates an up/down counter accordingly. The Fig. 1: Block diagram of automatic room light controller

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Fig. 2: Schematic diagram of automatic room light controller

Fig. 3: Timing waveforms

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commonly available in the market. These have three terminals for Vcc (+5V, here), ground, and the output signal, respectively. In the normal state, the output pin (pin 3) of this detector remains at high state, and when an IR light of correct modulating frequency is detected, its output pin goes low. The pin configuration of the IR modules may vary from one manufacturer to the other. (Pin configuration of module TSOP 1136 for 36 kHz used by EFY is shown in Fig. 2.) (Articles based on the IR sensor module have been published in Nov. 2000 (also in Electronics Projects Vol. 21) and some other previous issues of EFY. Readers may refer the same for more information about the module.) Since the IR transmitter in this circuit is continuously ‘on’, emitting IR light, in the normal condition, the output pins of both IR modules will be at low state. Therefore transistors T1 and T2 will remain cutoff. When a person enters

or leaves the room, the infrared light beams are interrupted one-by-one and the output of each IR sensor module, in turn, goes high, which results in conduction of associated transistors T1 and T2. Which transistor will turn ‘on’ first depends on whether the person is entering or leaving the room. In the circuit, two NE555 timer ICs (IC2 and IC3) wired as monostable multivibrators are used. The pulse width of the output waveform (on time) for these multivibrators is fixed at about 0.9 seconds by suitably selecting the values for the timing capacitors C5 and C6 in conjunction with their associated resistors R8 and R9. These monostable multivibrators get triggered when their trigger input pins (pin 2) go low. Thus the multivibrators are triggered only when the IR light beams are interrupted. Although the output pulse width of both the multivibrators is approximately the same, there is, however, a phase difference corresponding to the elapsed time between the successive interruptions of the IR beams. Refer to the waveforms shown in timing diagram of Fig. 3. Priority-detector logic circuit. The priority detector circuit uses three NAND gates, five inverter gates, and two differentiators. The timing diagram given in Fig. 3 helps in understanding as to how the priority-detector circuit detects a person going out of the room. At first the outputs from the monostable multivibrators are NANDed by gate N1 and its polarity is inverted again by gate N7. At the same time, the outputs of monostable IC3 and IC2 get

differentiated by the capacitor-resistor combinations of C7-R10 and C8-R11, respectively. Each differentiated output is passed via Schmitt inverter pairs of N5-N6 and N10-N9 to convert the differentiated pulses into rectangular pulses. The rectangular pulses obtained at the output of gates N6 and N9 are again NANDed with the output of gate N7 in NAND gates N2 and N3, respectively.

The rectangular pulse at pin 4 of NAND gate N2 ends before the output of gate N7 goes high and hence the output of NAND gate N2 stays high, while both inputs to NAND gate N3 are simultaneously high for the duration of rectangular output of gate N9. As a result, the output of gate N3 applied to countdown clock pin 4 of IC4 causes the counter to count down on its trailing edge (low-to-

Fig. 4: Actual-size, single-sided PCB layout for the circuit

Fig. 5: Component layout for PCB

high transition) and the output count goes down by one count. Similarly, when a person enters the room, pin 4 of counter IC4 remains high, while its pin 5 (count up) gets a low-going pulse resulting into counter output advancing by one count. Values of capacitors C7 and C8 and resistors R10 and R11 can be varied for optimum performance. Up/down counter. Up/down decade counter 74LS192 (IC4) is used as the counter. When the power is turned ‘on’, its outputs Q0 through Q3 are in the low state. Whenever a person enters the room, a low-going pulse is applied at its count-up pin 5, while its count-down pin 4 is held at logic 1 and its output count advances by one. Similarly, when the person leaves the room, a similar pulse is applied at its countdown input (pin 4) while its countup pin 5 is held at logic 1 and its output decreases by one. Thus the 4-bit output always represents the number of persons still inside the room. The output of the decade counter is connected to 7-segment decoder/driver IC6 (7447) that displays the number on common-anode 7-segment LED display (LT542). Magnitude comparator. The output of the up/down counter is also applied to 4-bit magnitude comparator that acts as zero detector, i.e. it detects whether the number of persons inside the room is greater than zero or not. The 4-bit output of the decade counter is always compared with a reference 4-bit number (0000), and if a match occurs, the output at pin 5 (P>Q) of the comparator goes low to represent an ‘empty room’ condition. In all other cases (when the number of persons in the room is greater than zero), P>Q output will be at high state. This output is given as one of the inputs to NAND gate N4 (followed by inverter gate N8). Thus, as long as the room is not empty, one of the inputs to N4 gate will be high. The second condition for the light to get switched ‘on’ is yet to be satisfied. Whether there is sufficient light in the room or not is checked by the light sensor circuit. Light sensor. The light sensor is wired around the opto-coupler MCT2E. The resistance of the LDR depends upon the amount of light in the room. An LDR with resistance below 5 kilo-ohm in normal light and more than 120k resistance in darkness is required. When there is ELECTRONICS PROJECTS Vol. 22

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Fig. 6: Proposed layout of IR transmitter and receiver pairs

sufficient ambient light, the transistor inside the opto-coupler is turned ‘on’ and the input of NAND gate (pin 3) is driven to low state. Thus the output of NAND gate remains at high state and that of inverter gate N8 at low. However, when the light is insufficient, the resistance of the LDR increases, turning off the transistor inside the opto-coupler. The sensitivity can be controlled by adding a high-valued variable resistance (about 680k) across the LDR. When both conditions are satisfied

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(that is one or more persons are inside the room and the ambient light is insufficient), the output of NAND gate goes ‘low’ and that of inverter gate N8 goes ‘high’ to turn on transistor T3, thereby energising relay RL1. A 230V, 100W electric bulb is connected via the relay to the AC mains. Once the relay gets energised, the LDR is effectively removed from the circuit (since the LDR is connected to the N/C contact of the two pole relay) to prevent the flickering of the lamp with changing resistance of the LDR.

Assembly and testing The full circuit, with the exception of the IR transmitter, can be assembled on a single general-purpose PCB. However, an actual-size, single-sided PCB for the circuit in Fig. 2 is shown in Fig. 4. The component layout for the PCB is shown in Fig. 5. The receiver-transmitter pairs are placed about a metre apart as shown in

Fig. 6. The distance between the two sensors (receiver modules) is about 40 cm. A steel pipe of 5mm diameter and 3cm length can be placed in front of the IR module in order to improve its directivity. After assembling the circuit, adjust preset VR1 (10k) until pin 3 of both the IR sensor modules go high (5V). If the circuit still does not function properly, adjust the distance between the sensors. The metal cabinets of the IR modules must be connected to ground. Note that the circuit works with a regulated +5V supply, except the power supply to the relay coil. The circuit has no off-time memory, and so its working is interrupted during power failure. Another disadvantage is that the circuit can count only up to 9. But it is quite unusual to have more than nine people in a normal living room. Take care about the IR sensor module pin connections. It may be damaged if connected wrongly. ❏

Intelligent Water Level Controller Sadhan Chandra Das

I

n coming years, the drinking water is going to be one of the scarce commodities. This would partly be attributable to our mismanagement of water supply and its wastage. In normal households, where pumps are used to fill the overhead tanks (OHT), it is usually observed that people switch on the pump and forget to switch it off even when the

tank has become full. As a result, water keeps overflowing until the household people notice the overflow and switch the pump off. As the OHT, in general, is kept on the topmost floor, it is not quite convenient to go up frequently and see the water level in the OHT. This problem can be solved by using the intelligent digital liquid level control-

ler circuit presented here. It has the following features: • It can automatically switch on the pump when the tank is empty and switch it off when the tank becomes full. • It can check the ground tank (sump tank) water level from which the water is pumped into the overhead tank (OHT). If the sump tank water level is below the

Fig. 1: Circuit diagram of water level controller ELECTRONICS PROJECTS Vol. 22

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Fig. 2: Power supply

Digital display circuit (refer Fig. 1.) This circuit comprises a quad 2-input XOR gate IC1 (CD4030) for sum outputs, decimal to BCD code converter using diode matrix of diodes D3 through D7, a BCD to 7-segment decoder/ driver IC2 (74LS47), Fig. 3: Construction details of probes for mineral water and common-anode type 7-segment display LTS 542R. When only the tip of sensor probe (cathode) No. 1 is in touch with the water, the voltage at pin 3 of IC1 becomes logic high (i.e. +5V), and hence voltage at line No. 1 (L-1) also becomes high. Now due to conduction of Fig. 4: Construction details of probes for non-conducting liquids diode D3, the BCD code 0001 (Q3 Q2 Q1 Q0) is predetermined level, the unit switches off generated and converted to equivalent the pump to protect the pump from dry7-segment code by IC2 (74LS47) to disrun, even though the overhead tank may play the decimal digit ‘1’. be completely empty. Similarly, when the tips of the both • It includes under- and over-voltage sensors 1 and 2 are in touch with water, cutout to switch off the pump if the voltage the voltage at pin 3 becomes logic low is not within specified low (200V) and high (0V) while the voltages at pin 4 and line (250V) limits. 2 (L-2) become logic high (i.e. +5V). Now • It includes a circuit for digital disdue to conduction of diode D6, the corplay of the overhead tank level to indicate responding BCD code 0010 is generated water levels 0 through 4 as per positions and decimal digit 2 is displayed on the of the tips of the sensors inside the over7-segment display. head tank. When the tank is completely empty, • The sensors used in this project have the outputs of all XOR gates of IC1 are a lifetime of more than five years.

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low and the display shows decimal digit 0. In this way the display circuit works to show decimal digits 0 through 4, corresponding to the level of the water, as defined by the position of the sensors at different heights. Here the resistors R9 through R12 and R19 through R21 have been used for passive pull-down. Controller circuit. The controller circuit is built around three quad 2-input NOR ICs (IC3 through IC5) to switch the pump motor on or off when certain conditions are fulfilled. The conditions to be met for switching-on/running of the pump are: 1. The mains supply should be within certain ‘low’ and ‘high’ cut-off limits (say between 200V AC and 250V AC). 2. The water level in the sump (ground tank) is above certain optimum level (2' in Fig. 1). 3. Water in the overhead tank (OHT) is below the minimum level. Once all the above-mentioned three conditions are satisfied, the pump motor would start running. The corresponding logic level at point A will be low (point B will also be low automatically—not being in touch with the liquid), point C will also be low and point D will be high. Once running, the pump will continue to run even when the water rises above the minimum level in the OHT (i.e. when point A subsequently goes high), provided the first condition is still fully satisfied and the water level in the sump has not fallen below that of sensor 1'. It will stop only when either the maximum specified level in the OHT has been reached or the water level in the sump has fallen below sensor 1' position. Here the NOR gate pairs of N2 and N3, and N6 and N7, form NOR-latches. When the ground tank (sump) water level is above the defined level 2', the voltage at pin 11 of gate N6 is low. So diode D12 cannot conduct. Also, if the mains voltage is within acceptable limits of 200-250V, the voltage at output pin 3 of gate N12 is high and the voltage at collector of transistor T2 is low. Diodes D8 and D11 are thus cut off. So the voltage at input pin 8 of gate N4 is pulled down to logic low level by passive pull-down resistor R18 (56 kilo-ohm). Now if overhead tank is empty, i.e. water level is below level 1, voltage states at input pins 1 of gate N2, and pins 12 and 13 of gate N1, are pulled down to logic low by passive pull-down resistors R13 and R14 respectively. Hence volt-

Parts List

Fig. 5: Actual-size, single-sided PCB for water level controller

Fig. 6: Component layout for the PCB

ages at output pin 11 of gate N1 and input pin 5 of gate N3 become logic high to force the output at pin 4 of gate N3 to be latched low. This logic level will not change until voltages at input pins 5 and 6 of gate N3 become low (0V) and voltage at pin 1 of gate N2 goes high (+5V). Since both inputs of gate N4 are low, hence its output at pin 10 goes logic high to drive transistor T1 into conduction. Relay RL1 is thus energised and the pump motor is switched ‘on’. The water level of the overhead tank starts rising. When the water level reaches the tip of the topmost sensor 5, voltage at

pin 1 of gate N2 goes high. Already, the voltage levels at pin 11 of gate N1 and input pin 5 of gate N3 are low. So the voltages at output pin 4 of gate N3 and input pin 9 of gate N4 become logic high to turn the output pin 10 of gate 4 to logic low level. Thus relay RL1 is de-energised, to switch the pump off. When line voltage is within the specified limits and ground water level goes below the defined level 1', the voltage at output pin 11 of gate N6 becomes logic high to make diode D12 conduct. As a result, the voltage at pin 8 of gate N4 becomes logic high to make its output pin

Semiconductors: IC1 - CD4030 quad 2-input XOR gate IC2 - 74LS47 BCD to 7-segment decoder/driver IC3-IC5 - CD4001 quad 2-input NOR gate IC6 - LM7812 regulator 12-volt IC7 - LM7805 regulator 5-volt T1-T2 - SL100 npn transistor D1-D15, D17-D20 - 1N4001 rectifier diode D16 - Red LED DIS1 - LTS542R 7-segment common anode display Resistors (all ¼-watt, ±5% carbon, unless stated otherwise): R1-R8 - 33-kilo-ohm R9-R18 - 56-kilo-ohm R19-R21 - 1.5-kilo-ohm R22, R24 - 2.2-kilo-ohm R23 - 1.2-kilo-ohm R25 - 1-kilo-ohm R26, R27 - 220-kilo-ohm R28-R34 - 330-ohm VR1, VR2 - 100-kilo-ohm preset Capacitors: C1-C4, C7 - 0.01µF ceramic disc C5 - 470µF, 35V DC electrolytic C6 - 2200µF, 35V DC electrolytic C8,C9 - 10µF, 25V DC electrolytic Miscellaneous: RL1 - 12V, 200-ohm 2 C/O relay X1 - 230V AC primary to (a) 0-15V, 750 mA, and (b) 0-12V, 100 mA secondary transformer S1 - Push-to-on button S2 - On/Off switch - IC sockets - Heat sinks for regulator ICs - SS304, 5mm dia. stainless steel rod for anode and 3mm dia. for all cathodes - of appropriate length - Multi-core feed wire

10 go low. Transistor T1 is cut off and the relay is kept disabled, even though the overhead tank is fully empty. The relay will be enabled only when the water level in the sump tank is above level 2'. When the ground tank water level is above level 2' but the line voltage is out of range, gate N12 output pin 3 goes low to cut off transistor T2, making diode D11 conduct. In this state the output of gate N6 and the output of gate N2 become logic low. Although diode D12 does not conduct, diode D11 conducts and the output of gate N4 goes low to cut off transistor T1. This disables relay RL1 and the pump remains off, even though the overhead tank is completely empty. Here two cathode sensors for sensing ground tank water level have been used ELECTRONICS PROJECTS Vol. 22

23

instead of one, to provide some hysteresis in the system. When ground water level is below level 1', the output of gate N6 becomes logic high (5V). When water level is above level 2', the output of gate N6 is logic low (0V). If the water level is in between levels 1' and 2', there is no change of state at output of gate N6, i.e. output remains at the last/previous state. Power supply (Fig. 2). The power supply circuit consists of step-down transformer X1 (having two secondaries with ratings of 12V, 100 mA and 15V, 750 mA), a bridge rectifier (using four 1N4001 diodes), a capacitor of 2200 µF for filtering purpose, regulator IC 7812 for feeding the anode probes as well as relay RL1, and regulator IC 7805 for feeding regulated +5V supply to all digital ICs, LEDs, and 7-segment display. The 12V secondary is used for sampling the mains.

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ELECTRONICS PROJECTS Vol. 22

One of its terminals is grounded while its other terminal, marked ‘G’, is connected to point ‘G’ of high/low cutout circuit in Fig. 1. The other secondary rated at 15V, 750 mA is used for deriving the regulated DC supplies required for operation of the circuit. Construction of sensors (Fig. 3). The highlight of the circuit are its electrodes (Fig. 3) used for mineral/conductive water, which are made of stainless steel (grade SS-304) rods. These electrodes have a life span of more than five years. Anode is a rod of 5 mm diameter and each of the cathodes is of 3 mm diameter, as shown in the figure. The cathodes and the anode should be long enough so that their soldered terminals are not in contact with water, even when the tank is full. The joints should be covered with insulation in such a way that

rain water does not come in contact with the soldered joints. One has to use orthophosphoric acid or zinc-chloride to make a soldered joint between stainless steel and conducting part of the flexible feed wire. The distance between the anode and the cathodes should not be more than 60 cm. Arrangement should be made in such a way that no electrode touches the other. The circuit can also be used for nonconductive liquids such as pure distilled water by using floats in conjunction with micro switches, as shown in Fig. 4. This arrangement can be used for distilled water plants, research laboratories, and for other nonconductive liquid level sensing applications. An actual-size, single-sided PCB for the circuits in Figs 1 and 2 is shown in Fig. 5, and the component layout is shown in Fig. 6. ❏

a unique Liquid Level Indicator Sadhan Chandra Das

A

separate alternative circuit of a unique liquid level indicator to provide a display in terms of the percentage of full-scale level in OHT is shown in Fig. 7. It can either be used to replace the digital display circuit included in Fig. 1 (by simply connecting the 10% and 100% sensor probes of Fig. 7, additionally, to points marked ‘A’ and ‘B’ respectively in Fig. 1, apart from connection of +5V and +12V supplies and ground points) or it can be used in conjunction with an audio alarm unit shown in Fig. 8

and the power supply circuit in Fig. 2 independantly. The latter configuration can be used when you do not desire to have automatic control for switching the pump mo- Fig. 8: Audio alarm unit

Fig. 7: Unique liquid level indicator ELECTRONICS PROJECTS Vol. 22

25

Fig. 9: Actual-size, single-sided PCB for the unique liquid level indicator

Fig. 10: Component layout for the above PCB

tor on and off but need only to be warned when water reaches 100% and also when its level drops to 10% so that you may manually switch the pump motor on or off, as the case may be. This level indicator can show the discrete levels in percentage from 0 to 100% with 10% resolution. An audio alarm circuit has been incorporated to generate audio alarm when the tank level reaches 100% and also when the level drops to 10%. The input to the audio alarm circuit

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ELECTRONICS PROJECTS Vol. 22

(Fig. 8) is tapped from line-1 and line-10 representing 10% and 100% levels respectively in Fig. 7. If, in place of displaying the liquid level in percentage, one wants to display only the digits 0 through 10, then 7-segment display DIS1 and LEDs (LED1 through LED4) for ‘%’ symbol can be removed. This circuit can be used for premises which have overhead tanks and the water supply is provided by municipalities or corporations etc.

Display circuit. The basic elements of the circuit, as shown in Fig. 7, comprise three quad 2-input XOR gates (IC1 through IC3) to get only the sum outputs, a hardwired decimal-to-BCD converter (using diodes D1 through D16), and a 74LS47 BCDto-7-segment decoder/driver (IC4). When the tip of sensor-1 is in touch with the water, the line (L-1) connected to pin 3 of IC1 (CD 4030) goes to logic 1 state (+5V). When the tips of sensors 1 and 2 both touch the water, pin 3 of IC1 goes to logic 0 (0V), while line L-2 connected to pin 4 of IC1 becomes high (+5V). Thus which one of the lines (L-1 through L-10) will be at logic 1 would depend on which last sensor (counted from bottom of the tank) is in touch with the water. If the tank is totally empty, all the lines, L-1 through L-10, would be at logic 0. These lines (L-1 through L-10) represent the decimal numbers 1 through 10. If line L-1 is at logic 1, BCD code 0001 is generated due to conduction of diode D9 only. Similarly, if line L-3 is at logic 1, BCD code 0011 is generated due to conduction of diodes D6 and D16. The voltages, corresponding to their BCD codes, are fed to the inputs of IC 74LS47 (7-segment decoder/driver) to drive 7-segment display DIS2. When line L-10 is high, display DIS3 is driven by transistor T1 (SL100) for decimal number 1. Since all the time the unit place digit of the percentage display is 0, the cathodes of corresponding segments of DIS1 have been permanently connected to 0V (ground) through current-limiting resistors of 330 ohms each. In this way the circuit displays 0 to 100 per cent of liquid level with 10 per cent resolution. One may or may not use diode D1. In

Parts List

Semiconductors: IC1-IC3 - CD 4030 quad 2-input X-OR gate IC4 - 74LS47 BCD to 7-segment decoder/driver IC5 - UM66 melody generator DIS1-DIS3 - LTS 542 common anode 7-segment display T1, T3, T4 - SL100 npn transistor T2 - BC 108 npn transistor D1-D16, D21, D22 - 1N4001 rectifier diode ZD1 - 3.1 volt zener diode LED1-LED4 - Red LED Resistors (all ¼-watt, ±5% carbon, unless stated otherwise): R1 - 3.3-kilo-ohm R2-R5 - 1.5-kilo-ohm R6-R24 - 330-ohm R25-R34 - 56-kilo-ohm R35-R44 - 33-kilo-ohm R45 - 100-kilo-ohm R46 - 2.7-kilo-ohm R47, R48 - 680-ohm Capacitor: C1 - 100µF, 25V electrolytic Miscellaneous: LS - 8-ohms speaker 7.5 cm dia - SS 304, 5 mm dia and 3mm dia stainless steel rods of appropriate length for anode and cathodes respectively. - Multi-core feed wire

this circuit the resistors of 56-kilo-ohm are connected across the inputs of XOR gates and ground, while resistors from R2 to R5 have been used for passive pulldown action. Audio alarm unit. Fig. 8 shows the circuit for audio alarm. The base of transistor T2 (BC108) is connected to the terminals of lines L-10 and L-1 via diodes D21 and D22 respectively and a common resistor of 100-kilo-ohm. When water touches the topmost sensor probe, transistor T2 conducts and transistor T3 is cut off. As a result 3.1V developed across zener ZD1 becomes available across pins 1 and 2 of melody generator IC7 (UM66). The amplified musical alarm is heard from the speaker. When the tank is neither 100% full nor it is above 10% (but less than 20%), transistor T2 cuts off while transistor T3 is saturated to make the voltage across pins 1 and 2 of IC7 at almost 0V, and hence no sound is produced by the unit. A separate parts list and actual-size PCB layout as well as component layout (Figs 9 and 10 respectively) are included after integrating the power supply of Fig. 2 with liquid level indicator circuit of Fig. 7 and audio alarm unit of Fig. 8. ❏

Readers’ comments: Q1. I have noticed, when the water level reaches the probe No 4, the C' segment LED of DIS 2 (LT542) does not glow. The same is the case even when the water level reaches probe No. 5 and probe No 6. Kindly suggest the corrective actions. M. Raja Bangalore Q2. I have constructed the circuit which is working perfectly. Instead of eleven roads, I want to use a stainless-steel hollow pipe that is sealed at one end and contains ten normally-open type

done for resistors R19, R20, and R21. The anode voltage should be +12V to +15V, which may or may not be regulated. You may also follow the modifications shown in Fig. 1 (component numbers shown for Intelligent Water-Level Controller) to correctively display the water-level. It is due to the fact that the output of the CMOS ICs (4030) are loaded by 1.5k resistors. EFY: The circuit works satisfactorily without resistors R2 to R5. A2. Sensors using a stainless steel hollow pipe cannot be used in this circuit. Imagine the float is in between 5th and 6th

Fig. 1: Modification to level controller

magnetic reed switches on the inside and one floating magnet on the outside as the sensor. Could you suggest me the changes required in the circuit if this type of sensor is used? J.P. Thakkar Through e-mail The author, Sadhan Chandra Das, replies: A1. Check IC1, IC2, and IC3 (4030), and the continuity of the wires connected to the sensors. If you find these alright, replace resistors R2 through R5 by 5.6k in the circuit before you switch on the power. If you wish to construct the circuit Intelligent Water Level Controller' published in February issue of EFY, the same replacement may be

reed micro switches, then no switch will be closed and the display will show only 0, although the water level is in between 5th and 6th sensors. Moreover, the number of wires from the reed switches remains the same. Nowadays for non-conductive liquid, coaxial or parallel-plate type capacitors are used with a suitable circuit that employs a frequency-to-voltage converter and displays the level of nonconductive liquid. But for conductive liquid like water, it is not advisable because due to electrolysis the sensors get damaged soon. The value of resistors R2 through R5 is 1.5k. Use 4.7k or 5.6k resistors instead to avoid loading effect of ICs 4030. ❏

ELECTRONICS PROJECTS Vol. 22

27

Interface Your Printer with 8085 Microprocessor Shaila Ghanti

I

t is very convenient to interface a printer to print 8085 programs. Here a simple hardware interface circuit with its driver software is described that would enable student to take printout of the 8085 programs in hexadecimal codes along with their memory locations in the format: xxxx DD, where XXXX is the 4-bit hexadecimal address and DD is 2-bit hexadecimal data. For most types of printers, the data to be printed is sent to the printer as

Fig. 1: Timing diagram

Fig. 2: System's block diagram

ASCII characters on eight parallel lines. The printer receives the characters to be printed and stores them in an internal buffer. When the printer detects a carriage return (odH), it prints out the first row of characters from the printer buffer. When the printer detects a second carriage return, it prints out the second row of characters. The process continues until the desired characters are printed. Transfer of ASCII codes from the microprocessor to a printer needs to be done on a handshake basis because the microprocessor can send characters much faster than the printer can print them. The printer must in some way let the mi-

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ELECTRONICS PROJECTS Vol. 22

croprocessor know that its buffer is full, and it cannot accept any more characters until it prints out some of the already stored characters. A common standard for interfacing with parallel printers is the Centronics interface.

Centronics interface

TABLE 1 Pin Assignments of Centronics Interface Connector Pin No. Signal Direction 2 Data bit 0 (D0) In 3 Data bit 1 (D1) In 4 Data bit 2 (D2) In 5 Data bit 3 (D3) In 6 Data bit 4 (D4) In 7 Data bit 5 (D5) In 8 Data bit 6 (D6) In 9 Data bit 7 (D0) In 1 Strobe (STR) In 14 Auto Feed (AF) In 36 Device Select (DSL) In 31 Initialise (INIT) In 11 Busy (BSY) Out 13 Select (SEL) Out 32 Error (ERR) Out 12 Paper end (PE) Out 19 to 30, 33 Ground —

Centronics printers usually have a 36-pin interface connector. The pin assignments of the significant pins of Centronics interface connector, used in this project, are given in Table 1. Fig. 1 shows the timing waveforms for transferring data characters to the printer using the basic handshake signals. Assuming that the printer has been initialised, first check the busy signal pin to see if the printer is ready which is used for connecting 8255 to the to receive data. If this signal is low (not printer, should normally have a 26-pin FRC busy), send an ASCII code on the eight connector to meet with the corresponding parallel data lines. After at least 0.5 µs, connector on the kit, and the other end assert the STROBE should have a 36-pin male Centronics consignal low to tell the nector to go into the corresponding connecprinter that a chartor on the printer. acter has been sent. Port A of 8255 is used for transferring The strobe signal the data to the printer. Port B is used for going low causes the checking the status signals coming from printer to assert its the printer. Port C is used for sending the Busy signal high. Afcontrol signals required to activate the ter a minimum time printer. The interface signals between of 0.5 µs the strobe signal can be raised 8255 and the printer should be connected high again. Note that the data must be as show in Table 1. held valid on the data lines for at least 0.5 (EFY Lab note. The maximum curµs after the strobe signal is made high. rent that an 8255 output pin can source When the printer is ready to receive the and sink is limited to 400 µA and 2.5 mA, next character, the BUSY signal will be respectively. To enhance this capability, low. The process continues. open-collector hex buffers/drivers 7407 The 8085 microprocessor is interfaced shown in Fig. 3 were used for all output to the printer through 8255 programmable peripheral interface device as Table II Port B of 8255—(Input) Status Signals shown in the block 32 13 11 diagram (Fig.2) and Cent. Pin no b2 PE ERR SEL BSY Comments the detailed inter- Signal B3 B2 BI B0 face diagram (Fig.3). Data 0 1 1 0 =06H (status OK) One end of the cable,

signal (DSL*) to select the printer. Read the status to find out whether the printer is selected and the Busy signal is low. Now send the ASCII character to print the character, followed by the STROBE* pulse for 0.5 µs. The process continues Fig. 4: Actual-size, single-sided PCB for the till the end of printer interface circuit the program. The end of the program is indicated using RST1 (CFH). The starting location of the program to be printed should be stored in D and E registers. The eight MSBs and eight LSBs of memory location should be stored in D register and Fig. 5: Component layout for the PCB Fig. 3: Schematic diagram of the printer interface circuit E register, report pins. For input port pins, there is no Parts List spectively. The complete software danger of overloading, and hence these program is given with comments as Semiconductors: ICI, IC2 - 7407 hex buffer/driver (open pins were connected directly from the necessary. collector type) printer to the kit.) (EFY Lab note. The original program Resistors (all ¼-watt, ±5% carbon, unless stated was tried many times, but we did not sucotherwise): ceed. Finally, the program was extensively R1-R12 - 1-Kilo-ohm (or use one-/twoPrinter driver program resistor networks modified and successfully run using Epson overview Miscellaneous: - Centronics connector and 9-pin printer. The program, along with cable During initialisation, some memory locaTables II and III showing the status and tions are kept aside to Table III store the ASCII equivaPort B of 8255—(Input) Status Signals lent of the characters that Cent pin no NU 14 31 1 NU 36 NU NU are to be printed. This is Signal AF INIT STR DSL Comments followed by configuration Data C7 C6 C5 C4 C3 C2 C1 C0 X 0 1 1 X 0 X X =30H (initialisation) of 8255 by X 0 0 1 X 0 X X =10H Printer sending the mode con- long delay initialisation trol world to its control X 0 1 1 X 0 X X =30H register. To initialise X 0 0 1 X 0 X X =20H the printer first send ini- Short delay strobe tialisation (INIT*) pulse X 0 1 1 X 0 X X =30H for a few microseconds. NU=Not Used Then send the select ELECTRONICS PROJECTS Vol. 22

29

Memory Location

Instructions

Code

7110 LXI H, 7000 21 7111 00 7112 70 7113 LXI D2A20 11 7114 20 7115 2A 7116 MOV A, H 7C 7117 CALL 70FC CD 7118 FC 7119 71 711A MOV A, H 7C 711B CALL7100 CD 711C 00 711D 71 711E MOV A,L 7D 711F CALL 70FC CD 7120 FC 7121 70 7122 MOV A, L 7D 7123 CALL 7100 CD 7124 00 7125 71 7126 MOV A, M 7E 7127 CALL 70FC CD 7128 FC 7129 70 712A MOV A, M 7E 712B CALL 7100 CD 712C 00 712D 71 712E INX H 23 712F MOV A, M 7E 7130 CPI CF FE 7131 CF 7132 JNZ 7116 C2 7133 16 7134 71 7135 MVIA, 43 3E 7136 43 7137 STAX D 12 7138 INX D 13 7139 MVI A, 46 3E 713A 46 713B STAX D 12 713C DCX D 1B 713D LXI H 2A20 21 713E 20 713F 2A 7140 MVI A, 82 3E 7141 82 7142 OUT 0B D3 7143 0B 7144 MVI A, 0B 3E 7145 0B 7146 OUT 0B D3 7147 0B 7148 CALL 7200 CD 7149 00 714A 72 714B MVI A, 05 3E 714C 05 714D OUT 0B D3 714E 0B 714F IN 09 DB 7150 09 7151 ANI 02 E6 7152 02 7153 CPI 02 FE 7154 02 7155 JNZ 714F C2

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ELECTRONICS PROJECTS Vol. 22

8085 Assembly Language Listing

Comments

Initialise memory locations to Store ASCII codes of the program.

To get the ASCII codes of address of memory location of the program to be printed using the subroutine.

To get the ASCII codes of the contents of the program to be printed using the subroutine.

Increment pointer to mem. location Move contents of mem, into acc. Whether it is end of the program. If not, start executing from 7116

If it is end of the program, transfer the code for CF.

Initialise mem. Pointer to block (2A20) where ASCII code of characters to be printed are stored. Initialise 8255 Write control word in control register of 8255 Reset printer.

Delay for a few microseconds.

Send SELECT signal.

Read the status of printer to find out whether the printer is selected.

IF printer is not selected, again read

Memory Location

Instructions

7156 7157 7158 MVI B, 04 7159 715A CALL 7220 715B 715C 715D INX H 715E DCR B 715F JNZ 715A 7160 7161 7162 MVI A, 20 7163 7164 OUT 08 7165 7166 MVI A, 09 7167 7168 OUT 0B 7169 716A MVI A, O8 716B 716C OUT 0B 716D 716E MVI C, 02 ter 716F 7170 CALL 7220 7171 7172 7173 INX H 7174 DCR C 7175 JNZ 7170 7176 7177 7178 MVIA, 0A 7179 717A OUT 08 717B 717C MVIA, 0D 717D 717E OUT 08 717F 7180 MVIA, 09 7181 7182 OUT 0B 7183 7184 MVIA, 08 7185 7186 OUT 0B 7187 MOV A,E 7188 XRA L 7189 JNZ 7158 718A 718B 718C MOV A,D 718D XRA H 718E JNZ 7158 718F 7190 7191 RST 1

Code

Comments

4F 71 06 04 CD 20 72 23 05 C2 5A 71 3E 20 D3 08 3E 09 D3 0B 3E 08 D3 0B 0E

the status of printer

02 CD 20 72 23 0D C2 70 71 3E 0A D3 08 3E 0D D3 08 3E 09 D3 0B 3E 08 D3 7B Ad C2 58 71 7A AC C2 58 71 CF

to print 2 codes. Use subroutine to transfer data. in polling mode.

Else counter of 4 is initialised in B register to print 4 digits of memory address (use subroutine to transfer data to printer in polling mode). Get next memory location check whether 4 characters are transferred.

send to (20) blank space to printer

to generate STROBE pulse to printer

Counter of 2 is initialised in C regis-

Get next memory location. Check whether 4 characters are transferred.

send LF code to printer

send CR code to printer

Check wheter the full program code are transferred to printer If not, continue to transfer next codes.

Else, stop executing the program

Subroutine for converting hexadecimal to ASCII codes 70FC RRC OF Rotate right 4 times to get 4 MSB. 70FD RRC OF 70FE RRC OF 70FF RRC OF 7100 ANI OF E6 Mask 4 LSBs 7101 OF 7102 CPI 0A FE Compare with 0A 7103 0A

Memory Location

Instructions Code Comments

7104 JC 7109 7105 7106 7107 ADI 07 7108 7109 ADI 30 710A 710B STAX D 710C INX D 710D RET

DC C6 70 C6 07 C6 30 12 13 C9

8085 Assembly Language Listing Memory Location

If it is less than 0A,

Add 07 to data Else add 30H to data to convert data into ASCII code.

Subroutine to transfer data in polling mode: 7220 MVI A 09 3E To generate the STROBE signal. 7221 09 7222 OUT 0B D3 7223 0B 7224 IN 09 DB Find whether the printer is not Busy 7225 09 7226 ANI 01 E6 7227 01 7228 CPI 00 FE 7229 00 722A JNZ 7224 C2 722B 24 722C 72

Instructions Code Comments

722D MOV A,M 722E OUT 08 722F 7230 MVI A,08 7231 7232 OUT 0B 7233 7234 MVI A,09 7235 7236 OUT 0B 7237 7238 MVI A,O8 7239 723A OUT 0B 723B 723C RET

7E D3 08 3E 08 D3 0B 3E 09 D3 0B 3E 08 D3 0B C9

Get the ASCII code from memory locations, send the data to printer to print. Send the strobe pulse with min 0.5µ duration.

Subroutine for delay: 7200 MVI C, FF 7201 7202 DCR C 7203 JNZ 7202 7204 7205 7206 RET

0E FF 0D C2 02 72 C9

Load C register with data FF. Decrement the contents of C reg. if the contents of C is not zero, goto 7202.

modified program used by EFY Addr. Hex code Label

Mnemonics

9000 310095 LXI SP, 9500H 9003 11209D LXI D,9D20H 9006 EB XCHG 9007 11209A LXI D, 9A20H 900A 7C X1 MOV A,H 900B CDFC90 CALL 90FCH 900E 7C MOV A,H 900F CD0091 CALL 9100H 9012 7D MOV A,L 9013 CDFC90 CALL 90FCH 9016 7D OV A,L 9017 CD0091 CALL 9100H 901A 7E MOV A,M 901B CDFC90 ALL 90FCH 901E 7E MOV A,M 90IF CD0091 CALL 9100H 9022 23 INX H 9023 7E MOV A,M 9024 FECF CPI CFH 9026 C20A90 JNZ X1 9029 3E43 VI A,43H 902B 12 STAX D 902C 13 NX D 902D 3E46 MVI A,46H 902F 12 TAX D 9030 1B CX D 9031 21209A XI H,9A20H 9034 3E82 VI A,82H 9036 D30B UT 0BH 9038 3E30 VI A,30H 903A D30A UT 0AH 903C 3E10 VI A,10H 903E D30A UT 0AH 9040 CD0092 ALL 9200H 9043 3E30 VI A,30H 9045 D30A UT 0AH

Remarks

Addr. Hex code

;Initialise stack pointer ;Store location where ;data to be printed starts ;into register pair DE ;Location where ASCII ;stored

9047 3E02 VI A,02H 9049 D308 UT 08H 904B CD5092 ALL 9250H 904E CD7092 ALL 9270H 9051 0604 X4: MVI B,04H 9053 CD249 X2: CALL 9224H 9056 23 INX H 9057 05 DCR B 9058 C25390 JNZ X2 905B 3E20 MVI A,20H 905D D308 OUT 08H 905F CD5092 CALL 9250H 9062 CD7092 CALL 9270H 9065 0602 VI B, 02H 9067 CD2492 X3: CALL 9224H 906A 23 INX H 906B 05 CR B 906C C26790 JNZ X3 906F CD9092 CALL 9290H 9072 7B MOV A,E 9073 AD XRA L 9074 C25190 JNZ X4 9077 7A MOV A,D 9078 AC XRA H 9079 C25190 JNZ X4 907C 3E03 MVI A,03H 907E D308 OUT 08H 9080 CD5092 CALL 9250H 9083 CD7092 CALL 9270H 9086 3E04 MVIA,04H 9088 D308 OUT 08H 908A CD5092 CALL 9250H 908B CD 7092 CALL 9270H 9090 76 HLT

;Convert data to be ;printed into ASCII

;End of data? ;ASCII code of C

;ASCII code of F

;Initialise mem. Pointer ;to start of ASCII codes ;Initialise 8255 ;Initialise Printer

;Call delay

Label

Mnemonics

;Subroutine for converting hex to ASCII 90FC 0F RRC 90FD OF RRC

Remarks ;ASCII code for start of text ;Call status ; Call strobe ;Counter of 4 for printing ;four digits of addresses ;of memory location

;Send blank space to printer ;Call status ; Call strobe ;Counter of 2 for printing ;two digits of data

;Call LFCR ;Check whether all data ;has been transfered for ;printing

;ASCII code for end of text ;Call status ; Call strobe ;ASCII code for end of ;transmission ;Call status ; Call strobe

;Rotate four times to get MSB

ELECTRONICS PROJECTS Vol. 22

31

Addr. Hex code 90FE 90FF 9100 9102 9104 9107 9109 910B 910C 910D

Label

OF OF E60F FE0A DA0991 C607 C630 X5: 12 13 C9

Mnemonics

Remarks

RRC RRC ANI 0FH PI 0AH C X5 DI 07H ADI 30H STAX D INX D RET

;Output Subroutine 9224 7E 9225 D308 9227 CD5092 922A CD7092 922D C9

MOV A,M OUT 08H CALL 9250H CALL 9270H RET

;Delay subroutine 9200 C5 9201 06FF 9203 0EFF X7: 9205 0D X6: 9206 C20592 9209 05 920A C20392 920D C1 920E C9

PUSH B MVI B,FFH MVI C,FFH DCR C JNZ X6 DCR B JNZ X7 POP B RET

;Status subroutine 9250 C5 9251 06FF X9: 9253 0EFF XS: 9255 DB09 9257 E60F 9259 FE06

PUSH B MVI B,FFH MVI C,FFH IN 09H ANI 0FH CPI 06H

control signals that have been used in program implementation, is included for the benefit of readers, who may try both the programs, if desired.) Address map of devices used: RAM locations used: 9000H to 92AEH (70FCH onwards used by author)

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ELECTRONICS PROJECTS Vol. 22

;Mask four bits of LSB

;Output one byte of data ;Call status ;Call strobe

;In port B ;Compare with 06H

Addr. Hex code 925B 925E 925F 9262 9263 9266 9267

Label

CA6692 0D C25392 05 C25192 C1 X11: C9

;Strobe subroutine 9270 3E20 9272 D30A 9274 C5 9275 0EFF 9277 0D X10: 9278 C27792 927B C1 927C 3E30 927E D30A 9280 C9

Mnemonics JZ X11 DCR C JNZ X8 DCR B JNZ X9 POP B RET

MVI A,20H OUT 0AH PUSH B MVI C,FFH DCR C JNZ X10 POP B MVI A,30H OUT 0AH RET

;Line feed and Carriage return Subroutine 9290 3E20 MVI A,20H 9292 D308 OUT 08H 9294 CD5092 CALL 9250H 9297 CD 7092 CALL 9270H 929A 3E0A MVI A,0AH 929C D308 OUT 08H 929E CD5092 CALL 9250H 92A1 CD7092 CALL 9270H 92A4 3E0D MVI A,0DH 92A6 D308 OUT 08H 92A8 CD5092 CALL 9250H 92AB CD 7092 CALL 9270 H 92AE C9 RET

Port A (output): 08H Port B (input): 09H Port c (output): 0AH Control word register: 0BH Important memory location: Stack pointer initialised: 9500H Data to be printed is stored at: 9D20H

Remarks

;ASCII code for space ;Call status ;Call strobe ;ASCII Code for Line feed ;Call status ;Call strobe ;ASCII code for Carriage ;Return ;Call status ;Call strobe

onward ASCII conversion of data to be printed starts at 9A20H. Data to end with CFH as the last byte. The actual-size, single-sided PCB layout of the printer interface circuit and the component layout are shown in Figs 4 and 5, respectively. ❏

MORSE PROCESSOR Junomon Abraham

Mo

rse code, introduced by Samuel Morse, is still used universally even though better modes of communication are now available Following are the main reasons for its preference over other means of communication. 1. It enables communication with distant stations, using low-power transmitters. 2. It avoids the problem of regional accents and pronunciation. 3. It has the ability to override noise as it occupies only a fraction of the bandwidth required for a radio telephony signal. The circuit presented here converts text into the corresponding Morse code, and vice versa. The high light of this circuit is that is can interpret Morse signals available in the form of sound from ham radio or any other source. It is very useful for not only learning but also for transmission and reception of Morse code. It can find application in ham radio, telegraphy, etc.

been used, which relieves the microprocessor from scanning the keyboard and display. Here, 25 keys, including SHIFT and CTRL keys, and six 7-segment common cathode character displays are used. Though 7-segment displays are not suitable for alphanumeric characters, these have been used here with some compromise for reducing the overall cost. (Note: The use of dot-matrix LCD display avoids the difficulty in displaying characters in 7-segment format. One can go for a microcontroller design, if needed.) The TABLE II Address Distribution Device Address EPROM 0000 to 03FF RAM 1000 to 17FF 8279: Command Port 21 Data Port 20

7-segment display pattern employed for different characters is shown in Table I. Two hardware interrupts, RST5.5 and RST6.5, are used for reading the key entries. These are driven by the IRQ line from the keyboard/display interface IC 8279. A buffer (IC8) is connected at the display output of 8279 to drive the 7-segment displays. The encoded scan lines (SL2-SL0) are decoded by an octal decoder 74LS138 (IC9), whose outputs drive the common cathode of displays via transistor switches. The keys are wired in such a way that those can be represented by the seven higher order rate of the keyboard data. Morse signals in the form of sound are converted to microprocessor-compatible signals. The arrangement comprises condenser microphone, preamplifier, and retriggerable monostable multivibrator

Hardware The circuit is configured around the basic 8085 microprocessor. For simplifying the overall design, a programmable keyboard/display interface 8279 chip has ELECTRONICS PROJECTS Vol. 22

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PARTS LIST Semiconductors: IC1 - 8085A microprocessor IC2 - 74LS373 octal D-type latches IC3 - 6116 RAM (2 kB) IC4 - 27C32 EPROM (4 kB) IC5 - 8279 keyboard/display decoder IC6, IC9 - 74LS138 3-bit binary decoder IC7 - 74LS123 retriggerable monostable multivibrator IC8 - 74LS244 octal bus driver IC10 - 7805 +5 volt regulator T1 - BC548 npn transistor T2 - BC549 npn transistor T3-T8 - BC558 pnp transistor Dl - 1N4148 switching diode LED1 - LED DIS1-DIS6- LTS543 common-cathode display Resistors (all ¼ watt, ±5% carbon, unless

and also acts as a stack. One can enter/store a maximum of approximately 1,750 characters in the RAM. This is adequate for normal applications. In case one needs to store lengthy text, one should use a larger capacity RAM. Battery backup may be used for avoiding loss of data due to power failure. The low-level address/ data lines of 8085 are demultiplexed using an octal transparent latch IC 74LS373. The address bits A12 and A13 are decoded by IC6 to generate chip select (CS) signals to various ICs. The address map of devices is indicated in Table II.

stated otherwise) Rl - 68-kilo-ohm R2 - 3.3-kilo-ohm R3 - 2.2-kilo-ohm R4 - 5.6-kilo-ohm R5 - 1-mega-ohm R6 - 15-kilo-ohm R7, R8 - 1-kilo-ohm R9-R16 - 68-ohm R17-R22 - 220-ohm R23 - 180-ohm VR1 - 2.2-kilo-ohm preset VR2 - 100-ohm preset Capacitors: C1 - 2.2uF, 16V electrolytic C2, C4, C6 - 0.luF ceramic disc C3 - 10uF, 16V electrolytic C5 - 0.001uF ceramic disc C7 - 10pF ceramic disc Miscellaneous: PZ1 - Piezo buzzer MIC - Condenser microphone S1-S26 - Tactile switches for keyboard XTAL - 6.144 MHz crystal

The software driver routines for the circuit, along with their Assembly language code, are listed in Appendix A. Basically, the following functions are performed by the software program: 1. Initialisation of the peripherals. 2. Reading the depressed key data and its storage in RAM. 3. Writing data into the display RAM in 8279. 4. Generation of Morse code. 5. Recognition of Morse code from its sound. 6. Giving proper messages at appropriate time. Since Morse code is a time-dependent code, the program contains many jump instructions. The program has been make interactive and user-friendly. The

74LS123 (IC7). The output of IC7 drives SID pin of 8085 and it is in ‘high’ logic state when a sound is detected by the microphone. The sensitivity of the amplifier can be adjusted by preset VR2. The converted Morse code drives a piezo buzzer via a transistor connected at the SOD line of 8085 microprocessor. Intensity of the sound can be controlled by potentiometer VR1. The firmware is stored in 27C32 (4k EPROM—only 1 kB is needed for the program). RAM 6116 stores the keyboard entries

34

Firmware

firmware is divided into the following modules: (a) booting, (b) keyboard, (c) transmit, (d) receive, (e) play, and (f) lookup table. The logic of the program can be generally understood from the Assembly language listing given in Appendix A. A brief description of each module is, however given below: (a) Booting: The section initializes stack pointer 8279 and the interrupts. It also fixes defaults speed for Morse code. It is the first module executed when you switch on the power supply. (b) Keyboard: When a key is pressed, IRQ pin of 8279 interrupts 8085. The ISR (interrupt service routine) reads the keyboard data and, if needed, does some manipulations. It also displays the entered characters in the 7-segment display. (Table III has been included by EFY for ready reference by the readers to know the hex data generated by 8279 when any key is either pressed alone or in combination with SHIFT or CTRL key.) (c) Transmit. This module converts each character in the RAM to its corresponding Morse code signals which are output through the SOD line. The speed of transmission or words per minute depends on the value entered in the setup menu. (d) Receive. The acquisition of Morse code is done by checking the presence of sound with time. The module continuously monitors the SID pin of 8085 microprocessor, where the

Table IV Lookup Table Chr/word 0 1 2 3 4 5 6 7 8 9 A B C D E F G H I *Notes:

ELECTRONICS PROJECTS Vol. 22

Address 0300 0304 0308 030C 0310 0314 0318 031C 0320 0324 0328 032C 0330 0334 0338 033C 0340 0344 0348

Hexcode Chr/word Address Hexcode Chr/word 3FAA0E00 J 034C IE A9 03 00 . 06 A9 0E 00 K 0350 70 E6 00 00 , 5BA5OEOO L 0354 38 59 03 00 ; 4F95 0E00 M 0358 55 3A 00 00 ? 66 55 0E 00 035C 46 - 6D55 0D00 7D 56 0D 00 N 0380 54 36 00 00 EOM* 07 5A0D00 O 0384 5C EA 00 00 WAIT* 7F 6A 0D 00 P 0388 73 69 03 00 BT* 6F AA 0D 00 Q 038C 67 9A 03 00 SK* 77 39 00 00 R 0390 50 D9 00 00 SELECt 7C 56 03 00 S 0394 6D D5 00 00 39 66 03 00 T 0398 78 0E 00 00 trAnst 5E D6 00 00 U 039C 3E E5 00 00 M oVEr 79 0D 00 00 V 03A0 2A 95 03 00 rECEIE 7165 03 00 W 03A4 6A E9 00 00 3D DA 00 00 X 03A8 52 96 03 00 SEtup 76 55 03 00 Y 03AC 6E A6 03 00 30 35 00 00 Z 03B0 4B 5A 03 00

1. EOM=End of message= 3 2. WAIT= 8 3. BT=Sentence separation= 4. SK=End of work= 3 3

Address 03B4 03B8 03BC 03C0 03C4 03C8 03CC 03D0 03D4 03D8 03DC 03E0 03E4 03E8 03EC 03F0 03F4 03F8 03FC

Hexcode 80 99 39 00 04 5A 3A 00 84 66 36 00 D3 A5 35 00 08 56 39 00 00 3F 00 00 OF 99 0D 00 7E 59 0D 00 49 56 0E 00 4F 95 39 00 6D 79 38 79 39 78 78 50 77 54 6D 78 55 00 5C 2A 79 50 50 79 39 79 30 79 6D 79 78 3E 73 00 00 00 00 00 00 00

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Size 215×100 mm

sound-converted logic level (depending on whether the sound is present or not) is available. It compares this logic level with a prefixed time value and accordingly decides whether the sound was due to dot or dash. Moreover, it displays characters corresponding to the entered Morse code. (e) Display. This module displays characters in the moving display format as per the entered message. The speed of movement is fixed to approximately three characters per second. (f) Lookup table (Table IV). This is a block of data, which contains the 7-segment data for every character and the data needed for Morse code generation or reception. Each character takes four EPROM locations. The first location indicates the 7-segment data, while the second and third locations hold the Morse data code. The fourth location is unused. (EFY Note. We have included Table IV showing the hex data generated by depression of any key alone or in combination with SHIFT or CTRL keys, for ready reference by the readers.)

Control-key functions Before going to the operating procedure, we have to know the functions of key associated with CTRL key. CTRL+SETUP (8EH). The default speed is initialized for approximately 5 words/minute. If you want to change this setting, you can do so by using this control key combination. When you press this combination k, the message ‘SEtUP’ is displayed. Here you can enter any one of the characters ranging from ‘1’ through ‘9’ and ‘A’ through ‘K’ to change the speed. Note that the minimum speed is associated with ‘K’ and the maximum with ‘1’. CTRL+CLEAR (98H). It clears the RAM content. CTRL+PLAY (84). CTRL+PLAY is used for displaying the RAM content in moving format. You can interrupt any process by pressing any control key that has no function. CTRL+CONT (86). It is used for continuing the play operation if it were interrupted.

Operating procedure 1. Switch on the power supply. A message

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ELECTRONICS PROJECTS Vol. 22

Fig. 2: Actual-size, single sided PCB for the Morse processor

APPENDIX ‘A’: 8085 ASSEMBLY LANGUAGE PROGRAM LISTING Addr. Opcode

Label

Mnemonics

Comments

Booting 0000 31FF17 LXI SP.17FFH Initialise stack pointer 0003 3E10 MVIA.10H Initialise 8279 0005 D321 OUT 21H 0007 3E40 MVI A.40H 0009 D321 OUT 21H 000B 3E0D MVI A.0DH 000D 30 SIM Activating RST6.5 000E 325017 STA 1750H Updating mode and position data 0011 211D00 LXI H.001DH 0014 225117 SHLD 1751H 0017 21246C LXIH.6C24H Fixing default setup 001A 227017 SHLD 1770H 001D 11DC03 LXI D.03DCH 0020 CDE000 CALL DISPLAY Display ‘SELECt’ 0023 FB El 0024 76 HLT Halt RST 5.5 002C C3F700

JMP 00F7H

Go to ISR of RST5.5

RST 6.5 0034 DB20 IN 20H Reading keyboard data from IC 8279 0036 F5 PUSHPSW Store it in the stack 0037 FE8A CPI8AH Checking CNTL+ RECEIVE key 0039 CA0002 JZ RECEIVE 003C FE8C CPI8CH Checking CNTL+ TRANSMIT key 003E CA8001 JZ KEYBOARD 0041 FE84 CPI 84H Checking CNTL+PLAY key 0043 CAD001 JZ PLAY 0046 FE86 CPI 86H Checking CNTL+ CONTINUE key 0048 CAD501 JZ 01D5H 004B FE98 CPI98H Checking CNTL+ CLEAR kev 004D C26000 JNZ 0060H 0050 210010 LXIH.1000H Clearing the RAM 0053 36C8 MVI M.C8H 0055 23 INXH 0056 7C MOV A, H 0057 FE17 CPI 17H 0059 DA5300 JC 0053H 005C 2A5117 LHLD 1751H Return to mode from 005F E9 PCHL where clearing action is called 0060 FE8E CPI8EH Checking CNTL+ SETUP key 0062 C27700 JNZ 0077H 0065 3E0E MVIA.0EH Activating RST5.5 0067 30 SIM 0068 11F403 LXI D.03F4H 006B CDE000 CALL DISPLAY Display the message ‘SEtUP’ 006E FB El 006F 76 HLT 0070 3E0D MV1A.0DH Activating RST6.5 0072 30 SIM 0073 2A5117 LHLD 1751H 0076 E9 PCHL Return to mode from where setup action is called

Addr. Opcode

Label

Mnemonics

0077 3A5017 LDA 175OH 007A B7 ORAA 007B CA8000 JZ 0080H 007E FB El 007F 76 HLT 0080 F1 POPPSW 0081 FE92 CPI92H 0083 C28900 JNZ 0089H 0086 2B DCXH 0087 2B DCXH 0088 C9 RET 0089 FE90 CPI90H 008B C8 RZ 008C FE80 CPI80H 008E C29600 JNZ 0096H 0091 110500 LXID.0005H 0094 19 DAD D 0095 C9 RET 0096 FE82 CPI 82H 0098 C2A000 JNZ 00A0H 009B 11F9FF LXI D.,FFF9H 009E 19 DAD D 009F C9 RET 00A0 FE88 CPI88H 00A2 CA1001 00A5 FE96 CPI 96H 00A7 C2BA00 JNZ 00BAH 00AA E5 PUSH H 00AB 46 MOV B,M 00AC 2B DCX H 00AD 70 MOV M,B 00AE 23 INX H 00AF 23 INX H 00B0 7C MOVA.H 00B1 FE17 CPI 17H 00B3 DAAB00 JC 00ABH 00B6 El POPH 00B7 2B DCX H 00B8 2B DCX H 00B9 C9 RET 00BA FE94 CPI 94H 00BC C2D100 JNZ 00D1H 00BF 2B DCX H 00C0 E5 PUSH H 00C1 46 MOVB.M 00C2 36C8 MVI M.C8H 00C4 23 INX H 00C5 7E MOV A,M 00C6 70 MOV M,B 00C7 47 MOV B,A 00C8 7C MOVA.H 00C9 FE17 CPI 17H 00CB DAC400 JC 00C4H 00CE E1 POPH 00CF 2B DCX H 00D0 C9 RET 00D1 FE7F CPI7FH 00D3 D2CF00 JNC 00CFH 00D6 07 RLC

Comments The following CNTL key functions are only for TRANSMIT mode Checking whether we were in the TRANSMIT mode

Getting key closure data which is stored in stack Checking CNTL+<—key Shifting the characters right by one place Checking CNTL+ßkey Shifting the characters by one place Checking CNTL+TABR key Shifting the characters left by six places

Checking CNTL+TABL key Shifting the characters right by six places

Checking CNTL+ START key JZ TRANSMIT Checking CNTL+DEL key

Delete one character in the left most position of the display

Checking CNTL+INS key Inserting a space for adding character

Checking whether key data is valid character

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Addr. Opcode

Label

00D7 77 00D8 C9

Mnemonics

Comments

Addr. Opcode

MOVM,A RET

Enter it into the RAM Return

0156 0159 015A 015D 015E 0161 0162 0163 0164 0165 0168

DISPLAY SUBROUTINE: 00E0 0E06 DISPLAY: MVI C,06H 00E2 1A LDAX D 00E3 D320 OUT 20H 00E5 13 INX D 00E6 OD DCR C 00E7 C2E200 JNZ 00E2H 00EA CDCOO1 CALL DELAY2 00ED CDC001 CALL DELAY2 00F0 C9 RET

Display six characters taken from lookup table Lookup table is pointed to by DE -reg pair

Wait for some time Return

VECTOR RST 5.5 00F7 DB20 IN 20H Reading key closure data from 8279 00F9 E63F ANI3FH 00FB 07 RLC 00FC 327017 STA 1770H Storing clot value 00FF 47 MOV B,A 0100 80 ADD B 0101 80 ADDB 0102 327117 STA 1771H Storing dash value 0105 C9 RET Return TRANSMIT SUBROUTINE: 0110 31FF17 TRANSMIT: LXI SP.17FFH 0113 7C MOVA.H 0114 FE17 CPI17H Checking end of mem. 0116 D2B301 JNC 01B3H 0119 1603 MVID.03H 011B 5E MOVE.M 011C 1A LDAX D 011D D320 OUT20H Display character in the RAM 011F F3 DI 0120 E5 PUSHH 0121 0EO4 MV1C.04H Morse code generation 0123 13 INXD 0124 1A LDAX D 0125 F5 PUSHPSW 0126 217017 LXIH.1770H 0129 E603 ANI 03H 012B FE01 CPI01H 012D CA4801 JZ 0148H 0130 23 INXH 0131 FE02 CPI02H 0133 CA4801 JZ 0148H 0136 FE03 CPI 03H 0138 CA5901 JZ0159H 013B Fl POPPSW 013C El POPH 0131 FB El 013E 7E MOVA.M 013F 23 INXH 0140 FECC CPICCH Checking end of message character’]’ 0142 C21001 JNZ0110H 0145 C3B301 JMP01B3H 0148 3ECD MVIA,CDH Setting SOD line 014A 30 SIM 014B 46 MOVB.M 014C CD7001 CALLDELAY1 Waiting 014F 05 DCRB 0150 C24C01 JNZ 014CH 0153 3E4D MVIA.4DH Resetting SOD line 0155 30 SIM

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ELECTRONICS PROJECTS Vol. 22

Label

217017 46 CD7001 05 C25A01 Fl 0F 0F 0D C22501 C32101

Mnemonics

Comments

LXI H.1770H MOVB.M CALL DELAY1 Waiting DCRB JNZ015AH POPPSW RRC RRC DCRC JNZ 0125H JMP 0121H

DELAY1 SUBROUTINE: 0170 E5 DELAY1: PUSH H 0171 21CF01 LXIH.01CFH 0174 2B DCXH 0175 7C MOVA.H 0176 B5 ORAL 0177 C27401 JNZ0174H 017A E1 POPH 017B C9 RET

Executing these instructions require approximately 3 msec

KEYBOARD SUBROUTINE: 0180 AF KEYBOARD: XRAA Updating mode and positing data 0181 325017 STA 1750H 0184 218001 LXIH.0180H 0187 225117 SHLD 1751H 018A 11E203 LXI D.03E2H Displaying message 018D CDE000 CALL DISPLAY ‘trAnSf for indicating the TRANSMIT mode 0190 31FF17 LXI SP.17FFH 0193 210610 LXIH.1006H Entering keyboard data to the RAM 0196 11FBFF LXI D.FFFBH 0199 19 DADD 019A 0E06 MVI C.06H 019C 1603 MVID.03H 019E 5E MOV E,M 019F 1A LDAX D 01A0 D320 OUT20H 01A2 23 INX H 01A3 0D DCRC 01A4 C29E01 JNZ 019EH 01A7 7C MOVA.H 01A8 FE17 CPI17H Checking end of mem. 01AA DAB301 JC 01B3H 01AD 11E803 LXI D.03E8H 01B0 CDE000 CALL DISPLAY If mem. is over display ‘MoVEr’ 01B3 FB El 01B4 76 HLT 01B5 C39601 JMP 0196H DELAY2 SUBROUTINE: 01C0 0E9F DELAY2: MVIC.9FH 01C2 CD7001 CALL DELAY1 01C5 0D DCRC 01C6 C2C201 JNZ 01C2H 01C9 C9 RET PLAY SUBROUTINE: 01D0 1603 PLAY: 01D2 210510 01D5 F3 01D6 23 01D7 7C

MVI D.03H LXI H.1005H DI INXH MOVA.H

Wait approximately 400 msec

Addr. Opcode

Label

Mnemonics

01D8 FE17 CPI 17H 01DA D2EB01 JNC 01EBH 01DD 5E MOVE.M 01DE 1A LDAX D 01DF D320 OUT20H 01E1 CDC001 CALLDELAY2 01E4 FB El 01E5 7E MOVA.M 01E6 FECC CPICCH 01E8 C2D501 JNZ 01D5H 01EB C3B301 JMP01B3H

Comments

Addr. Opcode

Checking end of mem.

0258 OF RRC 0259 OF RRC 025A B2 ORA D 025B 4F MOVC,A 025C ID DCR E 025D C22902 JNZ 0229H 0260 El POP H 0261 71 MOVM.C 0262 23 INX H 0263 C32302 JMP 0223H 0266 79 MOVA.C 0267 OF RRC 0268 OF RRC 0269 F6C0 ORIC0H 026B ID DCR E 026C CA7402 JZ 0274H 026F OF RRC 0270 OF RRC 0271 C36B02 JMP 026BH 0274 El POP H 0275 77 MOV M,A 0276 0638 MVI B.38H Comparing obtained 0278 21FD02 LXI H.02FDH moree code data with lookup data 027B 3A8017 LDA 1780H 027E 23 INXH 027F 23 INXH 0280 23 INX H 0281 23 INXH 0282 05 DCR B 0283 C29102 JNZ 0291H If given morse code is invalid, display V 0286 FE04 CPI 04H 0288 DA1D02 JC 021DH 028B 215C03 LXI H.035CH 028E C39F02 JMP 029FH 0291 BE CMPM 0292 C27B02 JNZ 027BH 0295 3A8117 LDA 1781H 0298 23 INXH 0299 BE CMP M 029A 2B DCX H 029B C27B02 JNZ 027BH 029E 2B DCX H 029F Dl POPD 02A0 7A MOV A,D 02A1 FE17 CPI 17H Checking end of mem. 02A3 D2D102 JNC 02D1H 02A6 7E MOVA.M 02A7 D320 OUT 20H Display the character 02A9 7D MOV A,L Store data, corresponding 02AA 12 STAXD to displayed character, in the RAM 02AB 0600 MVI B,00H 02AD 217017 LXI H.1770H 02B0 7E MOVA.M 02B1 07 RLC 02B2 23 INX H 02B3 86 ADD M 02B4 D2B802 JNC 02B8H 02B7 04 INRB 02B8 4F MOV C,A 02B9 0B DCX B 02BA CD7001 CALL DELAY1 02BD 20 RIM 02BE 07 RLC 02BF DA1B02 JC 021BH Check for space between 02C2 78 MOVA.B words 02C3 B1 ORAC 02C4 C2B902 JNZ 02B9H 02C7 AF XRAA 02C8 D320 OUT 20H Giving space in display 02CA 13 INX D 02CB 3EC8 MVI A,C8H 02CD 12 STAX D Store the apace data in the RAM 02CE C31B02 JMP021BH Repeat the process 02D1 76 HLT Halt

Displaying data in RAM Wait for some time

Checking end of mem. symbol’]’ Go to keyboard module

RECEIVE SUBROUTINE: 0200 3EFF RECEIVE: MVIA.FFH Updating mode and position data 0202 325017 STA 1750H 0205 210002 LXI H,0200H 0208 225117 SHLD 1751H 020B 11EE03 LXI D.03EEH 020E CDE000 CALL DISPLAY Display message ‘rECEIE’ 0211 11FA03 LXI D.03FAH 0214 CDE000 CALL DISPLAY Clear the display 0217 FB El 0218 110510 LXI D.1005H Morse code aquisition 021B 13 INXD 021C D5 PUSH D 021D 218117 LXIH.1781H 0220 3600 MVI M.00H 0222 2B DCX H 0223 E5 PUSH H 0224 0E00 MVI C,00H 0226 1E04 MVI E.04H 0228 61 MOVH.C 0229 0600 MVI B.00H 022B CD7001 CALL DELAY1 022E 04 INR B 022F 20 RIM Reading the SID pin 0230 07 RLC 0231 DA2B02 JC 022BH 0234 24 INR H 0235 3A7117 LDA 1771H 0238 BC CMPH Checking for the space between characters 0239 DA6602 JC 0266H 023C 78 MOV A,B 023D FE02 CPI02H 023F DA2902 JC 0229H 0242 2600 MVI H.00H 0244 1640 MVI D,40H 0246 3A7117 LDA 1771H 0249 OF RRC 024A B8 CMPB Checking for dot and dash 024B D25702 JNC 0257H 024E 7A MOV A,D 024F 07 RLC 0250 57 MOV DA 0251 00 NOP 0252 00 NOP 0253 00 NOP 0254 00 NOP 0255 00 NOP 0256 00 NOP 0257 79 MOVA.C Constructing morse code data

Label

Mnemonics

Comments

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‘SELECt’ will be displayed. By depressing the appropriate key, you can select any one of the following modes: (a) transmit, (b) receive, (c) setup, (d) play, (e) continue, and (f) clear. 2. Press CTRL+TRANSMIT keys for entering into the transmit mode. A message ‘trAnSt’ appears for a second, after which you can enter your message. 3. At the end of the message you have to enter ‘]’ symbol (by pressing SHIFT+] keys, i.e. 66H) for invoking the microprocessor. 4. By the use of arrow key ( or  or by TAB (TAB R or TAB L) keys, set the location in the message at which the transmission is to start. If you want to transmit the message from beginning depress CTRL+TRANSMIT keys again for getting into the first character. 5. Press CTRL+START keys for getting Morse code of the message. 6. You can go to any other mode by selecting the correspond mode before finishing the transmission or later. 7. For entering into the receive mode, press the CTRL+RECEIVE keys. You will see the ‘rECEIE’ message for one second. 8. Generate Morse code using a buzzer, voice, or some other source (such as ham radio and recorded tape). 9. The acceptance of sound will be indicated by LED1 for duration of ‘Dit’/’Dash’. If LED does not light, adjust the gain of the amplifier using potmeter VR2. 10. The converted data can be replayed by pressing the CTRL+PLAY keys. Note. 1. The clear and setup control keys can be used, at any time, if needed.

Construction PCB designed particularly for this circuit (as given in Fig. 2, with component layout shown in Fig. 3) is needed for making this circuit. IC bases are preferred for fixing the ICs. For continuous operation, provide a heat sink for the regulator IC. Since this is based on time comparison, it is necessary to use the correct frequency crystal (6.144 MHz). Fig. 3: Component layout for the PCB

Readers’ comments: Q1. In the software part of this project some steps are missing, due to which the processor is not functioning properly.

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ELECTRONICS PROJECTS Vol. 22

Going through the software, we found an error at the address 003CH, after which the processor doesn’t jump to the transmit mode and hangs up. Please correct the

anomalies in the software. Sidharth M. Modi Mumbai Q2. The project ‘Morse Processor’ pub-

lished in March issue correctly outputs in the initial stages but displays the message ‘transt’ on going to the transmit mode. After this when I press any key, it doesn’t take the input and doesn’t display the same in the 7-segment display. As a corollary, it’s also not giving the corresponding Morse code. Sunil Kumar B. Through e-mail Q3. How can I interface CRT display with 8085 microprocessor? Also provide me the complete circuit along with the IC numbers. Anwar Ali, Hyderabad

The author, Junomon Abraham, replies: A1. The firmware is correct and the same was duly tested at EFY. “You need not doubt the code at location 003CH, where it checks for the Ctrl+Transmit key. From here, it goes to the keyboard routine and feeds the message to the RAM. The actual conversion process starts when we press Ctrl+Start key, during which the processor goes to the transmit routine. A2. Make sure that you have loaded the look-up table (Table IV) in the EPROM and that the entered program

is absolutely correct. If the same conditions still persist, the problem is with your hardware. A3. It is possible to add a CRT controller to 8085 by changing the firmware. You can use the easily available 6845 CRT controller. The article based on this IC was published in the book Learn to Use Microprocessors by K. Padmanabhan published by EFY. The project ‘Video Display Add-On Board for the 8085 Kit’ based on 9364 chip was published as a technical article on page 21 in EFY’s Aug. ’83 issue. The same is also reproduced in Chapter XI of the above titled book.

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Access-Control System bhaskar banerjee

T

he easy-to-construct access control (code lock) circuit presented here incorporates the following unique features: (a) Many people can use the same system with their own unique 6-digit code. (b) A single-digit system code has been included, which is common to all users of the system. It can be easily changed with the help of DIP switches.

If any one or more of the six consecutive keyboard-entered digits do not conform to the predetermined code, an alarm generator sounds the alarm to indicate wrong code. If the result of final comparison of all the six digits is correct, a mono multivibrator, serving as lock driver for opening/closing a lock, gets activated for a fixed preset duration. The detailed description of individual units, as shown in Fig. 2, is as follows: Keyboard and keyboard encoder. Description The keyboard consists of 16 push-to-on The block diagram of the system shown type keys in a 4x4 matrix format. It can be in Fig. 1 provides an overall view of its made using data switches or one can use composition and working. A 16-digit keymembrane-type keyboard at some extra pad is used for sequentially entering six cost. The keys should be numbered in Hex Hex numbers, which are decoded by the as shown in the figure. keyboard encoder into their equivalent The encoder is built around 74C922 binary numbers and stored in separate (IC1), which is a 16-key keyboard encoder. data latches in binary form. It generates a 4-bit binary number correThe first three Hex numbers are used sponding to the key pressed; for example, as an address for an EPROM, which shorting pin 1 (R1) with pin 11 (C1) generstores a predetermined code at prefixed ates the binary equivalent of digit ‘0’. addresses allocated to separate users or Whenever a key is pressed, the signal used for separate purposes. The code data generated by this encoder IC is available output from EPROM (one byte/two nibbles) as logic ‘high’ output at pin 12 and is used at a specified address is compared with to activate a piezo-buzzer (PZ1) via tranthe next two keyboard entries in two 4-bit sistor T1 (BC547). The continuous tone of comparators that are cascaded together. PZ1 indicates that a key is pressed. The The resultant outputs of these two key-pressed signal is also used to store comparators are connected to the next data in the latches. comparator stage, in which the last The output from pin 12 is connected to keyboard digit (i.e. sixth Hex digit) is pin 13 of IC5 (CD4017 counter) for clockcompared with the system code selected ing at its trailing edge. On each clocking, by DIP switch. counter IC5 advances by one count and thereby stores data in separate data latches one after the other. IC1 also holds the last number at its output pins. Data latches. There are six data latches formed from three CD4508 ICs (IC2 through Fig. 1: Block diagram of the access-control system

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IC4). Each CD4508 contains two completely independent 4-bit data latches having a common power supply. The 6-digit code is stored in these latches. The 4-bit data bus originating from the output of IC1 is connected to data Parts List Semiconductors: IC1 - 74C922 16-key encoder IC2-IC4 - CD4508 dual 4-bit latch IC5 - CD4017 decade counter IC6 - 27C32 EPROM IC7-IC9 - CD4063 4-bit magnitude comparator IC10 - CD4528 dual retriggerable monostable IC11 - NE555 timer IC12 - CD4069 Hex inverter T1-T4 - BC547 npn transistor T5 - SL100 npn transistor T6 - 2P4M SCR D1, D2, D4 - 1N4148 switching diode D3 - 1N4007 rectifier diode LED1-LED3 - Red LED LED4 - Green LED Resistors (¼-watt ±5% carbon, unless stated otherwise) R1, R3, R4, R15, - 10-kilo-ohm R2, R5, R8, R21, R22 - 4.7-kilo-ohm R6 - 18-kilo-ohm R7 - 10-mega-ohm R9 - 2.2-mega-ohm R10, R11, R17-R20 - 1-kilo-ohm R12-R14 - 470-ohm R16 - 47-kilo-ohm R23 - 47-ohm Capacitors: C1, C7, C8, C12 - 0.1µF ceramic disc C2 - 2.2µF, 25V electrolytic C3, C5, C6, C9, C10 - 22µF, 25V electrolytic C4, C13 - 47µF, 25V electrolytic C11 - 470µF, 25V electrolytic Miscellaneous: S1 - Push-to-on switch S2 - Push-to-off switch - 4x4 keyboard matrix PZ1 - Continuous tone-type piezobuzzer RL1 - 9V, 200-ohm, 1 C/O relay S3 - 4-way DIP switch - Regulated 5V power supply etc

Fig. 2: Schematic diagram of access-control system

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Fig. 3: Actual-size, single-sided PCB layout for access-control system

input pins of all the six latches in parallel. For example, pin 17 (QA) of IC1 is connected to the corresponding pins 4 and 16 of all the latches as the LSB of 4-bit binary output from IC1. Initially, pin 3 of IC5 provides a high output to ‘clear’ and ‘store’ pins 1 and 2 of IC2A, thereby clearing its 4-bit register. When a key is pressed, the equivalent binary code is present at data input pins of all the latches. On releasing the key, pin 12 of IC1 changes its state from ‘high’ to ‘low’, thereby generating the required clock pulse for IC5. This clocking makes pins 3 and 2 of IC5 low and high, respectively, causing the binary data corresponding to the first Hex digit keyboard entry

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to be stored and available at the output of IC2A. Similarly, when the second key is pressed, new data is stored in IC2B without affecting the previously stored data in IC2A. The outputs from first three data latches are connected to address pins of EPROM 27C32 (IC6). The outputs from fourth and fifth data latches are connected to two 4-bit magnitude comparators IC7 and IC8 (CD4063), and the output from sixth data latch is connected to a similar 4-bit magnitude comparator IC9 for further processing. The memory. All 8-bit codes, except the 4-bit system code, are stored at different locations (addresses) in the EPROM

(IC6). Out of the six Hex digits, first five digits are used as personalised code, and out of these five digits, the first three are used to form an address for EPROM. The leftmost digit of the code is the MSD (most significant digit) and the third digit from left is the LSD (least significant digit) of the 12-bit wide address for IC6. The fourth and fifth digits from left are to be the same as the data stored in IC6 (beforehand) at that particular address. Thus, when a code is entered via the keyboard, the fourth and fifth digits are compared with the data stored at the address formed by the first three digits. (The EPROM can be programmed with the help of ‘Manual EPROM Programmer’, and may be replaced by an EEPROM for better reliability.) Code comparator. There are three 4-bit comparators (IC7 through IC9) used in the circuit, which are cascaded together to form a 12-bit comparator. Comparators IC7 and IC8 compare the 8-bit data output of EPROM with the corresponding fourth and fifth digits entered via the keyboard and stored in latches IC3B and IC4A. While IC7 compares the upper 4-bit output of IC6 with the contents of IC3B (i.e. the fourth digit from left), IC8 compares the lower 4-bit output of IC6 with the contents of IC4A (i.e. the fifth digit from left). Similarly, IC9 compares the last digit (i.e. the contents of IC4B) with the code entered/formed by 4-way DIP switch S3 (marked A through D), which is referred to here as the system code. This system code digit can be changed from time to time. The result of the comparison by the three comparators is finally available from IC9. If the entered code matches with the stored data, pin 6 of IC9 goes high, indicating a correct code. Otherwise, either of pins 5 and 7 goes high depending upon the magnitude of the data. Pins 5 and 7 are connected together via diodes D1 and D2 and used as the trigger for alarm circuit. The outputs from IC9 are available only after entering the last digit. Alarm generator. The alarm generator is built around a 555 timer (IC11). The logic ‘high’ output from pin 5 or pin 7 of IC9 triggers the SCR and applies Vcc supply to IC 555 to make it oscillate. The output from pin 3 of IC11 is used to drive transistor T2 (BC547) to generate a longduration alarm tone from PZ1. A common buzzer is used for key-press audio indicator and alarm generator to

gering, pin 6 of IC10 becomes high and remains in that state for a predetermined time period. The output at pin 6 of IC10 drives transistor T5 (SL100) to operate relay RL1. When the system is locked, red LED1 glows, and when it is unlocked, green LED4 glows. The other half of IC10 is used to keep the keyboard activated for a predetermined time. The keyboard is activated by pressing switch S1. This feature improves the security of the system.

Construction Data input/output pins are to be connected with utmost care because improper connection will force the system to work unpredictably. Also, care should be taken while using IC1, as it is quite costly. The points marked Vcc should be connected to the power supply directly. The system can be built on a generalpurpose PCB or a veroboard. A singlesided PCB layout for the circuit is, however, shown in Fig. 3, with its component layout shown in Fig. 4.

Operation

keep the cost low. The output from pin 3 of timer also drives LED2, which flashes at the output frequency of the astable oscillator.

MMV and lock driver. When a valid code is entered, pin 6 of IC9 becomes high and triggers monostable multivibrator CD4528 (IC10) via transistor T3. On trig-

Initially, when IC1 is disabled by IC10, no code can be entered. To activate the keyboard, press switch S1 momentarily. This will activate the keyboard for a predetermined time. The code should be entered within this time. Using the 4-way DIP switch S3, the system code can be changed at any time for extra security. If wrong code is entered, the buzzer sounds alarm and the red LED starts flashing. In this case, you can reset the circuit by a momentary depression of switch S2. It is to be noted that no display unit is used, to keep the code secret. But if you still prefer to have one, the same could be included. ❏

Readers’ comments: Q1. The construction project is very interesting and useful. However, how memory dump is to be programmed in EPROM IC6 is not given. Though different people would like to program different codes, at least one example should have been given to illustrate this.

Praveen Shanker, Haridwar A1. EFY: Though programming of EPROM is well explained by the author, here is an example of coding. Let us say address of the EPROM where a specific code is stored is 41A (Hex). It is equivatent to 0100 0001 1010, which is used as address All through AO of the EPROM.

Now assume data stored is B5 (Hex), i.e. 1110 0101 at the above mentioned addresses. Let the system code be E (Hex), i.e. 1010. For getting this system code close DIP switches B, C, D, and leave A open (in S3).Thus access code=41A B5E (Hex) or MSB.......................LSB 0100 0001 1010 1011 0101 1110 (Binary).

Fig. 4: Component layout for the PCB

ELECTRONICS PROJECTS Vol. 22

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Telephone Line-Interfaced Generic Switching system ajay subramanian and nayantara bhatnagar

Q

uite a few projects using DTMFto-BCD decoder ASIC MT 8870/ KT 3170 have appeared in EFY during the past few years. The project presented here also uses the same ASIC, but it is used here as part of a circuit in which a fairly advanced switching logic with adequate foolproofing and authentication is implemented. The major features of this circuit are: • Programmable password protection over a public network • Foolproof mechanisms for events such as time-out delays, incorrect password, and power-on initialisation • Expandable design The primary objective of this circuit is to make a fairly low-cost device for controlling up to a hundred household switches remotely over any public/private telephone network.

Description The block diagram of the system is shown in Fig. 1. It consists of the following three units: 1. The interface and control unit 2. The authentication unit 3. The main device selection and switching circuit The interface and control unit provides control signals and BCD data to the other two units. It handles interfacing with

the telephone line and also generates control signals for hanging up (HUP) and a universal reset pulse, which is used by the authentication circuit for its operation. Its design may be altered to achieve connectivity to another network, which is capable of providing certain control and data signal sequences. The authentication unit stores four presettable digits of code data and compares the same against the 4-digit DTMF code sent via the telephone lines before the time-out occurs. If the 4-digit code is found valid, the authentication unit issues an authorisation signal to the main device selection and switching unit. However if an incorrect password is entered, the device terminates the call by returning to the off-hook condition. The fifth DTMF digit determines the address of the group to be selected, while the sixth digit determines the device number that is to be selected within that group. The selected device can be switched on or switched off by a momentary depression of the telephone keypad switches marked * (code1011 binary) and # (code 1100 binary), respectively. Thus you can select any one of the hundred devices, divided into ten groups, to be switched on/ off, as desired—one at a time. The interface and control unit (Fig. 2). This unit performs the following

Fig. 1: Block diagram of telephone line-interfaced generic switching system

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functions: • Detects an incoming call. Counts up to a programmable number of rings and then simulates handset off-cradle condition. • Once off-hook, it must decode DTMF signals on the telephone line within a fixed time and generate appropriate BCD data and StD pulse for indicating a valid data condition. The positive edge of this StD pulse is used for subsequent operations. • Generates a universal Reset signal that includes a time-out and a power-onreset. This Reset signal is an active low pulse of programmable duration. • Generates a hang-up (HUP) signal on expiry of the time-out and uses this signal internally to take the device offline. When a call arrives, a 75-80V AC ring signal is available on the lines. This ring signal is coupled to optocoupler Opto-1 (MCT2E) via DC blocking capacitor C1 and current-limiting resistor R1. LED1 serves as a ring indicator and as an antiparallel diode to the in-built LED of the optocoupler for working with AC ring signal. The output of optocoupler triggers timer IC1, which is configured as a monostable retriggerable flip-flop to provide a pulse output to be used as a clock for decade counter IC2 (CD4017) with decoded outputs.

Fig. 2: Circuit diagram of the interface and control unit

The pulse-width of monostable should be slightly greater than 0.6 second to ensure that the pulse does not terminate during the 0.2-second pause between a pair of ring signals of 0.4-second duration. Thus the monostable produces one pulse for each ring (in fact, a pair), which clocks CD4017 counter. IC2 will freeze after counting a pre-programmed number of rings. This number is determined by its output pin which is tied to its pin 13. In Fig. 2 pin 9 (O8 output) is shorted to pin 13. Thus count of IC2 is frozen at the beginning of the eighth ring. The first pulse from IC1 also triggers the first stage of monostable multivibrator 74123 (IC5), which causes the Reset output to go high. As a result, CD4017 (IC2) is enabled (which was otherwise reset, when no ring signal was present). Also, the authentication circuit is enabled to receive BCD data and control signals, as and when generated by MT8870 (IC4). If the preset count is reached and

the call has not been answered yet (local telephone handset still on cradle), the counter (IC2) is frozen and ‘D’ flip-flop (IC3A) is set. This activates relay RL1 that places a 220-ohm load across the lines to simulate handset off-cradle condition and also enables MT8870 (IC4) by applying a ‘low’ at its inhibit (active high) pin 5. This causes the ring signal, in turn, to be taken off the telephone lines (by telephone exchange) and establish a connection (analogous to the maturity of a call). The circuit is now ready to receive signals from the remoteend telephone. In case the call is answered from the local telephone before the preset count of IC2 is reached, the ring ceases as the local telephone is in off-hook condition. Since there is no other way of re-triggering IC5, a time-out eventually occurs and the device reinitialises all units automatically. The device is also protected against activation by dialing from a parallel phone instrument, since the ring signal is necessary to power up the ASIC MT8870 (after

a pre-programmed number of rings). MT8870 (IC4) generates an StD pulse whenever fresh data is latched onto its outputs. This signal is used as a ‘data valid’ gate wherever appropriate. Also, when a key is pressed, an ESt (Early steering) pulse is generated at its pin 16, which lasts till the key is pressed. This ESt pulse is used for clocking IC12B in the authentication and control unit and retriggering monostable multivibrator 74123 (IC5), extending the duration of Reset pulse. This ensures that the circuit will operate as long as the user presses keys within preset time intervals, or else a time-out is decreed and the device is reset. The resetting process includes hangup (HUP) state, clearing the authentication circuit status, and consequently deactivating the main switching circuit, thus restoring the device to its initial state. (The flip-flops, which control devices in the main device selection and switching unit, are allowed to retain their states.) ELECTRONICS PROJECTS Vol. 22

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Fig. 3: The authentication unit circuit diagram

The power-on-reset circuit comprises resistor R5 and capacitor C10. It resets the device when power to the circuit is switched on. Since it is low for some time after power is switched on, it resets the flip-flop (IC3A) and decade counter CD4017 (IC2). Fig. 4 shows the relative timing waveforms pertaining to this unit. LED2 through LED5 are used to show the BCD output for the DTMF code received over the telephone lines (decoded after relay RL1 has energised). The authentication unit (Fig. 3). This circuit receives BCD data and StD control signal initially. It outputs authorisation (Auth) signal only when the correct security code has been entered. Control pulses can reach the ‘main switching unit’ only when this signal is low (implying that authentication of the four digit code sent over the telephone lines has been verified). Note that when a wrong code is received, IC9A clocks IC9B and a low is latched by IC9B. As a result the Q2 output of IC9B goes high and saturates transistor T2 in the interface and control unit and thereby shunts capacitor C10 to ground, thus simulating a power-on-reset condition. As a consequence CLR signal (at output of IC6A) is activated and the line interface circuit is initialised. Also, since the monoshot IC5 is cleared, Reset goes low (active) and resets the authentication unit also. When the Authentication unit is initialised, IC9A and 9B are set, which causes Q2 output of IC9 to be reset, and thus transistor T2 is cut off again. Capacitor C10 now charges through resistor R5 as it did when the circuit was initially switched on. The Reset signal is initially low. As

If the time-out period expires, the Reset pulse falls and the falling edge is used to trigger the second stage of monostable multivibrator 74123 (IC5). The complemented output 2Q of the second stage of IC5 is a HUP (hang-up) signal that clears relay driver flip-flop 74LS74 (IC3A), thus causing the device

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to hang up, and inhibit IC4. It resets CD4017 (IC2) counter that counts the number of rings. Also, as a Reset pulse goes low, it resets the authorisation circuit. The additional circuitry around the input of IC4 protects inbuilt op-amp input terminals within the chip.

Fig. 4: Signal waveforms of the interface and control unit

Fig. 5: Method of programming code using DIP switches

a result, this circuit is in its initialised state, wherein IC13 (CD4017) is reset and ICs 9A and 9B (7474) are set (i.e. their Q outputs are high and Q outputs are low). Also, IC12A has its CLR pin low and it is in reset state with its Q pin low. As stated earlier, the Auth signal is initially high. The password consisting of four 4-bit words is applied at the input pins D0-0 through D0-3 to D3-0 through D3-3 of 74LS244 ICs 15 and 14 respectively as shown in Fig. 3. These words may be programmed using thumbwheel switches or arrays of DIP switches with pull-down resistors as shown in Fig. 5. As soon as the device establishes a call (i.e. relay RL1 energises after a preprogrammed number of rings), the authentication unit (and not the main device selection and control unit) is activated due to a high Reset pulse that is generated as soon as the ring arrives and also on every pressing of a key due to (ESt) signal from IC4 via OR gate (IC7A), which triggers IC5. Now the caller is expected to enter the 4-digit password sequence from the remote telephone set in DTMF mode. Initially, only the first word (nibble) of the array of tri-state buffer drivers (ICs 74LS244) is enabled by O0 output of IC13 (CD4017). As a result, the first 4-bit programmed word is applied to the ‘P’

inputs of 4-bit comparator IC 74LS85 (IC8). LED7 through LED10 indicate the preset data present at ‘P’ inputs of the comparator. The other 4-bit ‘Q’ inputs for comparison are obtained from the BCD output decoded by IC4 upon pressing a DTMF telephone key at the remote end. This comparator result is available at pin 6 of IC8 before the arrival of StD pulse. On the arrival of StD pulse, the output of the comparator is latched into ‘D’ flipflop (IC9A). Initially, both flip-flops (IC9A and 9B) are set, as explained earlier. So the ‘CLK’ input of the second flip-flop (IC9B) is low. If at any instant, a low is latched into the first flip-flop IC9A (as a result of a failed match between the preset code and the code entered via the remote telephone set), the second flip-flop (IC9B) is clocked, and it latches a low at its Q output. This resets decade counter IC13 via inverter IC11F. The Q2 output of IC9B is normally low. But when a wrong password is entered, this output goes high. As a result, transistor T2 (2N2222) of the interface and control unit, grounds the power-onreset capacitor C10, as stated earlier. Thus the unauthorized call is terminated when the CLR signal (from the output of IC6A) is activated. As a result, IC2 and IC3A are reset (asynchronously) and the off cradle simulation circuit is deactivated. Also since the Reset signal is low, all other units are initialised. This feature ensures that a denial-of-service attack (wherein unauthorised agents engage the system and thus prevent authorised users from using it) is discouraged. However, if correct codes are entered, each time when a StD pulse arrives, it clocks CD4017 (IC13) counter so that the next word is applied at the input of the comparator. The result of the current comparison (high) is latched into the first ‘D’ flip-flop (IC9A). When the user presses all four keys in the correct sequence, the first flip-flop always latches a high and the second flip-flop is never clocked. At the end of the sequence, when the last digit is com-

pared and the result is latched, O4 output of CD4017 (IC13) goes high, and as a result, IC12A is clocked and latches a ‘high’ at its Q output and the input to inverter gate IC11E and ‘D2’ pin of flipflop IC12B goes high. Simultaneously, signal at the output of gate IC11E goes low. This low signal at pin 12 of IC10D AND gate disables the gate from accepting any further StD pulses. So the authentication unit is bypassed and subsequent BCD data and StD pulses are transmitted to the main switching unit. The ESt pulse associated with fifth BCD data, latches the high signal at D2 input of IC12 to its Q2 output, while its Q2 output (Auth) goes low to activate the main device selection and switching unit at the start of fifth code. When the Reset signal goes high, the output of inverter gate IC11F goes low. This enables IC13 (CD4017) again by taking its MR pin low. At the same time, the high ‘Preset’ signal at both the flip-flops (IC9A and IC9B) keeps them enabled. When the code is not entered within preset period, the Reset signal goes low on expiry of the time-out period, the circuit again goes back to its initial state by taking the preset pin on the flip-flops (IC9A and 9B) low and MR pin of CD4017 (IC13) high. Simultaneously, IC12A is cleared (its Q output goes low). As a result, Auth output goes high and the main device selection and switching circuit is initialised and deactivated. Since the initialised state is maintained as long as the Reset signal is low, any possibility of noise triggering is eliminated. Main device selection and switching unit (Fig. 6). This circuit receives StD control signal after a successful authentication of the four-digit code by the authentication unit. The AUTH and its inverse AUTH signals available on code authentication are used in this circuit for enabling various chips such as IC23 and IC24 (74LS195), IC25 (CD4017), IC27 through IC29 (74LS154), and StD gate IC19C (7408). A combinational logic circuit, comprising three 3-input NOR gates inside 7427 (IC16) and two inverter gates (IC17B and 17C) of 7404, has been used to discriminate between an address (numeric digit) and a switching signal (‘*’ for ‘on’ and ‘#’ for ‘off’). DTMF digit switches 1 through 9 and 0 (0 on the telephone keypad stands for decimal 10 and the decoded output ELECTRONICS PROJECTS Vol. 22

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from MT8870 is the equivalent binary number 1010) generate a logic-1, R_EN (register enable) signal, while keys marked ‘*’ and ‘#’ generate a logic-1, S_EN (switching enable) signal. Thus this combinational logic differentiates between register enable (R_EN) and device switching enable (S_EN) signals. The R_EN and S_EN outputs for various key depressions of the telephone keypad are shown in Truth Table. The combinational logic circuit is followed by the RCLK and SCLK generation circuitry comprising ICs 18, 19, 25, and 26, which allows the following functions to be performed: • After AUTH signal at Q (pin 8 of IC12B in Fig. 3) goes low (active), one can select a group and a device within the selected group by next two DTMF switch depressions on the telephone keypad, while a third key depression of ‘*’ or ‘#’ results into switching ‘on’ or ‘off’ of the desired device. • Multiple devices can be switched on/ off one after the other, once authorisation signal AUTH becomes active (low) without a system reset. • The system can be reset after or before switching ‘on’/‘off’ of the desired device with the help of remote telephone keypad. This feature can also be used for avoiding switching on/off of a device if the user perceives that he has selected a wrong device. When R_EN signal is logic 1, IC25 (CD4017) is clocked at the leading edge of StD pulse, while one of the 74LS195 registers (IC23 or IC24, as enabled by one of the Q outputs of IC25) is latched at the trailing edge of the delayed Std pulse (RCLK) as indicated by the direction of arrow on RCLK pulse in Fig. 6. The resistor-capacitor combinations R26-C11 and R25-C12 wired around Schmitt inverter gates A through D of IC26 (7414) provide the necessary delay for reliable latching of the data in IC23 and IC24. Resistors R27 and R28 across capacitors C11 and C12, respectively, serve as bleeders for discharging the respective capacitors. When S_EN signal is logic 1, clocking of 7474 ‘D’ flip-flops via active 74LS125 gates occurs corresponding to the leading edge of Std (SCLK) pulses, while the trailing edge resets IC25 via capacitor C14, to enable receiving of fresh group and device selection data. Group selection. When any of DTMF numeric keys 1 through 9 and 0 on the

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remote telephone keypad is depressed immediately after AUTH signal goes active low, R_EN signal goes to logic 1 (while S_EN is logic 0). As a result, Std pulse passing through NAND gates IC18B and IC18C clocks IC25 with its leading edge. IC25 is in reset condition before code authentication due to ‘high’ AUTH signal, and its Q0 (pin 3) is ‘high’. On clocking, shifting of ‘high’ state from Q0 to Q1 (pin 2) enables AND gate IC19B, while AND gate IC19 is still disabled. Thus the trailing edge of RCLK passes through IC19B to latch the MT8870-decoded data corresponding to the mentioned numeric key depression, which is available at the input of group select register IC24, at its output. This is the group select address. The group select address is applied to the address lines of 4-line-to-16-line decoder IC29 (group selector). In the normal telephone keypad, we use only ten numeric keys (1 through 9 and 0) and hence only ten outputs (Y1 through Y10) are available from IC29. The other six outputs Y0 and Y11 through Y15 are not used. Thus we can select any of the groups 1 through 10 via outputs marked Y1 through Y10 of IC29. The output corresponding to the address present at IC29’s input pins goes low (active). This low (active) output selects/enables another IC 74LS154 representing the corresponding group. (Please note that this is only a demo version circuit, wherein only two groups, out of ten possible groups, can be accessed using IC27 and IC28. Pin 19 of IC27 and IC28 can be connected to any of the group select pins Y1 through Y10 of IC29, as desired. Once connected, the specific group numbers will get allocated to IC27 and IC28.) Device selection within the selected group. The next DTMF number key depression (i.e. the sixth after energisation of relay RL1 or the second after the 4-digit authentication code) causes shifting of ‘high’ on pin 2 (Q1) of IC25 to pin 4 (Q2) in synchronism with the leading edge of StD pulse clocking IC25. As a result, AND gate IC19A is enabled while AND gate IC19B is disabled. The trailing edge of delayed StD pulse (RCLK) causes the data corresponding to the mentioned numeric key to be latched at the output of device select register IC23. This device select address is applied to address input pins of all group ICs (IC27 and IC28, here) in

Parts list Semiconductors: IC1 - NE555 timer IC2, IC13, IC25 - CD4017 decade counter IC3, IC9, IC12, IC21, IC22 - 7474 dual ‘D’ flip-flops IC4 - MT8870 DTMF decoder IC5 - 74123 dual retriggerable monostable multivibrator IC6 - 7411 triple 3-input AND gates IC7 - 7432 quad OR gates IC8 - 74LS85 4-bit magnitude comparator IC10, IC19 - 7408 quad 2-input AND gates IC11, IC17 - 7404 hex inverters IC14, IC15 - 74LS244 octal buffers/line drivers IC16 - 7427 triple 3-input gates IC18 - 7400 quad 2-input NAND gates IC20 - 74125 quad bus buffers IC23, IC24 - 74195 4-bit parallel access shift registers IC26 - 7414 hex Schmitt inverters IC27-IC29 - 74LS154 4-line to 16-line decoders Opto-1 - MCT2E opto-coupler T1,T2 - 2N2222 npn transistor D1,D2 - 1N4001 rectifier diode D3, D4 - 1N4148 switching diode ZD1, ZD2 - Zener diode 5.1V LED1-LED10 - Red LEDs Resistors (1/4W ± 5% carbon, unless specified otherwise) R1, R2, R5, R29 - 10-kilo-ohm R3, R12, R30 - 100-kilo-ohm R4 - 220-ohm R6-R9 - 51-kilo-ohm R10 - 39-kilo-ohm R11 - 56-kilo-ohm R13 - 330-kilo-ohm R14-R18 - 1.2-kilo-ohm R19 - 20-kilo-ohm R20, R27, R28 - 1-mega-ohm R21-R24, R31-R34 - 470-ohm R25,R26 - 1-kilo-ohm R31-R34 - 4.7-kilo-ohm Capacitors: C1 - 0.47µF, 160V polyester C2,C4-C6 - 0.01µF ceramic disc C3, C9, C13 - 10µF, 16V electrolytic C7, C8, C14 - 0.1µF ceramic disc C10 - 100µF, 16V electrolytic C11, C12 - 47µF, 16V electrolytic Miscellaneous: Xtal - 3.57946MHz quartz crystal RL1 - Relay 6V, 100-ohm, 1 C/O - 5V, 1A regulated power supply - Berg stick/FRC connectors - Ribbon cable etc.

parallel. However, since only one group IC is in selected condition (as explained earlier), the device control output corresponding to the device select address present at the active group input is pulled low. This active low output is used

Fig. 6: Main device selection and switching unit

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ELECTRONICS PROJECTS Vol. 22

Fig. 7: Actual-size, single-sided PCB for the circuits in Figs 2 and 3

Fig. 8: Actual-size, single-sided PCB for the circuit in Fig. 6

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ELECTRONICS PROJECTS Vol. 22

as the control signal for a corresponding tri-state gate of 74LS125 (IC20). We have shown only four gates, out of possible 100, in this circuit. The output pins of tri-state gates are connected to the clock inputs of the corresponding ‘D’ flip-flops (only four out of possible 100 are shown). The clock pins of IC21 and IC22 have been pulled high to avoid any noise triggering when tri-state buffers are in high-impedance state. Switching the selected device. Only one device corresponding to the digit in the registers—IC24 for group address and IC23 for device address—is enabled to be affected by the signal (‘*’ or ‘#’) as the seventh (or the third after authentication) code. On pressing DTMF keypad switch ‘*’ or ‘#’, the selected device is switched on or switched off depending on the key pressed. D0 bit of the decoded switching signals ‘*’ and ‘#’ is applied to data pins of all 7474 flip-flops in parallel. Only the data corresponding to the selected device gets clocked via the corresponding tri-state gate of 74LS125. The Q2 output of IC25 is still high when SCLK is generated and, as a result, AND gate IC18D is enabled to allow application of SCLK to all 74LS125 gates on depression of either ‘*’ or ‘#’ on the remote keypad. Switching takes place at the trailing edge of SCLK pulse, while the trailing edge of SCLK pulse causes resetting of IC25, thereby creating conditions that were unavailable just before the previous group selection. Subsequently, you can select any other (or the same) group and any other (or the same) device. You can switch on or off the selected device by following the same procedure. Switching on or off refers to Q output of the corresponding ‘D’ flip-flop (7474) going high or low, respectively. You may suitably use the flip-flop outputs to energise a relay or fire a triac or control the corresponding device/devices. If you press any number key (1 through 9 or 0) instead of ‘*’ or ‘#’ key on the DTMF keypad, IC25 will receive a clock pulse via AND gates IC18B and IC18C, and the ‘high’ state will shift from Q2 to Q3 (pin 7 of IC25). Since Q3 output is coupled to the base of transistor T2 via diode D4, it will result into a system reset. A system reset implies that you have to redial the local telephone number from remote telephone. When relay RL1 again energises, redial the four-digit authentica-

Fig. 9: Component layout for PCB-1

Fig. 10: Component layout for PCB-2

tion code, followed by group select, device select, and switch on (*) or switch off (#) codes, as explained earlier. Thus, after dialing two digits identifying the group and the device within that group, if we press a third numeric digit instead of ‘*’ or ‘#’ on the remote telephone keypad, a system reset can be achieved remotely. This feature can also be utilised to bypass switching operation if the user realises that he has selected a wrong group/device. Operation summary. The entire operation can be summarised as below: • Using the remote telephone keypad, dial the local number of the telephone to which the circuit is connected. • If the local handset is lifted before the programmed number of rings, a normal conversation can be ensured. • If the handset is not lifted before the programmed number of rings, wait for simulated off-hook status of the local telephone handset (indicating energisation of relay RL1). • Now dial the four digits of the preset authentication code in a proper sequence from the remote keypad within the preset duration. A system reset will occur in case the 4-digit code is not dialed within the preset duration or the code used is wrong, which causes de-energisation of the relay and creates conditions similar to on-hook state of the local telephone handset. So you will have to repeat all steps from the beginning. • If the 4-digit authentication code matches the preset code, you can dial the next two digits identifying the group and the device within that group selected for the purpose of switching on or off (or even as a dummy operation for the purpose of forcing a system reset). • Dialing ‘*’ from the remote telephone keypad will result into switching on of the selected device, while dialing ‘#’ will result into switching off of the selected device. (Dialing any number, 1 through 9 or 0, causes a system reset. Relay RL1 will be de-energised, and you will have to restart from the initial step.) You can proceed with the same procedure to switch on/off the next selected device. The procedure can be repeated for any number of devices (one-at-a-time) without affecting the status of non-selected devices. Testing. It is recommended that the circuit be built in stages, verifying proper operation at each stage. The main switchELECTRONICS PROJECTS Vol. 22

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ing circuit may be assembled conventionally, with logic operation tested

at various points. The authentication circuit is also selfcontained and may be assembled and debugged independently. H o we v e r, ca re must be taken while assembling the interface and control unit. The ASIC must be assembled first and tested for proper operation and output levels, followed by rigging and testing of monostable multivibrators in the 74123.

The ring pulse generator and decade counter, CD4017, comes next. Finally, interface connections between the various circuits should be made after verifying the proper functioning of each circuit in isolation. A single-sided PCB for the circuits in Figs 2 and 3, is shown in Fig. 7, while another single-sided PCB for the circuit in Fig. 6 is shown in Fig. 8. The component layouts for both the PCBs are given in Figs 9 and 10, respectively. Suitable connectors are provided to enable isolation and joining of individual circuits using jumpers/connectors, for easy testing and fault analysis during assembly. ❏

Readers’ comments: We have the following queries regarding the project: 1. The interface and control section. In IC2 (CD4017), where would be pins 3, 2, 4, 7, 10, 1, 5, 9, 6, 11, and 12 connected? Also show the connections of available pin 11 (RST) in IC5 (74123). 2. The authentication section. Show the connections of pins 1, 5, 6, 9, 10, 11, and 12 in IC13 (CD4017). 3. The selection and switching unit. (a) Pins 2 to 11 (Y1 to Y10) of IC27 (74LS154) and IC28 (74LS154) are connected in parallel. Pins 2 and 11 of IC27 are connected to pins 1 and 4 of IC20 (74LS125), respectively, and pins 2 and 11 of IC28 are connected to pins 10 and 13 of IC20 (74LS125), respectively. Pins 2 to 11 (Y1 to Y10) of IC29 (74LS154) are connected to the normal telephone keypad. Are all these connections correct? (b) After entering IC16 (7427), four data lines D0 through D3 go to IC23 (74LS195) and thereafter IC24 (74LS195) and then go out as shown in the PCB layout. Please explain which data goes in IC16 and where does it output in IC24. Are these connections related to PCB1? Ranjit Singh Amravati

EFY: 1. The interface and control section. Only one of the ten outputs of IC2 decade counter needs to be connected to its pin 13 [and pin 3 of IC3(A)] to freeze IC2 and create a condition simulating lifting of handset after waiting for a few rings. This is well explained in the article. The maximum number of rings can be nine, to avoid false ring condition on the line. RST and Reset outputs are not going to part II and have been used via IC6(A) in part I itself. The flags just indicate the label of these signals. 2. The authentication section. The mentioned output pins of IC13, excluding pin 10, are not used/connected. Pin 10 is Q3 and is used as clock for IC12(A). 3. The selection and switching unit. (a) The first four digits dialled from the remote telephone keypad are used for matching the authentication code. After authentication, the fifth digit selects the group. If the fifth digit is ‘1’ then IC27 (group 1) will be selected, and if the fifth digit is ‘0’ (code 1010 or ten decimal) then IC28 (group 10) will be selected. The sixth dialled digit selects the device inside the selected group. The seventh dialled digit (‘*’ for ‘on’ or ‘#’ for ‘off’) of the selected device or ‘0 through 9’ for circuit reset can be used as desired. The output pins reflecting the device

number in the selected group are not connected in parallel and only their address (input) pins are connected in parallel, which are controlled by deviceselect outputs from IC23. However, the selection of a group is dependent on IC29, which, in turn, is controlled by groupselect IC24. Output connections Y1 and Y10 from IC27 pertain to device Nos. 1 and 10, respectively, of group 1, while connections Y1 and Y10 from IC28 pertain to device Nos. 1 and 10, respectively, of group 10. Each device is controlled via 74LS125 gate in conjunction with one flip-flop from 7474 IC. In the circuit only two groups (1 and 10), with two devices (1 and 10) from each group, have been shown instead of all the ten groups and a hundred (ten per group) devices. You should be able to correlate as to which 74LS125 (IC20) gate and 7474 section (IC21 and IC22 pair) select which device out of which group. (b) Data lines D0 through D3 are coming from IC4 and go into pins labelled D0 through D3 of IC8, IC16, IC17, IC23, and IC24, respectively. They are thus related to PCB1. The Auth signal is correctly connected to IC27, IC28, and IC29 in Fig. 7 and there is no ambiguity.

Truth Table for Device Selection and Switching Keypad Decoded data input Switch and register Key from MT8870 enable outputs No. D3 D2 D1 D0 S_EN R_EN 1 0 0 0 1 0 1 2 0 0 1 0 0 1 3 0 0 1 1 0 1 4 0 1 0 0 0 1 5 0 1 0 1 0 1 6 0 1 1 0 0 1 7 0 1 1 1 0 1 8 1 0 0 0 0 1 9 1 0 0 1 0 1 0(10) 1 0 1 0 0 1 * (11) 1 0 1 1 1 0 #(12) 1 1 0 0 1 0

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ELECTRONICS PROJECTS Vol. 22

Programmable Melody Generator Vyjesh m.v.

A

number of melody generator circuits based on chips like UM3481, UM3482, UM34815A, UM66, etc have appeared in EFY. All these UMC chips contain preprogrammed masked ROM and are not field-programmable as such. Here is the detailed design of a typical melody generator circuit using different types of memories, including EPROM, RAM, and ROM (hard-wired). As soon as the power is switched on to UMXX series melody generators, a tune is heard, which stops after a while. When a switch on the melody generator is pressed, the second tune is heard. If the chip is capable of producing twelve tunes, each successive depression of the switch results in a new tune being played. After the twelfth tune has been played, the next depression of the switch causes the first tune to repeat, and so on. The circuit presented here can be programmed exactly the same way.

Basics of music Generally, an electronic organ or piano is played with both hands. Now imagine playing a 32-key organ with a single finger. In that case, only one key can be pressed at a time and hence only one note can be heard. Considering that the time taken by the finger to move from one

key to another is very short, the required notes can be played properly and hence the tune can be heard. The notes can also have breaks in between. This feature can be explained by considering five notes written in the following two ways: 1. Sa Re Ga Ma Pa 2. Sa---Re Ga---Ma Pa In the first case the notes are continuous. In the second case there are breaks (no sound), indicated by ‘—’ for a stipulated amount of time, in between Sa and Re as well as Ga and Ma. Each of the circuits explained in this project incorporates the break (no sound) feature. You should make sure that you have access to a musician before attempting any of the circuits. In addition, you would need a computer and a frequency meter or a digital multimeter. The computer is required to test the tunes, i.e. to make sure that the given notes match the tune of a given song. A brief on music from the software article ‘Generation of Indian Classical Music on a Microprocessor’ by Prof. V.V. Athani, published in April ’94 issue of EFY, is as follows: “Taking into account only one electronic organ (piano), the number of notes in music are only seven—Sa Re Ga Ma Pa Dha Ni. But these basic notes are divided into three octaves (refer Fig. 4), where each octave also has notes called

half notes. So each octave has twelve notes. On a piano keyboard, black keys in between white keys produce the average frequency of adjacent keys. For example, ‘Sa’ has a frequency of 595 Hz and ‘Re’ has a frequency of 668 Hz. When a black key in between them is pressed, a frequency of 631.5 Hz is produced. These black keys are called halfnote keys.” Here we have selected a total of 28 notes, including all notes from the middle octave, eleven notes from upper octave, and a few from the lower octave. All the 28 notes with their respective frequencies are given in Table I.

Software and testing of notes Before moving to the software program, let us see how the notes for a tune can be obtained. Give your musician the song for which you need notes. Write those notes in terms of Sa Re Ga etc, making sure that all the notes of the tune lie within the range of the 28 notes given in Table I. No sound in between the notes, including its duration, as also the duration of each particular note, should be taken into account. For example, if in a tune the time period of a note SA is 500 ms and that of Re 1500 ms, the two notes can be written as Sa Re Re Re. Similarly, no sound in between can be written as Sa Re-Re-Sa. The notes so obtained have to be converted into data characters. This can be done directly by using Table I; for example, Sa-Re Re Ga---Ma can be written as C-E E G---H. Execute the program

Fig. 1: Block diagram of EPROM-/RAM-based melody generator ELECTRONICS PROJECTS Vol. 22

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(refer Appendix ‘A’ for the source code of the program) and enter the delay value (say, 300). Now enter the first line of the tune and press ‘Enter’ key. The tune can be heard. This tune can be repeated by pressing ‘R’. If this tune needs to be changed, or a new tune is to be entered, press any key. In this way all the lines in a tune are tested line by line. After testing all the lines, enter all the lines of the tune once again and recheck the tunes until you are satisfied. Press ‘E’ to quit the program. Now convert the tunes (data characters) to hexadecimal values using Table I. These hexadecimal values are to be entered into EPROM/RAM at consecutive locations to get the tune.

Fig. 2: Main circuit of EPROM-/RAM-based melody generator

EPROM-/RAMbased melody generator

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Since most parts of the circuits for EPROM- and RAM-based melody generators are similar, the main circuits for both versions have been integrated in Fig. 2. Relevant changes have been described appropriately. The common block diagram for EPROMand RAM-based melody generators is shown in Fig. 1. A low-frequency oscillator followed by a binary counter is used to generate the addresses for EPROM/RAM. In the case of EPROM, the preprogrammed data output is directly coupled to two 1-of-16 decoders (one for upper nibble and the other for lower nibble of data). However, in RAM based-circuit, a keyboard

Fig. 3: Tone oscillator

Fig. 4: Piano keyboard

is deployed at the time of writing the data at specified locations (addresses) into the RAM. Thereafter the keyboard is detached and data output pins are connected to two 1-of-16 decoders, as in EPROM-based circuit. Only 28 outputs (out of 32 outputs) of the two decoders, with each representing a unique note, are used in conjunction with individual presets to control the oscillator’s frequency and thus the resulting sound from the loudspeaker

state (all outputs zero). The binary outputs of IC2 serve as the address for memory locations in the EPROM, where the data for the notes is stored. For the EPROM version, the pins of connector K2(F) are to be kept shorted to the corresponding pins of connector K3(M). Suffixes ‘F’ and ‘M’ within parentheses indicate female and male connectors, respectively. Data bits of the lower nibble (D0 through D3) are connected from EPROM to the address pins of 1-of-16 decoder IC4 (CD4514) and those of the higher nibble (D4 through D7) to the address pins of another 1-of-16 decoder IC5 (CD4514). The ‘Hex value’ column in Table I indicates that either the lower nibble or the upper nibble, or both nibbles, of stored hex data in memory will always be zero. It means that at least one of the

two CD4514 (IC4 and IC5) will have binary 0000 at its address input. The Q0 output of these ICs is not used for generating any note. The hex data 00 (i.e. 0000 0000) is, in fact, used for no sound. Similarly, hex values 01 (0000 0001) and 10 (0001 0000) are used for ‘Reset’ and ‘Stop-clock’ functions. The remaining 14 outputs from each of the two CD4514 (IC4 and IC5) are used together for generating one of the 28 notes corresponding to the hex data stored and the output from a specific memory location. The Q2 to Q15 outputs of IC4 and IC5 are connected via diodes D101 (and preset VR101) through diode D128 (and preset VR 128), respectively, to the tone oscillator circuit built around timer NE555 (IC101), as shown in Fig. 3. (EFY lab note. The numbering of diodes and other components of this circuit has been done for convenience.) IC101 is wired with presets to form an oscillator. At any time, only one of diodes D101 to D128, depending on the current note selected via EPROM’s addressed location, will be forward biased and its corresponding preset will form part of the oscillator circuit. Each preset is adjusted to a value (refer Table I) to obtain the frequency corresponding to the selected note. No sound (00 hex). Breaks are

EPROM-based circuit

In Fig. 2, NE555 timer (IC1) is wired in astable mode, which provides clock pulses for the 12-stage binary counter CD4040 (IC2). In the EPROM version, jumper J1 is used to permanently short pin 3 of IC1 and pin 10 of IC2, while there is no need to operate push-to-on switches S2 and S3 and you can leave them open (i.e. in off state). An 8-bit, 4k EPROM 2732 is used for IC3. Since its pin 21 is address A11, switch S6 is to be kept in position ‘a’ to connect it to O11 output of IC2. When clock pulses Fig. 5: Flow are fed to IC2, it starts chart of doorbell counting up from its reset

Fig. 6: Actual-size single-sided PCB-1 layout for circuit of Fig. 2 ELECTRONICS PROJECTS Vol. 22

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Fig. 7: Actual-size single-sided PCB-2 layout for circuit of Figs 3 and 11

Fig. 8: Component layout for PCB-1

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necessary in between the notes to make a tune sound perfect. The break period, termed as ‘no sound,’ is obtained by outputting hex value 00 from the EPROM. During this input to the two 1-of-16 decoder ICs (IC4 and IC5), the Q0 outputs of both decoder ICs go high. Since Q0 outputs are not connected to the tone oscillator circuit (or anywhere else), no note or sound is

Parts List (Common to EPROM, RAM and ROM) Semiconductors: IC101 - NE555 timer IC201 - 7805 +5V regulator D101-D128 - 1N4007 rectifier diode D201-D204 - 1N4001 rectifier diode Resistors (¼-watt ±5% carbon, unless otherwise stated) R101 - 5-kilo-ohm VR101-VR128 - Refer Table I VR129 - 10-kilo-ohm preset Capacitors: C101 - 0.1µF ceramic disc C102 - 0.22µF ceramic disc C103 - 10µF, 12V electrolytic C201 - 100µF, 25V electrolytic C202 - 1000µF, 16V electrolytic Miscellaneous: LS101 - 8-ohm, 4W loudspeaker X201 - 230V AC primary to 0-6V, 500mA sec. transformer (for EPROM and ROM) Semiconductors: IC1 - NE555 timer IC2 - CD4040 counter IC3 - (1) 2732 EPROM - (2) 6116 RAM IC4, IC5 - CD4514 1-of-16 decoder IC6 - CD4011 quad NAND gate T1-T8 - BC547 npn transistor D1-D64 - 1N4007 rectifier diode LED1-LED20 - Red LED Resistors (¼-watt ±5% carbon, unless otherwise stated) R1, R7 - 10-kilo-ohm R2 - 22-kilo-ohm R3, R8 - 470-ohm R4 - 1-mega-ohm R5, R9-R16 - 1-kilo-ohm R6 - 2.2-kilo-ohm R17, R18 - 100-ohm R19 - 330-ohm VR1 - 100-kilo-ohm preset Capacitors: C1 - 22µF, 12V electrolytic C2 - 0.1µF ceramic disc C3 - 0.01µF ceramic disc C4 - 0.22µF ceramic disc Miscellaneous: S1 - Push-to-off switch S2-S5 - Push-to-on switch S6 - SPDT switch J1, J2 - Jumper K1-K5 - Connectors

produced for hex value 00, and there is only time elapse. ‘No sound’ code is used as break between the notes. Reset (01 hex). When the data output of EPROM corresponds to 01 (hex), Q1 output of IC4 goes high. Since Q1 output of IC4 is connected to MR (master reset) pin 11 of counter IC2 via resistor-capacitor network R2-C3, IC2 is reset when data 01 hex appears at the output of EPROM. Stop-clock signal (10 hex). When

Fig. 9: Component layout for PCB-2

the data output of EPROM corresponds to 10 (hex), Q1 output of IC5 goes high, which after inversion by NAND gate N1 is applied to pin 4 of IC1 via normally-closed contacts of push-to-off switch S1. As a result, IC1 stops oscillating and producing clock pulses. The active ‘high’ Q1 output of IC5 is therefore referred to as stop-clock signal in this circuit. Pushing switch S1 at this stage removes logic ‘high’ input from NAND gate N1 and the clock oscillator starts oscillating. The flow chart for a doorbell given in Fig. 5 shows the order in which the data is entered/read. First, the data pertaining to the Fig. 10: Flow first tune is stored. Once chart for reall the notes (including adjustment

breaks/‘no sound’ periods) for the first tune are stored, a stop-clock data (10 hex) is stored at the end of tune-1 that stops after the first tune. Now on pressing push-to-off switch S1 momentarily, the clock advances to start the second tune (tune-2). Thus each tune is made to end with 10 hex code for stop signal. When all tunes of the doorbell are exhausted, the last stop-clock data is followed by a reset data (01 hex), so that one goes to the start of tune-1 (on reset), and the cycle repeats. For instance, the hexadecimal value of ‘SA’ is 70H (refer Table I) or binary 0111 0000, which means that binary data at the input of IC4 and IC5 is 0000 and 0111, respectively. As a result, only Q7 output of IC5 goes high. This output brings the associated preset resistor tuned to the frequency of SA (595 Hz) into the oscillator circuit. Simultaneously, data 0000 at the input pins of IC4 causes its Q0 pin to go high. But since Q0 is left

Fig. 11: Power supply

open, there is no effect. Similarly, when binary data corresponding to note SA (05 hex) is output by the EPROM, Q5 of IC4 and Q0 of IC5 go high. The Q5 output of IC4 brings into circuit the corresponding preset tuned to the frequency of SA (1190 Hz). The Q0 output of IC5 has no effect, as Q0 is open. In this way both the ICs (IC4 Fig. 12: Flow chart and IC5) function in of car-reverse horn accordance with data at their inputs to produce the corresponding notes. Power supply. The circuit shown in Fig. 11 is used to obtain the regulated 5V DC using IC 7805. The actual-size, single-sided PCB layouts for the circuits of Figs 2 and 3 (common for EPROM and RAM versions of the melody generator) are shown in Figs 6 (PCB-1) and 7 (PCB-2), respectively. The component layouts for PCBs of Figs 6 and 7 are shown in Figs 8 and 9, respectively. The power supply circuit (Fig. 11) has also been integrated in PCB-2. This circuit can be used as a doorbell, or even as a car-reverse horn. The flow chart for car-reverse horn is shown in Fig. 12. The necessary connections are shown in Fig. 13. When the circuit is used as a car-reverse horn, data flows from the next address location to where it stopped earlier. ❚

❚

Preset adjustment Connections to join the two PCBs should be made only after the adjustment of presets on PCB-2 using any of the following three procedures: Using frequency meter. Assemble all the components of PCB-2. Connect a probe to the Vcc using a crocodile clip at the other end. Switch on the 5V power supply and connect the output from the tone oscillator on the PCB to the frequency meter. Now connect the probe to the anode ELECTRONICS PROJECTS Vol. 22

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choose the main notes in the middle octave. Connect the probe to the respective diode of SA and tell the musician to adjust the variable resistor to the frequency of SA. Now connect the probe to the respective diode of RE and adjust the variable resistor to the frequency of RE, and so on. After adjusting main notes, adjust half notes. Fig. 13: Wiring connections for car-reverse horn (In Table I, music notes shown in small letters are half notes.) This method will of diode D101 and adjust preset resisbe successful only if the musician is well tor VR101 for 446 Hz (refer Table I). trained in music. In this way all the variable resistors Using digital multimeter. First, are adjusted one by one by connecting assemble only preset resistors VR101 +5V from the probe to the corresponding through VR128. Now adjust the variable diodes. resistors to their respective values (shown With the help of a musician. You in column 6 of Table I) using a digital can seek the help of a musician if you multimeter. Use the variable resistors don’t have access to a frequency meter or with maximum value as given in column a digital multimeter. Connect the output 7 of Table I. You can also use the values from the tone oscillator to the speaker shown in the circuit diagram of Fig. 3, but and switch on the power supply. First, Table I Music note

Frequency Data Hex of music character value note (Hz)

Variable resistor (preset) number

Variable resistor in-circuit value (ohm)

Maximum value of variable resistor

Lower octave Pa❚ dha❚ dha❚ ni❚ NI❚

446 1 472 2 500 3 530 a 561 b

20 vr101 30 vr102 40 vr103 50 vr104 60 vr105

8274 7740 7230 6744 6288

10k 10k 10k 10k 10k

595 c 70 vr106 630 d 80 vr107 668 e 90 vr108 707 f a0 vr109 749 g b0 vr110 794 h c0 vr111 841 i d0 vr112 891 j e0 vr113 944 k f0 vr114 1000 l 02 vr115 1062 m 03 vr116 1120 n 04 vr117

5850 5445 5055 4698 4356 4029 3726 3438 3165 2910 2655 2445

10k 10k 10k 5k 5k 5k 5k 5k 5k 5k 5k 5k

1190 o 1260 p 1335 q 1414 r 1498 s 1588 t 1682 u 1782 v 1888 w 2002 x 2122 y

2220 2016 1824 1644 1473 1308 1158 1014 876 747 624

5k 5k 2k 2k 2k 2k 2k 2k 1k 1k 1k

—

—

Middle octave sa re re ga ga ma ma pa dha dha ni ni Upper octave sa❚ re❚ re❚ ga❚ ga❚ ma❚ ma❚ pa❚ dha❚ dha❚ ni❚

❚

❚

❚

❚

❚

no sound

—

—

Reset  01 stop-clock  10

60

adjusting the variable resistors to lower values in the table may be very tedious. Any method may be used to adjust all the variable resistors. But after playing a tune, it may be felt that the tune doesn’t sound proper, even if it sounded right with computer. The reason can Fig. 14: LED indicator be that the re- circuit sistors were not properly tuned or it may be due to minute imperfections in output voltages from IC4 and IC5. These imperfections can be overcome by readjusting the resistors by the method given below. The imperfections can only be adjusted when data from the EPROM is heard. But, the notes of a tune will not be in an increasing frequency sequence. The sequence should be PA , dha , ----- to ----- DHA , ni . To do this, include at least two sets of sequence data from Table I with 2-3 bytes of gap in between successive sequences, after all the tunes, as shown in the flowchart of Fig. 10. This method of readjustment is used only to prevent disconnection of PCB of Fig. 7 from PCB of Fig. 6 and tuning the resistors again and again. Remove jumpers J1 and J2. Switch on the power supply. Press switch S4 to provide clock pulses for IC2. Say, if the EPROM contains 10 tunes, after the tenth tune release S4. Now keep pressing S2 momentarily until the first note of the sequence (PA ) sounds. Now connect the frequency meter at the speaker terminals (disconnect speaker if necessary) and adjust VR101 if the value of the frequency meter reading is not consistent with the value in the Table I. Press S2 again to adjust VR102, and so on. After the readjustment process insert jumpers J1 and J2 and press S3 to reset IC2.

ELECTRONICS PROJECTS Vol. 22

05 vr118 06 vr119 07 vr120 08 vr121 09 vr122 0a vr123 0b vr124 0c vr125 0d vr126 0e vr127 0f vr128 00

—

RAM-based circuit The only difference between the EPROMand RAM-based circuits is the use of RAM chip in place of EPROM and a key-

Fig. 15: Keyboard and probe for programming RAM

board for programming the RAM in RAM-based circuits. Besides, an LED panel is used for displaying the selected RAM address. Switch S2 is used to manually provide clock pulses to IC2. Similarly, switch S3 is used to manually reset IC2 before and after programming. Both switches (S2 and S3) are integrated into Fig. 2. The connector K1 in between IC2 and IC3 is used to connect to K5(M) connecter along with the associated LEDs as shown in Fig. 14. EPROM 2732 (IC3) is replaced with an 8-bit, 2k SRAM (6116). Pin 21

of 6116 is WE (write enable – active low). Switch S6 is to be kept in position ‘b’ while working with RAM. At the time of writing (programming) data into the RAM, there is no connection between connectors K2(F) and K3(M). Also, jumper J1 is removed. To program the RAM, K4(M) is to be mated with K2(F). After programming is over, K2(F) is connected to K3(M). IC6 (CD4011) contains four NAND gates, of which NAND gate N1 is used for stop-clock signals. It functions in the same manner as in an EPROM-based circuit.

The inputs of N1 are shorted and connected to the ground via resistor R7. So the output of N1 becomes high, which keeps IC1 oscillating. After a stop-clock (active ‘high’) signal appears at the input of NAND gate N1, its output goes low. When switch S1 is pressed, the output of N1 goes high and IC1 starts oscillating again. Gates N2 and N3 are used to provide read and write logic for RAM. In read condition, the output of N3 is at logic 0 because its inputs are at logic 1. Pressing of switch S5 provides ‘write’ condition, since the

Appendix ‘A’ #include <stdio.h> #include <dos.h> #include <stdlib.h> #include #include void play(char *str,int d); void main() { int f,d=200; char ch1[180],ch2; clrscr(); printf(“\n Enter delay value:”); scanf(“%d”,&d); while(1) { printf(“\n enter tune :”); scanf(“%s”,&ch1); play(ch1,d); a:ch2=getch(); if (tolower(ch2)==’r’) { play(ch1,d); goto a; } if (tolower(ch2)==’e’) exit(0);

} } void play(char *str,int d) { int i=0; while(str[i]!=’\0') { switch(str[i]) { case’1':sound(446); break; case’2':sound(472); break; case’3':sound(500); break; case’A’:sound(530); break; case’B’:sound(561); break; case’C’:sound(595); break; case’D’:sound(630); break; case’E’:sound(668); break;

case’F’:sound(707); break; case’G’:sound(749); break; case’H’:sound(794); break; case’I’:sound(841); break; case’J’:sound(891); break; case’K’:sound(944); break; case’L’:sound(1000); break; case’M’:sound(1062); break; case’N’:sound(1120); break; case’O’:sound(1190); break; case’P’:sound(1260); break; case’Q’:sound(1335); break; case’R’:sound(1414);

break; case’S’:sound(1498); break; case’T’:sound(1588); break; case’U’:sound(1682); break; case’V’:sound(1782); break; case’W’:sound(1888); break; case’X’:sound(2002); break; case’Y’:sound(2122); break; case’-’:nosound(); break; } delay(d); i++; } nosound(); }

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tor K4(M) should be connected to K2(F) during programming. The circles shown with the corresponding hex values are Fig. 16: Block diagram of ROM-based melody generator simple metallic contacts (or tabs) that avoid the use of a large number of output of gate N3 is at logic 1 and that of switches. To enter the hex data, the probe gate N2 at logic 0. is touched to the corresponding metallic LED connector. A separate male conneccontact tab. tor K5(M) is fabricated with LEDs as shown in The keyboard can be easily wired usFig. 14. This connector should be connected to ing a general-purpose board. To test the K1(F). The LEDs indicate addresses of memory keyboard after wiring, connect point ‘A’ to locations of RAM. Glowing of LED1 through the ground via a 100-ohm resistor (R18) LED11 together means that last RAM locaas shown in Fig. 15. Now touch each and tion is being addressed. (We are using a 2kB every tab one by one using the metallic RAM.) probe and verify that the data shown by Keyboard. The circuit diagram of keythe LEDs (LED13 through LED20) is board is shown in Fig. 15. Male connec-

Fig. 17: Schematic diagram of ROM-based melody generator

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consistent with the hex value shown on the tab/circle. After checking, disconnect resistor R18. Connector K3 should be soldered to the PCB by using a ribbon cable of adequate length, so that it could be easily connected to K2(F) after programming. The outputs from IC4 and IC5 go to preset-array part of the tone oscillator. Wiring is done similar to that in an EPROM version. Programming. Connect LED connector K5(M) to K1(F) and keyboard connector K4(M) to K2(F). Press switch S3 momentarily to reset IC2. No LED glows on the LED connector, indicating the initial address as zero. Now touch the tab marked ‘00’ with the probe. Press S5 momentarily and lift the probe. Glowing of no LED on the keyboard indicates that ‘00’ is entered in the initial memory loca-

tion. (It is good to enter ‘00’ in the first memory location.) Now get the hex dump values of the tunes. Press switch S2 to go to the next memory location, indicated by LED1 (corresponding to address line A0), on the LED connector strip. Touch the appropriate tab with the probe to enter the corresponding hexadecimal value at memory location 1. Press switch S5 and lift the probe. The data entered into memory location 1 is shown by keyboard LEDs in binary form. Hex data values (refer Table I) are such that any of the four LEDs corresponding to either D0 through D3 bits or D4 through D7 bits would glow to show the data entered. So it is easy to identify whether the data entered is correct or not. If necessary, make a table of binary data along with corresponding hex values. After entering all the tunes, disconnect keyboard from K2(F) and connect K3(M) to K2(F). Now connect external jumper J1 as shown in the circuit diagram. Switch S4 across jumper J1 terminals is not necessary but it may prove useful if any readjustment of variable resistors is needed (as in the case of EPROM), or for checking each and every tune one by one. The programming steps are summarised as below: 1. Press switch S3. 2. Touch tab 00 with the probe. 3. Press and release switch S5. 4. Lift the probe. 5. Press S2 to go to the next memory PARTS LIST Semiconductors: IC1 - NE555 timer IC2, IC3 - CD4017 decade counter IC4, IC5 - CD4069 hex inverters T1-T10 - BC547 npn transistor T11-T110 - BC558 pnp transistor Resistors (¼-watt ±5% carbon, unless otherwise stated) R1 - 10-kilo-ohm R2 - 100-kilo-ohm R3 - 680-ohm R4 - 1-mega-ohm R5 - 1-kilo-ohm R6 - 68-ohm Capacitors: C1 - 2.2µF, 12V electrolytic C2, C3 - 0.01µ ceramic disc Miscellaneous: S1 - Push-to-off switch

Fig. 18: Actual-size, single-sided PCB layout for the circuit

location. 6. Repeat from step 2 onwards for the next hex value programming. 7. After last data is entered, press S3. 8. Keep S4 pressed to check all the tunes that have been entered. 9. Connect jumper J1 if all tunes are entered. The data table (Table I), writing of musical notes, conversion of notes to hex values, preset-array alignment, and flow charts for door-bell and car-reverse tune are also applicable for the RAM version. Now we shall study a programmable melody generator using home-brewed ROM. There were only a few differences between the circuits of RAM- and EPROMbased programmable melody generators and as such we could integrate the common portion of the two circuits into a single schematic/PCB design. However,

the circuit of a ROM-based programmable melody generator is totally a new one. The ROM, as stated earlier, is home-built using discrete components, which can be used for storage of 100 bits (100 notes). The block diagram of the ROM-based melody generator is shown in Fig. 16. Note that the last block comprising variable resistor array is identical to that used in EPROM/RAM version (refer Fig. 3 in Part I of the article). The power-supply circuit shown in Fig. 11 can also be used for ROM-based melody generator. Thus PCB and component layouts shown in Figs 7 and 9 can be used without any modification in this system.

ROM-based circuit The circuit diagram of ROM-based melody generator is shown in Fig. 17. Here timer NE 555 (IC1) is wired as an astable ELECTRONICS PROJECTS Vol. 22

63

Fig. 20: Flow chart for repetitive playing of 99 notes (single tune)

clock’ and ‘Reset’ functions) from the 100-transistor array. Transistors T1 through T10 are used to switch on Vcc to transistors T11 through T110.

Operation

Initially, when power is switched ‘on’ to the circuit, IC2 and IC3 are in Reset condition. So only pin 3 (Q0) of IC2 and IC3 will be at ‘high’ logic. These high outputs are applied to the base of transistor T1 and the input of inverter N1. As a result, transistor T1 is switched on and +5V Fig. 21: Flow chart Vcc is available at the for repetitive emitter of transistor T1. playing of a number of This potential is extended tunes to the emitters of T11, T21, T31,…, T91

Fig. 19: Component layout for the PCB

multivibrator. The output pulses from IC1 are used as clock for decade counter CD4017 (IC2). The ten sequential outputs from IC2 are applied to npn BC547 transistors T1 through T10. Similarly, the outputs from another similar decade counter IC3 are connected to pnp BC558 transistors T11 through T110 via inverter gates N1 through N10 of IC3 and IC4 (CD4069). Each of these 100 transistors (T11 through T110) provides one bit for one note. The outputs are taken from the collectors of transistors and connected to the ‘variableresistor array and tone oscillator’ circuit. (Note: Collectors of transistors representing identical notes are shorted together.) As in the previous circuits of RAMand EPROM-based melody generators, here also ‘Stop-clock’ and ‘Reset’ signals are made available. You may program any/all of the hundred transistors T11

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through T110 for 28 notes as well as for the ‘Stop-clock’ and ‘Reset’ functions. However, both ‘Stop-clock’ and ‘Reset’ functions are optional, depending upon the number of tunes and number of notes. For ‘Stop-clock’ function, the output from a transistor is applied to inverter N11 whose output is connected to reset pin 4 of IC1. Similarly, for ‘Reset’ function, the output from a transistor is applied to pins 15 of IC3 and IC4. The collectors of transistors programmed for each specific note (including ‘Stop-clock’ and ‘Reset’ functions) are to be strapped (shorted) together for connection to the corresponding input points of the ‘variable-resistor array and tone oscillator’ circuit, while the ‘Stop-clock’ and ‘Reset’ lines are to be connected as shown in Fig. 17. We can have a maximum of 30 output lines (28 for the notes and two for ‘Stop-

and T101. Simultaneously, the output of inverter N1 will be at logic ‘0’, which is applied to the bases of pnp transistors T11 through T20. Since transistor T11 is the only transistor that has both Vcc at its emitter and nearly 0V at its base simultaneously, it gets forward biased and its collector is pulled toward its emitter voltage (Vcc). Thus initially, on powering the circuit, transistor T11 is activated and its collector goes high. The initial state lasts for a few seconds and as soon as IC1 generates a clock pulse (which is applied to the clock pin of IC2), Q1 (pin 2) of IC2 goes ‘high’ and pin 3 goes ‘low’, while no change takes place in IC3. Now transistor T2 is switched on. Since the base of transistor T12 is at low potential, the positive voltage will be available at its collector. Thus transistors T11 through T20 will be switched on and off sequentially with the arrival of each new clock pulse. At the beginning of tenth pulse, the carry-output pulse from pin 12 of IC2 is applied to clock pin 14 of IC3. Now pin 3 (Q0) of IC2 and pin 2 (Q1) of IC3 go ‘high’. Therefore, transistors T21 through T30 are now switched ‘on’ and ‘off’ in a sequential

fashion. In this way one out of 100 transistors is switched ‘on’ sequentially to produce an output to drive the ‘resistor-array tone oscillator’ according to the tune data. Thus when power is switched on, the tune is produced.

PCB layout and assembly The PCB design should ideally be doublesided for such types of transistor arrays. However to keep the cost down, we have included only a single-sided PCB layout, which is shown in Fig. 18 with its component layout in Fig. 19. First, assemble transistors T11 through T110. Solder the transistors, leaving a length from the PCB. Now take a thin, bare wire and connect the emitter leads of transistors T11, T21... T91 and T101 together from the components side. Similarly connect emitters of other rows of transistors. Suitable pads for the purpose have been provided on the PCB. Similarly the collectrors of transistors T1 through T10 may be connected together using bare wire from the components side. Now assemble all the remaining components.

Programming

In this circuit, programming means hard wiring. You should have a lot of patience to do all the hard wiring. No hexadecimal values are required. Before starting with

the wiring, label diodes D101 through D128 of ‘variable resistor array oscillator’ PCB (Fig. 7 and 9) in terms of their respective notes i.e. label D101❚ as PA❚, D102 as dha❚, ..., D128 as NI , and so on. Now starting from transistor T11 connect the transistor outputs (refer PCB of Fig. 18) to diodes D101 through D128 according to the tune note that each transistor (T11 through T110) sequentially represents. Extreme care should be taken while wiring, because if any error occurs, it will be very tedious to find out. Let us consider the example of five notes ‘SA RE—GA SA PA’. In this case programming can be done as under: Connect T11SAD6 T12RED8 T13NO connection, leave open (because the data is no sound) T14GAD10 T15SAD6 (Again to D6) T16PAD13 Reset. With this circuit a maximum of 100 notes are feasible. However if all notes are not utilised, Reset is necessary after the last utilised note. Because if the total number of notes is less than 98, for example, 86, then after 86th note there are 14 more bits to reach for an automatic reset to occur. (The circuit automatically resets itself after 100th bit.) So there is a big delay for the tune to get repeated. To skip the

delay, we use Reset. For this, the output from the next transistor after the last note is connected to point marked ‘RESET’ on PCB. When the pulse appears at pin 15 of IC2 and IC3, the circuit resets. Stop-clock. Stop-clock is used when more than one tune is to be programmed. If the clock is to be stopped, say, after the 1st tune, we use stop-clock. For this, the output from the next transistor after the last note of the tune is connected to the stop-clock point in the PCB. Please refer to flow charts of Fig. 20 and 21, which show occurance of automatic reset and use of stop-clock and reset functions. Housing. There is a lot of wiring in between the ROM circuit of Fig. 17 and the resistor-array oscillator. So the enclosure must have enough space for all the wires to fit properly without getting detached from the PCB while installing. [EFY note. To overcome this problem to some extent, a 28-pin (16+12) SIP connector (with pins projecting towards both sides of the PCB) may be used. This will obviate the need to run loose wires between ROM PCB and variable-resistor array oscillator PCB. Wires originating from the collectors of the transistor array may be connected to one side of the connector on ROM PCB itself and a ribbon cable with 28-pin SIP connector on both sides can be used between the two PCBs.] ❏

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Auto Control for 3-phase Motors d. dinesh

I

nduction motors widely used in workshops, irrigation pump sets, etc require a 3-phase supply. Normally, these motors are connected to 3-phase supply from electricity boards using thermal bimetal relays and relay contactors. Thermal relays protect the motor from overload. Relay coils having hold-on contacts with push-to-‘on’ and push-to-‘off’ switches are used for activating and deactivating the relay contacts. Single-phasing, line dropout, and reverse phasing are harmful for 3-phase motors. In the event of line dropout and single-phasing, the motor draws a heavy current from the existing phases, and during phase reversal the motor simply rotates in reverse direction. Further, an operator (attendant) for switching ‘on’/‘off’ the motor is always not possible, especially when the motor has to be operated round the clock. Also the protection provided by the thermal relay in the starter assembly is inadequate, since it involves some delay in activation. Thus some damage to the windings of the motor can take place, especially if overload conditions occur frequently. The circuit presented here incorporates the following features to overcome all the above-mentioned problems: • Electronic sensing of phase sequence with under-frequency cut-out. • Current sensing for single-phasing prevention. • Current sensing for overload cut-out.

• Automatic starting/tripping. • Programmable timer with battery backup to count the motor’s run time. • Latching circuit to prevent the motor from frequently starting and tripping. • Easy operation with just two switches for time set and reset. The phase-sequence detector protects the motor before starting, while the current-sensing circuit protects it during running. This double protection makes the motor operation really safe.

Circuit description

The schematic circuit diagram of induction motor controller is shown in Fig. 1. 3-Phase sequence checker. The voltage from each of the three phases is connected to optocouplers IC1 through IC3 via rectifier diodes D1 through D3. The outputs from the optocouplers are half-wave rectified DC pulses with a phase difference of 120° (during the conduction period of diodes), which are applied to a positiveedge-triggered, dual JK flip-flop IC4. When the red phase rises, the output of IC1 goes from ‘low’ to ‘high’, resulting in clearing of both flip-flops FF1 and FF2 through 0.1µF capacitor C1. While the red phase is still ‘high’, the yellow phase rises, resulting in the output of IC2 going ‘high’ and providing a clock pulse to FF1. As a result, Q output of FF1 goes ‘low’ (since J1 input of FF1 is already ‘high’ when the clock pulse arrives at CLK1 pin). Now, when the blue phase rises, the output of Table I IC3 goes ‘high’, while the output of IC2 is Phase sequence Signal OK LED RL1 already ‘high’, resulting in the output Q of Correct On On FF2 going ‘low’. Incorrect Off Off The above process Table II repeats once durMotor Core Core Primary Secondary ing each 50Hz cyHP size area Max SWG Turns S W G cle. If Q outputs Turns of both FF1 and (Max) amps FF2 are ‘low’, the 6 17 0.25 10 14 14 38 170 phase sequence is 20 23 0.56 22 11 9 38 110

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Parts List Semiconductors: IC1-IC3 - MCT2E optocoupler IC4 - CD4027 J-K flip-flop IC5, IC6 - NE555 timer IC7, IC9, IC10 - CD4017 decade counter IC8 - CD4060 14-stage counter and oscillator IC11 - 7805 5V regulator D1-D30 - 1N4007 rectifier diode ZD1, ZD2 - 3.3V zener diode LED1-LED4 - Red LED Resistors (1/4W ± 5% carbon, unless specified otherwise) R1-R3 - 100-kilo-ohm, 0.5 watt R4-R6, R16, R18-R23, R25, R30, R31, R38, R47, R49 - 4.7-kilo-ohm R7, R24 - 27-kilo-ohm R8-R10 R17, R26, R29, R32, R37, R39, R43, R44, R46, R48, R51-R53 - 10-kilo-ohm R11, R28, R34 - 1-kilo-ohm R12 - 220-kilo-ohm R13, R41 - 1-mega-ohm R14, R35, R36, R45, R50 - 470-ohm R15 - 470-ohm, 0.5 watt R27 - 180-kilo-ohm R33 - 2.2-kilo-ohm R40 - 22-kilo-ohm R42 - 82-kilo-ohm VR1 - 4.7-kilo-ohm preset VR2 - 47-kilo-ohm preset Capacitors: C1-C3, C6, C13 - 0.1 ceramic disk C4, C7, C11, C17 - 100µF, 63V electrolytic C5, C14-C16, C18, C19 - 10µF, 25V electrolytic C8, C10, C12 - 47µF, 25V electrolytic C9 - 1000µF, 63V electrolytic Miscellaneous: X1-X3 - Current-sensing transformers X4 - 0-230V AC primary to 12V-0-12V, 500mA secondary transformer S1 - ‘On’/‘off’ switch S2 - SPDT switch S3 - 7-way rotary switch - 1.5V X4 battery - Starter assembly - Cabinet

Fig. 1: Schematic diagram of auto control for 3-phase motor

correct and both diodes D28 and D29 are in blocking mode. The base of transistor T1 is pulled towards ground via resistor R11 and transistor T1 starts conducting.

As a result, IC5 is triggered and hence ‘sequence OK’ LED connected to pin 3 of IC5 via resistor R14, glows. IC5 is a popular 555 timer wired as a

retriggerable monoshot. Its time period is set at 25 milliseconds (approx.). If the monoshot is not retriggered within 25 milliseconds, the ‘sequence OK’ signal goes ELECTRONICS PROJECTS Vol. 22

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Fig. 2: Actual-size, single-sided PCB layout for the circuit

‘low’. The circuit operates smoothly at frequencies up to 42 Hz. If any of the phase fails, the phase sequence is disturbed, resulting in the output of IC5 going ‘low’ and ‘sequence OK’ LED goes ‘off’. The LED status in relation to the phase sequence is shown in Table I. The output of IC5 is also used for driving relay RL1 via transistor T2 (SL100). Normally-open (N/O) contacts of relay RL1 are wired in series with ‘off’ switch of starter assembly as shown in the Fig. 1. Thus when phase sequence is correct and the frequency is above 42 Hz, the relay is in energised state and it is feasible to switch on the starter by momentary energisation of relay RL2, whose N/O

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put of CD4017 (IC7) goes ‘high’, relay RL2 energises through transistor T9 (SL100). N/O contacts of RL2 are connected across ‘on’ switch of starter assembly, as stated earlier and the starter’s relay coil energises. The next clock pulse to IC7 deactivates relay RL2, but starter remains in ‘on’ state due to hold-on contact (the fourth contact of contactor in starter assembly). When Q9 (pin 11) of IC7 goes ‘high’, its CK pin 14 is muted due to conduction of transistor T8 (which pulls it to ground) to prevent further counting. The Q9 output of IC7 is also used in the motor ‘on’/‘off’ timer circuit, explained later. The supply to starter is connected through primaries of three small current transformers used for sensing the load in each phase. These transformers can be constructed using common EI laminations generally used for power transformers. Core number 23 or 17 may be employed as per details given in Table II. The secondaries of these transformers are connected to the current-sensing circuit wired around transistors T3 through T5. If any phase goes ‘off’, it cuts off the corresponding transistor and thereby provides forward bias to transistor T6. The outputs of transistors T3 through T5 are wired-OR via diodes D15, D16, and D17. Any excessive increase in load current (overload) results in forward biasing of transistor T7. The excess current limit can be set with the help of preset VR1. The conduction of transistors T6 and/or T7 causes their common collector junctions to be pulled low. This ‘low’ signal is coupled to transistor T2 via diode D30. As a result, relay RL1 deactivates to trip the starter and thus stop the induction motor. The above conditions are summarised in Table III.

contacts are wired in parallel with the ‘on’ switch of starter assembly. Auto-starter and current-sensing circuit. As soon as the phase sequence is detected to be correct (as Table III explained in the previous Truth Table of Current Sensing Circuit section), the output of IC5 T7 RL1 goes ‘high’. This output, Phase R Phase Y Phase B T6 (ON) (ON) R.B. R.B. Energised via resistor R15, is used to (ON) (ON) (OFF) F.B. R.B. De-energised reset IC7 and enable IC6, (ON) (ON) (OFF) (OFF) F.B. R.B. De-energised besides acting as a clock (OFF) (OFF) (ON) F.B. R.B. De-energised for decade counter IC10. (OFF) (ON) (OFF) F.B. R.B. De-energised (OFF) (ON) F.B. R.B. De-energised IC6 is an NE555 timer (ON) (ON) (ON) F.B. R.B. De-energised wired in astable mode to (OFF) In case of overloading provide clock pulses to decin any phase — X F.B. De-energised ade counter CD4017 (IC7). Note: R.B. = Reverse bias; F.B. = Forward bias; X = Don’t care Eventually, when Q8 out-

Fig. 3: Component layout for the PCB

Motor on/off counter and latch. Frequent start and stop operations subject the motor to lot of fatigue due to heavy currents, which may damage the motor. In this circuit, automatic restarting of motor is limited to three attempts for each power ‘on’, by using another decade counter CD4017 (IC10). It monitors each ‘on-off’ cycle of the motor by advancing the count of decade counter by one on every start. The clock for IC10 is obtained from the output of IC5 via resistor R15. This point i.e. the junction of resistor R15 and diode D30 is also used as supply point for transistors T6, T7, T12 and T13 as also for reset pin of timer IC6. On the third

start, pin 7 (Q3) goes ‘high’ and transistor T13 gets forward biased. As a result, CK pin 14 of IC10 is pulled low to stop any further clock to the decade counter, which thus gets latched and LED3 glows to indicate the latched state of the counter. Simultaneously, this ‘low’ signal causes transistor T2 to cut off and de-energise relay RL1. Thus the motor cannot restart automatically and only complete resumption of power can reset the latch. Motor on-off timer. A timer is provided to run the motor for a predetermined time. It counts run time of the motor and thereafter switches off the motor automatically. The signal from pin 11 (Q9) of IC7 is connected to the base of transis-

tor T11 via resistor R38 (as referred in ‘auto-starter and current-sensing circuit’). Thus the collector of transistor T11 goes ‘low’ to activate the oscillator circuit of CD4060 (IC8), while the motor is running. Prior to that, the oscillator circuit of CD4060 was inactive because its pin 11 was at logic ‘1’, being connected to +ve rails via resistors R39, R40 and diode D22. The frequency of oscillation is set by R-C network comprising 47µF capacitor C8 and resistor R42 in series with preset VR2. A timing of either 30 minutes or 60 minutes can be selected with the help of switch S2 for the output of ‘on’/‘off’ timer to go from ‘low’ to ‘high’ state. The output from the pole of switch S2 is connected to the clock input of decade counter IC9. The outputs of IC9 go ‘high’ sequentially after 30/60-minute time intervals, depending on the selection made via switch S2. Thus multiples of 30-/60-minute basic timing can be selected with the help of 7-way rotary switch S3. (The 7-way rotary switch may be substituted with decade thumb-wheel switch, if desired.) The output available at the pole of rotary switch S3 goes ‘high’ after the selected duration to forward bias transistor T12, which, in turn, causes de-energisation of relay RL1. Also, when the selected run time is over, the oscillator of IC8 (CD4060) gets inhibited because oscillator pin 11 of IC8 goes ‘high’ due to the feedback from the pole of switch S3 via resistor R43 and diode D23. LED1 glows to indicate that run time is over. To restart the motor, IC8 and IC9 can be manually reset by closing and then opening switch S1. The timer may be bypassed by keeping switch S1 closed. The timer section requires very low power in standby mode and is powered by four 1.5V cells as standby supply. A battery-low indicator is provided to warn the user about the low battery condition. Power supply. The normal DC power supply for the circuit is provided by a small step-down transformer X4 connected between R (red) phase and neutral, followed by rectifier and filter capacitor. The unregulated voltage is used for operation of the relays, while the 5V regulated supply is used for the remaining circuit.

Construction and testing An actual-size, single-sided PCB for the

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Fig. 4: Layout of cabinet for mounting transformer relays and the PCB

motor controller circuit of Fig. 1 is shown in Fig. 2, with its component layout shown in Fig. 3. It is recommended to use bases for ICs. Before connecting the circuit to starter assembly, a bench test is required for the adjustment of timer. Apply 3-phase power to the circuit. Observe pin 3 of IC5 (NE555), which should go ‘high’, provided the sequence is correct. Else, interchange any two phase wires. As ‘sequence OK’ signal at pin 3 of IC5 goes ‘high’, relay RL1 energises and IC6 (IC555) is activated. As a result, relay RL2 energises after a delay of 15 seconds for one second. Readers’ comments: Q1. Please clarify the following: 1. Which starter in the circuit starts the motor? 2. Is the starter manually operated or automatically? Ramaswamy Iyer Through e-mail Q2. When a 3-phase motor is started, it takes six times the rated current. So the current sense circuit will trip the motor during start-up. If we adjust the overload current setting for starting current, this will not trip the motor during normal running current through the load. Is there any initial bypass provided for overcurrent trip? G. Saravana Mohan Salem Q3. The contactor-type starter can be used for starting motors up to 10 HP. As I need to control motors of 15 to 20 HP, please clarify the following:

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Now adjust preset VR2 such that 30-minute-duration pulse train (time period 60 minutes) is available at pin 14 of IC8 (CD4060). Flip switch S2 to 30-minute position. Select the required run time using rotary switch S3. On completion of the selected run time, ‘time over’ LED should glow and the timer should stop. Relay RL1 Fig. 5: Creation of virtual neutral from 3-phaes 3-wires system should de-energise. all the wires to the starter point and the After resetting the timer with the load. Keep wiper contact of VR1 towards help of switch S1, relay RL1 should ground side and switch on the 3-phase energise once again. Then after a delay supply. Relay RL1 activates. After 5 secof 15 seconds, relay RL2 should again onds, relay RL2 also activates and the moenergise for one second. Now short tor starts running. Now slide the wiper of momentarily pin 14 of counter CD4017 VR1 and mark the position just before the (IC10) to ground thrice. On the third motor trips. (Remember that such trips touching, Q3 of IC10 will go ‘high’ and will be counted by latching counter.) LED3 will glow, followed by de-energiCaution. Some parts of this circuit sation of relay RL1. The mains should be contain live 3-phase voltages. So avoid interrupted completely to reset IC10. touching the circuit with bare hands. Current transformers X1 through X3, Note. In the case of non-availability step-down transformer X4, and relays of neutral terminal, assembler a circuit as RL1 and RL2 may be mounted side by shown in Fig. 5. Connect ‘N’ marked wire side in a compact box as shown in Fig. (shown in Fig. 1) to two more transformers 4. The PCB may be mounted over the X5 and X6 that are identical to X4. The transformers and relays using insulated secondaries of these transformers (X5 and spacers. Current transformers are to X6) are kept open, while the secondary of be connected before the starter relay X4 is connected to the power-supply circuit contacts. as shown in Fig. 1. Over-current adjustment can be done ❏ only after connecting the load. Connect 1. Can I use star-delta starter (which can reduce the starting current and can be used for motors up to 25 HP) instead of the contactor-type starter? If no, suggest a proper alternative as the starting current of up to 40A may affect other components. 2. Are there any current-reducing circuits used to withstand the high starting current while using the contactor-type assembly? 3. At the time of testing, what HP motor was used with contactor-type starter assembly? 4. The 12V, 300-ohm, 1 C/O relays (RL1 and RL2) specified in the circuit are not available. The available relays are 12V, 200-ohm, 1 C/O and 12V, 150-ohm, 1 C/O. So which relay should I use? What is the purpose of using VR2? Ramaswamy Iyer Through e-mail Q4. I am facing the following problems in the project:

1. In Table II, the turns ratio of current transformers (CTs) is 12 for both 6HP and 20HP motors. If the ratio is same, the secondary currents of CTs work out to be different, i.e. 1.8A for 20HP motor and 0.8A for 6HP motor. 2. In the phase-sequence indicator circuit, you have connected an RC (1-megaohm-0.1µF) combination to the reset pin of IC5 (NE555). In such a case, can the reset pin get a high input. Abhijeet S. Bhosle Through e-mail EFY: A1. 1. The starter comprises a contactor, an ‘on’ button (N/O), and an ‘off’ button (N/C). The contactor in Fig. 1 of the project uses three main N/O contacts connected to R, Y, and B phases and one auxiliary N/O contact, which is wired as shown in Fig. 1. The contactor coil is rated at 415V AC. At EFY, we used ML1 contactor from L&T to make the starter assembly.

2. You can manually operate the starter by making use of ‘on’ and ‘off’ buttons. In automatic operation you don’t have to use these switches. The circuit does it through relays RL1 and RL2 as per the logic explained in the project. The author, D. Dinesh replies: A2. A heavy current flows through the motor winding for a moment only. A certain delay is provided by capacitors C12 (47 µF) and C17 (100 µF) to account for this. A3. 1. One can use star-delta starter

for a higher HP motor, provided that star-to-delta changeover is done either on releasing the ‘on’ pushbutton or after a fixed time delay. A typical semi-automatic star-delta starter made by L&T is Mark1 type bearing catalogue No. SS96255. 2. A delay is provided by capacitor C12 to bypass the power-on surge current. 3. The circuit was tested using 6HP borewell pump. 4. If the specified relay is not available, one can also use 12V, 200-ohm, 1C/O relay.

A4. 1. Current transformers are used to sense the load current and these draw only a few milliamperes of current to bias the transistor. The bigger core is used to cater to the wire gauge. One can use a smaller core to hold this gauge by maintaining the specified turn ratio. 2. Pulses at pin 14 of IC08 (CD4060) should be adjusted to obtain 30-minute delay by varying preset VR2. Pin 4 of IC5 is wired through RC network for delayed reset at power-on (starting).

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Telephone Remote Control JUNOMON ABRAHAM

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elephone remote control implies control of devices at a remote location via a circuit interfaced to the remote telephone line/device by dialing specific DTMF (dual-tone multi-frequency) digits from a local telephone. The telephone remote control system described here has the following features: 1. It can control multiple channels/ relays. 2. It provides you feedback when the current is in energized state and also sends an acknowledgement indicating action w.r.t. the switching ‘on’ of each requested relay and switching ‘off’ of all relays (together). 3. It can selectively switch ‘on’ any one or more relays one after the other and switch ‘off’ all relays simultaneously.

Operation Instead of straightway proceeding with the circuit description, we shall start with the operation as this would help us in understanding the circuit better. The

A2 L L L L H H H H

Table I(a) Input Output A1 A0 Qn = addressed L L Q0 L H Q1 H L Q2 H H Q3 L L Q4 L H Q5 H L Q6 H H Q7

Table I(B) WR R Q Q addressed un-addressed L L = DATA hold L H = DATA L H L hold hold H H L L H = High; L = Low

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operation is as follows: 1. From the local telephone, dial the number of the remote telephone to which the circuit is connected. In a short while you will hear a musical note indicating that the circuit connected to the remote telephone is active. 2. Now if you want to switch ‘on’ a particular relay/device, press ‘*’ button on the telephone keypad followed by any one of digits 1 to 7 corresponding to the device/ relay number that you desire to switch ‘on’. The switching ‘on’ of the relay will be acknowledged/indicated by a musical note. Now you may keep the handset on the cradle. 3. If you want to switch ‘off’ the relays, press ‘*’ and them press key for digit 8. A musical note is heard, which indicated that all the relays have

Fig. 1: Schematic diagram of the telephone remote control

Fig. 2: Actualsize, single-sided PCB for the circuit

Fig. 3: Component layout for the PCB

been switched ‘off’. Keep the handset on cradle.

The Circuit At the remote telephone end, the ringing signal is detected by a high-input-impedance op-amp CA3140E that is wired as a comparator. Since the op-amp output is open-controller type, the output pin has been pulled Vcc via 10-kilo-ohm resistor R21, IC2 (NE556) comprised two timers (NE555 type) that have been configured as monostables.

When a ring is detected by IC1, its output triggers one of the timers in IC 556. The output of the timer after inversion by one of the NAND gates of IC3 (CD4011), enabled IC4 (CD4060) by taking its reset pin 12 ‘low’. (IC4 is an oscillator-cum-14bit binary counter.) As a result, IC4 starts counting when the ring signal strikes the input of the circuit. After some time, decided by the setting of preset VR3, Q12 output of IC4 goes ‘high’. This output coupled to pin 8 of a NAND gate inside IC3 will enable it. The detected ring signal (if the ring signal

PARTS LIST Semiconductors: IC1 - CA3140E op-amp IC2 - NE556 dual timer IC3 - CD4011 quad NAND gate IC4 - CD406014-stage counter/ oscillator IC5 - NE555 timer IC6 - UM66 melody generator IC7 - CM8870 DTMF-decoder IC8 - CD4099 8-bit addressable latch IC9 - 7805 regulator +5V T1 - BC548 npn transistor T2-T9 - BC547 npn transistor (only T2 and T6 shown) LED1, LED2 - Green LED LED3 - Yellow LED LED4 - Red LED D1, D2 - 1N4148 switching diode D3-D10 - 1N4007 rectifier diode (only D3 and D4 shown) Resistors (all 1/4-watt, ±5% carbon, unless otherwise stated) R1, R16. R17 - 150-kilo-ohm R2. R21 - 10-kilo-ohm R3 - 33-kilo-ohm R4 - 680-kilo-ohm R5 - 560-ohm R6, R10 - 22-kilo-ohm R7 - 1-mega-ohm R8, R15 - 390-ohm R9, R12 - 15-kilo-ohm R11 - 270-ohm R13, R14 - 3.3k-kilo-ohm R18 - 330-kilo-ohm R19, R22-R27- 4.7-kilo-ohm (R22-R27 not shown in the figure) R20 - 220-ohm VR1 - 10-kilo-ohm preset VR2 - 1-mega-ohm preset VR3 - 220-kilo-ohm preset VR4 - 470-kilo-ohm preset Capacitors: C1 - 0.22uF ceramic disk C2 - 220uF. 10V electrolytic C3 - 100uF, 10V electrolytic C4, C5, C8 - 0.0luF ceramic disk C6, C11, C12 - 0.1uF ceramic disk C7 - 10uF, 10V electrolytic C9 - 0.02uF ceramic disk CIO - 0.47uF, 100V polyester Miscellaneous: X PAL - 3.58MHz crystal RL-RL7 - 9V, 150-ohm 1C/0 relay (only RL4 shown)

is still persisting) applied to pin 9 of the same NAND gate (after inversion by another NAND gate) will pass through it to trigger the second monostable inside IC2 (NE556) as well as IC5 (NE555), which is again wired as a monostable. This arrangement avoids the circuit of being triggered by any transients or false ring signals on the telephone line. The output of the second monostable of IC2, available at its pin 9, drives transistor T2 and shunts the telephone line ELECTRONICS PROJECTS Vol. 22

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voltage drops to around 10 to 12 volts. This is equivalent to the lifting of the telephone handset of the remote telephone. As mentioned earlier, both IC5 and the second monostable of IC2 are triggered simultaneously. The output of monostable IC5 starts melody generator IC6 (UM66) and the musical note obtained from it is coupled to the telephone line. This informs the caller that the remote circuit is in energized state. As the remote circuit is in energized condition, the next step for the operator at local telephone is to press the ‘*’ button, which makes the local telephone to operate in the long-dialing mode. The digits that are pressed after pressing the ‘*’ button are converted to DTMF tones. The tone is decoded by IC7 and its three LSBs (covering binary equivalent of

decimal digits 0 through 7) are connected to the address inputs, while the MSB line is connected to reset pin 2 of IC8 (CD4099, an 8-bit addressable latch). When a valid DTMF tone is detected at the input of IC7, its pin 15 goes ‘high’ to enable IC8 after inversion by NAND gate of IC2. At the same time, it triggers IC5 for informing the caller that his key-press is accepted. Numbers 1 to 7 on the local keypad cause latching of the corresponding relays, while number 8 causes reset operation, which means that we can switch ‘on’ seven relays independently one by one and switch ‘off’ all relays simultaneously by pressing number 8. The output of IC8 drives the relays via the relay driver transistor. Truth tables I(A) and I(B) of CD4099 indicate relay operation.

Readers’ comments: Q1. In the circuit, if anyone makes a call to the connected telephone line and presses the consecutive switches, the unauthorised person can also switch the circuit on/off. Can the circuit be altered such that switching on/off the circuit is possible only after entering the authorisation code via telephone keypad?

Devjyoti Biswas Through e-mail The author, Junomon Abraham, replies: A1. It is possible to incorporate the facility as desired by you by using a microcontroller. The microcontroller will receive the signal from DTMF decoder and it will verify whether the correct password has been received. The same can

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Alignment 1. Connect the circuit to the telephone line. 2. Adjust preset VR1 so the ringing pulse causes LED1 to flicker. For better performance, set the voltage at pin 3 of IC1 at approximately 2 volts. 3. The time required to activate energise the circuit is adjusted by preset VR3 with the help of LED2. 4. The time available for remote switching action can be set by preset VR2 with the help of LED4. Indirectly, the setting of preset VR2 determines the charge that will have to be paid to the telecom department. 5. The period of the musical note can be controlled by the adjustment of VR4 with the help of LED3. ❏ be achieved with discrete ICs also, but the microcontroller method is better and flexible. EFY: Please refer to ‘Microcontroller-Based Access Control System’ and ‘Multichannel Access Control System’ projects published in October and November issues of EFY for implementation of the password authentication schemes used in such a system.

Microcontroller-based School Timer U. B. Mujumdar

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he basic requirements of a realtime programmable timer generally used in schools and colleges for sounding the bell on time are: • Precise time base for time keeping. • Read/write memory for storing the bell timings. • LCD or LED display for displaying real time as well as other data to make the instrument user-friendly. • Keys for data entry. • Electromechanical relay to operate the bell. We are describing here a sophisticated, yet economical, school timer based on Motorola’s 20-pin MC68HC705J1A microcontroller.

Description The pin assignments and main features of the microcontroller are shown in Fig.1 and the Box, respectively. The complete system is divided into four sections, namely, the time keeping section, the input section (keyboard), the output (display, indicators, and relay driving) section, and power supply and battery backup.

Fig. 1: MC68HC705J1A pin assignment

The time-keeping section. Accurate time-keeping depends on the accuracy of time base used for driving the microcontroller. In this project, the microcontroller is driven by AT-cut parallel resonant crystal oscillator that is expected to provide a very stable clock. A 3.2768MHz crystal provides a time base to the controller. The frequency (fosc) of the oscillator is internally divided by 2 to get the operating frequency (fop). This high-frequency clock source is used to control the sequencing of CPU instructions. Timer. The basic function of a timer is the measurement or generation of time-dependant events. Timers usu-

Main features of mc68h705j1a • 14 bidirectional input/output (I/O) lines. (All the bi-directional port pins are programmable as inputs or outputs.) • 10 mA sink capability on four I/O pins (PA0-PA3). • 1,240 bytes of OTPROM, including eight bytes for user vectors. • 64 bytes of user RAM. • Memory-mapped I/O registers. • Fully static operation with no minimum clock speed. • Power-saving stop, halt, wait, and data-retention modes. • Illegal address reset. • A wide supply voltage range from-0.3 to 7 volts. • Up to 4.0 MHz internal operating frequency at 5 volts. • 15-stage multifunction timer, consisting of an 8-bit timer with 7-bit pre-scaler. • On-chip oscillator connections for crystal, ceramic resonator, and external clock.

PARTS LIST Semiconductors: IC1 - 68HC705JIACP Microcontroller IC2 - CD4532 8-bit priority Encoder IC3 - 74LS138 3-line to 8-line decoder IC4 - 74LS47 BCD-to-7-segment decoder/driver T1-T3 - BC547/BC147 npn transistor T4-T7 - 2N2907 pnp transistor D1-D7 - 1N4007 diode ZD1 - 5.6V, 0.5 watt zener Resistors (1/4-watt, ±5% carbon, unless stated otherwise) R1 - 210-ohm, 0.5 watt R2 - 27-ohm R3, R12-R14, R24-R-27 - 1-kilo-ohm R4-R8 - 100-kilo-ohm R9-R11, R23, R29 - 10-kilo-ohm R15-R22 - 47–ohm R28 - 10-mega-ohm Capacitors: C1 - 350µF, 25V electrolytic C2, C3 - 1µF, 16V electrolytic C4, C5 - 27µF ceramic disk C6 - 0.1µF ceramic disk Miscellaneous: S1-S5 - Push-to-on switch (key) S6 - On/off switch PZ1 - Piezo buzzer RL1 - Relay 12V, 300-ohm, 1C/O XTAL - 3.2768MHz AT-cut crystal X1 - 230V AC primary to 12V-012V, 500mA secondary transforer DIS.1-DIS.4 - LTS542 common-anode display - 4 x 1.2V Ni-Cd cells

ally measure time relative to the internal clock of the microcontroller. The MC68HC705J1A has a 15-stage ripple counter preceeded by a pre-scaler that divides the internal clock signal by 4. This provides the timing references for timer functions. The programmable timer status and

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Fig. 2: Schematic diagram of the microcontroller-based school timer

control register (TSCR) is used for deciding the interrupt rate. It can be programmed to give interrupts after every 16,384, 3,2768, 65,536, or 131,072 clock

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cycles. In Table I, the control word is set to provide the interrupts after every 16,384 cycles. For a 32,768MHz crystal, the interrupt period will be 10ms. Thus,

timer interrupts will be generated after every 10 ms (100 Hz). That is, 100 interrupts will make 1 second. Now time-keeping becomes very simple. As we are having a precise 1-second time count, a real-time clock can be easily built. The MC68HC705J1A has a 64 byte RAM that is used for data storage, Real time (in terms of seconds, minutes, hours, days of a month, and months) is stored in this RAM. Thus an accurate real-time clock is generated. The input section. For setting the real-time clock and storing operating times, the timer requires to be programmed externally. Data is fed using the keyboard. Press-to-on type keys are interfaced to the microcontroller using an 8-bit priority encoder CD 4532. This encoder detects the key-press operation and generates the equivalent 3-bit binary data. Its truth table is shown in Table II. The priority encoder is interfaced to port A of the microcontroller. Various keys used in the timer, along with their functions, are described below: Time (4): For setting real time in minutes and hours. Bell (5): For setting the bell’s operating timings. Digit Advance (6): Data setting is done digitwise (hour’s digit followed by minute’s digit). The Digit Advance key shifts the decimal point to the right. Store (7): For storing the data (real time or bell time). Delete (3): For deleting a particular bell timing. Here, the figures within parentheses indicate the decimal equivalents of 3-bit binary data from the keyboard. Set and run modes. Data setting is possible only in set mode. Set mode or run mode can be selected by toggle switch S6. By using a lock switch for S6, the timer can be protected from unauthorized data entry/storage. In run mode if you press ‘Bell’ key once, the display shows the bell’s various operating timings one after the other, in the same order in which these had been previously stored. In case you want to discontinue seeing all the bell timings, you may press ‘Time’ key at any stage to revert back to the display of real time. The output section. Seven-segment displays are used for data display. As LEDs are brighter, these have been used

in the system. There are two techniques for driving the displays: (i) driving each display using a separate driver (like 74LS47 or CD4511) and (ii) using multiplexed displays. The first technique works well, but practically it has two problems: it uses a large number of IC packages and consumes a fairly large amount of current. By using multiplexed display both the problems can be solved. In multiplexing only one input is displayed at any given instant. But if you chop or alter

inputs fast enough, your eyes see the result as a continuous display. With LEDs, only one digit is lighted up at a time. This saves a lot of power and also components, making the system economical. Generally, displays are refreshed at a frequency of 50 to 150 Hz. Here, displays are refreshed at a frequency of 100 Hz (after every 10 ms). The display-refreshing program is an interrupt service routine program. BCD-to-7-segment decoder/driver 74LS47, along with transistor 2N2907, and 3-line-to-8-line decoder 74LS138 are used for driving common-anode displays. In multiplexed display, the current through the segments is doubled to increase the display’s brightness. 74LS47 is rated for sinking a current of up to 24 mA. As the current persists for a very small time in multiFig. 3: Power supply circuit for the school timer plexed display, it is peaky and can be as high as 40 mA per segment. The decimal point is controlled individually by transistor BC547, as 74LS47 does not support the decimal point. PA0 and PA1 bits of port A are used for controlling the electro-mechanical relay and buzzer, respectively. Power supply

and battery backup . T h e

Fig. 4: Actual-size single-sided PCB for the circuits in Figs 1 and 2

microcontroller and the associated IC packages require a 5V DC supply, while the relay and the buzzer require 12V DC supply. A simple rectifier along with zener diode-regulated power supply is used. The microcontroller is fed through a battery-

backed power supply, so that in the

case of power supply, so that in the case of power failure the functioning of the controller’s timer section is not affected. During power failure the timer is taken to ‘low power’ mode (called ‘wait’ mode). In this mode the controller draws a very small current. So small NiCd batteries can provide a good backup. A simple diode-resistance (27-ohm, ¼-watt) charger maintains the charge of the battery at proper charging rate.

Software Motorola offers Integrated Development Environment (IDE) software for programming its microcontroller and complete development of the system. The development board comes with Editor, Assembler, and Programmer software to support Motorola’s device programmer and software simulator. The ICS05JW in-circuit simulator and non-real-time I/O emulator for simulating, programming, and debugging code for a MC68HC705J1A/KJ1 family device. When you connect the pod to your host computer and target hardware, you can use the actual inputs and outputs of the target system during simulation of the code. You can also use the ISC05JW software to edit and assemble the code in standalone mode, without input/output to/from pod. The pod (MC68HC705J1CS) can be interfaced to any Windows 3.x-or Windows 95-based IBM computer using serial port. The software for the timer has been so developed that the system becomes as user-friendly as possible. The main constraint is read/write memory (RAM) space. As mentioned earlier, the microcontroller has only 64 byte RAM. About twenty bell operating timings are required to the stored. So the efficient use of RAM becomes essential. The software routines for the timer, along with their Assembly language codes, are listed in a folder. (Note: This folder, containing source code (.asm) and listing file (.lst) will form part of the EFY-CD provided with the August 2001 issue. As files are quite large, it is not feasible to include them here.) Basically, the following functions are performed by the software program: 1. Initialisation of ports and the timer. 2. Reading of keypressed data. 3. Storing of real time and bell ELECTRONICS PROJECTS Vol. 22

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Table I Timer Status and Control Register (TSCR) Bit 7 6 5 4 3 2 1 0 Signal TOF RTIF TOIF RTIE TOFR RTIFR RTI RTO Reset 0 0 0 0 0 0 1 1 TOF: Timer overflow flag RTIE: Real-time interrupt enable RTIF: Real-time interrupt flag RTI and RTO: Real-time interrupt select bit RTI RTO Interrupt period 0 0 fop ÷ 214 For 3.2768 MHz crystal 0 1 fop ÷ 215 Frequency of operation (fop) 1 0 fop ÷ 216 = 3.7268x106/2 = 1.638x106MHz 1 1 fop ÷ 217 For RTI=RTO=0 Interrupt period = 10ms (100Hz) Table II Truth Table for Priority Encoder CD4532 Keys E1 Store 1 Digit Adv. 1 Bell 1 Time 1 Delete 1

D7 1 X 0 1 0 0 0 0 0 0

D6 X X 1 0 0

D5 X X X 1 0

D4 X X X X 1

timings. 4. Comparison of real time and bell time. If the two match, the bell rings.

Fig. 5: Component layout for the PCB

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D3 X X X X X

D2 X X X X X

D1 X X X X X

D0 1 1 1 1 0

Q2 1 1 0 0 1

Q1 1 0 1 0 1

Q0

5. Display of data. 6. Time-keeping. For a user-friendly system, the associated software is required to perform many data manipulation tricks and internal branching. The operation and logic can be understood from the Assembly language listings. The software is mainly divided into the following modules: Keyboard. When a key is pressed, CD4532 sends the corresponding data. After reading the data, the controller decides on the action. ‘Set/ Run’ key (S6) is connected to port PA4. Bell. This part of the program is used for displaying the bell operating timings stored in the RAM. The operating timings are displayed one by one with a delay of 5 seconds between tow consecutive timings.

Set. The real time and bell timings are stored using this part of the software. Data is entered digitwise; for example, 08:30 a.m. will be stored a 0, followed by 8, followed by 3, and finally 0. Data is stored in 24-hour format. Data fed from the keyboard is converted into equivalent hex and stored in RAM. Any particular operating timing can be deleted from the memory using ‘Delete’ key, provided the timing is already stored in the memory. Run. Here the real time is compared with bell operating time. If the two match, the relay is operated. DataCon. This part of the software is used for finding out the decimal equivalent of hex data. The microcontroller manipulates the hex data and converts it into BCD format for display. Timer. The timer of the microcontroller is initialized to give an interrupt after every 10 ms. A real-time clock is generated using the interrupt. Also the display is refreshed during the interrupt service routine. For real-time systems battery backup is very essential, because power failure affects the time keeping. In interrupt service routine, the availability of power supply is checked. If the power is available, displays are refreshed and the timer operates normally. However, during the power-failure period, displays are off and system is taken to ‘low power’ mode. In this mode only the timer part of the microcontroller remains activated while operations of all other peripherals are suspended. This considerably reduces the power consumption. When the supply gets restored, the controller starts operating in normal fashion.

Operating procedure When the power is switched on, the display shows 12.00. Two settings are required in the timer: (a) setting of real time and (b) setting of bell operating timings. For setting real-time clock ‘Time’ key is used. Storing of real time. To store real time, say, 05:35 p.m., flip ‘Run’/’Set’ key (S6) to set mode. The display will show ‘0.000’. Press ‘Time’ key. Further pressing of ‘Time’ key will increment the data, like 0.000, 1.000, 2.000, and thereafter it will repeat 0.000, etc. To select the digit, press ‘Digit Advance’. This stores the present digit and the next digit is

selected as indicated by the decimal pointer. Data is stored in 24-hour format. The time to be stored is 17.35, of which the first digit will be 1.000. The second, third, and fourth digits can be stored in similar fashion. After the fourthdigit, press ‘Digit Advance’ key once more. The display will show 1735 (with no decimal). Now press ‘Store’ to store the data. Storing of bell timings. The procedure to store bell operating timings is similar to that of setting real time. The only difference is that here data is changed by ‘Bell’ key in place of ‘Time’ key. Any number of bell timings (<20) can be stored in the same fashion. If the number of bell operating timings exceeds 20, the timer

will not accept any new bell timings until one of the previously stored timings is deleted. Deletion of bell operating timings. For deleting a particular timing, first store this timing using the steps given above. Then press ‘Delete’ key to delete the specific data from the memory. Display of real time. If ‘Run’/’Set’ key is taken to run mode, real time will be displayed. Checking of bell operating times. For checking the bell operating times, press bell key in ‘Run’ mode only. The stored bell operating timings will be displayed one by one with a delay of 5 seconds between two consecutive timings.

Programming There are two ways to program the EPROM/OTPROM (one-time programmable ROM): 1. Manipulate the control bits in the EPROM programming register to program the EPROM/OTPROM on a byteby-byte basis. 2. Program the EPROM/OTPROM with Motorola’s MC68HC705J in-circuit simulator. The author has used the second method for programming the OTPROM. An actual-size, single-sided PCB for the circuits in Figs 2 and 3 is shown in Fig. 4, with its component layout shown in Fig. 5. ❏

Readers’ comments: Q1. I have assembled this circuit and found that pins D0 and D1 of IC 4532 are not properly terminated. Will this affect the keyboard data? Could you please tell me from where I can get the programmed controller? Deep Saraf Pune Q2. Does the circuit work by just assembling it with the IC (MC68HC705J1A) bought from the market, or do we have to install a software in it? From where can we get the software? Give a detailed procedure about how to install the software in the IC. Fig. 1: Modification of display circuit to operate 2.5cm/5cm display A. Rajasekaran Chennai ment kit and IDE (integrated development present circuit after omitting resistor R1. Q3. The circuit can be modified as shown environment) software available through Somnath Bera in Fig. 1 for using a brighter and bigger Motorola distributors/Internet. The same Through e-mail display. You can use this modification for can be purchased through our associate A1 and 2. EFY: Leaving two of the unone set of 2.5cm (1-inch) or 5cm (2-inch) Kits‘n’Spares. used input pins open will not affect the display. For 5V supply, use 7805 reguA3. EFY: The circuit sent by the reader circuit performance. The microcontroller lator in place of the zener diode in the had anomalies, which have been corrected. has to be programmed using a develop-

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Digital Capacitance-cumFrequency Meter pratap chandra sahu

He

re is an inexpensive circuit of a digital capacitance-cum-frequency meter that can measure capacitance in the range of 1 pF to 10,000 µF and frequency in the range of 0 to 100 kHz. With a slight modification, this circuit can be used as an article counter or a time meter. The principle. In a frequency counter, the unknown input is ANDed with a known time-base period, so that the numbers of cycles passed over the time-base period are counted. The time period can be measured similarly if a known frequency is gated with the unknown time period. The same instrument can also determine the time period of a periodic waveform or the time elapsed between two events. In this circuit, the capacitance measurement is nothing but the measurement of the time between two events in a charging capacitor. An R-C (resistor-capacitor) circuit works as a time generator and the time is directly proportional to capacitance value under suitable conditions. In the present case the condition being satisfied is that the time period (T) is equal to the product RxC, where R is the value of the charging resistor in ohms and C the capacitance value in farads. Capacitance measurement. One RxC time (seconds) is required to charge a capacitor to 63 per cent (approximately two-third) of its final value (applied voltage). Consider the following example: If C = 470 pF and R = 1 mega-ohm, then one RC time period T = 470x10–6 seconds = 470 microseconds. If we select the external frequency for the counter as 1 MHz (time period = 1 microsecond), the counter progresses by one count every microsecond and the counter reading is 470, as the gate will be open for 470 microseconds for the above-mentioned R and C under testing. We get the capacitance value directly from the readout of the counter in picofarads. Similarly, if we take R = 1 mega-ohm

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and external frequency = 1 kHz, we can read the value of the capacitor under test (CUT) directly in nanofarads. With R = 1 kilo-ohm and frequency = 1 kHz, we can read the value of the CUT directly in microfarads. Frequency measurement. This involves passing the unknown frequency signal for a known time base period through the counter. In a 4-digit counter with a time base of one second, the maximum display will be 9999, which means that we cannot read a frequency of more than 9999 Hz (≈10 kHz). However, if we reduce the time base to 0.1 second, the frequency reading can go up by a factor of ten to 99.99 kHz (≈100 kHz) as the time base virtually divides the input frequency by 10. For low-frequency measurement, we can increase the resolution by a factor of ten by increasing the time base period to 10 seconds, which is equivalent to the multiplication of the input frequency by a factor of 10.

Circuit and operation The capacitance measurement mode. During the capacitance measurement mode, switches S1 through S5 are kept slided towards position ‘C’. The unknown capacitor is placed across CUT terminals. Ganged switches SR1 and SR2 are used for capacitance measurement. Position 1 is used for capacitance range of 1 pF to 9999 pF (≈10 nF), position 2 for capacitance range of 1 nF to 9999 nF (≈10 µF), and position 3 for capacitance range of 1 µF to 9999 µF. Switch SR1 selects 1 mega-ohm charging resistor in its positions 1 and 2, while switch SR2 selects a frequency of 1 MHz in position 1 and a frequency of 1 kHz in position 2 for the counter operation. In position 3, 1-kilo-ohm charging resistor R6 is selected by SR1, while SR2 selects 1 kHz as the frequency for counter operation. Ganged rotary switches SR3 and SR4 are used for frequency measurement mode

Parts List Semiconductors: IC1 - NE555 timer IC2, IC3 - CA3140 high-input impedance op-amp IC4 (A-D) - 7408 AND gate IC5 - MM74C925 4-digit counter/7segment driver IC6 - 74LS121 monostable MV IC7-IC9 - 74LS90 decade counter (divide-by-10) IC10 - 7476 JK flip-flop IC11 - 7805 regulator +5V D1-D5 - 1N4007 rectifier diode D6 - 1N4148 switching diode LED1 - Red LED T1-T5 - BC547B npn transistor T6 - BS107 FET Resistors (all ¼ watt, ±5% carbon, unless stated otherwise) R1 - 2.2-kilo-ohm R2, R5 - 1-mega-ohm R3, R8, R24 - 4.7-kilo-ohm R4, R20 - 10-kilo-ohm R6, R7, R18 R21 - 1-kilo-ohm R9-R16 - 220-ohm R17 - 20-kilo-ohm R19 - 100-kilo-ohm R22, R23 - 560-kilo-ohm VR1 - 1-kilo-ohm preset Capacitors: C1 - 15µF, 25V electrolytic C2 - 0.01µF ceramic disk C3 - 10nF ceramic disk C4 - 10µF, 250V electrolytic C5 - 1000 µF, 25V electrolytic C6 - 100µF, 25V electrolytic C7, C8 - 22 pF ceramic C9 - 0.01µF ceramic Miscellaneous: X1 - 230 AC primary to 9-0-9 volt, 500mA secondary transformer XTL - 1MHz quartz crystal S1-S5 - Slide switch S6, S7 - Push-to-on switch SR1-SR2 - Ganged 3-way, 2-pole rotary switch SR3-SR4 - Ganged 3-way, 2-pole rotary switch DIS1-DIS4 - LT543 common-cathode, 7-segment display

only. (EFY note. As decimal indication is not required during capacitance measurement, one might have an additional ‘off’ position for SR3/SR4 ganged rotary switch.)

Fig. 1: Circuit diagram for digital capacitance-cum-frequency meter

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IC1 is a monostable multivibrator based on timer NE555 and is meant for capacitance measurement only. In normal condition, the low output of the monostable turns on the FET (BS107) switch. So the capacitor under test gets shorted via the FET switch. As and when triggered by the momentary push-to-on operation of start switch S6, the monostable provides a pulse

of 15-second duration. As soon as its output goes high, it switches off FET switch. Simultaneously, it takes pin 5 of AND gate IC4A high.

Now let us examine the conditions at IC2 and IC3 (both CA3140 op-amps). The voltage across CUT, after being buffered by IC2, is fed to the inverting input of IC3 wired as a comparator. The non-inverting

Fig. 2: Internal block diagram and functional description for IC 74C925

Fig. 3: Actual-size, single-sided PCB layout for digital capacitance-cum-frequency meter

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input of IC3 is biased at 0.63Vcc, which is set accurately by 1-kilo-ohm preset VR1. Now the capacitor begins to charge. As soon as the voltage across the capacitor crosses 0.63Vcc (i.e. 3.15 volts with Vcc = 5 volts), the output of IC3 goes low. Thus the output of IC3 and also that of AND gate IC4A remains high until the capacitor charges to 63 per cent of Vcc in one RC time. Latch-enable (LE) pin 5 of counter IC5 (74C925) connected to pin 6 of IC4A remains high to pass the clock selected via rotary switch SR2 and coupled to CL (clock) pin 11 of IC5 via AND gate IC4B. It goes low after one CR time to latch its count as the output of IC3 goes low. Thus the number of cycles from the frequency source passed over one CR time is recorded in the counter and gets displayed. For precise generation of 1MHz frequency, a 1MHz crystal oscillator is wired around Schmitt inverter gates N3 and N4. The oscillator output is routed via AND gate IC4C to slide switch S2 and rotary switch SR2 position 1. In capacitance (C) position of switch S2, this signal, after division by three decade counters IC7, IC8, and IC9 (7490), which are common to both frequency and capacitance meter modes, provides 1kHz signal at pin 12 of IC9, which, in turn, is extended to positions 2 and 3 of switch SR2. (Note. The outputs of IC10 are not used during capacitance measurement. IC10 comes into play only during the frequency measurement as explained later.) The NE555 timer used as a monoshot ensures the capacitance measurement in an easy and automatic manner. The LED connected to AND gate IC4D glows during the charging of the capacitor. During the measurement of high-value capacitances, it may take several seconds to charge to 0.63Vcc. For low-value capacitances, the LED glows for just a moment after pressing start switch S6. If the LED goes off after the start button is pressed, it indicates that the measurement is over. You can reset NE555 timer using switch S7 if you want to make another measurement. If this switch is not provided/operated, you would have to wait for at least 15 seconds until NE555 timer becomes normal. Alternatively, you will have to switch off the complete circuit and then switch it on again. Frequency counting. In place of 1MHz oscillator, a 100Hz full-wave rectified (pulsating DC) after being shaped by

Fig. 4: Component layout for the PCB

Schmitt inverter N2, is used as the master clock to provide the required time bases. The voltage divider network of resistors R20 and R21 protects gate N2 against high voltage. R21 is test selected to get proper 100 Hz rectangular wave form at the output Readers’ comments: Q1. A provision to include inductance measurement by using an FET-based tank oscillator circuit (as shown in Fig. 1) would enhance the utility of the circuit. The tank circuit could be tuned by trimmer CT for a frequency of, say, 1 MHz with a standard inductor of 1 µH. Decade dividers can be used for other ranges for direct readout.

Fig. 1: FET-based tank oscillator circuit

of gate N2. This 100Hz signal is divided by decade counters IC7, IC8, and IC9 to obtain 10Hz, 1Hz, and 0.1Hz frequencies. The frequency selected via rotary switch SR3 is then divided by 2 by JK flip-flop 7476 (IC10) so as to provide a gate time of 0.1 second, 1 second, or 10 seconds in positions 1, 2, and 3, respectively, of switch Praveen Shankar Haridwar Q2. I have the following queries: 1. Are ICs DM74LS121 and SN74LS121 the same? Can I use DM74LS121 or SN74LS121 as IC6? 2. Can I replace 15µF, 25V capacitor C1 with a 22µF, 25V capacitor? Chang Heen Loong Through e-mail The author, Pratap Chandra Sahu, replies: A1. Such a tank oscillator is not suitable for this circuit as the frequency in such a circuit is inversely proportional to the value of inductance and there is an offset frequency. The circuit proposed by the reader could, however, be used in conjunction with moving coil type

SR3. Q output of IC10 is used to enable counter IC5. The resetting of counter-cum-display IC5 is accomplished by the narrow output pulse from IC6 (74121), which is generated by the leading (rising) edge of Q output of IC10 connected to its B input (pin 5) via switch S5. Thus at the beginning of each counting period, IC5 is reset. IC5 (74C925) is TTL-compatible with a multiplexed 4-digit, 7-segment display driver. Its internal block diagram and functions are described in Fig. 2. The maximum frequency display in positions 1, 2, and 3 of ganged switches SR3 and SR4 is limited to 99.99 kHz, 9.999 kHz, and 999.9 Hz. The decimal point position is fixed by switch SR4. Calibration and testing. Connect a multimeter to the non-inverting terminal of IC3 and set the point at 0.63Vcc = 3.15 volts using 1-kilo-ohm preset VR1. To test the capacitance meter, use a 470pF polystyrene capacitor with one per cent tolerance. Precaution. Try to screen the mains transformer from the input. Place the transformer at a place where the chances of its interference with the input are minimal or nil. While measuring the frequency, the frequency source under test should not be touched or loaded to avoid affecting its frequency due to stray capacitance associated with the test leads. An actual-size, single-sided PCB for the circuit of Fig. 1 is shown in Fig. 3, with its component layout shown in Fig. 4. ❏ frequency meter after suitably calibrating its scale. A2. EFY: 1. Yes, ICs DM74LS121 and SN74LS121 are similar and you may use either of these two ICs as IC6. 2. You can replace 15µF, 25V capacitor C1 with a 22µF, 25V capacitor by changing the value of resistor R2 (1 mega-ohm) such that R2 x C1 product remains the same, in order to retain the same output pulsewidth. Since IC1 (NE555) is a timer IC, configured as a monostable, the output at pin 3 is approximately 16 seconds as per the following relationship: Pulsewidth = 1.1(R2 x C1) seconds So, if the capacitor value is changed from 15 µF to 22 µF, the resistor value needs be changed from 1 mega-ohm to 680 kilo-ohms so that R2 x C1 product remains almost the same. ELECTRONICS PROJECTS Vol. 22

83

Fluid-Level Controller with Indicator bhaskar banerjee

T

he fluid-level controller circuit presented here allows you to set the lower and upper fluid levels at the desired specific positions between two extreme levels. The total fluid level is divided into ten equal parts. Any two of these ten positions may be defined as ‘low’ and ‘high’ level, respectively. The system shows the preset levels on the two 7-segment displays and the current fluid level at any instant on a 10-LED bar graph indicator. The same circuit could also be used for controlling temperature in a similar fashion.

The circuit The main part of the circuit as shown in Fig. 1 is dot/bar graph driver LM3914 (IC1). This IC is linearly scaled and is intended for use in LED voltmeter application where the number of illuminated LEDs indicates the value of input voltage. It contains a floating 1.2V reference source between pins 7 and 8 that may be used as the reference input for the IC. The voltage from the sensor is fed to the input of IC1 at pin 5. The output of the sensor may vary

Fig. 1: Schematic diagram of fluid-level controller with indicator

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from ground level (0V) to supply voltage. Thus the reference voltage source should be externally preset, which is feasible with the help of IC1. This IC can also display the input voltage on a linear scale using ten LEDs in the bar graph or the dot mode. Here we have used the bar graph mode. The outputs of IC1 are active‘low’ and hence they sink current to illuminate LEDs. Inverters are used between the outputs of IC1 and the inputs of IC3 and IC4 to invert the active-‘low’ outputs of IC1. There are ten outputs available from IC1,

of which only five are used here. One may use up to eight outputs of IC1 since IC3 and IC4 (4051) are 1-of-8 data selectors. (Note. If 4067 were used in place of 4051, all the ten outputs could be used. It is also Fig. 2: Optical sensor possible to get more than ten outputs by cascading LM3941 ICs.) Using this circuit, the maximum fluid level can be divided into four equal parts giving five different level readings from ‘0’ (empty/low level) to ‘4’ (full/high level). Thus the five levels are empty, onefourth, half, three-fourth, and full. This division is meant only for controlling the level, while all levels including the intermediate levels are constantly displayed on LED bar graph. The lower level can be set anywhere between 0 and 3 in steps of 1 and high level can be set between 1 and 4. The fluid level can be maintained between any two levels by using IC3 and IC4. IC3 selects the high level and gets inputs of levels 1, 2, 3, and 4, while IC4 selects the low level and gets inputs of levels 0, Parts List Semiconductors: IC1 - LM3914 bar/dot display driver IC2 - 4069 hex inverter IC3, IC4, IC5 - 4051 8-channel analogue multiplexer IC6 - 4520 dual binary counter IC7 - 555 timer IC8 - 4081 quad 2-input AND gate IC9, IC10 - 4511 BCD-to-7-segment latch/decoder/driver LED1, 3, 5, 7, 9 - Green LED LED2, 4, 6, 8, 10, 11 - Red LED Resistors (all ¼-watt, ±5% carbon unless stated otherwise): R1-R10, R16-R31 - 470-ohm R11-R15 - 10-kilo-ohm R32-R33 - 47-kilo-ohm R34 - 1-kilo-ohm VR1 - 10-kilo-ohm preset Capacitors: C1, C2 - 22µF, 25V electrolytic C3, C4 - 10µF, 25V electrolytic C5 - 1µF ceramic disk Miscellaneous: DIS1, DIS2 - Common-cathode 7-segment display S1, S2 - Push-to-on switch

1, 2, and 3. All other unused input pins of IC3 and IC4 are grounded. The selection takes place according to the binary word preset at the select Fig. 3: Sensor usinput pins (pin ing float operated potmeter 9, 10, and 11) of IC3 and IC4. The required binary word is generated by a dual divide-by-16 counter IC6 (4520). (IC6 can be replaced by a divide-by-10 counter 4518, if desired.) Half of IC6 is used for high level and the other half for low level. IC6 gets its counting pulse from a 555 timer (IC7) used for generation of approximately 1Hz pulse train. The high level is set by pressing switch S1, while the low level is set by pressing switch S2. IC6 is reset when the power is

switched on. This power-on-reset function is realised using capacitors C1 and C2, and resistors R12 and R13. The part of IC6 connected to high-level selector also gets reset when the count is 5 (101 binary). This reset pulse is generated using AND gates of IC CD4081. The selected minimum and maximum levels are displayed by two 7-segment displays DIS1 and DIS2 that are controlled by two BCD-to-7-segment decoders 4511 (IC9 and IC10, respectively). The outputs of IC3 and IC4 are fed to the select input pins of IC5 (4051). The output of IC5 is fed back to one of its select inputs through an inverter. IC5 determines the control logic. The pump (or the heater in temperature controller) should be ‘on’ when the fluid (or temperature) level is below the minimum level and should remain ‘on’ until the maximum level is reached. It must not start if the fluid level falls below the maximum level but remains above the minimum level.

Fig. 4: Actual-size, single-sided PCB layout for fluid-level controller with indicator ELECTRONICS PROJECTS Vol. 22

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Fig. 5: Component layout of PCB

This function is realised by IC5 that can operate a pump (or an alarm, or a flow valve, or a heater, as required) according to this control logic. For this, the input lines of IC5 are set to appropriate logic levels, which must not be disturbed. Sensor. To control the fluid level (say,

water level in a tank), an optical sensor as shown in Fig. 2 may be used. This optical sensor consists of a small filament lamp (generally used in torch or an IR LED as light source) and an LDR or a photodiode as the sensor. The filament lamp may be powered using the same step-down

Readers’ comments: Q1. Please explain the detailed working of the circuit. Also elaborate as to how to arrange the LDR and filament lamp in the tank? Please give details, how water level will be controlled by LDR? Is there any reflection of light from water surface? Ajit Through email A1. EFY: The optical sensor section (LDR and filament lamp) can be fixed rigidly on the bottom side of the tank lid/cover using M-seal or Fevi Quick or similar compound. Alternatively, you may mount them on a wooden strip and secure the strip to the

bottom side of the tank cover. The working of the LDR to control the water level is explained below. The light rays from the lamp are reflected from the water surface and fall on LDR1. The orientation and the intensity of light source are the deciding factors for incidence of adequate reflected light on the LDR for proper control of water level control. No direct light should be allowed to fall on the LDR. Fix a suitable opaque screen closer to LDR, between the light source and the LDR. For any given orientation of the light source and the position of the LDR, the

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transformer that is used to power the circuit. Alternatively, a separate step-down transformer may be used for the purpose, but taking into account the voltage and current ratings of the lamp. One may also use the sensor described in ‘Digital Water Level Meter’ in Circuit Ideas section of the February 2000 issue of EFY (also in Electronics Projects Vol. 21). Use that sensor (VR4) as part of a voltage divider network as shown in Fig. 3. If the circuit is used as a temperature controller, a temperature sensor using the popular LM35 IC may be built (refer Circuit Ideas published in March 1993 issue of EFY or Electronics Projects Vol. 14). Operation. The lower or the minimum level is set by pressing switch S2 and the upper or the maximum level by pressing switch S1. The two switches should be kept pressed until the required level is displayed. For example, if the lower level is selected 1 and the upper level 3, the pump (or heater or a flow valve) will start when the fluid falls below level 1 and will stop when the fluid reaches level 3. Assembly and testing. The circuit may be built on a veroboard. However, an actual-size, single-sided PCB and its component layout are shown in Figs 4 and 5, respectively. Switches used, should be of good quality. After assembling, the circuit may be tested using a voltage divider (potentiometer) that could be varied between ground and positive supply. While testing, set preset VR1 to increase or decrease the reference voltage taking into account the maximum output available from the actual sensor. In case of power failure, there should be proper battery back-up. Otherwise, the system will not behave as desired. Red and green LEDs are arranged in alternate fashion to make the bar display look attractive. ❏

reflected light intensity received by the LDR will be low when the water level is low and it will increase as the water level in the tank rises. (The intensity at the LDR depends on the total length of the path travelled by light.) Thus the resistance of LDR is high when the level of water is low and its resistance decreases as the water level increases. The intensity of light is indicated by the LEDs and 7-segment display in Fig. 1. of the article. VR2 (preset) is used to vary the sensitivity of LDR1 so as to obtain a predetermined LED/7-segment display when a specified level is reached.

MGMA—A Mighty Gadget with Multiple Applications a. jeyabal

M

GMA, pronounced as migma, is a versatile and multi-purpose gadget. It can be used for a range of applications, from a simple toy to domestic and workbench applications. It measures time, compares light output, temperature, resistance and capacitance, etc. You can use this gadget in a number of ways, depending on your imagination and creativity. Basically, MGMA is a resistancecapacitance-controlled oscillator that counts the pulses for a specific period. If any transducer, such as light-dependent resistor (LDR) or heat-dependent resistor (thermistor), is connected to it, the display shows the value corresponding to its resistance. Contact or break (normally open or closed) type transducers can also be used with MGMA.

Fig. 1: Block diagram of the MGMA circuit

Fig. 1 shows the block diagram of the MGMA circuit. Block 1 is an oscillator that is controlled by block 2. Block 2 contains another oscillator whose frequency is much lower than that of the former. The differentiator circuit in block 3 resets the decade counters periodically. Blocks 4 and 5 count the pulses, which, in turn, are displayed by blocks 6 and 7. Digit 9 in tens counter is decoded by block 8, and its output disables the counting process and triggers the aural indicator in block 9. Block 10 comprises the regulated power supply to run the gadget.

Circuit Oscillator. In Fig. 2, Schmitt trigger input NAND gate N1 of IC1 (CD4093), capaci-

tor C1, and potmeter VR1 form the oscillator circuit. Let us presume that capacitor C1 is in discharged state and pin 2 of gate N1 is in high state. As the input pin is low, output pin 3 is high and capacitor C1 starts charging through potmeter VR1. When the voltage across capacitor C1 reaches above half of the supply voltage, input pin 1 of gate N1 goes high and output pin 3 goes low. Now capaciParts List Semiconductors: IC1 - CD4093 quad 2-input Schmitt trigger NAND gate IC2, IC3 - CD4033 decade counter/ 7-segment decoder IC4 - 7805 +5V regulator T1 - BC557 pnp transistor T2 - SL100 npn transistor D1-D7 - 1N4148 switching diode D8, D9 - 1N4001 rectifier diode LED1 - Red LED Resistors (all ¼-watt, ±5% carbon, unless stated otherwise) R1, R6-R9 - 100-kilo-ohm R2 - 220-kilo-ohm R3 - 470-kilo-ohm R4 - 3.3-kilo-ohm R5, R10, R11 - 330-ohm VR1 - 1mega-ohm pot., linear VR2 - 47-kilo-ohm pot., linear Capacitors: C1, C3 - 0.001µF ceramic disk C2 - 4.7µF, 10V tantalum C4 - 1000µF, 25V electrolytic C5, C6 - 0.1µF ceramic disk Miscellaneous: X1 - 230V AC primary to 9-0-9V AC, 100mA secondary transformer S1, S2 - Push-to-on switch S3 - SPST switch, 230V AC DIS1, DIS2 - LT543 7-segment, commoncathode type LED display SOC1 - SOC4 - Earphone socket SOC5 - DC IN socket PZ1 - Piezo-buzzer - IC bases, knobs, mains chord, cabinet - Banana-type earphone plugs

ELECTRONICS PROJECTS Vol. 22

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Fig. 2: Schematic diagram of MGMA

tor C1 discharges through potmeter VR1. When the voltage across capacitor C1 falls below half of the supply voltage, pin 1 of gate N1 goes low and the output pin goes high. Now capacitor C1 starts charging again and the cycle repeats itself. The pulses from the output of gate N1 reach counter IC2 through resistor R1. Switch S1 is provided to stop the counting manually by grounding the Table Count 0 1 2 3 4 5 6 7 8 9

88

Decoded output of IC CD4033 a b c d e 1 1 1 1 1 0 1 1 0 0 1 1 0 1 1 1 1 1 1 0 0 1 1 0 0 1 0 1 1 0 1 0 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 0

ELECTRONICS PROJECTS Vol. 22

f 1 0 0 0 1 1 1 0 1 1

pulses through R1 when switch S1 is pressed. Counter and display. The output of the oscillator is connected to clock input pin 1 of IC2 (CD4033, a decade counter for unit digits). The carry-out pin 5 of IC2 is connected to the clock input of decade counter IC3 that is meant for ten’s digits. The segment outputs of both IC2 and IC3 go to the respective seven segments of DIS1 and DIS2 (LT543) for displaying the number of pulses. Lamp-test (LT) pin 14 of both IC2 and IC3 is grounded g CO through 100-kilo-ohm resistor 0 1 R8. The test-point (TP) may 0 1 be used to check the display. 1 1 When a high-level voltage (5V) 1 1 is applied to the test-point, all 1 1 segment outputs go high and 1 0 the display shows 88. 1 0 The display is blanked out 0 0 when the number to be dis1 0 played is 0, provided the ripple 1 0 blanking input (RBI) pin 3 is

held low. So on reset, only DIS1 (unit digit) will show zero as RBI pin 3 of IC3 is grounded. Switch S2 is provided to reset the counter manually. Current-limiting resistors R5 and R10 provided with DIS2 and DIS1, respectively, are used to reduce the component count and ensure the proper operation of digit-9 decoder circuit. Display controller and differen-tiator. For accurate reading of the counter, it must be reset periodically and the pulses must be counted for a specific period. For this an oscillator circuit comprising gate N2, diodes D1 and D2, resistor R2, potmeter VR2, and capacitor C2 is used. This oscillator also works like the previous one, but its charging and discharging paths are separated by diodes D1 and D2. Its ‘on’ time (high-level output) can be controlled by potmeter VR2. When output pin 4 of gate N2 goes from low to high state, the differentiator circuit comprising capacitor C3 and resis-

and it outputs clock pulses. These pulses are counted by IC2 and IC3 and displayed on DIS1 and DIS2, respectively. So the oscillator around gate N1 is enabled and disabled during the high and low states, respectively, of the output of gate N2. The counters retain their last count for reading until the output goes high once again. This reading time is about 2 to 3 seconds, which is set by resistor R2. Any increase in the value of R2 will increase the reading time and vice versa. Resistor R3 connected in parallel across capacitor C3 is used to discharge it quickly and diode D3 is used to block the DC voltage (when switch S2 is pressed) going to gates N1 and N3, and other parts of the circuit. Digit 9 decoder and aural indicator. It is very useful to sound an alarm for a certain readFig. 3: Actual-size, single-sided PCB pattern suggested for ing or otherwise, say, for a the circuit in Fig. 2 particular temperature or light output or resistance value, etc. A permanent number 90 is chosen for simplicity of the decoding circuit. When the display shows 90, the counter must be disabled and the buzzer enabled. From the table of decoded outputs of IC 4033 it is found that for number 9, at least one of the segment outputs is low (a, b and f are high, while e is low). For number 8, segment e is inverted by transistor T2. As RBI pin 3 of IC3 is grounded, all the segment outputs go low for 0. The clock-enable (CE) pin 2 of IC3 is pulled up by resistor R7. Pin 2 is also connected to a, b, f and e segment outputs of IC3 through diodes D5, D6, D7, and transistor T2, respectively, that altogether act as AND Fig. 4: Component layout for the PCB tor R9 produces a sharp pulse that resets counters IC2 and IC3. At the same time, gate N1 is enabled as output pin 4 of gate N2 is connected to input pin 2 of gate N1,

gate and bring the CE pin to ground for numbers 0 through 8. When the number is 9, the segment outputs a, b, and e are high, except the segment output e, which is inverted by transistor T2. As a result, CE pin of IC2 goes high and the counters are disabled. Simultaneously, this high-level output is inverted by gates N3 and N4. The inverted output from gate N4 forward biases transistor T1 to drive the piezobuzzer, while the inverted output from gate N3 grounds the resetting pulses. Diode D4 prevents the high output of N3 from reaching the reset pins of IC2 and IC3. Power supply. In 5V DC power supply shown at the bottom in Fig. 2, IC 7805 (IC4) is employed for better regulation. DC input/output socket (SOC5) is provided to operate the gadget with external 9V battery. LED1 acts as a power-on indicator.

Construction Figs 3 and 4 show suggested actual-size, single-sided PCB layout and component layout, respectively, for the circuit in Fig. 2. Solder the components in the order of IC sockets, jumpers, resistors, capacitors, diodes, LED, and transistors. Then connect the rest of the components through wires. Fig. 5 shows the proposed front-panel layout of MGMA. Before connecting VR1 and VR2 to the PCB, mark the dials using a digital multimeter. Both dial 1 and dial 2 (refer Fig. 5) are calibrated in terms of resistance for the variable resistance values of 1 mega-ohm in case of VR1 and 47 kiloohm in case of VR2, respectively, using a digital multimeter. (Note. There may be dead-ends on both ends of the potmeter, and it may vary in construction from manufacturer to manufacturer.) Mark the dials for every ten units for easy reading and setting.

Applications For high-resistance and low-resistance transducers, use earphone-type sockets SOC1 and SOC3, respectively. For low-capacitance and high-capacitance testing, use earphone-type sockets SOC2 and SOC4, respectively. For SOC1 and SOC2, the reading will decrease for the increasing value of resistance and capacitance, and vice versa for SOC3 and SOC4. Strength-0-meter. This game reELECTRONICS PROJECTS Vol. 22

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Fig. 5: Proposed front-panel layout of MGMA

Fig. 6: Connections for water level monitor

quires two small rods or prods. Connect them to an earphone plug using a pair of wires half a metre long. Then insert the plug into SOC1. Hold the rods in each hand between forefinger and thumb. Adjust dials 1 and 2 such that the buzzer beeps. Then rotate dial 1 slightly in the anti-clockwise direction to read around 70, a point where the buzzer is silent. Now ask your friends one by one to grip the rods firmly. The winner is the one who sounds the buzzer or scores higher on the meter. This depends on how hard one holds the rod, the internal resistance of the body, and dampness of the fingers. Plant tender. You can use MGMA to indicate the time of watering in order to avoid excessive watering of plants. For this, insert two metal strips on both sides of the plant. Connect them to an earphone plug using wires and insert the plug into socket SOC3. Since soil-resistance increases with loss of water, the alarm can be set/activated for a specific moisture level. Adjust dials 1 and 2 such that the buzzer sounds when the plant needs to be watered. The buzzer stops in a short while on sprinkling water over the soil supporting the plant. The next time the buzzer will sound automatically when the plant needs to be watered.

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Game of quick hands. This game requires an earphone plug with its two terminals shorted. Inserting this plug into SOC4 grounds input pins 5 and 6 of Schmitt NAND gate N2 via the shorted plug in SOC4. Since output pin 4 is always in high state, its periodic action of disabling gate N1 is no longer there. Connect a 0.1µF capacitor to SOC2 using an earphone plug. Since its capacitance value is higher than that of capacitor C1, the frequency of the oscillator decreases. The display shows a reading on momentarily pressing start/reset button S2 and then quickly depressing stop button S1. Adjust the dial to read 50 in the display. Now tell your friends to press button S2 momentarily and then S1. One who scores less is more quicker than the others, and hence the winner. Water-level monitoring. Five resistors R12 through R16 are connected in series and the junctions of the resistors are extended to the five levels of the water tank using wires (refer Fig. 6). A reference rod is also fitted with its lower end just below level 1. Plug-in a dummy resistor of 100k into SOC1 and rotate dial 1 to the zero-resistance position. Adjust dial 2 to read 55 in the display. Cover the unit digit with an opaque tape, so that only the ten’s digit is visible. Now remove the dummy resistor. Connect the other end of five-resistor ladder and the reference probe to SOC1. The display will show the water levels from one-fifth to five-fifth of the tank, depending on the actual level at that time. Measuring resistance. The idea is simple. First, VR1 (dial 1) is excluded from the circuit by rotating it to zero reading. Then an unknown resistor is connected to SOC1 and dial 2 is adjusted to read a number just below 90. Now VR1 (dial 1) is reinstated and rotated to display the same reading. As dial 1 is marked for resistance values, the position of dial 1 indicates the value of unknown resistor. With MGMA, up to 2-mega-ohm resistor can be measured. Connect the unknown resistor to SOC1 using crocodile clips. Rotate dial 1 to the zero-resistance position without touching the resistor, otherwise your body resistance will get included in the measurement. Adjust dial 2 such that the display reads around 90. The resistor is open if the display shows

0, and shorted if you’re unable to set the reading near 90. Remove the unknown resistor. Without disturbing dial 2, slowly rotate dial 1 to get the same reading. Now dial 1 shows the value of unknown resistor. If the resistor value is less than 40k, use SOC3 and repeat the same procedure with dial 2 instead of dial 1 for accurate measurement. The resistance value can be read from dial 2. Checking and measuing capacitance. Using MGMA you can measure capacitances from 0.001 µF to 5 µF. First check for the usability of the unknown capacitor. Adjust dials 1 and 2 to read 50 in the display. Now check the unknown capacitor using SOC2 for unipolar or SOC4 for electrolytic/tantalum capacitors with the inner and the outer terminals of the socket for positive and negative terminals of the capacitor. If there is no change in the reading it means the capacitor is shorted and a higher reading implies it is good. To find the value of an unknown nonelectrolytic (unipolar) capacitor, connect the same to SOC2. Adjust dials 1 and 2 to read a number around 80 in the display. Now, without disturbing the dials insert the known capacitors one by one in SOC2. The unknown capacitor value is equal to the value of the known capacitor for which the display shows the same reading or near the number 80. The procedure is same for electrolytic and tantalum capacitors, except that SOC4 is to be used in place of SOC2, ensuring that the inner and the outer terminals of the socket are used for positive and negative terminals of the capacitor, respectively. Testing a diode. Rotate dial 1 to high-resistance position and adjust dial 2 such that the display shows a flickering 45. Test the diode in SOC3 using an earphone plug in the same manner as mentioned earlier. Interchange the leads and test again. A shorted diode will not make any change in the reading, while a good one gives a reading of around 60 and 90 in both the tests. And for the open diode, the display shows 90 in both the tests. While checking the diodes, a parallel resistance of 100k is required across the diode. Our body resistance may also do. Other utilities. Heat alarm, fire alarm, security alarm, strain gauge, intruder alarm, rain alarm, number game, timer, and many other circuits can be realised using this MGMA circuit. ❏

TRAFFIC AND STREET LIGHT CONTROLLER rajesh gupta

T

his circuit of an adjustable traffic and street light controller can control the timings of four sides of traffic lights separately. It can also control the changeover from continuous traffic light mode to blinking yellow light mode (at night), and from blinking yellow light mode to continuous traffic light mode (during day). In addition, this circuit also controls the automatic switching off/on of the streetlights in the mornings and evenings with flexible settings—defining the morning and evening time. In order to prevent false triggering of streetlight circuitry due to some shadow or light on the sensor, some time delay is taken into consideration before sending the control

signal for streetlight operation. Its operation does not require any software and hardware knowledge. This circuit can also be adopted for synchronisation with the signals of adjacent traffic lights by introduction of appropriate delay in traffic light signals’ timings.

The circuit The circuit has two parts—the first for generation of control signals for streetlight and traffic light modes and the second for generation of four sides of traffic light signals. The circuit for streetlight and traffic

Fig. 1: Block diagram of traffic and street light controller

light modes (Part I) controls the switching time of streetlights in evenings and mornings and the time to changeover from PARTS LIST Semiconductors: IC1 - LM358 op-amp IC2 - 7404 Hex inverters IC3, IC6, IC12 - NE555 timer IC4 - 74LS93 4-bit binary counter IC5 - 74LS164 8-bit serial shift register IC7-IC9 - 7476 dual JK master-slave flip-flop IC10 - 7400 Quad 2-input NAND gates IC11 - 7410 Triple 3-input NAND gates IC13 - 7408 Quad 2-input AND gates IC14-IC17 - 7402 Quad 2-input NOR gates T1-T6 - SL100 npn transistor D1-D14 - 1N4007 rectifier diode LED1, LED3, LED6, LED9, LED12 - 3mm red LED LED2, LED5, LED8, LED11 - 3mm green LED LED4, LED7, LED10, LED13 - 3mm yellow LED Resistors (all ¼-watt, 5% carbon, unless stated otherwise): R1, R2, R18-R21 - 2.2-kilo-ohm R3-R5, R8, R12-R17, R22-R25 - 100-kilo-ohm R6 - 47-kilo-ohm R7, R9, R11 - 10-kilo-ohm R10 - 100-ohm R26 - 47-ohm R27 - 22-kilo-ohm R28 - 6.8-kilo-ohm VR1, VR2, VR4-VR7 - 1-mega-ohm preset VR3 - 100-kilo-ohm preset VR8 - 10-kilo-ohm preset Capacitors: C1 - 220µ, 10V electrolytic C2, C4, C6 - 0.01µ ceramic C3, C5 - 6.8µ, 10V electrolytic Miscellaneous: LDR1 S1 - Push-to-on switch

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Fig. 2: Schematic diagram for the traffic and street light controller

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continuous traffic light mode to blinking yellow light mode (at night), and from blinking yellow light mode to continuous traffic light mode (in daytime). Thus it decides the mode of operation. The circuit for four sides of traffic lights (Part II) also controls the time allowed for each side of traffic. It is further classified into continuous traffic light mode (for day) and blinking yellow light mode (for night). Part I Circuit. The block diagram of the circuit for signal generation for streetlight and traffic light modes is shown above the dotted line in Fig. 1. A natural light-dependent voltage and a reference voltage, which determines the evening and morning times are connected individually to the two inputs of a comparator. Low and high states of the comparator output decide morning and evening timings, respectively. The output of comparator is properly delayed for obtaining the signals for streetlight and traffic light modes. In the detailed circuit diagram shown above the dotted line in Fig. 2, a natural light-dependent voltage is obtained at the junction of light-dependent resistor LDR1 and resistor R7. Resistor R6 is used in parallel with LDR1 to limit the variation of the LDR. Light-dependent voltage and variable reference voltage are connected to the inverting and non-inverting terminals respectively of comparator IC1(a). In the evening, voltage at the inverting terminal of the comparator decreases with time due to the increasing resistance of LDR1. At a particular natural light intensity (determined by variable reference voltage, which can be adjusted with the help of preset VR8), it becomes less than the voltage at the non-inverting terminal. This drives the comparator into positive saturation region. Similarly, in the morning the comparator goes into negative saturation region at the same natural light intensity. In this way, the comparator gives high voltage (logic 1) for evening and low voltage (logic 0) for morning. IC1(b), with the non-inverting terminal biased at about 1/3rd Vcc, is simply used as an inverter (though wired as comparator). The inverted output of comparator IC1(a) is coupled to transistor T1 through resistor R4, while its direct output is coupled to transistor T2 via resistor R5. It is observed that transistor T1 is cut off in the evening and Vcc is applied to pin 7 of timer IC3 (wired in astable

multivibrator mode) via resistor combination RA1 (=R2+R3+VR1), while in the morning T2 is cut off and Vcc is applied to pin 7 of IC3 via RA2 (=R1+R8+VR2). In other words, the time period of IC3 is dependent on RA1 from the evening and RA2 from the morning. The diode pair of D1 and D2 or D4 and D5 is used to effectively isolate pin 7 of IC3 from being pulled towards ground via the conducting transistor (T2 in the evening and T1 in the morning). Time period of 555 clock in astable mode can be determined from the following relationship: T = RA (C/1.44) + 2 RB (C/1.44) where RA = RA1 from the evening and RA = RA2 from the morning, while RB = R9 = 10 kilo-ohm and C = C1 = 220 µF. Clock-1 output of IC3 is connected to 4-bit negative-edge-triggered counter 74LS93 (IC4). Period of output QD of IC4 is 16 times the clock-1 time period. This QD output (low for first eight clock-1 cycles and high for the next eight clock-1 cycles, and repeating thereafter) of IC4 is connected to the clock input of an 8-bit (positive-edgetriggered) serial shift register 74LS164 (IC5). The output of IC1(a) forms the data (D) input for the shift register. The data (D) at QA output is available after eight clock-1 cycles, while that at QH is available after 120 clock-1 cycles. Thus morning/evening (low/high) data is available at QA and QH outputs after 8 and 120 clock-1 cycles, respectively. Note that the clock-1 period itself differs for morning data and evening data. Streetlight indicator (LED1) is connected to QA output of shift register IC5.

The evening data (high) from comparator IC1(a) passes to the streetlight after eight clock cycles of clock-1. This delay is taken into consideration in order to prevent false signals to the streetlight due to some shadow or light on the sensor. The delayed high QH output provides the control signal for night to the second part of circuit and changes continuous traffic light mode to blinking yellow light mode. In this way the time at which night functioning of traffic light starts can be adjusted by choosing appropriate time period for clock-1 by adjusting the value of RA1. Similarly, the time at which day functioning of traffic light starts (stop blinking yellow light mode and start continuous traffic light mode) can be adjusted by RA2. Low (delayed morning signal) and high (delayed evening signal) QH outputs go to the second part of circuit for selecting the mode of traffic light. Table I summarises the functioning of the circuit for signal generation for streetlight and traffic light modes. Part II Circuit. The block diagram of the circuit for signal generation for four sides of traffic lights is given below the dotted line in Fig. 1. Here, the 4-bit and 2-bit counters are joined together to form a 6-bit counter. Outputs of the 2-bit counter, representing two MSB digits, are connected to a decoder that has two control inputs and four outputs. The decoder activates one of the four outputs depending upon the input (00 or 01 or 10 or 11) of 2-bit counter. Each output of the decoder can drive clock-2 at a different frequency. These four outputs are connected to the four sides of traffic lights and select each side one after

Table I Functional Summary of Part I Circuit Time Output Output at Output at Activated RA of IC1(a) QA of IC5 QH of IC5 Resistance Evening HIGH LOW LOW RA1 After 8 cycles of clock-1 HIGH HIGH LOW RA1 (Delay time for streetlight) After 120 cycles of clock-1 HIGH HIGH HIGH RA1 (Delay time for night) Morning LOW HIGH HIGH RA2 After 8 cycles of clock-1 LOW LOW HIGH RA2 (Delay time for streetlight) After 120 cycles of clock-1 LOW LOW LOW RA2 (Delay time for day) Evening HIGH LOW LOW RA1 Delay times and evening/morning times are adjustable. A: Continuous traffic light mode B: Blinking yellow light mode

Street Traffic Light Light (LED1) Mode OFF A ON A ON

B

ON OFF

B B

OFF

A

OFF

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Fig. 3: Actual-size, single-sided PCB for the circuit in Fig. 2

Fig. 4: Component layout for the PCB

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another. The time in which the preceding 4-bit counter counts from 0000 to 1111 (16 counts) is the time allowed for each side of traffic lights. First, when the 4-bit counter counts from 0000 to 0001 (two counts), yellow light of the selected side will turn ‘on’. From count 0010 to 1101 (12 counts), green light will turn ‘on’. Again from 1110 to 1111 count (two counts), yellow light will turn ‘on’. Meanwhile, in the other three sides of traffic lights that are

not selected by the decoder, red light will be ‘on’. Similar operation will repeat for each of the selected side in its turn. Reset pin of clock-3 and clear pins of the 6-bit counter are controlled by output QH from IC5 of Part I. At night, QH will go high and the 6-bit counter will clear, while clock-3 becomes active. As a result, yellow lights of the four sides of traffic light will blink simultaneously. The detailed circuit diagram is given below the dotted line in Fig. 2. The

Fig. 5: Connections for vehicular traffic lights and pedestrians’ signals

Fig. 6: The traffic and street light controller

active-‘low’, clear input signal for the 6-bit counter (formed by dual J-K flip-flops inside IC7 through IC9) is provided from the output of NOR gate E1, whose one input is connected to QH output of shift register IC5 of Part I and the other input is connected to the output of inverter gate N3. The input of inverter gate N3 is connected to push-to-on reset switch S1. Thus the 6-bit counter will clear when QH output is high or the reset button is pressed. The reset key, when pressed, also causes counter IC4 and shift register IC5 of Part I to be cleared. QH output of IC5 is connected to reset pin 4 of clock-3 (IC12). The output of this clock is connected to inverter gate N4. Low QH (during day) activates the 6-bit counter and deactivates clock-3. Due to this, the output of inverter gate N4 will be high during the day. This output is connected to one of the inputs of four AND gates H1 through H4. Each of these AND gates is a part of one side of traffic light circuit. NAND gates B1, B2, and B3 are connected to the outputs of flip-flops F2, F3, and F4 of the 6-bit counter. The final output of this circuit (the output of gate B2) will be high whenever the first four bits of the counter are 1110 or 1111 or 0000 or 0001 (14 or 15 or 0 or 1), otherwise it will be low. Accordingly, inverter N5 output will be low for the above contents of the counter and high for the remaining contents (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13). The output of NAND gate B2 and its complement (the output of inverter N5) are connected to NOR gates X2 (=E2, J2, K2, and M2) and X3 (=E3, J3, K3, and M3) of the each side of traffic light, respectively. Other inputs of X2 and X3 NOR gates are common. The last two flip-flops (F5 and F6) of the 6-bit counter are connected to four NAND gates G1 through G4 in such a way that the output of G1, G2, G3, and G4 will be low when last two counter bits are 00 (0), 01 (1), 10 (2) and 11 (3), respectively. For example, when last two bits of counter contents are 01 (1), only output of NAND gate G2 will be low and others (G1, G3 and G4) will be high. The complements of these four NAND gate outputs (obtained from collectors of transistors T3 through T6) are connected to the four RA resistors of 555 clock-2. Other terminals of these four resistors are connected to the anodes of diodes D8, D10, D12, and D14, while their cathode ELECTRONICS PROJECTS Vol. 22

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Table II Daytime Functions of Part II Circuit Counter Decoder output Activated RA Glowing LEDs contents G1 G2 G3 G4 resistance 000000 - 0 1 1 1 RA3 4,6,9,12 (Yellow light of 1st side and 000001 red light of other sides) 000010 - 0 1 1 1 RA3 2,6,9,12 (Green light of 1st side and 001101 red light of other sides) 001110 - 0 1 1 1 RA3 4,6,9,12 (Yellow light of 1st side and 001111 red light of other sides) 010000 - 1 0 1 1 RA4 3,7,9,12 (Yellow light of 2nd side and 010001 red light of other sides) 010010 - 1 0 1 1 RA4 3,5,9,12 (Green light of 2nd side and 011101 red light of other sides) 011110 - 1 0 1 1 RA4 3,7,9,12 (Yellow light of 2nd side and 011111 red light of other sides) 100000 - 1 1 0 1 RA5 3,6,10,12 (Yellow light of 3rd side and 100001 red light of other sides) 100010 - 1 1 0 1 RA5 3,6,8,12 (Green light of 3rd side and 101101 red light of other sides) 101110 - 1 1 0 1 RA5 3,6,10,12 (Yellow light of 3rd side and 101111 red light of other sides) 110000 - 1 1 1 0 RA6 3,6,9,13 (Yellow light of 4th side and 110001 red light of other sides) 110010 - 1 1 1 0 RA6 3,6,9,11 (Green light of 4th side and 111101 red light of other sides) 111110 - 1 1 1 0 RA6 3,6,9,13 (Yellow light of 4th side and 111111 red light of other sides) Note. The two MSB digits determine the side, while the next four digits determine the time for which the mentioned LEDs are ‘on’.

terminals are all connected to pin 7 of 555 clock-2 (IC6). This is analogous to the fashion in which RA1 and RA2 have been connected in Part I in the clock-1 circuit. When last 2-bit counter contents are 00, RA3 (=R21+R25+VR7) will become active and other three resistors RA4, RA5, and RA6 will become inactive. Therefore the time period of clock-2 of the 6-bit counter will be dependent upon RA3. Similarly, when last 2-bit counter contents are 01 or 10 or 11, the time period of clock-2 will be dependent upon RA4 (=R20+R24+VR6), RA5 (=R19+R13+VR5), and RA6 (=R18+R12+VR4), respectively. The output of NAND gate G1 is connected to the common input of NOR gates E2 and E3 of the first side of traffic light and complements of the outputs of other three NAND gates G2, G3, and G4 are connected to one of the inputs of NOR gates J1, K1, and M1, respectively. The other inputs of these NOR gates are connected to QH output of IC5. Red and green lights are connected to the outputs of NOR gates X4 (=E4, J4, K4, and M4) and X2 (=E2, J2, K2, and M2), and yellow light is connected to AND gate of each side of the traffic light. During daytime, the outputs of AND gates (which are connected to yellow lights) will be the same as the outputs

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of NOR gates X3(=E3, J3, K3 and M3) of each side, because one of the inputs of AND gates is high in daytime. Low QH (during daytime) forces NOR gates J1, K1, and M1 to work as the inverter gate for the other inputs. Therefore the common input of NOR gates X2 and X3 of sides 1, 2, 3, and 4 will be the same as the output of NAND gates G1, G2, G3, and G4, respectively. Let us suppose that initially the contents of the 6-bit counter are 000000. When the counter counts up from 000000 to 001111, the output of NAND gate G1 will be low and that of other NAND gates G2, G3, and G4 high. Due to this, RA3 will be active and the time period of clock-2 of the counter will be according to RA3. The high output of NAND gates G2, G3, and G4 forces the output from NOR gates J2, K2, M2 and J3, K3, M3 to low state. These low outputs are input to NOR gates J4, K4, and M4, due to which the output of these gates will be high. It means yellow and green lights will be ‘off’ and red light will be ‘on’ in the remaining three sides of the traffic light. Due to the low output of NAND gate G1 (which is connected to the common input of NOR gates E2 and E3 of first side), the output of NOR gates E2 and E3

of first side will depend on the output of the three-NAND gate circuit (comprising gates B1, B2, and B3). When the 6-bit counter counts from 000000 to 000001, the output of the threeNAND gate circuit will be high, which is connected to NOR gate X2 of each side and its complement is connected to NOR gate X3 of all sides. Due to this, the output of NOR gate E3 will be high and those of NOR gates E2 and E4 low. In short, during the count period 000000 to 000001 yellow light of the first side of traffic light and red light of the other three sides will be ‘on’. When the counter counts up further from 000010 to 001101, the output of the three-NAND gate circuit will be low and its complement will be high. Due to this reason, the output of NOR gate E2 will go high and that of NOR gates E3 and E4 low. Therefore, when counter contents increment from 000010 to 001101, green light of first side and red light of all the other sides will be ‘on’. Again from 001110 to 001111, the output of three-NAND gate circuit will go high, due to which yellow light of first side and red light of the other sides will turn ‘on’. The time in which the counter counts from 000000 to 001111 can be adjusted by RA3. The functioning of the other three sides of the traffic light is similar. Daytime functional summary of the circuit for signal generation for four sides of traffic light is given in Table II. Change in RB resistance (VR3+R11) of clock-2, being common for all sides, will change the time allowed for each side of traffic light by an equal amount. At night, QH output of IC5 will be high, due to which the 6-bit counter will clear and clock-3 will start working. The output of NOR gates J1, K1, and M1 and NAND gate G1, and the complement of the output of the three-NAND gate circuit will be low. This forces the output of NOR gate X3 of each side to high state. This high output will turn off all the red lights and give high signal to one of the inputs of AND gates H1 through H4. The other input of these AND gates is connected to the complement of clock-3, due to which all the four sides of yellow light will blink. The four sides of traffic light signals can be used for driving vehicular traffic signals for straight, right, and left turns and pedestrian’s signals. Fig. 5 shows one of such possible connections of vehicu-

lar and pedestrian’s signals. The complete circuit in model form is shown in Fig. 6. Actual-size, single side PCB for the circuit shown in Fig. 2 is given in Fig. 3 with its component layout in Fig. 4.

Calibration Set preset VR8 in such a position that the output of comparator IC1(a) switches from one state to the other at a particular intensity of natural light. Variable resistors VR1 and VR2 can be calibrated on a time scale using the following relationships: VR1 = (1/120) (1.44 TNight/220) 106 – (122.2) 103 VR2 = (1/120) (1.44 TDay/220) 106 – (122.2) 103

where TDay and TNight are delay times in seconds (time interval between switching of comparator IC1(a) and when the traffic light switches its mode) corresponding to day and night, respectively. Variable resistors VR4 through VR7 can be calibrated on a time scale by the following relationship: VR (4,5,6,7) = (1/16)(1.44 T/6.8) 106 – (122.2) 103 – 2 VR3 where T is the time allowed (in seconds) for the side of traffic light in which the corresponding variable resistance is connected. Possible enhancements. Stepper motor-driven wiper can be used for cleaning the dust over the light sensor during night time. Control signal for this can be

obtained from the shift register. Also, time-controlling variable resistors VR4 through VR7 of Part II can be replaced by LDRs with a small light source whose light intensity varies according to the strength of traffic on each side. Implementation of this system requires traffic-sensing sensors. This system will change the time of each side of traffic light according to the strength of traffic. Further, the present circuit being only a demonstration model uses LEDs for lights. To drive high-wattage lights, one can easily boost the signals used for driving the LEDs to operate solidstate or electomechanical relays. ❏

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Lead-Acid Battery Charger with Active Power Control m.k. chandra mouleeswaran

H

igh-power lead-acid battery chargers usually employ constant voltage charging method. In such chargers the charging is monitored against the battery terminal voltage.

Constant voltage at a constant current results in a very large initial current in a ‘flat’ battery and a very low current in a partially charged battery. To overcome this problem, the charger should be made

Fig. 1: Schematic diagram of lead-acid battery charger

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to vary the charging current in accordance with the existing terminal voltage of the battery. In the circuit presented here the charging current is adjusted against the

terminal voltage in such a way that any battery with any level of charge can be connected to the charger without requiring any manual adjustment. The charging voltage is held constant, while an appropriate charging current range is automatically selected at successive different battery terminal voltages. And when the battery gets fully charged, the charger switches over to tricklecharge mode. The circuit consists of the Fig. 2: Charging current versus battery terminal voltage

Fig. 3: Actual-size, single-sided PCB for battery charger

Fig. 4: Component layout for the PCB

following sections: 1. The DC power supply section. 2. The series DC voltage regulation section. 3. The battery status indication-cumcharge current regulation section. The DC power supply section. The 230V AC mains supply is connected to a step-down transformer with a secondary rating of 24V AC, 5A through DPDT toggle switch S1. When switch S1 is in ‘off’ position, the availability of mains supply is indicated by green LED1. When switch S1 is toggled to ‘on’ position, red LED2 glows to indicate that the charger is ‘on’. The four 15-kilo-ohm resistors R1, R2 and R3, R4 in the path of LED1 and LED2, respectively, are rated at 1 watt each. The output from the secondary of transformer X1 is rectified by the bridge rectifier comprising 1N5408 diodes D3 through D6, rated at 800V, 3A. The rectified output is smoothed by three capacitors C1, C2, and C3 before being applied to the rest of the circuit. The 4.7-kilo-ohm resistor R6 acts as a bleeder resistance. LED7 indicates that DC is available at the output of this section. The series DC voltage regulation section. This section is configured around power Darlington transistor TIP142 (T1) that functions in conjunction with transistor T3 (BC549) and preset VR2 to regulate the output voltage from the DC voltage regulator section. Since zener diode ZD1 conducts only after the output voltage reaches 15 volts, the output voltage needs to be adjusted in the vicinity of 15 volts with the help of preset VR2. When transistor T3 conducts fully, the base of transistor T1 is pulled towards ground via resistor R8 and it stops conducting after the output voltage exceeds a specific value. Transistor T2 (also a BC549) helps in current limit adjustments. Low-value, high-wattage resistors R15 (shunted by R14) through R19 connected in series form a currentlimiting resistor network at the output of transistor T1. This resistor network limits the charging current depending on the energisation/ de-energisation state of relays RL1 through RL4 that select the current range. The resistors are either ELECTRONICS PROJECTS Vol. 22

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Parts List Semiconductors: IC1 - LM324 quad op-amp T1 - TIP142 power Darlington transistor T2, T3 - BC549 npn transistor T4-T7 - 2N2222A npn transistor D1, D2, D7-D11 - 1N4007 rectifier diodes D3-D6, D12 - 1N5408 rectifier diodes LED1 - Green LED LED2 - Red LED LED3 - Bright yellow LED LED4, LED5 - Bright green LED LED6, LED7 - Bright red LED ZD1 - 15V, 1W zener diode ZD2 - 6.8V, 1W zener diode Capacitors: C1, C2 - 2200µF, 40V electrolytic C3 - 1000µF, 40V electrolytic C4 - 470µF, 25V electrolytic C5 - 100nF ceramic Resistors (all ¼-watt, ±5% carbon unless stated otherwise) R1-R4 - 15-kilo-ohm, 1W R5 - 2.2-kilo-ohm R6 - 4.7-kilo-ohm, 0.5W R7, R10, R12 - 1-kilo-ohm R8 - 100-ohm R9 - 470-ohm R11 - 4.7-kilo-ohm R13 - 47-ohm R14-R15 - 0.66-ohm, 3W wirewound or fusible R16 - 0.67-ohm, 3W wirewound or fusible R17 - 0.20-ohm, 3W wirewound or fusible R18 - 0.47-ohm, 3W wirewound or fusible R19 - 1.0-ohm, 1W wirewound R20-R23 - 470-ohm, MFR 0.5% or 0.1% R24 - 820-ohm, MFR 0.5% or 0.1% R25 - 10-kilo-ohm, MFR 0.5% or 0.1% R26-R28 - 1.2-kilo-ohm R29 - 1.5-kilo-ohm VR1-VR2 - 2.2-kilo-ohm preset VR3 - 10-kilo-ohm preset VR4 - 15-kilo-ohm preset Miscellaneous: RL1-RL4 - 24V DC, 500-ohm relay contacts at 10A DC X1 - 230V AC primary to 0-24V, 5A secondary transformer S1 - DPDT toggle switch F1 - 750mA cartridge glass fuse

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Table LED/Relay Operation and Charging Resistance Battery LED/Relay status voltage LED3 LED4 LED5 LED /RL1 /RL2 /RL3 /RL4 <10.5V Off Off Off Off 10.5V On Off Off Off 11.5V On On Off Off 12.5V On On On Off 13.5V On On On On * 0.5A is taken as the trickle charging current.

shorted or added by respective relay contacts RY1 through RY4 depending on the charging current requirement from the regulator. The battery status indication-cumcharge current regulation section. In this circuit, a quad op-amp LM324 (IC1) is wired as a four-stage comparator to indicate the battery voltage with the help of four LEDs (LED3 through LED6), while at the same time selecting and driving corresponding relays to set the charging current range. The 6.8V reference voltage developed across zener diode ZD2 is proportionately applied to the inverting terminals of comparators A1 through A4, while the sampled battery voltage is proportionately applied to the non-inverting terminals of all the comparators. Preset VR3 may be adjusted to obtain the reference voltages as shown in Fig. 1. Preset VR4 may be adjusted by applying an external fixed voltage of 10.5V, 11.5V, 12.5V, or 13.5V across the battery’s screw terminals, ensuring that the corresponding LEDs (and relays) light up (energise) in accordance with the table. In the charge characteristic curve of Fig. 2, it can be seen that the terminal voltage is compared by the comparators against the preset values and the charging current is selected accordingly. Thus a battery of any charge level can be connect-

Charging resistance

Preset current

1 ohm 0.33 ohm 0.53 ohm 1 ohm 2 ohms

1A 3A 2A 1A 0.5A*

edand left unattended under the control of this charger circuit. When the battery is flat with terminal voltage below 10.5 volts, the initial charging current is selected at just one ampere because a higher initial charging current may cripple both the battery and the charger. A higher charging current is selected only when the battery has reached a safe level of terminal voltage. Later, as the battery starts charging and its terminal voltage starts rising, the charging current is decreased in proper steps. Upon reaching the full voltage of 13.5 volts, the charger switches to the trickle charge mode with resistor R19 coming into the charging path. Optionally, one can switch off the charger on energisation of relay RL4 by just removing resistor R19 from the circuit. Whenever the terminal voltage level of the battery goes low, the charger automatically resumes charging. Figs 3 and 4 show the actual-size, single-sided PCB and the component layout, respectively, of the charger circuit. Note. To ensure proper functioning of the circuit, use good-quality relays and precise-value resistors (R14 through R24) with tolerance as mentioned in the Parts List. Connect the metal housing of the charger circuit to the earth line of the AC mains supply for personal safety. ❏

Amplitude Measurement of Sub-Microsecond Pulses anil kumar maini

A

pulse or a repetitive train of pulses is one of the most frequently encountered electronic signals, and the conventional way to determine its peak amplitude is to have an oscilloscope display of the waveform. An oscilloscope that has the required bandwidth to correctly display submicrosecond-wide pulses is an expensive instrument, and is often beyond the reach of most electronics enthusiasts, hobbyists, and small-scale units. The circuit presented here allows you to measure the peak amplitude of a single pulse as well as of a repetitive train of pulses with a conventional multimeter. The circuit is capable of measuring peak amplitude of pulses as narrow as 100 nanoseconds (ns) up to a maximum of 100V amplitude. There is practically no limit on the maximum value of the pulse width. It can also be used to measure the peak amplitude of a repetitive pulsed waveform as long as the time interval between two successive pulses is greater than 100 microseconds (µs).

The circuit The pulse under measurement is fed to the input of a cascaded arrangement of two unity-gain peak detection stages built around IC1 and IC2 using high-speed op-amps AD829, as shown in Fig. 1. The op-amp has a guaranteed unity-gain bandwidth of 120 MHz and a slew rate of 230 V/µs, and it is capable of driving highly capacitive loads. This makes it ideal for receiving input pulses as narrow as 100 ns. D1 and D2 (1N914) are high-speed switching diodes having a response time of the order of 2 ns to 3 ns. The input pulse gets stretched to about 10 µs at the output of the first peakdetection stage built around IC1 and to about 100 µs at the output of the second peak-detection stage built around IC2. With switch S1 open, the circuit can

Fig. 1: Circuit for measuring sub-microsecond pulses ELECTRONICS PROJECTS Vol. 22

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Photograph of author’s prototype

switch causes division of the input voltage by a factor of 10 due to the arrangement of resistors R1 through R3. Fig. 2: Waveforms at various points of the circuit The peak amplitude of receive input pulses greater than 100 mV the stretched pulse at the output of the (which is the same as the reference voltage second peak detector is the same as set for comparator IC3, LM319) but less the input pulse peak amplitude. This than or equal to 10 volts. output amplitude is halved by resistors With switch S1 closed, the input pulse R9 and R10 before feeding the same to amplitude may be anywhere between the analogue input of IC5 (ADC-type 1 volt and 100 volts. The closure of the AD0808). This ensures that for the maxi-

Fig. 3: Actual-size, single-side PCB layout for the circuit

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mum input pulse amplitude of 100 volts, the ADC analogue input is limited to 5 Parts List Semiconductors: IC1, IC2 - AD829 op-amp IC3 - LM329 comparator IC4 - 74121 monostable multivibrator IC5 - AD0808 analogue-to-digital converter IC6 - DAC0808 digital-to-analogue converter IC7 - LF356 op-amp IC8 (N1-N3) - 74HCT04 hex inverter IC9 (N4-N7) - 7400 NAND gate D1, D2 - 1N914 high-speed switching diode ZD1 - 2.5V zener diode Resistors (all ¼-watt, ±5% carbon, unless stated otherwise): R1, R2 - 18-kilo-ohm R3, R16 - 1-kilo-ohm R4 - 12-kilo-ohm R5, R6 - 22-kilo-ohm R7, R8 - 15-kilo-ohm R9-R12 - 100-kilo-ohm R13 - 470-ohm R14 - 220-kilo-ohm R15, R18, R19, R24, R25 - 10-kilo-ohm R17 - 2.2-kilo-ohm R20, R21 - 2.7-kilo-ohm R22 - 4.7-kilo-ohm R23 - 33-kilo-ohm VR1 - 50-kilo-ohm preset Capacitors: C1, C2, C4, C5, C7-C9, C11-C17 C19 - 0.1µF ceramic disk C3, C10 - 0.001µF ceramic disk C6 - 0.01µF ceramic disk C18 - 56 pF ceramic disk Miscellaneous: S1, S2 - On/off switch (SPST) Meter - Multimeter

Operation

Fig. 4: Component layout for the PCB

volts, which is the maximum amplitude it can accept. The output of the first peak detector stage after a division by a factor of 2 by the arrangement of resistors R11 and R12 feeds comparator LM319 (IC3). The leading edge of the pulse output from the comparator coincides with the leading edge of the input pulse. The leading-edge comparator output triggers monoshot 74121 (IC4) to produce a 1µs pulse (as determined by timing components R1 7-C10) at its Q-output, with its leading edge coinciding with the leading edge of the input pulse. The monoshot output is passed through an appropriate NAND gate logic circuit built around 7400 (IC9) and

it acts as the start-of-conversion pulse for analogue-to-digital converter IC5 (ADC0808). The NAND logic is used here to incorporate the reset feature. The clock generator circuit for IC5 is built around 74HCT04 (IC8) to provide 1MHz clock. The clock frequency is decided by R24, R25, and C18. The latched digital output from IC5 feeds the corresponding inputs of DAC0808 (IC6). The DAC output, which is a latched DC current, is converted into a proportional voltage in the current-to-voltage circuit built around op-amp LF356 (IC7). This DC voltage is connected to the multimeter for indication of peak amplitude of the input pulse to the circuit. Potentiometer VR1 is meant for calibration.

Every time there is a pulse at the input, there is a stretched pulse appearing at the analogue input of the ADC, with its leading edge coinciding with the leading edge of the input pulse. Fig. 2 shows waveforms available at various test points marked A, B, C, D, and E in the circuit shown in Fig. 1. Also, there is a start-of-conversion pulse appearing at the relevant input of the ADC. The conversion starts at the trailing edge (test point E) of this pulse 1 µs after the leading edge of the input pulse. Since the stretched pulse is about 100µs wide, the peak amplitude of the pulse 1 µs later is almost the same as the actual peak amplitude. At the same time, this small delay ensures that the analogue input is already present on the relevant input at the start of conversion. The latched digital output from the ADC feeds the corresponding inputs of the DAC0808 (IC6) as stated earlier. The output of the DAC, after conversion into the proportional voltage by LF356 (IC7), is fed to the multimeter (set to appropriate DC voltage scale) for measurement of peak pulse amplitude. Potentiometer VR1 is used for calibration. The display holds the peak amplitude of the last pulse until it is reset using switch S2 or it is updated by another pulse at the input. The accompanying photograph shows the assembled circuit that the author used for performance evaluation. An actual-size, single-side PCB for the circuit is shown in Fig. 3 and its component layout in Fig. 4. ❏

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Automatic Submersible Pump controller k.c. bhasin

A

number of construction projects as well as circuit ideas for water-/ fluid-level control have appeared in EFY over the years, but so far no dedicated project has appeared for automating the control of submersible water pumps. Looking into the demand for such a project from readers, we present here a circuit for

2850 rpm typically. The ESP body is made of cast iron or stainless steel. For low and medium range, one can use 3-phase or split-phase (also referred to as 2-phase) supply. ESPs of 3 HP or higher rating invariably use 3-phase supply. Let us consider a typical case of 1.5HP

Fig. 1: Line diagram of control panel for manual operation of ESP motor

automating the operation of an electrical submersible pump (ESP) based on the minimum and the maximum levels in the overhead tank (OHT). This circuit can be interfaced to the existing manual control panel of an ESP and can also be used as a standalone system after minor additions.

ESP basics Electrical submersible pumps are singleor multiple-stage radial-flow pressure series impeller pumps that are close coupled to the motor for low and medium heads. These find applications in domestic, industrial, irrigation, air-conditioning, and various other systems. The ESPs are classified by the bore diameter (which generally varies from 100 mm to 200 mm), horse-power (from about 0.5 HP to 40 HP), and discharge rate (typically 120 litres per minute for 0.5 HP to about 2000 litres per minute for 40 HP). These are run at a fixed speed, which is

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the run capacitor value can be calculated using the simple thumb rule (70 µF per HP), while the start capacitor value may be determined from Table I. Manual operation of ESP motor (Fig. 1). The control panel comprises an isolator switch, push-to-on single-/dualsection ‘start’ button, push-to-off ‘stop’ button, a triple-pole moulded case circuit breaker (MCCB) for motor protection with magnetic trip and resetting facility (with an adjustable current range of 12 to 25 amperes), start and run capacitors, amperemeter, voltmeter, neon indicators, etc. (Note. The MCCBs used for motor control are termed as motor circuit protectors (MCPs). These are classified/catalogued by number of poles, continuous ampere rating, and magnetic trip range (current). For details, you may visit Cutler-Hammer’s Website or contact Bhartia Cuttler-Hammer dealers.) Fig. 1 shows a simplified control panel diagram, along with ESP motor wiring. The ‘start’ pushbutton (green), which is normally open, and the ‘stop’ pushbutton (red), which is normally closed, are in

ESP with 100mm bore diameter, using a split-phase motor. The motor draws a running current of 10 to 11 amp, while Motor rating Start capacitor value (µF) the starting current is around 2.5 to three in HP 230V AC (working) times the running current value. 275V AC (surge) To obtain a higher initial torque, the 1/6 20-25 run winding is connected in series with a 1/5 30-40 1/4 40-60 parallel combination of 120-150µF, 230V 1/3 60-80 AC bipolar paper electrolytic capacitor 1/2 80-100 and 72µF, 440V AC run-mode capacitor. 3/4 100-120 After two or three seconds of running, 1 120-150 1½ 150-200 when the motor has picked up sufficient 2 200-250 speed, the start capacitor goes out of the circuit because of the opening of the centrifugal switch inTruth Table for relay operation side the motor, while the run Water level Relay operation (2.5 – 3 sec.) Pump motor capacitor stays in the circuit in tank RL1 (stop) RL2 (Start) operation permanently. For ESPs that Below No Yes Starts don’t have an integral cen- low level trifugal switch arrangement, Above a dual-section start switch low level (explained later) can be used but below high level No No Remains on to perform the function of the Reaches centrifugal switch. high level Yes No Stops For the split-phase motor,

Fig. 2: Circuit diagram for automatic control of ESP motor via control panel (Fig. 1)

series with the live or phase line. The isolator switch is normally in ‘on’ position. When ‘start’ button is momentarily pressed, the contactor energises via the closed contacts of ‘off’ button. One of the contact pairs of the contactor is used as the hold contact to shunt ‘on’ button and provide a parallel path to the contactor coil, which thus latches. The supply to the motor gets completed via the other N/O contacts of the contactor and the pump motor starts. When the motor gains sufficient speed (around 80 per cent of the normal running speed), the

centrifugal switch opens to take the start capacitor out of the circuit and only the run capacitors (2x36 µF) permanently stay in series with one of the two stator windings of the ESP motor. In case the ESP is not provided with an integral centrifugal switch, a second section in ‘start’ button (shown in light shade in Fig. 1) can be used to shunt points ‘E’ and ‘F’. Since this switch section has no hold on contacts, the start capacitor will go out of circuit as soon as ‘start’ button is released. The motor can be switched off by momentarily depression of

‘off’ button, which interrupts the supply to the contactor coil. To interface the control circuit shown in Fig. 2, we use circled points A and B (in parallel with ‘on’ button) and C and D (formed by disconnecting one of the wires going to ‘off’ button terminal, i.e. in series with ‘off’ button). Points E and F will be used if the ESP does not have an integral centrifugal switch. It may be recalled, by referring to Fig. 1 of the project ‘Auto Control for 3-phase Motor’ published in EFY’s June issue (same EP Vol. 22), that wiring of ‘on’ and ‘off’ buttons of 3-phase (4-wire system) and split-phase motors are identical. Hence the control circuit described here can equally be used for 3-phase motors of up to about 10 HP. For motors of higher HP, one must use star-delta type starter configuration.

The circuit

Fig. 3: Actual-size, single-sided PCB layout for Fig. 2

As shown in Fig. 2, the 230V AC mains (tapped from the same points from which it is fed to the control panel of Fig. 1) is stepped down to 12V-0-12V by transformer X1. The rectified output smoothed by capacitor C1 is used for operation of ELECTRONICS PROJECTS Vol. 22

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Fig. 4: Component layout for the PCB

heavy-duty 24V, 250-ohm relays RL1 and RL2 having contact rating of 30 amp. The relay contacts identified by letters ‘A’ through ‘F’ in Fig. 2 are to be connected to identically marked points in Fig. 1. Note that point C in Fig. 1 is created by breaking the connection going to point D on the ‘stop’ switch. We have used relay RL1 with single changeover contacts. If you need higher current rating, use relays with double changeover contacts by interconnecting N/C, N/O, and pole of one set to the corresponding terminals of the other set. The circuit, except for the relay drivers, is operated with regulated +12V supply developed across capacitor C2. The +12V supply is fed to the common probe in the overhead tank/storage tank via 10-kilo-ohm resistor R1 and diode D9. Low-level and high-level probes are connected to the input of CMOS inverter gates N3 and N1, respectively, via 10-kiloohm resistors. The final low-level output at pin 10 of gate N5 goes high when the water level in the overhead/storage tank is below the low-level probe. The final high-level output at pin 4 of gate N2 goes high as soon as the water touches the high-level probe. Both IC1 and IC2 have been configured as monostables with a pulse width of about 2.5 to 3 seconds. This period is found to work optimally for ‘start’ and ‘stop’ switch operation of the manual control panel. The respective monostables for low level (IC2) and high level (IC1) get triggered via transistors T2 and T1 when the final output at pin 10 of gate N5 or pin 4 of gate N2, respectively, goes high. The connection of reset pins of IC2 and IC1 to the outputs of gates N1 and N2, respectively, ensures that no false triggering of monostables takes place due to the noise generated during changeover of

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relay contacts, and also that the two relays never operate simultaneously. In the case of mains failure, the pump stops if it was already running. When the mains supply resumes, the pump starts only when the water goes below the low level. In such a situation, you can restart the motor by manual operation of ‘start’ button on the control panel. The connections for the ammeter and the voltmeter, not shown in Fig. 1, can be made easily. Connect the voltmeter across the incoming live and neutral lines, and insert the ammeter in series with the stop switch by breaking the live line connection after the stop switch. Transformer, relays, switches, fuse, and neon indicator (with integral resistor) are to be mounted on the cabinet.

Precautions

The following are the vital points to be borne in mind during wiring, assembly, and installation: 1. One-watt resistor R18 should be mounted leaving some space below it. 2. Use multistrand insulated copper wires of 15-amp rating for taking connections from relay terminals and terminate them on a tag block, marking each terminal properly. Similarly, terminate the points to be extended to the OHT/storage tank on a tag block (TB) using 25-28SWG wire, marking them suitably. 3. Mount the relays inside the body of a suitable metallic enclosure. The enclosure should be properly earthed via the earth lead of the mains. Also mount the step-down transformer inside the same enclosure/cabinet. Use a TB for incoming live, neutral, and earth connections from the mains (to be taken from the manual control panel of ESP motor). 4. After assembly, position the cabinet as close to the manual control panel

Parts List Semiconductors: IC1, IC2 - NE555 timer IC3 - CD4049 hex inverter/buffer T1, T2 - BC548 npn transistor T3, T4 - BD139/SL100 npn transistor D1-D4, D7-D9 - 1N4007 rectifier diode D5, D6 - 1N4001 rectifier diode ZD1 - 12V, 1W zener diode Resistors (all ¼-watt ±5% carbon unless stated otherwise) R1, R3, R5, R7, R9, R12, R14 - 10-kilo-ohm R2, R6, R11, R15-R17 - 1-kilo-ohm R4, R13 - 220-kilo-ohm R8, R10 - 330-kilo-ohm R18 - 330-ohm Capacitors: C1 - 470µF, 63V electrolytic capacitors C2 - 470µF, 25V electrolytic capacitors C3, C7 - 47µ, 25V electrolytic capacitors C4, C6 - 0.01µF ceramic disk C5, C8 - 10µF, 25V electrolytic capacitors Miscellaneous: X1 - 230V AC primary to 12V-012V, 1amp Secondary transformer L1 - NE2 (neon bulb with inbuilt resistor) S1 - On/off switch F1 - 3amp fuse RL1 - 24V, 250-ohm, 1 c/o relay, 30A contact rating

of ESP motor as possible and extend connections from tag blocks for relay and power supply to the corresponding points, as explained earlier, using cables of correct ratings. 5. For probes, use stainless steel rods of about 10cm length and 5 to 8 mm diameter with arrangement for screwing the telephone-type 25/26 SWG wire to be used for extending the probes’ connections to the circuit. Teflon-insulated wires are, however better as they would last longer. The joint may be covered by epoxy. 6. The probes can be hung from the lid of the tank to appropriate levels using the same wire. Make sure that the common probe goes up to the bottom of the tank/storage tank. 7. All the wires from tank to the TBs in the cabinet should be routed in such a way that they do not interfere with any mains wiring. The length of the wires hardly matters as the CMOS gates used for terminating the wires from probes have very high input impedance. ❏

Transistor Curve Tracer a. saravanan

T

ransistor is the basic component of all electronic equipment. A good design of electronic circuitry requires proper knowledge of the characteristics and parameters of transistors. Due to such factors as changes in doping level of impurities and physical dimensions, production imperfections, and environmental (ambient temperature, humidity, etc) changes, no two transistors can have the same characteristics. Transistor is an active device and even a very small change in its parameters causes a large drift in its operation. This affects the overall efficiency and the reliability of an equipment. Hence for an efficient, reliable, and trouble-free design/ operation of the electronic equipment, the designer must know the characteristics and parameters of each transistor used in the equipment. The manufacturer provides generalised family characteristics of transistors bearing specific part numbers. These characteristics are drawn under specific test conditions such as 25oC temperature and 10mA collector current IC. But as the

circuit designed may need to be operated at different conditions (for example, at an ambient temperature of 40°C and collector current of 10 mA), the manufacturer’s data is no longer adequate. The manual procedure to draw the characteristics of a transistor is tedious and cumbersome. Further, using the manual procedure, it is not feasible to draw the dynamic characteristics of a transistor. The transistor curve tracer circuit presented here enables one to draw the input and output characteristics of npn transistors in common-emitter configuration on a cathode ray oscilloscope (CRO). It can be constructed and calibrated by the designer himself. The circuit can be upgraded to draw the characteristics of both npn and pnp transistors, field effect transistors (FETs), metal-oxide semiconductor field effect transistors (MOSFETs), unijunction transistors (UJTs), silicon-controlled rectifiers (SCRs), TRIACs, etc. In general, it can be upgraded for any two- or three-terminal analogue electronic device that has a

Fig. 1: Block diagram for tracing transistor output characteristics

Fig. 2: Block diagram for tracing transistor input characteristics

single control terminal unlike op-amps.

Block diagram The transistor curve tracer is built around the ramp generator and the current-tovoltage converter. The ramp generator produces a linear ramp that is applied to the transistor under test either as the collector-emitter voltage (VCE) or the base-emitter voltage (VBE). The ramp is also used to deflect the electron beam horizontally (along x-axis) on the screen of the CRO. Similarly, the current-to-voltage converter converts either the collector current (IC) or the base current (IB) into a proportional voltage that is used to deflect the electron beam vertically (along y-axis) on the screen. The signal conditioning and switching circuits, along with the ramp generator and current-to-voltage converter, make a complete curve tracer for the input and output characteristics of an npn transistor. Output characteristics (Fig. 1). The ramp and clock generator generates a linear ramp and 1 kHz clock pulses. The ramp is amplified by the ramp buffer amplifier to 0 to 5 volts. This amplified ramp is applied to the collector of the transistor under test as the collector-emitter voltage (VCE) through the current-to-voltage converter. The current-to-voltage converter gives an output voltage proportional to collector current IC that is applied to the CRO to deflect the beam in y-axis. The 0-5V ramp output is applied to the CRO to deflect the beam in x-axis. Hence we can trace the output characteristics of the transistor with the collector-emitter voltage (VCE) on x-axis and IC on y-axis. To trace the output characteristic graph for various base current (IB) values, the generator’s clock output fed to the counter is incremented for each clock pulse. The count sequence is 000, 001, 010, 011, 100, 101, 110, and 111 (0 to 7 decimal). After 111, the counter resets ELECTRONICS PROJECTS Vol. 22

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Fig. 3: Circuit diagram of transistor curve tracer

automatically to 000 and the sequence repeats. The lower three bits of the counter are applied to the base-current control circuit. The base-current control circuit sets IB in eight discrete 100µA steps, i.e. 0 µA, 100 µA, 200 µA, 300 µA, 400 µA, 500 µA, 600 µA, and 700 µA. Adjust the step width (100 µA) using a potentiometer such that the output characteristics of various npn transistors with various current gains (β) are traced/accommodated. Input characteristics (Fig. 2). Here again, the ramp and clock generator generates a linear ramp and 1kHz clock pulses. The ramp is amplified by the ramp buffer amplifier to 0-5V. This amplified ramp is attenuated and amplified as required to get 0-1V ramp and applied to the base of the transistor under test as the base-emitter voltage (VBE) through the current-to-voltage converter. The current-to-voltage converter gives an output voltage proportional to base current IB that is applied to the CRO to deflect the beam in y-axis. The 0-1V ramp output is applied to the CRO to deflect the beam in x-axis. Hence we can trace the

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input characteristics of the transistor with VBE on x-axis and IB on y-axis. To trace the input characteristics graph for various VCE values, the clock output of the generator is fed to the counter and switching circuit. The counter counts the number of pulses in the binary form. Q0 output of the counter is used as the collector-emitter voltage control that toggles VCE with 0 volt and 10 volts for every clock pulse. Thus we can trace the input characteristics for VCE = 0 volt and VCE = 10 volts.

The circuit The transistor curve tracer circuit (Fig. 3) comprises power supply, ramp and clock generator, ramp buffer and offset null, current-to-voltage converter, counter, base current control, and switching sections. 1. The power supply section. The circuit operates on ±12V regulated power supply. The input AC mains supply is stepped down by transformer X1 to deliver a secondary supply of 15-0-15V AC at 1 ampere. The output of the transformer is rectified by a bridge rectifier. The 1000µF,

35V capacitors act as filters to eliminate ripples and provide unregulated DC output voltage. The unregulated dual DC voltage is converted by three-terminal ICs AN7812 and AN7912 into ±12V regulated power supply. (Note. Connect 0.1µF decoupling capacitors between the supply terminals and ground of every IC in order to suppress unwanted noise signals in the supply voltage.) 2. The ramp and clock generator section. The ramp and clock generator uses a constant current source (LM334) and a capacitor, in conjunction with timer NE555 (IC3) wired as an astable multivibrator, to generate a linear ramp. The control terminal of timer 555 (pin 5) is held at a reference voltage of 5 volts by a zener diode so that the upper threshold (VUTP) is at 5 volts and the lower threshold (VLTP) at 2.5 volts. The output current from IC LM334 can be controlled with the help of potentiometer VR1. This current charges the capacitor linearly in the form of a linear ramp. As soon as the voltage across the capacitor exceeds the upper threshold volt-

Fig. 4: Actual-size, single-side PCB layout for transistor curver tracer

age (VUTP), the output of timer 555 changes its state and goes low. This activates the discharge terminal (pin 7) of timer 555 and hence the capacitor quickly discharges through the timer. As the voltage across the capacitor drops below the lower threshold voltage (VLTP), the output of timer 555 changes its state and goes high to disable the discharge terminal and further discharging of capacitor stops. Once again the capacitor gets charged linearly through the constant current source and the sequence repeats. Thus the potential across the capacitor is a positive linear ramp between 2.5 volts and 5 volts. The ramp frequency can be controlled by varying the charging current using potentiometer VR1. (EFY Lab note. During lab testing, we used AD590 temperature transducer in place of LM334H as the constant current source, and the method of using the same is shown in Fig. 3 within dotted lines.) 3. The ramp buffer and offset null section. Since the output impedance of the ramp source is very high, we cannot load it. Also, a DC offset voltage equal to the lower threshold voltage (VLPT = 2.5V) is present in the ramp output. In order to nullify the offset voltage of the ramp and to source the current from the ramp, use a buffer amplifier. An op-amp in noninverting amplifier configuration is used to achieve this function. As the input impedance of the noninverting amplifier is very high, it will not load the ramp source. Also, it is possible to nullify the DC offset voltage present in the ramp output with the help of ramp offset adjustment preset VR2. By adjusting feedback preset VR3, the output of ramp buffer can be set to deliver a linear 0-5V ramp. This output is used as VCE for the transistor under test to source the collector current (IC). To draw the input characteristics of the transistor, the base-emitter voltage (VBE) should be varied linearly. For this we require a linear 0-1V ramp with sufficient current sourcing capability. In order to achieve this, a ramp attenuator (voltage divider) and an amplifier are used. The 0-5V ramp output of ramp buffer is attenuated by the potential divider network (comprising resistors R4 and R5) followed by an op-amp (IC5) connected in non-inverting configuration. The gain of the op-amp can be adjusted using preset VR5 connected in the feedback path. In order to nullify the offset voltage of

Fig. 5: Component layout for the PCB ELECTRONICS PROJECTS Vol. 22

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the op-amp, balancing preset VR4 is connected between the offset null terminals of the op-amp. The output of the op-amp is 0-1V linear ramp, which is used as the base-emitter voltage (VBE) for sourcing the base current (IB) of the transistor under test. 4. The current-to-voltage converter section. The spot on the CRO screen is deflected in proportion to the potential applied to its input. Hence in order to deflect the beam along y-axis, which is the current axis (collector current IC in

the transistor output characteristics and base current IB in the transistor input characteristics), the current component is to be converted into a proportional voltage. The current to be measured is passed through series resistor R7 of 10-ohm, ±1% MFR (metal film resistor). Potential drop Vout across the resistor, according to the Ohm’s law, is proportional to current I through it and is given by the following relationship: V = IR

Fig. 6: Waveforms at various points in the circuit

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where Vout = 10xI Hence, there is a potential drop of 10 mV per mA of the current through the circuit. We cannot apply this small floating potential directly to the CRO for a significant deflection. Therefore we use a differential amplifier to have an output voltage with respect to the ground that is proportional to the current though the circuit. The differential amplifier has a gain of 100 that can be fine-tuned with the help of gain adjust preset VR7 in the feedback path. The current-to-voltage converter converts the current of 1 mA into a potential difference of 1 volt that can be applied to the CRO to deflect the beam in vertical axis. In order to nullify the offset voltage of the op-amp, connect a balancing preset to the offset null terminals of the op-amp. 5. The counter section. The base current (IB) is to be changed in discrete steps for every ramp to enable the transistor’s output characteristics for various IB values simultaneously on the CRO screen. In the counter circuit, the output of timer 555 (IC3) from pin 3 is a square wave that intimates the end of ramp. This output is used as clock pulse for the counter wired around CMOS binary/decade, up/down IC MC14029B or CD4029B (IC7). IC7 is wired as a 3-bit binary upcounter so that the output of the counter (Q2, Q1, and Q0) is incremented by binary 1 for every clock pulse. The count sequence is 000, 010, 011, 100, 101, 110, and 111, i.e. 0 through 7 decimal. After 111, the counter is automatically reset to 000, and once again the count sequence repeats. Hence we get eight discrete logic levels, and accordingly we can set the base current (IB) using a base current control circuit. Similarly, to draw the input characteristics of the transistor under test for various collector-emitter voltage (VCE) values, the collector-emitter voltage (VCE) is to be changed for each ramp. The least significant bit (Q0) of the counter is used to toggle the collector-emitter voltage (VCE) from 0 volt to 10 volts. Thus we can view the input characteristics of the transistor for VCE= 0 volt to VCE= 10 volts simultaneously on the screen of the CRO. 6. The base current control section. This section receives the input from the counter circuit and varies the base current (I B ) of the transistor. The output of counter IC7 in series with a high-value resistor acts as the constant current source.

Fig. 7: Actual output curves on CRO (shown without retrace)

Fig. 8: Actual input curves on CRO (shown without retrace)

The high-level outputs of the counter are fairly constant at 10 volts. When we connect a resistor of 100 kilo-ohms in series with Q0 output of the counter, it supplies a constant current of 100 µA during its logic 1 state. Similarly, when we connect a resistor of 50 kilo-ohms (two 100 kilo-ohm resistors in parallel) in series with Q1 output of the counter, it supplies a constant current of 200 µA during its logic 1 state. Using 25-kilo-ohm resistor in series with Q2 output we can get a constant current source of 400 µA. When more than one current source are connected in parallel, the result is similar to having a current source equal to the sum of individual source currents. If we use the base current (IB) setting as it is for a transistor with large current amplification factor (α), its collector current (IC) gets saturated for much smaller values of IB and only two or three traces appear on the screen of the CRO. To get the maximum number of traces, reduce the base current by increasing the series resistor values through IB SET potentiometer VR8. With the help of VR8, we can adjust the base current in incremental steps from 10 µA to 100 µA. (Note. Connect two 100-kilo-ohm resistors in parallel to get 50-kilo-ohm resistor. Similarly, connect four 100-kilo-ohm

resistors in parallel to have 25-kilo-ohm resistor. This method has been shown in Fig 3.) 7. The switching section. Certain circuits are common in tracing both the output characteristics and input characteristics. The ramp and clock generator, ramp buffer and amplifier, and counter circuits are retained at their places for both output and input characteristics. But to trace the output characteristics the current-to-voltage converter is to be connected in the collector of the transistor under test and to trace the input characteristics it is to the connected in the base of the transistor (refer Figs 1 and 2 for output and input characteristics, respectively). To have minimum complexity, the collector and the base circuits of the transistor are switched suitably using a changeover switch on the front panel. The switching details are obvious from the circuit diagram in Fig. 3.

Construction Wire the circuit on a 2.5mm, IC-type general-purpose printed circuit board (PCB) as shown in Fig. 3. The use of glass-epoxy PCB is recommended. An actual-size, single-side PCB for the circuit is shown in Fig. 4, with its component layout shown in Fig. 5. Carefully solder all the components and use sockets for ICs. All range resistors used should be stable, close-tolerance type (preferably MFRs). Preferably use linear-type IB SET potentiometer and mount it on the front panel of the instrument. Enclose the circuit board, power transformer, and other circuit components in a metal box having approximate dimensions of 22x17x7.5 cm. Extend input and output leads to the corresponding points in the circuit. Terminate the outputs for connection to the CRO in BNC(F) connectors.

Calibration After construction, check the circuit thoroughly for short circuits, breaks, and open circuits on the PCB. After switching on the instrument, let it warm up for a few minutes before commencing with the calibration. Calibration procedure of the circuit is as follows: 1. Check and ensure ±12V regulated voltage with respect to ground. 2. Connect a CRO to shorted pins 2

and 6 of timer 555 (ramp output). A linear ramp with positive slope is observed on the screen of the CRO. By adjusting frequency control potentiometer VR1, set the frequency of the ramp at 1 kHz (refer waveform 1 in Fig. 6). 3. Connect the CRO to the output of ramp buffer. Adjust preset VR2 to nullify the DC offset voltage in the output of ramp buffer. Adjust preset VR3 to set the amplitude of ramp output to 0 to 5 volts (refer waveform 2 in Fig. 6). 4. Connect CRO at the output of ramp attenuator and amplifier. Adjust preset VR4 to nullify the DC offset voltage in the output of ramp buffer. Adjust preset VR5 to set the amplitude of ramp output to 0 to 1 volt (refer waveform 3 in Fig. 6). 5. Calibrate the current-to-voltage converter by connecting a 1-kilo-ohm. 1% metal film resistor between the collector and emitter terminals of the transistor under test. Connect the output of the current-to-voltage converter to a CRO. By observing the ramp waveform on the screen of the CRO, nullify DC offset voltage using preset VR6 and adjust the amplitude of the observed ramp waveform to 0-5 volts with the help of preset VR7. Calibrate the current-to-voltage converter to convert 1 mA of current into 1 volt (refer waveform 4 in Fig. 6). Then check the clock output by connecting the CRO to pin 3 of timer 555 (refer waveform 5 in Fig. 6). 6. Verify the outputs of the counter by using a dual-trace oscilloscope. Connect one input channel of the CRO with clock pulses at pin 3 of IC3 and the outputs at pins 6, 11, and 14 of counter IC7 to the other input of the CRO sequentially (refer waveforms 5, 6, 7, and 8 in Fig. 6). 7. Short-circuit the base-emitter terminals of the transistor under test. Select input/output characteristics switch S2 to output characteristics position and connect the CRO to the output of the currentto-voltage converter. By adjusting IB SET potentiometer VR8 on the front panel of the instrument, check proper operation of the base-current section by observing stair-case ramp of varying amplitude on the screen of the CRO (refer waveform 9 in Fig. 6).

Operation After calibration, the instrument is ready for use to trace the input and output characteristics of npn transistors. Follow the operating procedure given below every ELECTRONICS PROJECTS Vol. 22

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Parts List Semiconductors: IC1 - 7812, +12V regulator IC2 - 7912, –12V regulator IC3 - NE555 timer IC4, IC6 - µA741 op-amp (IC OP-07 op-amps can be used in place of µA741 with advantage) IC7 - MC14029B/CD4039 binary/ decade up-/down-counter IC8 - LM334H/AD590 temperature sensor D1-D4 - 1N4007 rectifier diode ZD1 - 5V zener diode Resistors (all ¼-watt, ±1% MFR, unless stated otherwise): R1, R5, R6, R8, R9 - 1-kilo-ohm R2, R4 - 22-kilo-ohm R7 - 10-ohm R3, R10, R11 (A,B), R12(A-D) - 100-kilo-ohm VR1 - 1-kilo-ohm preset VR2 - 2.2-kilo-ohm preset VR3, VR4, VR5, VR6 - 10-kilo-ohm preset VR7 - 150-kilo-ohm preset VR8 - 1-mega-ohm potmeter Capacitors: C1-C4, C9 C5, C6 C7, C8 C10

- 0.1µF ceramic disk - 1000µF, 35V electrolytic - 100µF, 25V electrolytic - 0.01µF ceramic disk

Miscellaneous: X1 - 230V AC primary to 15V-0-15V AC, 500mA secondary transformer S1 - On/off switch S2 - DPDT switch

time to get correct traces of input and output characteristics of the transistor: 1. Connect the x-axis and y-axis BNC pins of the transistor curve tracer to the

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corresponding inputs of the CRO. 2. Plug in the AC cord of both the CRO and the transistor curve tracer and switch them on. 3. Set the CRO inputs to ground. 4. Allow warm-up time of at least 10 minutes for the circuit components to get stabilised. 5. Set the CRO for X-Y mode of operation. 6. Adjust intensity and focus controls to get a sharp spot on the screen of the CRO. 7. Set the volts/div control of x-axis to 0.5 volt/div. 8. Set the volts/div scale of y-axis to 2 volts/div. 9. Adjust the position controls of the CRO to position the spot on the left bottom of the screen ((0,0) position in the graph). 10. Set the inputs for DC coupling to the CRO. 11. Connect the transistor whose characteristics are to be traced to the transistor curve tracer, ensuring correct pin configuration. 12. Set the selector switch for input/ output characteristics to the output characteristics position. 13. Release the CRO inputs from ground and switch them over to connect inputs. 14. Now view the output characteristics of the transistor. Fine-tune the IB set potentiometer to get eight traces on the screen of the CRO. 15. To trace input characteristics of the transistor, change the input/output characteristics selector switch to the input

characteristics position. 16. Set the volts/div control of x-axis to 0.1 volt/div and observe the input characteristics likewise. Figs 7 and 8 show a typical transistor’s output and input characteristics, respectively, on the CRO screen (without retrace).

Conclusion To draw the characteristics of pnp transistors, insert an inverter circuit in the ramp path of collector-emitter voltage VCE and base-emitter voltage VBE, and invert the output of the current-to-voltage converter. By using a potential divider and buffer amplifier circuit in place of the base-current control circuit you can draw the characteristics of FETs and MOSFETs. To trace the forward characteristics of diodes, connect the anode of the diode to the base terminal and the cathode to the emitter terminal. Set the transistor curve tracer to draw input characteristics, and the CRO screen displays the forward characteristics of the diode. Similarly, with simple add-on circuits to the motherboard, you can draw the characteristics of UJTs, SCRs, TRIACs, etc. Thin and faint retrace lines visible along with the characteristic traces can be removed by connecting a retrace blanking circuit to the Z-mod input of the CRO. Almost all CROs exceeding 30MHz bandwidth have the Z-mod input facility. ❏

Tripping Sequence Recorder-Cum-Indicator r.g. thiagraj kumar and s. ramaswamy

I

n applications like power stations and continuous process control plants, a protection system is used to trip faulty systems to prevent damages and ensure the overall safety of the personnel and machinery. But this often results in multiple or cascade tripping of a number of subunits. Looking at all the tripped units doesn’t reveal the cause of failure. It is therefore very important to determine the sequence of events that have occurred in order to exactly trace out the cause of failure and revive the system with minimal loss of time. The circuit presented here stores the tripping sequence in a system with up to eight units/blocks. It uses an auxiliary relay contact point in each unit that closes whenever tripping of the corresponding unit occurs. Such contact points can be identified easily, especially in systems using programmable logic controllers (PLCs). This circuit records tripping of up to eight units and displays the order in which they tripped. A clock circuit, however fast, cannot be employed in this circuit because the clock period itself will be a limiting factor for sensing the incidence of fault. Besides, it may also mask a number of

events that might have occurred during the period when the clock was low. Hence the events themselves are used as clock signals in this circuit. Fig. 1 shows the block diagram of the tripping sequence recorder-cum-indicator. The inputs derived from auxiliary relay contacts (N/O) of subunits or push-to-on switches are latched by RS flip-flops when the corresponding subunits trip, causing the following four actions: 1. The latch outputs are ORed to activate audio alarm. 2. The latch outputs are differentiated individually and then ORed to provide clock pulses to the counter to increment the output of the counter that is initially preset at 1 (decimal). 3. Each individual latch output activates the associated latch/decoder/driver and 7-segment display set to display the number held at the output of the counter, which, in fact, indicates the total number of trips that have taken place since the last presetting. 4. LEDs associated with each of the latch, decoder, and driver sets remain lit to indicate the readiness of the sets to receive the tripping input. LEDs associated with the tripped unit go off.

Fig. 1: Block diagram of tripping sequence recorder-cum-indicator

The circuit IC1 and IC2 (CD4043) Quad NOR RS flip-flops in Fig. 2 are used to capture and store the information pertaining to the tripping of individual units. Reset pins of all the eight flip-flops and sub-parallel enable (PE) pin 1 of BCD up-/down-counter CD4510 (IC3) are returned to ground via 10-kilo-ohm resistor R22, while set pins of all RS flip-flops are returned to ground via individual 10-kilo-ohm resistors R14 through R21. Initially, all the eight Q outputs of IC1 and IC2 are at logic 0. The auxiliary relay contacts of the subunits, which are depicted here by push-to-on switches S1 through S8, connect the set terminal of the corresponding stage of RS flip-flop to +12V whenever tripping of a specific subunit occurs. This makes the output of the associated flip-flop go high. Thus whenever Parts List Semiconductors: IC1, IC2 - CD4043 quad NOR RS latch IC3 - CD4510 BCD up-/downcounter IC4-IC11 - CD4511 BCD-to-7-segment latch/decoder/driver T1-T11 - BC547 npn transistor T12-T19 - BC557 pnp transistor D1-D16 - 1N4007 rectifier diode DIS1-DIS8 - LT543 common-cathode 7-segment display Resistors (all ¼-watt, ±5% carbon, unless stated otherwise): R1-R11, R13-R38 - 10-kilo-ohm R12, R39-R46 - 1-kilo-ohm R47-R102 - 470-ohm Capacitors: C1-C8 - 0.01µF ceramic disk Miscellaneous: S1-S8 - Push-to-on switch or relay contacts (N/O) S9 - Push-to-on switch PZ1 - Piezobuzzer - 12V, 500mA power supply

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Fig. 2: Schematic diagram of tripping sequence recorder-cum-indicator

a sequence of tripping of subunits occurs, the corresponding outputs (1Q to 8Q) go high in the order of the tripping of the associated subunits. All the eight Q outputs are connected to the corresponding latch-enable inputs of BCD latch-cum-decoder-driver ICs (CD4511). These Q outputs are also ORed using diodes D1 through D8 to activate an audible alarm and also routed to a set of differentiator networks (comprising capacitors C1 through C8 and resistors R2 through R9). A differentiator provides a sharp pulse corresponding to the tripping of a subunit. All such differentiated pulses are ORed via diodes D9 through D16 and coupled to the counter stage Fig. 3: Actual-size, single-side PCB of the main control portion of tripping sequence controller-cumformed by IC3 (CD4510, a syn- indicator circuit

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Fig. 4: Component layout for Fig. 3

Fig. 5: Actual-size, single-side PCB for latch decoder/driver and display circuit of one subunit

chronous up-/down-counter with preset) after amplification and pulse shaping by transistor amplifier stages built around transistors T2 and T3. These pulses serve as clock to count the number of trippings that occurred after a reset.

Operation Let us assume that three units, say, E, H, and A (fifth, eighth, and first), tripped in that order following a fault. When the system is reset (before any tripping), the outputs of all RS flipflops (1Q through 8Q) are low. This LE* active-low makes latches IC4 through IC11 transparent and as the counter is preset to 1 (since P1 input is high while P2, P3, and P4 are low) with the help of switch S9, all the latches hold that ‘1’ and their decoded ‘b’ and ‘c’ segment

Fig. 6: Component layout for Fig. 5

outputs go high. However, the common-cathode drive is absent in all the 7-segment displays because driver transistors T4 through T11 are cut off due to the low outputs of all RS flip-flops and hence the displays are blank. At the same time, the low outputs of all RS flip-flops (1Q through 8Q) forward bias pnp transistors T12 through T19 associated with LED1 through LED8 of each of the displays. As a result, all these LEDs glow, indicating no tripping. Now when unit E trips, output 5Q of RS flip-flop IC2 goes high to provide the base drive to common-cathode drive transistor T8. This, in turn, activates DIS5 (fifth from left in Fig. 2) to display ‘1’, indicating that unit E tripped first. The corresponding LED5 goes off as transistor T16 is cut off. Also, latch IC8 is disabled due to logic 1 on its pin 5 and therefore it does not respond to further changes in its BCD

data input. Simultaneously, the buzzer goes on to sound an audible alarm, indicating the emergency situation at the plant. The differentiator formed by C5 and R6 responds to the low-to-high transition of 5Q and generates a short pulse. This pulse passes through diode D13 and transistors T2 and T3 and reaches clock pin of counter IC3. The counter counts up and its output becomes 0010 (decimal 2). As mentioned earlier, all the display units other than E have the drive signal on segments a, b, g, e, and c now but are off because of the missing common-cathode drive. When the next subunit H trips, output 8Q experiences a low-to-high transition and the corresponding display (DIS8) shows digit ‘2’. The above sequence of operation holds true for any further subunit tripping—with the displayed digit incrementing by one for each sequential tripping. In the prototype, LEDs D17 through D24 were fixed below the corresponding 7-segment displays pertaining to subunits A through H to provide a visual indication that these units are ready to respond to a tripping. The circuit works satisfactorily with twisted-pair wires of length up to 5 metres. In electrically noisy environments, the length of the cable has to be reduced or a shielded twisted-pair cable can be used. An actual-size, single-side PCB layout for the main control portion of the tripping sequence recorder-cum-indicator circuit is shown in Fig. 3 and its component layout in Fig. 4. The PCB layout for the indicator set comprising IC4, DIS1, transistors T4 and T12, LED1, etc is shown in Fig. 5 and its component layout in Fig. 6. The indicator set of Fig. 5 can be connected to the main PCB of Fig. 3 using Bergstrip type SIP (single-inline-pin) connectors as per requirements. This tripping sequence recorder-cumindicator circuit can also be used in quiz games to decide the order in which the teams responded to a common question. For this, provide push-to-on switches on the tables of individual teams and a master reset to the quiz master. Modify the alarm circuit suitably with a retriggerable monostable stage so that the audible alarm stops after a short interval. ❏ ELECTRONICS PROJECTS Vol. 22

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SECTION B : CIRCUIT IDEAS

Electronic Starter for Single-Phase motors sarat chandra das

A

novel single-phase electronic starter circuit meant for 0.5HP and 1HP motors is presented here. It incorporates both overload and shortcircuit protections. A special currentsensing device has been added in this starter to sense the current being drawn by the motor. If the motor jams due to bearing failure or defect in the pump or any other reason, it would draw much higher current than its normal rated current. This will be sensed by the current-sensing device, which will trip the circuit and protect the motor. Some other reasons for the motor drawing higher current are as follows: (a) Windings damaged or short-circuit between them. (b) Shorting of motor terminals by mistake. (c) Under voltage or single phasing occuring in the mains supply source (normally, a 440V AC, 3-phase with neutral four-wire system). The main components used in the circuit comprise a specially wound sensing transformer X1, another locally available step-down transformer X2, single-changeover relay RL1, two double-changeover relays (RL2 and RL3), and other discrete components shown in the figure. The mains supply to the motor is routed in series with the primary of transformer X1 via normally-open contacts of relay RL3. The primary of transformer X1 is connected in the neutral line. To switch on the supply to the motor, switch S1 is to be pressed momentarily, which causes the supply path to the primary of transformer X2 to be completed via N/C contacts of relay RL1. Relay RL2 gets energised due to the DC voltage developed across capacitor C2 via the bridge rectifier. Once the relay energises, its N/O contacts RL2(a) provide a short across switch S1 and supply to the primary of transformer X2 becomes continuous, and

hence relay RL2 latches even if switch S1 is subsequently opened. The other N/O contacts RL2(b) of relay RL2, on energisation, connect the voltage developed across capacitor C2 to relay RL3, which thus energises and completes the supply to the motor, as long as current passing through primary of transformer X1 is within limits (for a 1hp motor). When the current drawn by motor exceeds the limit (approx. 5A), the voltage developed across the secondary of transformer X1 is sufficient to energise relay RL1 and trip the supply to relays RL2 and RL3, which was passing via the N/C contact of relay RL1. As a result, the supply to the motor also trips.

The contact rating for relays RL1 and RL2 should be 5 amperes, while contact ratings of relay RL3 should be 10 to 15 amperes. Transformer X1 can be wound using any suitable size CRGO core. (One can use a burntout transformer core as well.) The primary comprises 30 to 31 turns for use with 1HP motor and additional eight turns, if you are using a 0.5HP motor. Fuses F1 and F2 are kit-kat type. The ‘on’ pushbutton is normally-‘off’ type, while ‘off’ pushbutton S2 is of normally-‘on’ type. Capacitors C1 and C2, apart from smoothing the rectified output, provide necessary delay during energisation and de-energisation of relays. Diodes across

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relays are used for protection as freewheeling diodes. Starters for 0.5HP and 1HP motors are not easily available in the market. Readers’ comments:  I would like to have some doubts cleared: 1. In the circuit, AC supply was indicated by phase and neutral. If phase and neutral are changed, what will be the result? 2. What is the suggested core size of transformer X1 tested by the author? 3. Can we use ferrite core transformer instead of CRGO core? J.V.K. Naidu Eluru The author, Sarat Chandra Das, replies: 1. The mains supply is taken from 3-phase, 4-wire system. When you are using any domestic appliance such as fan, TV, etc, you

Users are therefore compelled to use 10-amp rated circuit breaker for such motors. A mechanical starter or auto starter would turn out to be costlier than the

Fig. 1: Suggested ‘El’ core dimensions

do not make sure as to which is phase and neutral in the socket (when using a 2-pin plug top). Nothing happens when phase and neutral are interchanged. (EFY: Nothing untoward happens as long as supply in the gadget is first routed to a transformer. In some appliances neu-

circuit given here, which works very reliably. Parts used in this circuit are easily available in most of the local markets.

tral and ground are shorted together and thus the body of the gadget becomes ‘live’ and one could get a shock on touching the gadget. One must use a 3-pin plug and socket system in which live and neutral conventions are adhered to as per ISI specifications/Electricity Act and rules.) 2. The suggested core size is shown in Fig. 1. I suggest the use of a 220V/6V-06V, 500mA to 1A rating step-down transformer core used in battery eliminators (new or burnt), which is available in the local market. 3. Ferrite core is better than CRGO, but it is costlier and not easily available in the local market, while CRGO core is easily available. Technically, turns ratio in winding will change.

Modem ‘On’/‘Off’ Indicator t.k. Hareendran

H

ere is an interesting, low component-count, and easy-to-build electronic circuit for the Internet surfers. This circuit, using two LEDs, indicates the modem status, i.e. whether it is in use or not. The incoming telephone line terminating on a master phone is shunted by a metal oxide varistor. The circuit is configured around the popular timer chip NE555, which is wired as an astable multivibrator. When power is applied to the circuit, the astable starts working as usual. However, LEDs D2 and D3 connected to its output pin 3 would not glow as transistor T1 is in off condition and hence resistor R4’s bottom end is hanging in high impedance state. However, when the modem is working, voltage drop across preset VR1 illuminates the LED inside the optocoupler (IC2). As a result, transistor T1 gets sufficient base-bias through activated transistor inside opto-coupler via resistor R3. Consequently, LEDs

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D1 and D2 start blinking at the bistable IC1’s frequency determined by the values of resistors R1 and R2 and capacitor C1. A 9V, 0.5A AC adapter can be used to power the circuit. Finally, one minor adjustment is required for successful operation of the gadget. For this, first switch on the supply to the gadget and then switch ‘on’ the modem. Now adjust the

wiper of preset VR1 very slowly until the LEDs start blinking. Memorise the wiper position and fix it in this position using a good-quality glue/compound. After construction, fix the complete circuit in a suitable and attractive cabinet with one LED in its front panel. Keep the whole unit near the modem and fit another LED near the master telephone with the label ‘Modem in Use’.

Touch-Select Audio source saravanan j.

O

ften you need to connect output from more than one source (preamplifier) such as tape recorder/player and CD (compact disc) player to audio power amplifier. This needs disconnecting/connecting wires when you want to change the source, which is quite cumbersome and irritating. Here is a circuit that helps you choose between two stereo sources by simple touch of your hand. This circuit is so com-

pact that it can be fixed within the audio power amplifier cabinet and can use the same power supply source. The circuit uses just two CMOS ICs and a few other componenets. The ICs used are MC14551/CD4551 (quad 2-channel analogue multiplexer) and CD4011 (quad 2-input NAND gate). When touchplate S1 is touched (its two plates are to be bridged using a fingertip), gate N1 output (IC1, pin 3) goes high while the output of gate N2 at pin 4 goes low. This

causes selection of CD outputs being connected to the power amplifier input, which is indicated by lighting of LED1. When touch-plate S2 is touched, the outputs of gates N1 and N2 toggle. That is, IC2 pin 3 is pulled ‘low’ while its pin 4 goes ‘high’. This results in selection of tape recorder outputs being connected to the input of power amplifier. This is indicated by lighting of LED2. Pin 9 is the control pin of IC2. In the circuit, the state of multiplexer switches is shown with pin 9 ‘high’ (CD source selected). When pin 9 is pulled ‘low’, all the switches within the multiplexer change over to the alternate position to select tape player as source. EFY Lab note. Although one can connect pin 7 (VEE) of IC2 to ground, but for operation with preamplifier signals going above and below ground level, one must connect it to a negative voltage (say, –1V to –1.5V) to avoid distortion.

Precision attenuator with digital control anantha narayan

W

hen instruments are designed, an analogue front-end is essential. Further, as most equipment have digital or microcontroller interface, the analogue circuit needs to have digital control/access. The circuit of a programmable attenu-

ator with digital control is described here, where digital control can be a remote dip switch, or CMOS logic outputs of a decade counter (having binary equivalent weight of 1, 2, 4, and 8, respectively), or I/O port of a microcontroller like 80C31. The heart of this circuit is the popu-

lar OP07 op-amp with ultra-low offset in the inverting configuration. A dual, 4-channel CMOS analogue multiplexer switch CD4052 enables the change in gain. An innovative feature of the circuit is that the ‘on’ resistance (around 100 ohms) of CD4052 switch is bypassed so ELECTRONICS PROJECTS Vol. 22

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op-amp. (b) Output The output can be connected to a 7107/7135-based DPM or any other analogue-to-digital converter or op-amp stage. Use a buffer at the output if the output has to be loaded by a load less than 1 meg-ohm. Use an inverting buffer if input leads have to have polarity where ground is the inverting terminal. (For details, see next circuit.) (c) CD4052 CMOS switch The on-resistance (100-ohm approx.) comes in series with the op-amp output source resistance, which produces no er

that no error is introduced by its use. Resistors R1 to R6 used in the circuit should be of 0.1 per cent tolerance, 50 ppm (parts per million) if you use 3½-digit DPM, i.e. ±1999 counts (approx. 11 bits). But for 4½-digit DPM (approx. 14 bits), you may need to have trimpots (e.g. replace 1k-ohm resistor R6 by a fixed 900-ohm resistor in series with a 200-ohm trimpot) to replace R3, R4, R5, and R6 gain selection resistors for proper calibration to required accuracy. However, for testing or trials, use 1 per cent 100ppm MFR resistors. The expected errors will be around 1 per cent. To keep parts count (hence cost) to a minimum, the common or ground is used as the positive input terminal and one end of resistor R1 as the negative. This is so because the op-amp inverts the polarity as it is used in inverting configuration. This does not matter as the equipment will be isolated by the power supply transformer and all polarities are relative. In case you want the common to be the negative, you will

have to add some stages (IC4 and IC5 circuitry shown in precision amplifier circuit described later). The OP07 pinout is based on standard single op-amp 741. Any other op-amp like CA3140, TLO71, or LF351 can be used but with offset errors in excess of 1 per cent, which is not tolerable in precision instrumentation. The OP07 has equivalent ICs like µA741 and LM607 having ultra-low offset voltage (<100µV), low input bias current (<10nA), and high input impedance (>100M), which are the key requirements for a good instrumentation op-amp for use with DC inputs. The following design considerations should be kept in mind: (a) Input: 500V max Since ¼W resistors can withstand up to 250V, resistors R1 and R2 in series are used for 1 meg-ohm with 500V (max) input limit. These resistors additionally limit the input current as well. Diodes D1 and D2 clamp the voltage across input of op-amp to ±0.5V, thereby protecting the

Truth Table (Control input VS attenuation) X,Y (ON-switch (2) (1) Gain Pair) B A (Attenuation) X0,Y0 0 0 1/1000 X1,Y1 0 1 1/100 X2,Y2 1 0 1/10 X3,Y3 1 1 1

ror at output. Caution. The circuit does not isolate, it only attenuates. When high voltage is present at its input, do not touch any part of the circuit. (d) Digital control options (i) A and B can be controlled by I/O port of a microcontroller like 80C31 so that the controller can control gain. (ii) A and B can be given to counters like 4029/4518 to scroll gain digitally. (iii) A and B can be connected to DIP switch. (iv) A and B can be connected to a thumbwheel switch. Notes. 1. Digital input logic 0 is 0V and logic 1 is 5V. 2. All resistors are metal film resistors (MFR) with 1% tolerance, unless specified otherwise. 3. C2 and C3 are ceramic disk capacitors of 0.1µF = 100nF value.

Precision Amplifier with Digital Control anantha narayan

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his circuit is similar to the preceding circuit of the attenuator. Gain of up to 100 can be achieved

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in this configuration, which is useful for signal conditioning of low output of transducers in millivolt range.

The gain selection resistors R3 to R6 can be selected by the user and can be anywhere from 1 kilo-ohm to 1

meg-ohm. Trimpots can be used for obtaining any value of gain required by the user. The resistor values shown in the circuit are for decade gains suitable for an autoranging DPM. Resistor R1 and capacitor C1 reduce ripple in the input and also snub transients. Zeners Z1 and Z2 limit the input to ±4.7V, while the input current is limited by resistor R1. Capacitors C2 and C3 are the power supply decoupling capacitors. Op-amp IC1 is used to increase the input impedance so that very low inputs are not loaded on measurement. The user can terminate the inputs with resistance of his choice (such as 10 meg-ohm or 1 meg-ohm) to avoid floating of the inputs when no measurement is being made. IC5 is used as an inverting buffer to restore polarity of the input while IC4 is used as buffer at the output of CD4052, because loading it by

resistance of value less than 1 megohm will cause an error. An alternative is to make R9=R10=1 meg-ohm and do away with IC4, though this may not be an ideal method. Truth Table (Control Input vs Gain) X,Y (On-switch (2) (1) Gain Pair) B A (Av.) X0,Y0 0 0 1/10 X1,Y1 0 1 1 X2,Y2 1 0 10 X3,Y3 1 1 100

Gains greater than 100 may not be practical because even at gain value of 100 itself, a 100µV offset will work out to be around 10 mV at the output (100µV x 100). This can be trimmed using the offset null option in the OP07, connecting a trimpot between pins 1 and 8, and connecting wiper to +5V supply rails. For better performance, use ICL7650 (not

pin-compatible) in place of OP07 and use ±7.5V instead of ±5V supply. Eight steps for gain or attenuation can be added by using two CD4051 and pin 6 inhibit on CD4051/52. More steps can be added by cascading many CD4051, or CD4052, or CD4053 ICs, as pin 6 works like a chip select. Some extended applications of this circuit are given below. 1. Error correction in transducer amplifiers by correcting gain. 2. Autoranging in DMM. 3. Sensor selection or input type selection in process control. 4. Digitally preset power supplies or electronic loads. 5. Programmable precision mV or mA sources. 6. PC or microcontroller or microprocessor based instruments. 7. Data loggers and scanners.

Random Number Generator Based Game k. udhaya kumaran

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his electronic game is simulation of one-arm bandit game. Electronics hobbyists will find it very interesting. When toggle switch S1 is in

‘run’ position, all segments of 7-segment displays (DIS1 through DIS3) will light up. On turning toggle switch S1 from ‘run’ to ‘stop’ position, displayed digits will con-

tinue advancing and the final display is unpredictable. Thus the final number displayed in DIS1 through DIS3 is of random nature. The speed with which ELECTRONICS PROJECTS Vol. 22

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the number in 7-segment display keeps changing on flipping switch S1 from ‘run’ to ‘stop’ condition slowly decays before stopping with a random number display. To play this game, one has to obtain three identical numbers in displays DIS1 through DIS3. The contestant would score 1 (one) point if he manages to get a final display of ‘000’, 2 points for getting ‘111’ display, 3 points for ‘222’,… and so on—up to ten points for ‘999’. He should try to score maximum possible points in fixed numbers of attempts (say, 20 to 25 attempts). Apart from using this circuit as a game for entertainment, one can use it as random number generator for any other application as well. The decay time with the given component values is around 15 seconds before the display could stop at a final random number. The circuit comprises clock oscillator built around NE555 timer IC4, threestage clock pulse counter built using three CD4033 ICs (IC1 to IC3), and three 7-segment LED displays (DIS1 to DIS3). In clock oscillator circuit, NE555 timer IC4 is used in a similar way as a free-running astable multivibrator, the only difference being the additional capacitor C1 introduced between pin No. 7 of IC4 and junction of resistors R22 and R24. When toggle switch S1 is in ‘run’ position, both terminals of capacitor C1 are shorted by switch S1 and timer IC4 works as a free-running astable multivibrator. The operating frequency is in the vicinity of 35 kHz, determined by the value of timing components. When toggle switch S1 is flipped from ‘run’ to ‘stop’ position, capacitor C1 is introduced in the discharge path of pin No. 7 of IC4 and junction of resistors R22 and R24. At the same time, capacitor C4 comes in parallel with

timing capacitor C3 to change the operating frequency of the astable from around 35 kHz to around 65 Hz. Now capacitor C1 slowly starts charging as it is connected in the discharge path of the timing capacitors C3 and C4. The clock frequency of IC4 gradually reduces and after 15 seconds, when capacitor C1 is sufficiently charged, the oscillating frequency gradually drops and finally it stops oscillating. Thus, pin 3 of IC4 becomes low. Second part of the circuit comprises three cascaded ICs, IC1 through IC3 (CD4033 decade upcounter cum 7-segment decoder). In conjunction with three 7-segment displays (DIS1 to DIS3), these form a 3-digit clock counter. The clock counting speed is dependant upon the clock pulse frequency of IC4. It is connected to clock input pin 1 of IC1 while chip enable pin 2 of IC1 to IC3 are held low. Thus all clock counter ICs advance

by 1 for every positive clock transition. Reset pin 15 of all counter ICs is held low through resistor R25. Thus reset facility is not used in this circuit. Due to persistence of vision, one cannot distinguish 0-9 counting in DIS1 to DIS3 when the clock frequency is high. All 7-segment displays appear to show digit 8, while the red LED1 remains lit continuously, indicating clock counter is in running condition. On sliding toggle switch S1 from ‘run’ to ‘stop’ position, the counting speed of individual digits falls immediately due to the clock frequency changing to around 65 Hz. Now, the counting speed will be 65 Hz for DIS3, 6.5 Hz for DIS2, and 0.6 Hz for DIS1. This speed of individual digit counting slowly decays, until the counter stops and LED1 stops blinking, and the final count (random numbers) are displayed in DIS1, DIS2, and DIS3.

9-Line Telephone Sharer Dhurjati Sinha

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his circuit is able to handle nine independent telephones (using a single telephone line pair) located at nine different locations, say, up to a

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distance of 100m from each other, for receiving and making outgoing calls, while maintaining conversation secrecy. This circuit is useful when a single telephone

line is to be shared by more members residing in different rooms/apartments. Normally, if one connects nine phones in parallel, ring signals are heard in all

the nine telephones (it is also possible that the phones will not work due to higher load), and out of nine persons eight will find that the call is not for them. Further, one can overhear others’ conversation, which is not desirable. To overcome these problems, the circuit given here proves beneficial, as the ring is heard only in the desired extension, say, extension number ‘1’. For making use of this facility, the calling subscriber is required to initially dial the normal phone number of the called subscriber. When the call is established, no ring-back tone is heard by the calling party. The calling subscriber has then to press the asterik (*) button on the telephone to activate the tone mode (if the phone normally works in dial mode) and dial

extension number, say, ‘1’, within 10 seconds. (In case the calling subscriber fails to dial the required extension number within 10 seconds, the line will be disconnected automatically.) Also, if the dialed extension phone is not lifted within 10 seconds, the ring-back tone will cease. The ring signal on the main phone line is detected by opto-coupler MCT2E (IC1), which in turn activates the 10-second ‘on timer’, formed by IC2 (555), and energises relay RL10 (6V, 100-ohm, 2 C/O). One of the ‘N/O’ contacts of the relay has been used to connect +6V rail to the processing circuitry and the other has been used to provide 220-ohm loop resistance to de-energise the ringer relay in telephone exchange, to cut off the ring. When the caller dials the extension number (say, ‘1’) in tone mode, tone re-

ceiver CM8870 (IC3) outputs code ‘0001’, which is fed to the 4-bit BCD-to-10 line decimal decoder IC4 (CD4028). The output of IC4 at its output pin 14 (Q1) goes high and switches on the SCR (TH-1) and associated relay RL1. Relay RL1, in turn, connects, via its N/O contacts, the 50Hz extension ring signal, derived from the 230V AC mains, to the line of telephone ‘1’. This ring signal is available to telephone ‘1’ only, because half of the signal is blocked by diode D1 and DIAC1 (which do not conduct below 35 volts). As soon as phone ‘1’ is lifted, the ring current increases and voltage drop across R28 (220-ohm, 1/2W resistor) increases and operates opto-coupler IC5 (MCT2E). This in turn resets timer IC2 causing: (a) interruption of the power supply for processing circuitry as well as the ring signal relays RL1 through RL9, and (b) removal of loop resistance R16, via the second contact of relay RL10. As a result, the telephone line voltage shoots up to 48V, DIAC1 and diode D1 connected in series with phone 1 conduct within a few milliseconds, and phone 1 comes into operation. The telephone exchange does not interpret this as break in off-hook condition, since some delay margin is set at exchange. When phone ‘1’ is busy, the other eight phones will not work, since line voltage will again drop to 10V and the other diacs will not conduct. Thus conversation secrecy will be maintained. The other extensions also work in a similar manner when another extension number is dialed and its corresponding relay energises to extend the 50Hz ring to another extension. The 24V, 50Hz ring signal derived from transformer X1 is sufficient for working with phones of Beetel and ITI make, but for Pretel and some other makes, it may be necessary to increase the ring voltage to about 30 volts or even higher. ELECTRONICS PROJECTS Vol. 22

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Electronic Card Lock System priyank mudgal

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his circuit of electronic card lock system is much simpler and cheaper than other similar circuits that have appeared in earlier issues of EFY. The circuit is configured around an addressable 1 of 16 demultiplexer CD4514B (IC1). Any number in binary form, when available at input pins 2,

3, 21, and 22 (address pins A0 through A3), makes corresponding output go logic high, thus turning on the appliance through relay contacts. Up to 15 appliances can be switched on/off (one at a time). Output Q0 (pin 11) can be used for visual indication, to show that circuit is active. A 40W bulb illuminates LDR1

result, transistor T1 conducts and extends positive supply to the collectors of transistors T2 through T5. Then, depending upon the holes blocked/punched in the inserted card, any combination of emitters of transistors T2 through T5 turns logic ‘high’ (transistors’ output corresponding to blocked LDRs only goes ‘high’). These outputs connected to address input pins A0 through A3 of IC1 Table I Appliance LDR2 LDR3 LDR4 LDR5 no. 1 - * * * 2 * - * * 3 - - * * 4 * * - * 5 - * - * 6 * - - * 7 - - - * 8 * * * 9 - * * 10 * - * 11 - - * 12 * * - 13 - * - 14 * - - 15 - - - Blocked hole corresponding to selected binary address. * Punched holes corresponding to LDR position on card

to LDR5 constantly. This pulls down bases of transistors T1 through T5 to ground. LDR1 ensures that card is properly inserted into the card slot. When the card is correctly inserted, it covers the hole/opening for LDR1 and thus blocks the light from falling on LDR1. As a

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switch on the corresponding appliance (one out of 15). The card used should be of opaque plastic. It should be able to withstand some heat from the bulb, even though the appliance remains ‘on’ only for the period for which the card is in the slot. The card has a triangular notch that shows correct orientation/direction of insertion of card and prevents false operation. LDRs can be placed in a line, or randomly, to increase security. The order in which holes should be punched for each appliance is given in Table I. Two illustrations, one each for card-2 and card-5, are shown in the accompanying figures. An elevation and plan/top view of the gadget is also shown in the figures.

Pulsed Operation of a CW Laser Diode dr. alika khare

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ere a simple low-cost technique for converting a CW laser diode at 670 nm wavelength to pulsed laser up to a frequency of 500 kHz is presented. A low-power pulsed radiation source is very important for any laboratory involved in optical pulsed systems—laser,

pulsed discharges, optical communication, fibre-optic sensors, image processing, etc—where one is required to check the frequency response of the detection system or optical simulation of an optical source or local networking using optical fibre cable. Fast-speed LED offers the solution for such requirements, but because of very low power and large divergence, its use remains limited. On the other hand, a pulsed diode laser offers a very good solution for this problem. Commercial systems are usually expensive. However, a CW diode laser operating at 670 nm can easily be pulsed up to a frequency of 500kHz with low-cost technique, using a function generator and an inexpensive push-pull amplifier interface circuit. The block diagram of the system is shown in Fig. 1. A 3mW CW diode laser at 670 nm with voltage and current rating of 3V at 100mA, respectively, is used. The source (a function generator) is capable of delivering square pulses of 3V amplitude, which are amplified by a complementary symmetry push-pull circuit shown in Fig. 2. The output of the amplifier is connected to the diode laser for pulsed operation. The laser is focused onto a photodiode terminated with 50-ohm resistor (Fig. 1). The output of photodiode is displayed on

digital storage oscilloscope and it is also connected to the PC for getting a hard copy. Up to a frequency of around 20 kHz, the threshold voltage for laser oscillations is around 2.4V. For frequencies greater than 20 kHz, the threshold for laser oscillations depends on the operating frequency and is higher than 2.4V. The behaviour of laser pulses up to 10 kHz is nearly similar. Laser output at a typical frequency of 2 kHz is shown in Fig. 3, at various voltages (2.6V, 3.4V, and 4V). The input waveform ‘A’ is shown at the bottom of the figure. For a driving pulse of about 3V (which is the normal operating voltage for CW operation), the laser pulse becomes flat after a delay of approximately 40 µs (time taken to build up the laser oscillations to its maximum amplitude). Above 3V, probably population inversion is developed much above threshold, before the laser oscillations build up into the cavity, and so we observe the sharp peak in laser output (for more details, refer Laser Fundamentals book by W. T. Silfvast, published by Cambridge University Press), exponentially decaying to a steadystate value with a time constant depending on the initial peak intensity and the carrier life time in the excited state. After the

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input pulse is over, the oscillations die down within 5 µs. Therefore above 3V, up to a frequency of 10 kHz, the laser is operated in quasi CW mode. In the frequency range of 10 kHz to 50 kHz, the laser output keeps on increasing, even during the flat portion of the input current pulse, and falls down to zero during the off period of the driving pulse. Fig. 4

shows the laser waveforms at 50 kHz, 100 kHz, 200 kHz, 300 kHz, and 500 kHz, respectively. All these pulses were recorded at around 4V. In this range of frequencies, the duration for which voltage is on/off is of the order of less than 5 µs, and so the driving pulses switch off before the termination of laser oscillations. Therefore the laser output shows a modulation

with the DC component in it. Beyond 500 kHz, it is difficult to observe laser oscillations even at voltages higher than 4V. Lab note. Tests conducted at EFY using laser diode of laser torch (rated for <5mW) with identical inputs at 2 kHz did not show any marked departure of output waveform (square wave) from the input square wave.

Generation of 1-sec. pulses spaced 5-Sec. Apart Praveen shanker

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his circuit using a dual-timer NE556 can produce 1Hz pulses spaced 5 seconds apart, either manually or automatically. IC NE556 comprises two independent NE555 timers in a single package. It is used to produce

timer 1 in NE556 triggers by itself. The output of the first timer is connected to trigger pin 8 of second timer, which, in turn, is connected to a potential divider comprising resistors R4 and R5. Resistor R1, preset VR1, resistor R2,

two separate pulses of different pulse widths, where one pulse initiates the activation of the second pulse. The first half of the NE556 is wired for 5-second pulse output. When slide switch S2 is in position ‘a’, the first timer is set for manual operation, i.e. by pressing switch S1 momentarily you can generate a single pulse of 5-second duration. When switch S2 is kept in ‘b’ position, i.e. pins 6 and 2 are shorted,

preset VR2, and capacitors C2 and C5 are the components determining time period. Presets VR1 and VR2 permit trimming of the 5-second and 1-second pulse width of respective sections. When switch S2 is in position ‘a’ and switch S1 is pressed momentarily, the output at pin 5 goes high for about 5 seconds. The trailing (falling) edge of this 5-second pulse is used to trigger the second timer via 0.1µF capacitor

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C6. This action results in momentarily pulling down of pin 8 towards the ground potential, i.e. ‘low’. (Otherwise pin 8 is at 1/2 Vcc and triggers at/below 1/3 Vcc level.) When the second timer is triggered at the trailing edge of 5-second pulse, it generates a 1-second wide pulse. When switch S2 is on position ‘b’, switch S1 is disconnected, while pin 6 is connected to pin 2. When capacitor C2 is charged, it is discharged through pin 2 until it reaches 1/3Vcc potential, at which it is retriggered since trigger pin 6 is also connected here. Thus timer 1 is retriggered after every 5-second period (corresponding to 0.2Hz frequency). The second timer is triggered as before to produce a 1-second pulse in synchronism with the trailing edge of 5-second pulse. This circuit is important wherever a pulse is needed at regular intervals; for instance, in ‘Versatile Digital Frequency Counter Cum Clock’ construction project published in EFY Oct. ’97 (or Electronics Projects Vol. 18), one may use this circuit in place of CD4060-based circuit. For the digital clock function, however, pin 8 and 12 are to be shorted after removal of 0.1µF capacitor and 10-kilo-ohm resistors R4 and R5.

High-/Low-voltage Cutout with Timer dr d.k. kaushik

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his inexpensive circuit can be connected to an air-conditioner/ fridge or to any other sophisticated electrical appliance for its protection. Generally, costly voltage stabilisers are used with such appliances for maintaining constant AC voltage. However, due to fluctuations in AC mains supply, a regular ‘click’ sound in the relays is heard. The frequent energisation/de-energisation of the relays leads to electrical noise and shortening of the life of electrical appliances and the relay/stabiliser itself. The costly yet fault-prone stabiliser may be replaced by this inexpensive high-low cutout circuit with timer. The circuit is so designed that relay

RL1 gets energised when the mains voltage is above 270V. This causes resistor R8 to be inserted in series with the load and thereby dropping most of the voltage across it and limiting the current through the appliance to a very low value. If the input AC mains is less than 180 volts or so, the low-voltage cut-off circuit interrupts the supply to the electrical appliance due to energisation of relay RL2. After a preset time delay of one minute (adjustable), it automatically tries again. If the input AC mains supply is still low, the power to the appliance is again inter-

rupted for another one minute, and so on, until the mains supply comes within limits (>180V AC). The AC mains supply is resumed to appliance only when it is above the lower limit. When the input AC mains increases beyond 270 volts, preset VR1 is adjusted such that transistor T1 conducts and relay RL1 energises and resistance R8 gets connected in series with the electrical appliance. This 10-kilo-ohm, 20W resistor produces a voltage drop of approximately 200V, with the fridge as load. The value and wattage of resistor R8 may be suitably chosen according to the electrical appliance to be used. It is

tor T3 remains cut off (with its collector remaining high) until the mains supply falls below the lower limit, causing its collector voltage to fall. The collector of transistor T3 is connected to the trigger point (pin 2) of IC1. When the input is more than the lower limit, pin 2 of IC1 is nearly at +Vcc. In this condition the output of IC1 is low, relay RL2 is de-energised and power is supplied to the appliance through the N/C terminals of relay RL2. If the mains supply is less than the lower limit, pin 2 of IC1 becomes momentarily low (nearly ground potential) and thus the output of IC1 changes state from ‘low’ to ‘high’, resulting in energisation

practically observed that after continuous use, the value of resistor R8 changes with time, due to heating. So adjustment of preset VR1 is needed two to three times in the beginning. But once it attains a constant value, no further adjustment is required. This is the only adjustment required in the beginning, which is done using a variac. Further, the base voltage of transistor T2 is adjusted with the help of pre-set VR2 so that it conducts up to the lower limit of the input supply and cuts off when the input supply is less than this limit (say, 180V). As a result, transis-

of relay RL2. As a result, power to the load/appliance is cut off. Now, capacitor C2 starts charging through resistor R6 and preset VR3. When the capacitor charges to (2/3)Vcc, IC1 changes state from ‘high’ to ‘low’. The value of preset VR3 may be so adjusted that it takes about one minute (or as desired) to charge capacitor C1 to (2/3)Vcc. Relay is now de-energised and the power is supplied to the appliance if the mains supply voltage has risen above the lower cut-off limit, otherwise the next cycle repeats automatically. One additional advantage of this circuit is that both relays are deenergised when the input AC mains voltage lies within the specified limit and the normal supply is extended to the appliance via the N/C contacts of both relays. ELECTRONICS PROJECTS Vol. 22

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Automatic heat detector Sukant Kumar Behara

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his circuit uses a complementary pair comprising npn metallic transistor T1 (BC109) and pnp germanium transistor T2 (AC188) to detect heat (due to outbreak of fire, etc) in the vicinity and energise a siren. The collector of transistor T1 is connected to the base of transistor T2, while the collector of transistor T2 is connected to relay RL1. The second part of the circuit comprises popular IC UM3561 (a siren and machine-gun sound generator IC), which can produce the sound of a fire-brigade siren. Pin numbers 5 and 6 of the IC are connected to the +3V supply when the relay is in energized state, whereas pin 2 is grounded. A resistor (R2) connected across pins 7 and 8 is used to fix the frequency of the inbuilt oscillator. The output is available from pin 3. Two transistors BC147 (T3) and BEL187 (T4) are connected in Darlington configuration to amplify the sound from UM3561. Resistor R4 in series with a 3V zener is used to provide the 3V supply to UM3561 when the relay is in energised state. LED1, connected in series with 68-ohm resistor R1 across resistor R4, glows when the siren is on.

To test the working of the circuit, bring a burning matchstick close to transistor T1 (BC109), which causes the resistance of its emitter-collector junction to go low due to a rise in temperature and it starts

Readers’ comments:  I have tried to construct the circuit, which failed to respond even after supplying heat for a long time. How can one check the transistor if the relay does not operate? Can you suggest me an alternative for transistor T1? Please help. Saket Through e-mail  In the project, on heating the base of transistor T1, the circuit does not work. I was even more surprised to see that the circuit had been ‘EFY Tested.’ Manish Poudwal Through e-mail

EFY: In reply to Saket: The circuit has been tested and there is nothing wrong in the circuit. To test the transistor, take a lighted matchstick (or candle) close to the BC109 (or the BC549). (For higher amplification, use BC109C or BC549C.) The relay should energise via the amplifier comprising complementary pair of transistors T1 and T2. When you gradually withdraw the candle or burning matchstick, the brightness of the LED will gradually decrease and it will finally go off. We have also tried the circuit with

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Pin Designation SEL1 SEL2 No Connection No Connection +3V No Connection Ground No Connection Do not care +3V

conducting. Simultaneously, transistor T2 also conducts because its base is connected to the collector of transistor T1. As a result, relay RL1 energises and switches on the siren circuit to produce loud sound of a firebrigade siren. Sound Effect Lab note. We have added a table to enable readers to obtain Police Siren Fire Engine Siren all possible sound effects by returning pins 1 and 2 as suggested Ambulance Siren Machine Gun in the table.

transistor BC108 in place of BC109, and it worked well. In reply to Manish Poudwal: All the circuits are undoubtedly tested and records maintained. If you have not been able to achieve the results, there is likely to be some flaw. You may have used a silicon transistor for AC188. Note that AC188 is a germanium transistor (in metal can). Please check. Response from the reader: As guessed correctly by you, I was using a silicon transistor AC188 instead of germanium transistor. Now the circuit does work.

Musical ‘Touch’ Bell Sukant Kumar Behara

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ere is a musical call bell that can be operated by just bridging the gap between the touchplates with one’s fingertips. Thus there is no need for a mechanical ‘on’/‘off’ switch because the touch-plates act as a switch. Other features include low cost and low power consumption. The bell can work on 1.5V or 3V, using one or two pencil cells, and can be used in homes and offices. Two transistors are used for sensing the finger touch and switching on a melody IC. Transistor BC548 is npn type while

transistor BC558 is pnp type. The emitter of transistor BC548 is shorted to the ground, while that of transistor BC558 is connected to the positive terminal. The collector of transistor BC548 is connected to the base of BC558. The base of BC548 is connected to the washer (as shown in the figure). The collector of BC558 is connected to pin 2 of musical IC UM66, and pin 3 of IC UM66 is shorted to the ground. The output from pin 1 is connected to a transistor amplifier comprising BEL187 transistor for feeding the

loudspeaker. One end of 2.2-mega-ohm resistor R1 is connected to the positive rail and the other to a screw (as shown in the figure). The complete circuit is connected to a single pencil cell of 1.5V. When the touch-plate gap is bridged with a finger, the emitter-collector junction of transistor BC548 starts conducting. Simultaneously, the emitter-baser junction of transistor BC558 also starts conducting. As a result, the collector of transistor BC558 is pulled towards the positive rail, which thus activates melody generator IC1 (UM66). The output of IC1 is amplified by transistor BEL187 and fed to the speaker. So we hear a musical note just by touching the touch points. The washer’s inner diameter should be 1 to 2 mm greater than that of the screwhead. The washer could be fixed in the position by using an adhesive, while the screw can be easily driven in a wooden piece used for mounting the touch-plate. The use of brass washer and screw is recommended for easy solder-ability.

Readers’ comments:  The circuit starts ringing (without touching the screw) when connected to 3V. On disconnecting points 1 and 2 (kept open), I still received the ring. Why so? A. Vaidhyanathan

Pollachi The author, Sukant Kumar Behara, replies: You can rectify this snag by changing transistor T1 (BC148) with a new one. On touching the base of transistor T1, its

emitter-collector junction starts conducting. But you’ve mentioned that even without touching the base of transistor T1, the bell starts ringing, which means that the emitter-collector junction of the transistor has got shorted internally.

Non-contact liquid-level controller R.G. Thiagaraj Kumar

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FY readers are quite familiar with liquid-level controllers. But the one presented here is different. Usually, transducers using electric con-

duction, or variation in resistance or capacitance principle, are employed for level sensing. In conduction type of sensors, the elec-

tric current passes through the liquid. The corrosion of contacts is a major problem while using DC excitation. The cost and the size are the two restrictive factors ELECTRONICS PROJECTS Vol. 22

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in using AC excitation. Further, passing current through the liquid in combustive environments is not permissible. In resistance type sensors, the resistance is altered through some mechanical arrangement, which means a large operating force is required, which may be a problem in small tanks. In capacitive transducer type, the construction cost is high. In the present project, an easy but effective liquid-level controller is presented using the magnetic principle. It is non-contact type and hence can be used in almost all applications, irrespective of whether the liquid is conductive or not. Two reed switches (with glass enclosure) and a ring

magnet (normally used in loud-speakers) form the sensor unit. The reed switches used are normally-open type and they close when placed (and oriented properly) in a magnetic field. The electronic circuit is a simple bistable multivibrator wired around the common 555 timer IC. It can be set or reset by the closure of reed switches. The output of the multivibrator drives the relay, which controls the AC mains supply to the pump motor or any other controller (such as a solenoid-operated valve). The reed switches are connected as shown in the figure. These are put in a closed (non-conductive) tube, which is then placed in the tank. The ferrite ring

magnet is put inside the float, and it moves up and down along the tube depending upon the level of the liquid in the tank. When the level of the liquid in the tank is low, the magnet comes closer to reed switch S2. As a result, switch S2 is closed + and the bistable multivibrator sets. This actuates the relay, thereby starting the pump to fill the tank. The level of the liquid in the tank starts increasing. When the level of the liquid in the tank is high enough, the ring magnet comes close to reed switch S1, and it closes. The bistable multivibrator now resets and the pump is switched off. This process is repeated and the tank gets filled automatically. Switches S1’ and S2’ are used for testing the circuit or when the reed switches are nonfunctional. A neon bulb is used to indicate the presence of the AC supply in the plug. An optional piezobuzzer is used to raise an audible alarm when the relay energises. If you desire to display the level of the liquid in the tank, additional reed switches would need to be placed inside the tube at different levels (say, 1/4th, 1/2, 3/4th, and near-overflow level). They can be connected between the LEDs and the supply via current-limiting resistors for level indication. The LEDs can be arranged in a model tank diagram printed on the front panel of the controller. The LED corresponding to the level of the liquid in the tank would glow in this arrangement. The selection of float material is to be done carefully to avoid chemical reaction and/or pollution of the liquid. Teflon floats are suitable for most applications.

Readers’ comments:  The circuit is indeed very effective and accurate, while being very simple and straightforward. Congratulations to the author! Based on this circuit, I successfully arranged a number of reed switches using a 24-lead flat cable inside a PVC tube,

for monitoring a vehicle’s fuel tank level. Since the idea of using an electric conduction method is out of question with petrol, I was agonisingly pondering over various alternatives. The idea given by Mr Kumar is what exactly I was looking for. M.K. Chandra Mouleeswaran Tamil Nadu

 We are undergraduate students from a leading University of Sri Lanka and have constructed the project successfully. The IC should function when both switches S1 and S2 are open and the AC supply is switched on, but we found that it becomes on automatically. Also explain the purpose of using diode D3 and capaci-

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tor C2 in the circuit. Menaha Shah and Colleagues Through e-mail EFY: During lab testing (with DC supply) we haven’t noticed any such fault. The stated fault could occur only if the

gap between reed switch contacts is too small. Try placing 0.1µF capacitor across resistors R1 and R2 to bypass any noise surge at switching on as it may trigger the timer. Diode D3 acts as a free-wheeling diode

to dissipate the stored energy in the relay coil when the transistor suddenly cuts off and the magnetic field collapses. Capacitor C2 at control pin 5 of timer 555 is used as a filter capacitor when the timer is used in noisy environments.

High-Power Bicycle Horn T.K. Hareendran

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n interesting circuit of a bicycle horn based on a popular, low-cost telecom ringer chip is described here. This circuit can be powered using the bicycle dynamo supply and does not require batteries, which need to be replaced frequently. The section comprising diodes (D1 and D2) and capacitors (C1 and C2) forms a half-wave voltage-doubler circuit. The output of the voltage doubler is fed to capacitor C3 via resistor R1. The maximum DC supply that can be applied to the input

terminals of IC1 is 28V. Therefore zener diode ZD1 is added to the circuit for protection and voltage regulation. The remainder of the circuit is the

tone generator based on IC1 (KA2411). The dual-tone output signal from pin 8 of IC1 is fed to the primary of transformer X1 (same as used in transistor radios) via capacitor C6. The secondary of X1 is connected to a loudspeaker directly. In case you are interested in connecting a piezoceramic element in place of the loudspeaker, remove capacitor C6, transformer X1, and the loudspeaker. Connect one end of the piezoceramic disk to pin 5 of IC1 and the other end to pin 8 of IC1 through a 1/4W, 1-kilo-ohm resistor. IC1 KA2411 is also available in COB style, with the same pin configuration. Both packages work equally well. However, to get the best results with the COB package, change values of resistors R2 through R4 to 330-kilo-ohm, capacitor C4 to 0.47µF, 63V electrolytic (positive end to pin 3 of IC1), and C5 to 0.005µF, 63V. This bicycle horn project can also be used as a telephone extra ringer by just removing all components on the left side of capacitor C3 and connecting the circuit shown in Fig. 2 to the terminals of capacitor C3.

AC mains phase-sequence indicator M.K. Chandra Mouleeswaran

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mains phase-sequence indicator serves as a hand-tool in checking electrical wiring, especially the wiring of three-phase AC motors.

The basic idea of the circuit is that when any (say, Y) of the three phases (RYB), taken as a reference phase, is at negative-going zero voltage, its leading

phase (say, R) is positive while its lagging phase (B) is negative, and these states can be easily verified. The circuit comprises two main parts. ELECTRONICS PROJECTS Vol. 22

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The first part comprises transistorised multivibrator, decade countercum-LED driver, and LED arrays arranged in a specified fashion, as shown in the figure. The second part comprises the phasesequence detector followed by phase-sequence sensor operated flip-flop and LED switching transistors.

The astable multivibrator section provides clock pulses in the 10 to 1000Hz range to the decade counter and the LED array section. The LEDs are grouped into two parts to form two distinctive indicators. These two groups are successively driven by Q0 to Q4 and Q5 to Q9 outputs of IC1. Only one of the two groups’ LEDs will turn on sequentially, depending on which of the two transistors (transistor T3 or transistor T4) is on, which, in turn, is dependent upon the phase sequence of the three-phase supply. This becomes clear from the following explanation of the second part of the circuit. The three phases (R, Y, and B) are brought to an artificial neutral at the junctions of resistors R17 through R19 (each 22 kilo-ohm, 2-watt) to serve as the common reference. As stated earlier,

for a given phase sequence, when phase R is at its negative-going zero, phase B is negative. So data-input pin 5 of the flip-flop (IC2) is logic ‘high’ (due to non-conduction of transistor T5). Meanwhile, clock-input pin 3 of the flipflop goes from low to high due to phase R (refer waveforms for condition 1, as observed by EFY Lab). The ‘high’ at data pin appears at the q output (pin 1) while Q output remains ‘low’ as long as the

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phase sequence is clockwise. Therefore the Q output drives transistor T3 to extend the ground path for green LEDs D1 through D10 to show a clockwise-rotating LED ring. When any of the two phases gets interchanged (say, after a maintenance work at the power-house or repair/replacement of a 3-phase transformer), the conditions are reversed (refer waveform for condition 2, as observed by EFY Lab), and Q become ‘high’ and red LEDs D11 through D20 are switched on (sequentially)

by transistor T4 to show an anticlockwiserotating ring. While testing for the phase sequence, there is no need to keep the device on for a long time. A push-to-on read switch can be used during the phase-sequence testing. If the device is to be used for long periods, use a high-capacity battery in place of PP3 battery. Also replace 2W resistors R17 to R19 with 5W fusible-type resistors. The frequency of the astable multivibrator is unimportant, except that the

speed of the LED ring must be easily visible. Zener diodes ZD1 and ZD2 are used for protection of transistors T5 and T6, respectively. Precautions. 1. Never use an AC mains adaptor-type power supply in place of the battery. 2. Correctly position LEDs D1 through D20 in the ring for its proper viewing. 3. Assemble resistors R11 to R19 on the PCB at a slightly elevated level using ceramic beads for proper dissipation of heat.

Luxurious toilet/ bathroom facility A.R. Gidwani

Ag

ed persons in the house and guests often fumble while search ing for the toilet and bathroom switches at night. Also, very few of us take care to switch off the lights of toilets/bathrooms after using them. The circuit given here helps to overcome both the problems. The figure shows two symmetrical

circuits (one each for toilet and bathroom) sharing common power supply and a melody generator-cum-audio warning unit. The reed switches S1 and S2 are of normally-open type, operated by permanent magnets appropriately fixed to the doors of bathroom and toilet, respectively. When the doors of bathroom and toilet are closed, the reed switches

are also closed, and vice versa. (Door is assumed in closed condition with nobody inside bathroom/toilet, i.e. reed switch is activated.) The operational features of the circuit are: • Lamp and exhaust fan are switched on when the door is opened. • Soft music is played continuously

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until the door is closed from inside/outside. • With a person inside the room, lamp and fan remain on, until the door is reopened. They go off when the door is reopened. • Visual indication of whether the toilet/bathroom is occupied/vacant is given by two bicolour LEDs fixed on a panel, which may be fitted near the door with corresponding ‘toilet’/‘bathroom’ labels on them. Here the LED colour turns from ‘green’ to ‘red’ if the room gets occupied, and vice-versa. • If the door is opened once, and not closed back within 10 seconds, the lamp and fan are automatically switched off, thus conserving electricity. But the music remains on as a reminder that the door is not closed. • For cleaning of bathroom/toilet with doors kept open, a parallel on/off switch is included on the switchboard to bypass the relay contacts and manually control the switching on/off of the light and exhaust fan. (This is the service mode.) In this case, the music remains on as long as the door remains open. In case of failure of the unit, the same on/off switch can be used as usual until the circuit is repaired. • Due to battery backup facility, even with power failure, when a person is inside, the door status is maintained. However, the lamp and fan will be on only on mains resumption.

• Also, when a person leaves the room during power failure, with door closed, the lamp and fan are kept off on resumption of power. (Intelligent-mode!) • However, the circuit can be fooled by opening and closing the door within 10 seconds, without entering inside. In this case, the lamp and fan will continue to be on and would require reopening and closing of the door to bring the circuit to order. This problem can be prevented to some extent by using a hydraulic door opener, which would approximately take 10 seconds to close the opened door. A delay period of 10 seconds is deliberately chosen for letting the person inside the toilet/bathroom in normal case! IC1 is a dual positive edge-triggered ‘D’ type flip-flop. IC1(a) gets triggered when bathroom door (and switch S1) is opened and hence IC1(b) toggles, as Q output of IC1(a) is connected to clock input pin of IC1(b). As a result, relay RL1 energises through transistor T3, thereby switching on the lamp and exhaust fan. (Please refer to Fig. 2, the separate wiring diagram of lamp and exhaust fan via the N/O contacts of the relay.) Simultaneously, pin 2 (Q1) of IC1(a) goes low, switching transistor T5 ‘on,’ which switches on melody generator IC4, letting out a sweet audio tune via transistor T6 and loudspeaker. In normal condition, when someone opens the bathroom door and gets inside within preset time of IC3(a) (10 seconds here), and closes the door from inside, the music stops with lamp and fan ‘on.’ Now, in case someone opens the door before or after use, and forgets to shut it, the lamp and exhaust fan are switched off after 10 seconds but the music remains ‘on’ as a reminder that the door is to be closed. This happens due to mono multivibrator (MMV)

IC3(a), which resets pin 10 of IC1(b) through transistor T1 after 10 seconds. (This period can be adjusted by varying the values of resistor R11 and/or capacitor C7.) It should be noted here that although IC3 is used as ‘MMV,’ it is triggered here with a positive pulse through its pin 4 (reset pin) rather than its pin 6 (trigger pin). This arrangement makes it unique for setting and resetting IC3 through pin 4, and resetting IC1(a) through pin 5 of IC3 and transistor T1. Battery backup facility ensures memory backup during power failure. Power supply uses a normal 2-diode full-wave rectifier circuit, which needs no further explanation. The purpose of using bi-colour LED1 and LED2 is that, initially when the door is closed these emit green light—as the green LED part gets the supply via resistor R15—to indicate that bathroom/ toilet is vacant. When bathroom/toilet is occupied, transistor T3/T4 conduct to light up the red LED part as well. Melody generator IC4 (UM66) is switched on through diodes D3/D4 and transistor T5, which conducts when IC1(a) pin 2 or IC2(a) pin 2 goes low. When transistor T5 conducts, zener ZD1 breaks down and supplies regulated 3.9V to IC4, to produce a melodious tune via transistor T6 and the speaker. As most toilets and bathrooms are ‘attached’ nowadays, only a single circuit is required, and the circuit can be wired on a general-purpose veroboard. A small modification of the circuit, by adding additional SPST switch S3, as shown in Fig. 2, needs to be done inside the wooden switchboard box. This permits the user to operate the lamp and fan during cleaning of the toilet or for bypassing the circuit, when bathroom or toilet undergo repair work.

EEPROM W27C512 (Winbond) Eraser j.p. Sharma

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PROMs (electrically erasable PROMs) are generally erased by ultraviolet rays, and it takes half an hour or so to erase the data in an

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ERPOM. Nowadays a special EEPROM from Winbond is available in the market, which is being used in telecommunication due to its low cost.

The simple, low-cost circuit presented here takes only 100 ms to erase old programs electrically. The programming voltage VPP for the mentioned IC is 12.7V,

unlike the 28xxxx series EEPROMs that can be written to or read like a RAM, in situ. Multiple ICs connected in parallel can be erased simultaneously using the given circuit. The circuit requires 15V to 20V DC. Timer IC3 (LM555) is used for generation of clock pulses of 200 millisecond time period with an ‘on’ time of 100 milliseconds. Pulse time is achieved by using presets VR1 and VR2 and capacitor C1. The ‘on’/‘off’ time of pulse may be set with the help of an oscilloscope or by taking appropriate values of presets VR1 and VR2 (in-circuit resistances) and capacitor C1 using the following relationship: On time=0.69VR1xC1=Off Time= 0.69VR2xC1=100 milliseconds IC1 (7812) and IC2 (7805) are voltage regulator ICs that are used to obtain regu-

lated 14V DC and 5V DC, respectively, required for operation of the circuit. The clock with time period of 200 milliseconds is fed to IC4 (CD4017). In this IC, the output is available successively only at one of the output pins with a delay of 200 milliseconds when reset pin 15 is low. The 14V DC is made available via transistors T1 and T2 to pins 22 and 24 of IC5 (27C512, which is the IC under erasure). As soon as push-to-on switch S1 is pressed momentarily, it resets IC4 by application of 5V DC to reset pin 15, and the output at pin 3 (Q0) goes high. This high output is shifted to the next output pin with the successive clock pulse received at pin 14. Q5 output from pin 1 of IC4 is inverted using transistor T3 and is given to chipenable pin 20 of IC5, when 14V DC is

already available at pins 22 and 24 of IC5. All address pins, except pin 24 (A9), are set low and all data pins (11 through 13 and 15 through 19) are at high level (+5V). Then pins 22 and 24 are pulsed low for 100 milliseconds. Immediately all data (cells) are set high. (Data output is high only in erased condition.) At the end when Q9 output of IC4 goes high, transistor T4 conducts, pulling its collector low. LED1 glows to indicate completion of erasure. Simultaneously, pin 4 of timer IC3 is taken low to stop generation of further clock pulses until IC4 is reset. Insert the next IC to be erased in IC5 socket (preferably use a ZIF socket) and reset IC4 by pushing switch S1 momentarily. It takes only 100 milliseconds to erase the EEPROM IC.

Readers’ comments:  Please clarify the following: 1. Why all address pins, except 24 (A9), are connected to 0V (GND)? 2. If there are more than A15 address pins (as in 27C010, 27C020, 28C020 etc), which address pin willbe connected to Vcc (+5V)?

3. Is it possible to set pulse time with DTM? 4. Is it possible to erase EEPROM other than Winbond make? Angika Electronics, Bhagalpur The author, J.P. Sharma, replies: 1. The given configuration is mandatory as per the manufacturer’s catalogue for

erasing W27C512 EEPROM. 2. 27C010 and 27C020 are UV erasable EPROMs (with less than A15 pins). For 28C020, refer the datasheet. 3. A monostable low pulse of 100 milliseconds can be set with DTM. 4. You can erase similar EEPROMs electrically.

Intelligent Electronic Lock k. udhaya Kumaran

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his intelligent electronic lock circuit is built using transistors only. To open this electronic lock,

one has to press tactile switches S1 through S4 sequentially. For deception you may annotate these switches with

different numbers on the control panel/ keypad. For example, if you want to use ten ELECTRONICS PROJECTS Vol. 22

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switches on the keypad marked ‘0’ through ‘9’, use any four arbitrary numbers out of these for switches S1 through S4, and the remaining six numbers may be annotated on the leftover six switches, which may be wired in parallel to disable switch S6 (shown in the figure). When four password digits in ‘0’ through ‘9’ are mixed with the remaining six digits connected across disable switch terminals, energisation of relay RL1 by unauthorised person is prevented. For authorised persons, a 4-digit password number is easy to remember. To energise relay RL1, one has to press switches S1 through S4 sequentially within six seconds, making sure that each of the switch is kept depressed for a duration of 0.75 second to 1.25 seconds. The relay will not operate if ‘on’ time duration of each tactile switch (S1 through S4) is less than 0.75 second or more than 1.25 seconds. This would amount to rejection of the code. A special feature of this circuit is that pressing of any switch wired across disable switch (S6) will lead to disabling of the whole electronic lock circuit for about one minute. Even if one enters the correct 4-digit password number within one minute after a ‘disable’ operation, relay RL1 won’t get energised. So if any unauthorised person keeps trying different permutations of numbers in quick successions for energisation of relay RL1, he is not likely to succeed. To that extent, this electronic lock circuit is foolproof. This electronic lock circuit comprises disabling, sequential switching, and relay

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latch-up sections. The disabling section comprises zener diode ZD5 and transistors T1 and T2. Its function is to cut off positive supply to sequential switching and relay latch-up sections for one minute when disable switch S6 (or any other switch shunted across its terminal) is momentarily pressed. During idle state, capacitor C1 is in discharged condition and the voltage across it is less than 4.7 volts. Thus zener diode ZD5 and transistor T1 are in nonconduction state. As a result, the collector voltage of transistor T1 is sufficiently high to forward bias transistor T2. Consequently, +12V is extended to sequential switching and relay latch-up sections. When disable switch is momentarily depressed, capacitor C1 charges up through resistor R1 and the voltage available across C1 becomes greater than 4.7 volts. Thus zener diode ZD5 and transistor T1 start conducting and the collector voltage of transistor T1 is pulled low. As a result, transistor T2 stops conducting and thus cuts off positive supply voltage to sequential switching and relay latchup sections. Thereafter, capacitor C1 starts discharging slowly through zener diode ZD5 and transistor T1. It takes approximately one minute to discharge to a sufficiently low level to cut-off transistor T1, and switch on transistor T2, for resuming supply to sequential switching and relay latch-up sections; and until then the circuit does not accept any code. The sequential switching section com

prises transistors T3 through T5, zener diodes ZD1 through ZD3, tactile switches S1 through S4, and timing capacitors C2 through C4. In this three-stage electronic switch, the three transistors are connected in series to extend positive voltage available at the emitter of transistor T2 to the relay latch-up circuit for energising relay RL1. When tactile switches S1 through S3 are activated, timing capacitors C2, C3, and C4 are charged through resistors R3, R5, and R7, respectively. Timing capacitor C2 is discharged through resistor R4, zener diode ZD1, and transistor T3; timing capacitor C3 through resistor R6, zener diode ZD2, and transistor T4; and timing capacitor C4 through zener diode ZD3 and transistor T5 only. The individual timing capacitors are chosen in such a way that the time taken to discharge capacitor C2 below 4.7 volts is 6 seconds, 3 seconds for C3, and 1.5 seconds for C4. Thus while activating tactile switches S1 through S3 sequentially, transistor T3 will be in conduction for 6 seconds, transistor T4 for 3 seconds, and transistor T5 for 1.5 seconds. The positive voltage from the emitter of transistor T2 is extended to tactile switch S4 only for 1.5 seconds. Thus one has to activate S4 tactile switch within 1.5 seconds to energise relay RL1. The minimum time required to keep switch S4 depressed is around 1 second. For sequential switching transistors T3 through T5, the minimum time for which the corresponding switches (S1 through S3) are to be kept depressed is 0.75 seconds to 1.25 seconds. If one operates these switches for less than 0.75 seconds, timing capacitors C2 through C4 may not get charged sufficiently. As a consequence, these capacitors will discharge earlier and any one of transistors T3 through T5 may fail to conduct before activating tactile switch S4. Thus sequential switching of the three transistors will not be achieved and hence it will not be possible to energise relay RL1 in such a situation. A similar situation arises if one keeps each of the mentioned tactile switches depressed for more than 1.5 seconds. When the total time taken to activate switches S1

through S4 is greater than six seconds, transistor T3 stops conducting due to time lapse. Sequential switching is thus not achieved and it is not possible to energise relay RL1. The latch-up relay circuit is built around transistors T6 through T8, zener diode ZD4, and capacitor C5. In idle state, with relay RL1 in deenergised condition, capacitor C5 is in

discharged condition and zener diode ZD4 and transistors T7, T8, and T6 in nonconduction state. However, on correct operation of sequential switches S1 through S4, capacitor C5 is charged through resistor R9 and the voltage across it rises above 4.7 volts. Now zener diode ZD4 as well as transistors T7, T8, and T6 start conducting and relay RL1 is energised. Due to conduction

of transistor T6, capacitor C5 remains in charged condition and the relay is in continuously energised condition. Now if you activate reset switch S5 momentarily, capacitor C5 is immediately discharged through resistor R8 and the voltage across it falls below 4.7 volts. Thus zener diode ZD4 and transistors T7, T8, and T6 stop conducting again and relay RL1 de-energises.

Stable 455kHz BFO for SSB Reception D. Prabaharan

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ost Indian amateur radio operators prefer to operate on SSB (single sideband) and CW because these carry the signal over a long distance for a given transmitter power. Broadcast receivers are not meant to directly receive Morse code transmission on SSB and CW. Short-wave listeners require some arrangement to receive the same. One such arrangement comprises a simple IF BFO (beat frequency oscillator), which is an RF oscillator of conventional type. The output of BFO is heterodyned to beat with another frequency to obtain a resultant frequency (difference of the two frequencies) lying in the audio range (about 1 kHz). BFO can be used to get an audio note from CW reception and also to resolve SSB signals. An SSB signal is transmitted without carrier signal. In ordinary receivers, it does not produce speech with sufficient

clarity. When BFO signal is heterodyned with SSB signal, this RF acts like a carrier and the signal is well resolved. The BFO circuit comprises transistors T1 and T2, which are connected in a straightforward two-stage, directcoupled, common-emitter configuration. The input and output are in phase and

positive feedback between the two is provided by ceramic filter CF1. A significant amount of feedback is provided only at the operating frequency of the filter, which is 455 kHz. So the circuit oscillates at this frequency. The ceramic filter gives good frequency stability and requires no adjustment in order to produce the correct frequency. This BFO is meant for single-sideband reception only. There is no need to connect BFO to receiver. Tune your BC receiver to any SSB signal, and then on keeping BFO just close to it, you may notice some hissing noise in your receiver. Match BFO frequency to your receiver’s IF, which may be between 452 and 460 kHz, until you get clear sound. If the BFO signal is too strong, increase the distance between BFO and receiver.

Auto Shut-off For Cassette Players And Amplifiers arthur louis

H

ere are two simple, low-cost circuits that can be used to shut off the mains supply to any audio or video equipment (such as

tape recorder, CD player, and amplifier). These circuits are helpful to those in the habit of falling asleep with their music system on.

The circuits will also protect the equipment from getting damaged due to high-voltage spikes whenever there is a resumption of power after a break. This ELECTRONICS PROJECTS Vol. 22

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is possible because the equipment will get switched off automatically under such conditions but will not get switched on automatically on resumption of mains supply. The circuit in Fig. 1 can be used to shut off any cassette player that has a reliable auto-stop mechanism. Whenever switch S1 is pressed momentarily, it extends the supply to the step-down transformer of the tape recorder and charges capacitor C1 through diode D1. This, in turn, makes transistor T1 conduct and energise relay RL1 to provide a parallel path to switch S1, so that supply to the step-down transformer continues even when switch S1 is released. When any button on the cassette player is pressed, the capacitor charges through diode D2. This ensures conduction

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of transistor T1 and thus the continuity of operation of cassette player. However, whenever the auto-stop mechanism functions at the end of a tape, the leaf switch gets opened. This cuts the charging path for the capacitor and it starts discharging slowly. After about one minute, the relay opens and interrupts main power to the transformer. The time delay can be increased by increasing the value of capacitor C1. If the appliance used is a two-in-one type (e.g. cassette player-cum-radio), just connect another diode in parallel with diodes D1 and D2 to provide an additional path for charging capacitor C1 via the tape-to-radio changeover switch, so that when radio is played the relay does not interrupt the power supply.

The other circuit, shown in Fig. 2, functions on the basis of the signal received from preamp of the appliance used. In this circuit, opamp µA741 is wired in inverting opamp configuration. It amplifies the signal received from the preamp. Timer NE555 is used to provide the necessary time delay of about one minute. Preset VR1 is used to control the sensitivity of the circuit to differentiate between the noise and the signal. Resistor R4 offers feedback resistance to control the gain of the opamp. By increasing or decreasing the value of resistor R4, the gain can be increased or decreased, respectively. The preset time delay of timer NE555 (which is about one minute) can be increased by increasing the value of C4. Initial energisation of relay RL2 and charging of capacitor C4 take place on depression of switch S3 in the same manner as charging of capacitor C1 (refer Fig. 1) on depression of switch S1. As a result, pins 2 and 6 of NE555 go high and the output of timer goes low to switch off mains supply from the relay to step-down transformer X2 of the appliance. Bleeder resistor R6 is used to discharge capacitor C4. Now if signals are received from the preamplifier, these are amplified by µA741 and fed to the base of transistor T2, which keeps capacitor C4 charged through resistor R5. When there is no signal, T2 will not conduct and the capacitor slowly discharges through R6. The output of 555 goes high to switch off the relay and thus the mains supply to transformer X2. Switch S2 can be depressed momentarily if the device needs to be manually switched off. Note. The 12V supply should be provided to the circuit from the equipment’s power supply. Opamp µA741 should be driven from the preamplifier of the gadget used, and not from its power amplifier output. Switches S1 and S2 are 2-pole push-to-on switches. These can also be fabricated from 2-pole on-off switches, which are widely used in cassette players, by removing the latch pin from them.

House Security System malay banerjee

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ere is a low-cost, invisible laser circuit to protect your house from thieves or trespassers. A laser pointer torch, which is easily available in the market, can be used to operate this device. The block diagram of the unit shown in Fig. 1 depicts the overall arrangement for providing security to a house. A laser torch powered by 3V power-supply is used for generating a laser beam. A combination of plain mirrors M1 through M6 is

used to direct the laser beam around the house to form a net. The laser beam is directed to finally fall on an LDR that forms part of the receiver unit as shown in Fig. 2. Any interruption of the beam by a thief/trespasser will result into energisation of the alarm. The 3V powersupply circuit is a conventional full-wave rectifier-filter circuit. Any alarm unit that operates on 230V AC can be connected at the output. The receiver unit comprises two identical step-down transformers

(X1 and X2), two 6V relays (RL1 and RL2), an LDR, a transistor, and a few other passive components. When switches S1 and S2 are activated, transformer X1, followed by a full-wave rectifier and smoothing capacitor C1, drives relay RL1 through the laser switch. The laser beam should be aimed continuously on LDR. As long as the laser beam falls on LDR, transistor T1 remains forward biased and relay RL1 is thus in deenergised condition. When a person crosses the line of laser beam, relay RL1 turns ON and transformer X2 gets energised to provide a parallel path across N/C contact and the

pole of relay RL1. In this condition, the laser beam will have no effect on LDR and the alarm will continue to operate as long as switch S2 is on. When the torch is switched on, the pointed laser beam is reflected from a definite point/ place on the periphery of the house. Making use of a set of properly oriented mirrors one can form an invisible net of laser rays as shown in the block diagram. The final ray should fall on LDR of the circuit. Note. LDR should be kept in a long pipe to protect it from other sources of light, and its total distance from the source may be kept limited to 500 metres.

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Simple WaterLevel IndicatorCumAlarm pradeep g.

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his water-level indicator-cumalarm circuit is configured around the well-known CMOS inputcompatible, 7-channel IC ULN2004 Darlington array. As the water level rises in the tank, it comes in contact with probes P1 through P7 and thereby makes pins 7 through 1 high, sequentially. As a result, the corresponding output pins 10 through 16 go low one after the other, and LED1 through LED7 light up in that order.

When water comes in contact with the final probe P7, it results in sounding of

the piezo-buzzer connected to output pin 16 along with LED7.

Precision Inductance and Capacitance Meter P. Thangavel

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his circuit for measurement of inductance and capacitance can be used to test whether the values of inductors and capacitors quoted by the manufacturer are correct. The principle used in the circuit is based on the transient voltages produced across inductors and capacitors connected as series R-L and R-C networks, respectively, across a constant voltage source. The time constant for R-C and R-L networks is given by the relationships t=RxC and L/R, respectively, where resistance R is in ohms, capacitance C in Farads,

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inductance L in Henries, and time t in seconds. The voltage across capacitor in R-C network rises exponentially to 0.632 of the applied voltage and voltage across inductor in R-L network degrades exponentially to 0.368 of the applied voltage in one RxC and one L/R time (referred to as time constant T of the combination), respectively. When the inductor/capacitor under test is connected across terminals A and B shown in the circuit, it is discharged through the normally-closed

contacts of two-way push-to-on/off switch S1. When switch S1 is pushed, the capacitor’s voltage begins to grow (or the inductor’s voltage begins to drop). Simultaneously, the output of timer 555 IC, which is wired as an astable multivibrator, is passed through NOR gates N1 and N2 and applied to the counter circuit. When the time constant (one CxR or one L/R, as the case may be) reaches, gate N2 is inhibited as its pin 2 goes high and the counter circuit freezes. Mode switch S2 is to be kept in position ‘a1’ for capaci-

tance measurement and in position ‘a2’ for inductance measurement. As series resistance R1 is 1 kilo-ohm, the capacitance value is given by the relationship C=Tx10–3 while the inductor value is given by the relationship L=Tx103. The time period (1/frequency) of timer 555 (IC2) is adjusted for 1 ms and 1 µs in ‘b1’ and ‘b2’ positions, respectively, of the range switch. The values of capacitors and

inductors covered in each Displayed range, together value with displayed values, are Capacitance in µF and inductance shown in the in H table. From the table it is obviCapacitance in nF ous that this and inductance circuit can in mH measure capacitance from 1 nF to 9,999 µF and inductance from 1 mH to 9999 H. While presets VR1 and VR2 are to be adjusted for the in-circuit value of 1.717 kilo-ohm each, the in-circuit value of preset VR3 is close to 4.7 kilo-ohm. If a regulated +5V is not used, the measurement of capacitance and inductance will be imprecise. Given below are some important points to be taken care of:

1. The position of modeselect switch S2 and range-select switch S3 should be changed before switch S1 is pressed. 2. If the circuit is allowed to function until it displays a constant value, the maximum time taken for measurement will be 10 seconds. 3. When modeselect switch S2 is in position a1, capacitances can be measured, and when it is in position a2, inductances can be measured. 4. When range-select switch S3 is in position ‘b1’, the output of 555 IC will have a time period of 1 ms (frequency = 1 kHz), and when it is in position ‘b2’, the output of 555 IC will have a time period of 1 µs. (EFY lab note. The guaranteed frequency of NE555 is limited to 500 kHz, and hence it may not be possible to get 1µs period. One may therefore use a 2nF capacitor to get a period of 2 µs and multiply the displayed value by 2, in b2 range.) 5. Use a breadboard for connecting inductors or capacitors across terminals A and B. 6. Using both the ranges for measuring an inductor or capacitor enables one to obtain the accurate value. For example, a 4.7µF capacitor will display only 4 µF when measured in range b1 , while in b2 range it will display 4700 nF (or 4.7 µF). 7. Don’t press switch S1 before inserting the capacitor or the inductor between terminals A and B.

Readers’ comments:  I have asssembled the circuit, which doesn’t work at all. It is showing different values all the time. Even when the capaci-

tor and the inductor aren’t connected for measurement, it still shows some values. I have checked all the ICs and found them all in good condition. Is there any misprint

in the said circuit idea? Kindly tell me the cause of this problem. Asif Draboo Bemiba, Jammu & Kashmir

555 IC Time period 1 ms (Switch S3 in position b1) 1 µs (Switch S3 in position b2)

Table Capacitance Inductance range range C=Tx10–3 L=Tx103 When T=1 ms, When T=1 ms, L=1H C=1 µF When T=9999 ms, When T= L=9999 H 9999 ms, C=9999 µF When T=1 µs, When T=1 µs, C=1 nF L=1 mH When T=9999 µs, When T=9999 µs, C=9999 nF L=9.999H =9.999 µF =9999 mH

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The author, Thangavel Ponnusamy, replies: Please go through the article throughly. There are some rules to be followed to get proper performance from the instrument: 1. The circuit should be switched on only after connecting the capacitor or the inductor for measurement. If you connect

the capacitor or the inductor (for measurement) after the circuit is powered on, you are bound to get wrong results. 2. Check the position of mode switch, which needs to be set in appropriate mode before switching on the circuit. No switch should be changed when the circuit is powered on. For large-value inductors

and capacitors, the results will be stable only after a few seconds, so wait until the results stabilise. 3. Ensure that you are using good capacitors and inductors for measurement. I hope the above tips will help you to get yor instrument working.

Under-/Over-Voltage Beep for Manual Stabiliser K. Udhaya Kumaran

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anual stabilisers are still popular because of their simple construction, low cost, and high reliability due to the absence of any relays while covering a wide range of mains AC voltages compared to that handled by automatic voltage stabilisers. These are used mostly in homes and in business centres for loads such as lighting, TV, and fridge, and in certain areas where the mains AC voltage fluctuates between very low (during peak hours) and abnormally high (during non-peak hours). Some manual stabilisers available in the market incorporate the high-voltage auto-cut-off facility to turn off the load when the output voltage of manual stabiliser exceeds a certain preset high voltage limit. The output voltage may become high due to the rise in AC mains voltage or due to improper selection by the rotary switch on manual stabiliser. One of the major disadvantage of using a manual stabiliser in areas with a wide range of voltage fluctuations is that one has to keep a watch on the manual stabiliser’s output voltage that is displayed on a voltmeter and keep changing the same using its rotary switch. Or else,

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the output voltage may reach the preset auto-cut-off limit to switch off the load without the user’s knowledge. To turn on the load again, one has to readjust the stabiliser voltage using its rotary switch. Such operation is very irritating and inconvenient for the user. This under-/over-voltage audio alarm circuit designed as an add-on circuit for the existing manual stabilisers overcomes the above problem. Whenever the stabiliser’s output voltage falls below a preset low-level voltage or rises above a preset high-level voltage, it produces different beep sounds for ‘high’ and ‘low’ voltage levels—short-duration beeps with short intervals between successive beeps for ‘high’ voltage level and slightly longer-duration beeps with longer interval between successive beeps for ‘low’ voltage level. By using these two different types of beep sounds one can readily readjust the stabiliser’s AC voltage output with the help of the rotary switch. There is no need of frequently checking voltmeter reading. It is advisable to preset the highlevel voltage 10V to 20V less than the required high-voltage limit for auto-cut-off operation. Similarly,

for low level one may preset lowlevel AC voltage 20V to 30V above minimum operating voltage for a given load. The primary winding terminals of step-down transformer X1 are connected to the output terminals of the manual stabiliser. Thus, 9V DC available across capacitor C1 will vary in accordance with the voltage available at the output terminals of the manual stabiliser, which is used to sense high or low voltage in this circuit. Transistor T1 in conjunction with zener diode ZD1 and preset VR1 is used to sense and adjust the high-voltage level for beep indication. Similarly, transistor T2 along with zener ZD2 and preset VR2 is used to sense and adjust low voltage level for beep indication. When the DC voltage across capacitor C1 rises above the preset high-level voltage or falls below the preset low-level voltage, the collector of transistor T2 becomes high due to non-conduction of transistor T2, in either case. However, if the DC voltage sampled across C1 is within the preset high- and low-level voltage, transistor T2 conducts and its collector voltage gets pulled to the ground level. These changes in the collector voltage of transistor T2 are used to start or stop oscillations in the astable multivibrator circuit that is built around transistors T3 and T4. The collector of transistor T4 is connected to the base of

buzzer driver transistor T5 through resistor R8. Thus when the collector voltage of transistor T4 goes high, the buzzer sounds. Preset VR3 is used to control the volume of buzzer sound. In normal condition, the DC voltage sampled across capacitor C1 is within the permissible window voltage zone. The base of transistor T3 is pulled low due to conduction of diode D2 and transistor T2. As a result, capacitor C2 is discharged. The astable multivibrator stops oscillating and transistor T4 starts conducting because transistor T3 is in cut-off state. No beep

sound is heard in the buzzer due to conduction of transistor T4 and non-conduction of transistor T5. When the DC voltage across capacitor C1 goes above or below the window voltage level, transistor T2 is cut off. Its collector voltage goes high and diode D2 stops conducting. Thus there is no discharge path for capacitor C2 through diode D2. The astable multivibrator starts oscillating. The time period for which the beep is heard and the time interval between two successive beeps are achieved with the help of the DC supply voltage, which is

low during low-level voltage sampling and high during high-level voltage sampling. The time taken for charging capacitors C2 and C3 is less when the DC voltage is high and slightly greater when the DC voltage is low for astable multivibrator operation. Thus during low-level voltage sensing the buzzer beeps for longer duration with longer interval between successive beeps compared to that during high-voltage level sensing. This circuit can be added to any existing stabiliser (automatic or manual) or UPS to monitor its performance.

Ultra-Sensitive Solidstate Clap Switch Pradeep G.

H

ere is the circuit of a highly sensitive clap switch that can be operated from a distance of up to 10 metres from the microphone. Signals picked up by the microphone are amplified by transistors T1, T2, and T3. Diode D1 detects clap signals and the resulting positive voltage is applied to the base of transistor T4. The output from

transistor T4 is further amplified by transistor T5, whose output is used to trigger a monostable multivibrator wired around the 555 timer (IC1). The output of IC1 is used as a clock for decade counter 4017 (IC2) that is wired as a divide-by-two counter. For each successive clap, transistor T6 conducts and cuts off alternately. As a result, for each

clap signal, the lamp is either switched ‘on’ or ‘off’. Triac 8T44A (or ST044) can drive load of up to 4-amp rating. The 12V DC for operation of the circuit is directly derived from the mains using rectifier diode D2, current-limiting resistor R16, and 12V zener ZD1 shunted by filter capacitor C7.

15-Step Digital Power Supply naveen thariyan

H

ere is a simple circuit to obtain variable DC voltage from 1.25V to 15.19V in reasonably small

steps as shown in the table. The input voltage may lie anywhere between 20V and 35V.

The first section of the circuit comprises a digital up-down counter built around IC1—a quad 2-input NAND ELECTRONICS PROJECTS Vol. 22

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schmitt trigger (4093), followed by IC2— a binary up-down counter (4029). Two gates of IC 4093 are used to generate up-down logic using push buttons S1 and S2, respectively, while the other two gates form an oscillator to provide clock pulses to IC2 (4029). The frequency of oscillations can be varied by changing the value of capacitor C1 or preset VR1. IC2 receives clock pulses from the oscillator and produces a sequential binary output. As long as its pin 5 is low, the counter continues to count at the rising edge of each clock pulse, but stops counting as soon as its pin 5 is brought to logic 1. Logic 1 at pin 10 makes the counter to count upwards, while logic 0 makes it count downwards. Therefore the counter counts up by closing switch S1 and counts down by closing switch S2. The output of counter IC2 is used to realise a digitally variable resistor. This section consists of four N/O reed relays that need just about 5mA current for their operation. (EFY lab note. The original circuit containing quad bilateral switch IC 4066 has been replaced by reed relays operated by transistorised switches because of unreliable operation of the former.) The switching action is performed using BC548 transistors. External resistors are connected in parallel with the reed relay contacts. If particular relay contacts are opened by the control input at the base of a transistor, the corresponding resistor across the relay contacts gets connected to the circuit. The table shows the theoretical output for various digital input combinations.

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Table Binary output 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

Equivalent dec no. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

LED4 R14 (W) Shorted Shorted Shorted Shorted Shorted Shorted Shorted Shorted 1500 1500 1500 1500 1500 1500 1500 1500

LED3 R13 (W) Shorted Shorted Shorted Shorted 820 820 820 820 Shorted Shorted Shorted Shorted 820 820 820 820

The measured output is nearly equal to the theoretically calculated output across regulator IC3 (LM317). The output voltage is governed by the following relationship as long as the input-to-output differential is greater than or equal to 2.5V: Vout = 1.25(1+R2'/R1') Where, R1' = R15 = 270 ohms (fixed)

LED2 R12 (W) Shorted Shorted 470 470 Shorted Shorted 470 470 Shorted Shorted 470 470 Shorted Shorted 470 470

LED1 R11 (W) Shorted 220 Shorted 220 Shorted 220 Shorted 220 Shorted 220 Shorted 220 Shorted 220 Shorted 220

R2' (W) 0 220 470 690 820 1040 1290 1510 1500 1720 1970 2190 2390 2540 2790 3010

Vout (V) 1.25 2.27 3.43 4.44 5.05 6.06 7.22 8.24 8.19 9.21 10.37 11.39 11.99 13.01 14.17 15.19

and R2' = R11 + R12 + R13 + R14 = 220 + 470 + 820 +1500 ohms = 3,010 ohms (with all relays energised) One can use either the binary weighted LED display as indicated by LED1 through LED4 in the circuit or a 74LS154 IC in conjunction with LED5

through LED20 to indicate one of the 16 selected voltage steps of Table I. The input for IC4 is to be tapped from points marked ‘A’ through ‘D’ in the figure. This arrangement can be used to replace the LED arrangement at points A, B, C, and D. This 74LS154 IC is a decoder/demultiplexer that senses the output of IC2 and accordingly activates only one of its 16 outputs in accordance with the count value. LEDs at the output of this IC can be arranged in a circular way along side the corresponding voltages.

Working When the power is switched on, IC2 resets itself, and hence the output at pins 6, 11, 14, and 12 is equivalent to binary zero, i.e. ‘0000’. The corresponding DC output of the circuit is minimum (1.25V). As count-up switch S1 is pressed, the binary count of IC2 increases and the output starts increasing too. At the highest count output of 1111, the output voltage is 15.19V (assuming the in-circuit resistance of preset VR2 as zero). Preset VR2 can be used for

trimming the output voltage as desired. To decrease the output voltage within the range of 1.25V to 15.2V, count-down switch S2 is to be depressed. Notes. 1. When relay contacts across a particular resistor are opened, the corresponding LED glows. 2. The output voltages are shown assuming the in-circuit resistance of preset VR2 as zero. Thus when the in-circuit resistance of preset VR2 is not zero, the output voltage will be higher than that indicated here.

Microphone for Computer Vyjesh M.V.

B

uying a microphone for a computer is costly. Especially when there is a need to have two microphones—one for modem and another for sound card—or if the present microphone is not working properly and needs to be replaced, you are likely to feel the burden of extra cost. Here is a low-cost microphone circuit that comes within your budget. All sound cards and modems have a socket for microphone that is in compatible with stereo jack pins. The stereo socket takes condenser microphone as input and

provides the necessary positive voltage for a condenser microphone. Before building the full circuit, connect three wires to the jackpin, switch on the computer, and insert the jack pins; into the socket of the

sound card. With the help of a multimeter, find out the positive terminal out of the three wires. There exists a potential difference of 4V or so between the positive and ground terminals. The third terminal will obviously be for the signal input. The positive terminal is used for biasing the condenser microphone. After identifying all the terminals, connect them as shown in the accompanying circuit diagram.

Versatile Zener Diode Tester k. udhaya kumaran

Z

ener diodes available in the market are specified according to their breakdown voltage as well as tolerance. The tolerance may vary from 5 per cent to 20 per cent. The circuit of a versatile zener diode tester presented here enables you to verify the specified breakdown voltage and tolerance values. In addition, you can check the dynamic impedance of

a zener diode. The dynamic impedance characteristics of a zener diode determine as to how well the zener diode regulates its own breakdown voltage. Thus this circuit can be used to compare the dynamic impedance characteristics of zener diodes from a lot and segregate/categorise them accordingly.

For full-fledged zener diode testing you will have to refer to the manufacturer’s datasheet to check zener diode parameters such as zener voltage, power, and current (maximum/nominal) ratings. In addition, temperature coefficient and dynamic impedance have also to be checked if zener diode is to be used for critical functions such as voltage reference ELECTRONICS PROJECTS Vol. 22

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Table I Minimum and Maximum Test Current Values Zener diode values IT(min) IT(max) 3.3V to 4.3V 10mA 15mA 4.7V to 18V 5mA 10mA 20V to 39V 2mA 4mA Note: Zener diode power ratings are 250 mW, 400 mW, and 500 mW.

Table II Minimum and Maximum Test Current Values Zener diode values IT(min) IT(max) 3.3V to 12V 10mA 15mA 13V to 27V 5mA 10mA 30V to 43V 2mA 5mA 47V to 75V 1.5mA 3mA 82V to 120V 1mA 2mA Note: Zener diode power rating is 1 watt.

for digital voltmeters, control systems, and precision power-supply circuits. However, for a common hobbyist it is not necessary to check zener diodes critically, and only checking its dynamic impedance characteristic is sufficient. Dynamic impedance implies the degree of change in a zener diode’s voltage with the change in current. Expressed in ohms, it equals the small change in zener voltage divided by the corresponding change in zener current (centered around the test current figure prescribed in datasheets by manufacturers). From datasheets it is observed that test current value is high for low-voltage zener diodes and low for higher-voltage zener diodes. However, the dynamic impedance value will be low for low-voltage zener diodes and vice versa for higher-voltage zener diodes. To test 3.3V to 120V zener diodes by the practical dynamic impedance method, you need to have a variable voltage (0 to above 120V) and current (1 mA to 150 mA) supply source. Designing this type of

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power supply is quite complicated and is prone to damage if excess current is drawn accidentally. The zener diode tester circuit presented here has been designed considering the above factors. It is capable of testing zener diodes of breakdown voltage ratings of upto 120V and wattage ratings of 250 mW, 400 mW, 500 mW, and 1W. The circuit can be deployed in quicktest mode as also in quality-test mode of operation. In quick-test mode, you can perform a rough check of zener diode’s breakdown voltage up to 47 volts. In quality-test mode, you can check dynamic impedance characteristic for zener diodes from 3.3V to 120V. Commonly available step-down transformers X1 and X2 (230V AC primary to 9V AC, 750 mA sec. each) are connected back-to-back as shown in the figure. A bridge rectifier followed by filter capacitor C1 converts the output from X2 transformer to DC. Neon lamp L1 indicates the presence of higher DC voltage (220V approximately) across capacitor C1, which is used to test various zener diode values from 3.3V to 120V. An advantage of using this highvoltage circuit is that the current gets restricted to a low value. It delivers only 3 mA (approx.) when testing zener diodes with higher breakdown values (e.g. 120V zener diode), but while testing zener diodes of low breakdown values, such as 3.3V, it delivers a current slightly above 20 mA. Such power-supply characteristics suit our requirement, as stated earlier. Since a small current is used for testing of zener diodes, there is no danger of zener diodes getting damaged during testing using the dynamic impedance method. Before using the circuit, check DC voltage across test terminals A and B without connecting any zener diode and then flip toggle switch S2 to quick-test position. DC voltage available across terminals A and

B will be around 200V DC. Now put toggle switch to quality-test position. DC voltage can now be adjusted from 6V DC to 200V DC (approx.) with the help of potentiometer VR1. After these preliminary checks, the circuit is ready for operation. To test zener diode by quick-test method, connect zener diode across terminals A and B and flip switch S1 to ‘on’ position. Note down DC voltage in digital multimeter M2, which is the rough breakdown voltage. In quick-test method you can test zener diode values up to 47 volts safely. For higher-value zener diodes you will have to increase the value of resistor R3 suitably. If zener diode presents a short, digital multimeter M2 will read ‘0’ volts. To perform quality test on the same zener diode, turn switch S1 ‘off’ and remove zener diode from across terminals A and B. Now turn switch S1 ‘on’ and adjust potentiometer VR1 to obtain DC voltage (on digital multimeter) across terminals A and B equal to the one found during quick test method. Now keep potentiometer VR2 in mid position and connect zener diode across terminals A and B. (Note. Before testing zener diode, refer Table I and Table II for the minimum test current (ITmin) and maximum test current (ITmax) required for various zener diode values, depending upon their wattage rating.) Test current is adjusted using potentiometer VR2 and measured using meter M1 (A 0-25mA analogue milliampere meter or a 0-20mA digital multimeter can be used.) Now adjust potentiometer VR2 and note down changes in zener voltage during ITmin and ITmax conditions. If the required current is not available, increase DC voltage by adjusting potentiometer VR1 suitably. While changing test current from ITmin to ITmax, the voltage variation across zener diode should be less than 1 volt for lower-value zener diodes and a few volts for higher-value zener diodes. A voltage variation of more than this value indicates that zener diode is not properly regulating. When comparing zener diodes of same values, the zeners showing less voltage deviation would regulate better.

DTMF Proximity Detector k.s. sankar

A

DTMF-based IR transmitter and receiver pair can be used to realise a proximity detector. The circuit presented here enables you to detect any object capable of reflecting the IR beam and moving in front of the IR LED photodetector pair up to a distance of about 12 cm from it. The circuit uses the commonly available telephony ICs such as dial-tone generator 91214B/91215B (IC1) and DTMF decoder CM8870 (IC2) in conjunction with infrared LED (IR LED1), photodiode D1, and other components as shown in the figure. A properly regulated 5V DC power supply is required for operation of the circuit. The transmitter part is configured around dialer IC1. Its row 1 (pin 15) and column 1 (pin 12) get connected together via transistor T2 after a power-on delay

(determined by capacitor C1 and resistors R1 and R16 in the base circuit of the transistor) to generate DTMF tone (combination of 697 Hz and 1209 Hz) corresponding to keypad digit “1” continuously. LED 2 is used to indicate the tone output from IC3. This tone output is amplified by Darlington transistor pair of T3 and T4 to drive IR LED1 via variable resistor VR1 in series with fixed 10-ohm resistor R14. Thus IR LED1 produces tone-modulated IR light. Variable resistor VR1 controls the emission level to vary the transmission range. LED 3 indicates that transmission is taking place. A part of modulated IR light signal transmitted by IR LED1, after reflection from an object, falls on photodetector diode D1. (The photodetector is to be

shielded from direct IR light transmission path of IR LED1 by using any opaque partition so that it receives only the reflected IR light.) On detection of the signal by photodetector, it is coupled to DTMF decoder IC2 through emitterfollower transistor T1. When the valid tone pair is detected by the decoder, its StD pin 15 (shorted to TOE pin 10) goes ‘high’. The detection of the object in proximity of IR transmitterreceiver combination is indicated by LED1. The active-high logic output pulse (terminated at connector CON1, in the figure) can be used to switch on/off any device (such as a siren via a latch and relay driver) or it can be used to clock a counter, etc. This DTMF proximity detector finds applications in burglar alarms, object counter and tachometers, etc.

Stepper Motor Control Jaydip appasaheb dhole

A

simple, low-cost hardwired step per motor control circuit that can be used in low-power applications,

such as moving toys etc is presented here. The circuit comprises a 555 timer IC configured as an astable multivibrator

with approx. 1Hz frequency. The frequency is determined from the following relationship: ELECTRONICS PROJECTS Vol. 22

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Frequency = 1/T = 1.45/(RA + 2RB)C Where RA = RB = R2 = R­3 = 4.7 kilo-ohm and C = C2 = 100 µF. The output of timer is used as clock for two 7474 dual ‘D’ flip-flops (IC2 and IC3) configured as a ring counter. When power is initially switched on, only the

first flip-flop is set (i.e. Q output at pin 5 of IC2 will be at logic ‘1’) and the other three flip-flops are reset (i.e. their Q outputs will be at logic ‘0’). On receipt of a clock pulse, the logic ‘1’ output of the first flip-flop gets shifted to the second flipflop (pin 9 of IC2). Thus with every clock

pulse, the logic ‘1’ output keeps shifting in a ring fashion. Q outputs of all the four flip-flops are amplified by Darlington transistor arrays inside ULN2003 (IC4) and connected to the stepper motor windings marked ‘A’ through ‘D’ in the figure. The common point of the winding is connected to +12V DC supply, which is also connected to pin 9 of ULN2003. The colour code used for the windings is shown in the figure. When the power is switched on, the control signal connected to SET pin of the first flip-flop and CLR pins of the other three flip-flops goes active ‘low’ (because of the power-on-reset circuit formed by R1-C1 combination) to set the first flip-flop and reset the remaining three flip-flops. On reset, Q1 of IC2 goes ‘high’ while all other Q outputs go ‘low’. External reset can be activated by pressing the reset switch. By pressing the reset switch, you can stop the stepper motor. On releasing the reset switch, the stepper motor again starts moving further in the same direction.

Low-Cost Intercom Pradeep G.

T

150

he intercom circuit described here uses two transistors, an audio transformer, and a few passive

ELECTRONICS PROJECTS Vol. 22

components in addition to condenser microphone and low-wattage speaker (refer Fig. 1). The complete unit can be made on a general-purpose veroboard. The microphone signals are amplified by a two-stage transistor amplifier, while the speaker is driven through an audio output transformer (similar to the one used in transistor radios). When ring button (pushto-on switch S1) is pressed, capacitor

C3 gets connected between the base of transistor T2 and the top end of primary winding of audio output transformer. As a result, the amplifier circuit wired around transistor T2 gets converted into a Hartley oscillator and produces an audible tone for call-bell. To build a two-way intercom set, make two identical units with the speaker of each circuit installed near the other unit as shown in Fig. 2.

High-power Car Bat tery Eliminator T.K. Hareendran

T

o operate car audio (or video) system from household 230V AC mains supply, you need a DC adaptor. DC adaptors available in the market are generally costly and supply an unregulated DC. To overcome these problems, an economical and reliable circuit of a high-power, regulated DC adaptor using reasonably low number of components is presented here. Transformer X1 steps down 230V AC mains supply to around 30V AC, which is then rectified by a bridge rectifier comprising 1N5406 rectifier diodes D1 through D4. The rectified pulsating DC is smoothed by two 4700µF filter capacitors C1 and C2. The next part of the circuit is a seriestransistor regulator circuit realised using high-power transistor 2N3773 (T1). Fixedbase reference for the transistor is taken from the output pin of 3-pin regulator IC1 (LM 7806). The normal output of IC1 is raised to about 13.8 volts by suitably biasing its common terminal by components ZD1 and LED1. This simple arrangement provides good, stable voltage reference at a low cost. LED1 also works as an output indicator. Finally, a crowbar-type protection

Readers’ comments:  The way of biasing of transistor T1 seems to be wrong. As per the datasheet, the transitor T1 should be of pnp type and connected as shown in Fig. 1 here. Chittaranjan Parida Cuttack The author, T.K. Hareendran, replies: As clearly mentioned in the text, transistor T1 works as a series regulator transistor. In conventional circuits, one ordinary zener diode is used to provide a fixed-base bias to T1. The circuit published is an improved version and the fixed three-terminal voltage

x1

GND

circuit is added. If the output voltage exceeds 15V due to some reason such as component failure, the SCR fires because of the breakdown of zener ZD2. Once SCR fires, it presents a short-circuit across the

Fig. 1: Reader’s modification to battery eliminator circuit

regulator IC1 is used to increase the efficiency and lifetime. The schematic of a conventional series regulator is shown in Fig. 2. The circuit indicated by you, based on NSC datasheet, is in fact a current booster. Here T1 plays the

unregulated DC supply, resulting in the blowing of fuse F1 instantly. This offers guaranteed protection to the equipment connected and to the circuit itself. This circuit can be assembled using a small general-purpose PCB. A goodquality heat-sink is required for transistor T1. Enclose the complete circuit in a readymade big adaptor cabinet as shown in the figure.

Fig. 2: Schematic of a conventional series regulator

role of a bypass transistor. At low load currents, all the load current is provided by IC1 and whenever the load current is increased beyond a preset value (decided by R) transistor T1 comes into picture and supplies all the excess current. ELECTRONICS PROJECTS Vol. 22

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Automatic Plant Irrigator priyank Mudgal

T

he circuit presented here waters your plants regularly when you are out for a vacation. The circuit comprises a sensor part built using only one op-amp (N1) of quad op-amp IC LM324. Op-amp N1 is configured here as a comparator. Two stiff copper wires are inserted in the soil containing plants. As long as the soil is wet, conductivity is maintained and the circuit remains off. When the soil dries out, the resistance between the copper wires (sensor probes A and B) increases. If the resistance increases beyond a preset limit, output pin

1 of op-amp N1 goes ‘low’. This triggers timer IC2 (NE 555) configured as a monostable multivibrator. As a result, relay RL1 is activated for a preset time. The water pump starts immediately to supply water to the plants. As soon as the soil becomes sufficiently wet, the resistance between sensor probes decreases rapidly. This causes pin 1 of op-amp N1 to go ‘high’. LED1 glows to indicate the presence of adequate water in the soil. The threshold point at which the output of op-amp N1 goes ‘low’ can be changed with the help of preset VR1.

To arrange the circuit, insert copper wires in the soil to a depth of about 2 cm, keeping them 3 cm apart. When the soil gets dried, adjust VR1 towards ground rail until LED1 turns off and relay RL1 is energised. The motor starts pumping the water. LED1 glows up as the water reaches the probes. For small areas a small pump such as the one used in air coolers is able to pump enough water within 5 to 6 seconds. The timing components for IC2 are selected accordingly. The timing can be varied with the help of preset VR2. The circuit is more effective indoors if one intends to use it for long periods. This is because the water from reservoir (bucket, etc) evaporates rapidly if it is kept in the open. For regulating the flow of water, either a tap can be used or one end of a rubber pipe can be blocked using M-seal compound, with holes punctured along its length to water several plants.

Simple telephone ring tone generator K. Udhaya Kumaran, VU3GTH

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ere is a simple telephone ring tone generator circuit designed using only a few components. It produces simulated telephone ring

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tone and needs only DC voltage (4.5V DC to 12V DC). One may use this circuit in ordinary intercom or phonetype intercom. The sound is quite

loud when this circuit is operated on +12V DC power supply. However, the volume of ring sound is adjustable. The commonly available 14-stage

+

binary ripple counter with built-in oscillator (CMOS IC CD4060B) is used to generate three types of pulses, which are available from pin 1(O11), pin 3 (O13), and pin 14 (O7), respectively. Preset VR1 is adjusted to obtain 0.3125Hz pulses (1.6-second ‘low’ followed by 1.6-second ‘high’) at pin 3 of IC1. At the same time, pulses available from pin 1 will be of 1.25 Hz (0.4-second ‘low’, 0.4-second ‘high’) and 20 Hz at pin 14. The three output pins of IC1 are connected to base terminals of transistors T1, T2, and T3 through resistors R1, R2, and R3, re-

spectively. Transistors T1 through T3 are cascaded in such a way that the positive voltage available at the emitter of transistor T1 is extended to the collector of Transistor T3 when the outputs of all the three stages are low. As a result, transistors T1 through T3 are forward biased for 0.4, 1.6, and 0.025 seconds, respectively and reverse biased for similar durations. Using a built-in oscillator-type piezobuzzer produces around 1kHz tone. In this circuit, the piezo-buzzer is turned

‘on’ and ‘off’ at 20Hz for ring tone sound by transistor T3. 20Hz pulses are available at the collector of transistor T3 for 0.4-second duration. After a time interval of 0.4 second, 20Hz pulses become again available for another 0.4-second duration. This is followed by two seconds of nosound interval. Thereafter the pulse pattern repeats itself. Refer the figure that indicates waveforms available at various points including the collector of transistor T3. Preset VR2 can be used for adjusting the amplitude of the ring tone.

Dual-input high-fidelity audio mixer Prasad J.

Th

e circuit described here is based on the superior characteristics of dual-gate MOSFET (metaloxide semiconductor field-effect transistor). It exhibits a very high input impedance that lends for good sensitivity and very less loading of the input signal source. Low cross-modulation characteristic leads to minimal distortion of the output with respect to the input signals. Also, the MOSFET offers low feedback capacitance and high transconductance. All these advantages make the MOSFET the most effective for high-quality mixer and converter applications. This dual-input audio frequency mixer circuit employs a single dual-gate

MOSFET 3N200. One may, however, substitute it with any other dual-gate

MOSFET such as 3N187 and BF966. (It is to be noted that BF966 is not gate-protected and hence calls for suitable precaution in handling it.) The audio frequency (AF) input from the first channel (CH1) is applied on gate 1 (G1) of the MOSFET through 500-kiloohm potentiometer VR1. The AF input from the second channel (CH2) is ELECTRONICS PROJECTS Vol. 22

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applied on gate 2 (G2) of the MOSFET through another 500-kilo-ohm potentiometer VR2. Potentiometers VR1 and VR2 serve as gain controls for the mixer inputs. Gate 1 receives the negative bias resulting from the voltage developed by the current passing through resistor R1 that is in series with the source. Gate 2 receives the positive bias produced across resistor R3 by the voltage divider formed by resistors R3 and R4. The mixed common output signal developed across drain load resistor R2 is coupled to the output through capacitor C5. This output can be, in turn, fed to any audio amplifier system for further

amplification. The input impedance at each signal input is approximately 500 kilo-ohm, which is determined largely by the resistance of potentiometers VR1 and VR2. Higher input impedance may be obtained by substituting higher-resistance potentiometers, but this will lead to the pickup of stray signals. The current drain of this circuit at 6V DC is less than 3 mA. The open-circuit voltage gain is 10 for each channel. The maximum amplitude of input signals at gates G1 and G2 is 0.1V RMS. Signals of higher amplitudes are reduced by the adjustment of potentiometers VR1 and VR2, hence evading the output signal peak-clipping. The corresponding output

signal amplitude is 1V RMS. The entire circuit can be built on a general-purpose PCB or veroboard. The complete assembly is shielded using a metal container. The two input jacks should be fixed on the opposite sides of the container against the output jack. This simple circuit can be utilised for various combinations of devices at the input end. A few examples are two microphones, two audio players, or one audio player and one microphone, etc. Note. Adequate precautions should be taken to prevent the destruction of MOSFET due to static electricity. The use of a grounded tip for the soldering iron is recommended.

Unipolar/bipolar triangular and bipolar square wave generator Yogesh Kataria

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he circuit given here is capable of generating unipolar and bipolar triangular waves as well as bipolar square waves. In unipolar mode, the output frequency is double that of bipolar mode (using identical component values). When switch S1 is closed, the circuit generates bipolar triangular as well as bipolar square waves, and when switch

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S1 is open, it generates unipolar triangular and bipolar square waves—both having double the frequency in the first case. Op-amp 301 acting as a comparator produces bipolar square wave with output swinging between +Vcc and –VEE. The square wave output is fed to op-amp 741 that is configured as an integrator to produce a triangular waveform. Figs 2 and 3 show the waveforms with switch S1 in closed and open positions, respectively, using 0.047µF capacitor C and in-circuit value of preset VR1 as 28 kilo-ohm. The circuit is capable of working on a few hertz to around 250kHz.

Anti-theft security for car audios T.K. Hareendran

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his small circuit, based on popular CMOS NAND chip CD4093, can be effectively used for protecting your expensive car audio system against theft. When 12V DC from the car battery is applied to the gadget (as indicated by LED1) through switch S1, the circuit goes into standby mode. LED inside optocoupler IC1 is lit as its cathode terminal is grounded via the car audio (amplifier) body. As a result, the output at pin 3 of gate N1 goes low and disables the rest of the circuit. Whenever an attempt is made to remove the car audio from its mounting by cutting its connecting wires, the optocoupler immediately turns off, as its LED cathode terminal is hanging. As a result,

the oscillator circuit built around gates N2 and N3 is enabled and it controls the ‘on’/‘off’ timings of the relay via transistor T2. (Relay contacts can be used to energise an emergency beeper, indicator, car horns, etc, as desired.) Different values of capacitor C2 give different ‘on’/‘off’ timings for relay RL1 to be ‘on’/‘off’. With 100µF we get approximately 5 seconds as ‘on’ and 5 seconds as ‘off’ time. Gate N4, with its associated components, forms a self-testing circuit. Normally, both of its inputs are in ‘high’ state. However, when one switches off the ignition key, the supply to the car audio is also disconnected. Thus the output of gate N4 jumps to a ‘high’ state and it provides a differentiated short pulse to forward bias

transistor T1 for a short duration. (The combination of capacitor C1 and resistor R5 acts as the differentiating circuit.) As a result, buzzer in the collector terminal of T1 beeps for a short duration to announce that the security circuit is intact. This ‘on’ period of buzzer can be varied by changing the values of capacitor C1 and/or resistor R5. After construction, fix the LED and buzzer in dashboard as per your requirement and hide switch S1 in a suitable location. Then connect lead A to the body of car stereo (not to the body of vehicle) and lead B to its positive lead terminal. Take power supply for the circuit from the car battery directly. Caution. This design is meant for car audios with negative ground only.

Readers’ comments:  The oscillator circuit built around gates N1, N2, and N3 is not working. The circuit works like a timer when pin 2 of MCT2E is connected to the body of the audio. There is no change in the performance when pin 2 is left hanging. But when pins 2 and 4 of MCT2E are shorted, the circuit starts working.

Also clarify the use of R3 in the circuit and show pin connections of MCT2E. D. Mohan Kumar, Trivandrum The author, T.K. Hareendran, replies: Carefully recheck your assembled circuit. No modification/corrections Fig. 1: MCT2E pin are required in the published de- configuration

sign. The fault indicated may be due to a faulty optocoupler (MCT2E). Resistor R3 (100 kiloohm) is used to pull down the input of gate N1 (wired as inverter) in active state. The pin configuration of MCT2E is shown in Fig. 1.

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PC-based dial clock-cumelectronic roulette Vijaya Kumar P.

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his hardware-cum-software project is meant to control hardware through software. The hardware using LEDs to simulate both dial clock and electronic roulette is rather simple. Of the two 4-line-to-16-line decoders used in the circuit, the first (IC1) drives ‘hour LEDs’ and the other (IC2) drives ‘minute LEDs.’ These decoders are interfaced directly to the PC’s printer port provided on its backside. Data output lines D0 to D3 (pins 2 through 5 of 25-pin ‘D’ connector) of the printer port are connected to four address inputs of the decoder used for minute

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display, while data output lines D4 to D7 (pins 6 through 9) are connected to four data inputs of the decoder used for hour display. Since the outputs of these decoders are active-low, the positive terminals of LEDs are made common. This obviates the need to use additional inverters. In accordance with 4-bit binary address at inputs A through D of decoders, only one of the 16 outputs at a time goes active-low to light the corresponding LED. Since a dial clock requires only 12 LEDs, only 12 of 16 outputs of 74154 decoders are used in this circuit. Only the

minute decoder (IC2) is used for electronic roulette. The dial clock and electronic roulette functions, which can be selected via the software program, are explained below: Dial clock. When dial clock is selected, system time is displayed on the LED panel. The hour-indicating LED glows continuously, while minute-indicating LED blinks for each odd second (i.e. 1, 3, 5, ...., and so on). The clock incorporates hourly chime and alarm setting features. Chime and alarm sound can be distinguished from the duration for which it will sound. Electronic roulette. Roulette is a game of chance that basically comprises a circular wheel divided into a number of sectors that are numbered serially and a pointer. There exists a relative motion between the pointer and the wheel. The rotation is initiated by mechanical means. The wheel is allowed to stop itself and the number indicated by the pointer decides the winner. This game can also be arranged electronically by using sequential running lights, which will simulate the rotating wheel, and making them to stop at random position. The chance of a number to be winner is 1 out of 12 in the PC-based electronic roulette explained here. The software for dial clock and electronic roulette is written in ‘C’ language. For simulation of dial clock, the software uses gettime () function to read time from the computer, which is then stored in a variable. This time is written into

the parallel-port as 8-bit binary number by using outportb () function. To make minute-indicating LED to blink, minute variable is multiplied by (second % 2). The multiplication result comes out to be 1 for odd seconds and 0 for even seconds. i=sc%2; mn=mn*i; The binary equivalent of minute variable is written into data pins D0 to D4 of the parallel-port. Hour variable is multi-

plied by 16 to write the binary equivalent of hour into data pins D5 to D7. Outportb (0x0378,ho*16+mn); Simulation of roulette wheel is quite simple. The software uses a decimal number (1 through 13) generator whose binary equivalents are written into data pins D0 to D4 of the parallel port using outputb () function. The roulette can be reset by initialising decimal number generator that simulates running lights.

#include <stdio.h> #include <dos.h> #include <stdlib.h> #define PORT 0x0378 main() { int k=0; clrscr(); gotoxy(30,10); printf(“1.(D)ial Clock\n”); gotoxy(30,12); print(“2.(R)un Electronic Roulette \n”); gotoxy(30,14); printf(“3.(E)xit\n”); do { k=getch(); k=touchper(k); if(k==’D’) { Aclock(0,0,0); } if(k==‘R’) { Roulet(); } } while(k!=‘E’); clrscr(); print(“By Vijaya kumar.P,3rd Sem,E&C, K.V.G.C.E,Sullia\n”); printf(“Dedicated to Father of Electricity Michael Faraday who is my favourite Scientist.\n”); exit(0); } Aclock(int shor,int smin,int ssec) { int ho,sc,mn,mnt,k,i=0; struct time tim; clrscr(); do { gettime(&tim); gotoxy(30,8); ho=tim.ti_hour; mn=tim.ti_min; sc=tim.ti_sec; mnt=mn;

if(ho>12) { ho=ho-12; } if(ho==0) { ho=12; } i=sc % 2; mn=mn*i; /*Making minute LED to blink*/ mn=mn/5; outportb(PORT,ho*16+mn); printf(“hour:min:sec = %2d:%02d:02d\n”,ho,mnt,sc); gotoxy(30,10); printf(“1.(G)oto MAIN MENU\n”); gotoxy(30,12); printf(“2.(S)et Alaram\n”); if(shor==ho&&smin==mnt&&ssec==sc) { alarm(15); } if(mnt==0&&sc==0) { alarm(1); } if(bioskey(1))/* To check Whether any keyis pressed */ k=getch(); k=toupper(k); if(k==‘S’) { setala(); } } while(k!=‘G’); { outportb(PORT,0); main(); } } setala() /*Function to set Alarm*/ { int hrs,mns,scs; clrscr(); printf(“Enter hour\n”); scanf(“%d” ,&hrs); printf(“Enter Minute\n”);

The decimal number generator can be stopped at random for play. The speed of running can be adjusted by using delay () function. The delay time has to be selected appropriately, as it should not be either too low or too high. Keeping the delay time very low is undesirable, since it will cause continuous glowing of LEDs. Similarly, a very high delay time is also undesirable, since the player can stop the wheel at his winning position.

DialCLK.C scanf(“%d” ,&mns); printf(“Enter seconds\n”); scanf(“%d” ,&scs); Aclock(hrs,mns,scs); } alarm(int beps) /*Function to produce beeping sound*/ { int i; for(i=0;i
Long-Range Cordless Burglar Alarm t.k. hareendran

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his long-range cordless burglar alarm circuit makes use of a cordless telephone (CLT) unit with paging facility and a few low-cost discrete components. The circuit is so simple that even a novice can easily construct it without any difficulty. When the ‘page’ button on a CLT is

pressed and held in that position, the handset starts beeping to indicate that somebody is calling. This function is used here to build the gadget. The system consists of three sub-assemblies: 1. Wireless beeper. The handset of the CLT. 2. Infrared transmitter. A number

Fig. 1

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Fig. 2

of IR transmitter circuits based on the well-known 555 chip have been published earlier in EFY. Just select one circuit with a modulating frequency of 36 to 38 kHz and assemble it on a veroboard. After that, enclose it in a proper cabinet. (EFY note. A typical IR transmitter circuit used during testing is shown in Fig. 1.) 3. Infrared receiver-cum-control unit. The circuit diagram of this unit is shown in Fig. 2. Front end of this block is TSOP1736 infrared receiver module. This module can demodulate 36kHz modulated IR beam to produce an active-similar ‘low’ output. You may also use any other module, provided it has an active-‘low’ output. The modulated IR beam from the transmitter is received by the receiver module and its output at pin 2 goes ‘low’. The rest of the circuit is in sleep mode as it does not get power for its operation. The SCR here plays the role of an electronic switch.

Fig. 3

When the infrared beam is interrupted, the output of the receiver module goes ‘high’ to apply a forward bias to the base of transistor T1. As a result, the gate of SCR gets sufficient forward bias to conduct (and

latch). The astable multivibrator built around IC1 starts working to control the ‘on’/‘off’ relay timings. Diode D1 prevents the relay from latching and diode D2 works as a free-wheeling diode. Normally open (N/O) contacts of the relay are used to close the ‘page’ button contacts until the circuit is reset by pressing pushto-off switch S1 (N/C type). One may replace switch S1 with a key-lock switch to avoid its unauthorised operation. The astable circuit helps the handset user to distinguish between a normal paging call and an intrusion warning alarm. After construction, fix the transmitter and receiver modules at opposite sides in the door frame as shown in Fig. 3. Carefully open the CLT and solder two wires to the ‘page’ button terminals with their free ends connected to the relay contacts (N/O). Now your cordless burglar alarm with a wireless monitoring range of about 500 metres (actual range is based on the CLT’s paging range) is ready to detect an intruder. EFY note. The author has successfully tested his prototype with the following CLT makes: 1. Panasonic KX-T 3611 BH (made in Japan) 2. Panaphone WT-3990 (made in China) 3. Citizen JRT-5400 (made in India)

Water-Level Controller joydeep kumar chakraborty

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n most houses, water is first stored in an underground tank (UGT) and from there it is pumped up to the ELECTRONICS PROJECTS Vol. 22

overhead tank (OHT) located on the roof. People generally switch on the pump when their taps go dry and switch off the pump

when the overhead tank starts overflowing. This results in the unnecessary wastage and sometimes non-availability of

water in the case of emergency. The simple circuit presented here makes this system automatic, i.e. it switches on the pump when the water level in the overhead tank goes low and switches it off as soon as the water level reaches a pre-determined level. It also prevents ‘dry run’ of the pump in case the level in the underground tank goes below the suction level. In the figure, the common probes connecting the underground tank and the overhead tank to +9V supply are marked ‘C’. The other probe in underground tank, which is slightly above the ‘dry run’ level, is marked ‘S’. The low-level and high-level probes in the overhead tank are marked ‘L’ and ‘H’, respectively. When there is enough water in the underground tank, probes C and S are connected through water. As a result, transistor T1 gets forward biased and starts conducting. This, in turn, switches transistor T2 on. Initially, when the overhead tank is empty, transistors T3 and T5 are in cut-off state and hence pnp transistors T4 and T6 get forward biased via resistors R5 and R6, respectively. As all series-connected transistors T2, T4, and T6 are forward biased, they conduct to energise relay RL1 (which is also connected in series with transistors T2, T4, and T6). Thus the supply to the pump motor gets completed via the lower set of relay contacts (assuming that switch S2 is on) and the pump starts filling the overhead tank. Once the relay has energised, transistor T6 is bypassed via the upper set of contacts of the relay. As soon as the water level touches probe L in the overhead tank, transistor T5 gets forward biased and starts conducting. This, in turn, reverse biases transistor T6, which then cuts off. But since transistor T6 is bypassed through the relay contacts, the pump continues to run. The level of water continues to rise. When the water level touches probe H, transistor T3 gets forward biased and starts conducting. This causes reverse biasing of transistor T4 and it gets cut off. As a result, the relay de-energises and the pump stops. Transistors T4 and T6 will be turned on again only when the water level drops below the position of L probe. Presets VR1, VR2, and VR3 are to be adjusted in such a way that transistors T1, T3, and T5 are turned on when the water level touches probe pairs C-S, C-H,

and C-L, respectively. Resistor R4 ensures that transistor T2 is ‘off’ in the absence of any base voltage. Similarly, resistors R5 and R6 ensure that transistors T4 and T6 are ‘on’ in the absence of any base voltage. Switches S1 and S2 can be used to switch on and switch off, respectively, the pump manually. You can make and install probes on your own as per the requirement and facilities available. However, we are describing here how the probes were made for this prototype. The author used a piece of nonmetallic conduit pipe (generally used for domestic wiring) slightly longer than the depth of the overhead tank. The common wire C goes up to the end of the pipe through the conduit. The wire for probes L and H goes along with the conduit from the outside and enters the conduit through two small holes bored into it as shown

in Fig. 2. Care has to be taken to ensure that probes H and L do not touch wire C directly. Insulation of wires is to be removed from the points shown. The same arrangement can be followed for the underground tank also. To avoid any false triggering due to interference, a shielded wire may be used. ELECTRONICS PROJECTS Vol. 22

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Invisible Broken Wire Detector k. udhaya kumaran, vu3gth

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ortable loads such as video cameras, halogen flood lights, electrical irons, hand drillers, grinders, and cutters are powered by connecting long 2- or 3-core cables to the mains plug. Due to prolonged usage, the power cord wires are subjected to mechanical strain and stress, which can lead to internal snapping of wires at any point. In such a case most people go for replacing the core/cable, as finding the exact location of a broken wire is difficult. In 3-core cables, it appears almost impossible to

detect a broken wire and the point of break without physically disturbing all the three wires that are concealed in a PVC jacket. The circuit presented here can easily and quickly detect a broken/faulty wire and its breakage point in 1-core, 2-core, and 3-core cables without physically disturbing wires. It is built using hex inverter CMOS CD4069. Gates N3 and N4 are used as a pulse generator that oscillates at around 1000 Hz in audio range. The frequency is determined by timing components comprising resistors R3 and R4, and capacitor C1. Gates N1 and N2

are used to sense the presence of 230V AC field around the live wire and buffer weak AC voltage picked from the test probe. The voltage at output pin 10 of gate N2 can enable or inhibit the oscillator circuit. When the test probe is away from any high-voltage AC field, output pin 10 of gate N2 remains low. As a result, diode D3 conducts and inhibits the oscillator circuit from oscillating. Simultaneously, the output of gate N3 at pin 6 goes ‘low’ to cut off transistor T1. As a result, LED1 goes off. When the test probe is moved closer to 230V AC, 50Hz mains live wire, during every positive half-cycle, output pin 10 of gate N2 goes high. Thus during every positive half-cycle of the mains frequency, the oscillator circuit is allowed to oscillate at around 1 kHz, making red LED (LED1) to blink. (Due to the persistence of vision, the LED appears to be glowing continuously.) This type of blinking reduces consumption of the current from button cells used for power supply. A 3V DC supply is sufficient for powering the whole circuit. AG13 or LR44 type button cells, which are also used inside laser pointers or in LED-based continuity testers, can be used for the circuit. The circuit consumes 3 mA during the sensing of AC mains voltage. For audio-visual indication, one may use a small buzzer (usually built inside quartz alarm time pieces) in parallel with one small (3mm) LCD in place of LED1 and resistor R5. In such a case, the current consumption of the circuit will be

around 7 mA. Alternatively, one may use two 1.5V R6- or AA-type batteries. Using this gadget, one can also quickly detect fused small filament bulbs in serial loops powered by 230V AC mains. The whole circuit can be accommodated in a small PVC pipe and used as a handy broken-wire detector. Before detecting broken faulty wires, take out any connected load and find out the faulty wire first by continuity method using any multimeter or continuity tester. Then connect 230V AC mains live wire at one end of the faulty wire, leaving the other end free. Connect neutral terminal of the mains AC to the remaining wires at one end. However, if any of the remaining wires is also found to be faulty, then both ends of these wires are connected to neutral. For single-wire testing, connecting neutral only to the live wire at one end is sufficient to detect the breakage point. In this circuit, a 5cm (2-inch) long, thick, single-strand wire is used as the test probe. To detect the breakage point, turn on switch S1 and slowly move the test probe closer to the faulty wire, beginning with the input point of the live wire and proceeding towards its other end. LED1 starts glowing during the presence of AC voltage in faulty wire. When the breakage point is reached, LED1 immediately extinguishes due to the non-availability of mains AC voltage. The point where LED1 is turned off is the exact brokenwire point. While testing a broken 3-core rounded cable wire, bend the probe’s edge in the form of ‘J’ to increase its sensitivity and move the bent edge of the test probe closer over the cable. During testing avoid any strong electric field close to the circuit to avoid false detection.

Readers’ comments: ❏ I congratulate the author for giving a smart, useful, and compact circuit of ‘Invisible Broken Wire Detector’ in August issue. I have the following queries regarding this circuit:

1. How do gates N1 and N2 sense the presence of 230V AC field around the live wire? 2. What is meant by buffering weak AC voltage? 3. Why only 1kHz oscillator is used

for the detection of signal at pin 10 of IC 4069? 4. While searching broken points in faulty wires, why the remaining wires are neutralised? Rajeev Mehndiratta, Rohtak

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❏ We used the circuit as continuity tester as follows: • Keep the finger in the positive terminal of the battery. • Touch the wire end that is to be checked. • Keep the test probe on the other end of the wire. • If the wire is good, the LED glows. Otherwise, the LED does not glow. In this regard, we require some clarification on the following: 1. In case the probe touches on AC 230V, what will happen to the circuit? 2. Shall we get a shock if we place our hand on the positive terminal of the battery? P. Ramesh and S. Ponkumar Tuticorin In reply to Rajeev Mehndiratta, the author, K. Udhaya Kumaran, states: 1. CMOS gate has a very high input impedance. When the test probe is placed closer to 230V AC electrical field, some AC voltage is induced in the test probe,

which is sufficient to make the output gate of N1 go low and the output gate of N2 go high. Thus during every positive cycle, pin 10 of gate N2 goes high, which activates the oscillator circuit. 2. As diode D1 conducts during every positive half cycle of AC, the DC voltage available at the cathode of diode D1 provides a very small source current, which is not able to control the oscillator circuit directly. When gates N1 and N2 are used as buffer (booster), the DC voltage available at pin 10 of IC1 is capable of enabling/inhibiting the oscillator circuit. 3. The 1kHz oscillator circuit is not used to detect signals at pin 10 of IC1, but it is used for providing audio-visual indication during sensing of 230V AC field. During rest of the period, the oscillator remains inhibited. 4. While connecting 230V AC phase point to the faulty wire, there is a possibility of voltage induction into the wires that are in close proximity to the faulty

wire. This may lead to false detection of broken wire point or no detection. By connecting remaining wires to neutral, there is no likelihood of false detection. Reply to P. Ramesh and S. Ponkumar: In the prototype circuit, single strand wire with sleeve was used as the probe, so there is no chance for the probe to come in direct contact with the mains AC voltage. However, nothing will happen if you use a naked wire as a probe that is in direct contact with mains live wire. If you touch the positive terminal of the battery, you won’t get any lethal electrical shock because resistance between the test probe and ground (resistor R1 + your body resistance) is more than 1 mega-ohm. As described by you using my circuit as continuity tester, some time testing continuity of long wire may not be reliable because my circuit show wrong continuity indication even if in between any part of wire are partially contacted or long wire may act as probe and sense any neighbouring electrical field.

PC-BASED MULTI-MODE LIGHT CHASER vijaya kumar p.

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or those who want to use their PC for various electronic functions, here is a circuit that converts a PC to a multi-mode light chaser. The advantage of this light chaser over other light chasers is that users can define their own patterns (designs) of running lights by altering the source program that requires a simple hardware. The program given here produces 24 different patterns of running light. The circuit shown in Fig. 1 is mainly used to physically isolate the PC hardware from the mains supply and to make it capable of driving 230V loads. The PC’s parallel port (LPT1) provided on its back is used to interface with the circuit. LPT port is terminated in a 25-pin ‘D’ type female connector. Its pin configuration is shown in Table I. Triacs are used to drive 230V bulbs.

Table I Pin Configuration Pin Description 1 *Strobe 2 Data bit 0 3 Data bit 1 4 Data bit 2 5 Data bit 3 6 Data bit 4 7 Data bit 5 8 Data bit 6 9 Data bit 7 10 Acknowledge 11 *Busy 12 Paper end 13 Select 14 *Auto feed 15 Error 16 Initialise 17 *Select input 18-25 Ground Note: *indicates that pins are internally (hardware) inverted.

Triac BT136 used here can take up a load of up to 800 watts. If you want to drive higher loads, BT136 (4A) can be replaced with triacs of higher current ratings, like BT139 (16A). Since we are using triacs to drive 230V bulbs, the mains supply would also appear on the PC. Optocouplers have been used to isolate the PC from 230V mains supply. The circuit can be assembled on a general-purpose dotted PCB and can be linked to the PC’s LPT port (female) usDecimal number 1 2 4 8 16 32 64 128

Binary equivalents 00000001 00000010 00000100 00001000 00010000 00100000 01000000 10000000

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input decimal number from the keyboard. The outportb() function directly outputs the binary equivalent of the decimal number to LPT1 (Port ID is 0x0378). For example, if the number entered is 15, its binary equivalent 00001111 is written into data pins D0 to D7 of the parallel port. To understand the logic of multi-mode light chaser, first consider the following simple sequential running light program:

miniature lamps, 50 miniature lamps must be connected in series and the net combination of 50 bulbs in series should be connected to each channel (channel 1 through channel 8). Since LEDs require very small current, parallel ports can directly drive LEDs. Software can be tested using simple hardware as shown in Fig. 2. C language provides a built-in outportb() function to output binary data to a hardware port. To understand this, let us consider the following program:

ing a 25-pin male ‘D’ type connector along with a 25-core cable. Instead of connecting 230V bulbs you can connect small 6.2V miniature lamps, which are easily available in electrical shops. Connections are shown in Fig. 3. While using 6.2V

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#include <dos.h> main() { int i; printf(“Input a decimal number”); scanf (“%d”,&i); outportb() (0x0378,i); }

The above program is used to output a binary equivalent of the decimal number entered. Scanf function is used to take the

#include <stdio.h> #include <dos.h> #include <math.h> main() { int temp=0,i,ch,PORT = 0x0378; printf(“Press x to exit”); run: for(i=0;i<8;i++) { tempb=pow(2,i); outportb(PORT,temp); /* outputs BINARY no. to LPT1 */ delay(2000); /*using delay to control speed */ if(bioskey(1))/*To check whether any key is pressed */ ch=getch(); ch=toupper(ch); if(ch==’X’) { exit(0); } } goto run; }

In the above program, ‘for’ loop is used to increment ‘i’ in steps of 1 up to 7. As these values of ‘i’ get substituted in temp=pow(2,i), numbers temp= 20=1, 21=2, 22 =4, 23=8, 24=16, 25=32, 26=64, and 27=128 are generated. The outportb() function is used to write binary equivalents of 1, 2, 4, 8, 16, 32, 64, and 128 to data pins D0 to D7, with D0 as the least significant digit and D7 as the most significant digit. The binary equivalents of numbers obtained by incrementing powers of 2 up to 7 are given below:

It is clear from the table that the resulting binary numbers will produce

the running light effect. Delay function defines the speed of running. ‘Go to’

statement is used to take control unconditionally to ‘for’ loop, so as to repeat the running process. By changing the formula of producing binary number patterns, one can get different actions. The multi-mode light chaser program is divided into a number of cases. Each case will produce two or more actions. These cases are made to switch automatically using switch statement and one ‘for’ loop. Further, by changing the delay time, one can increase or decrease the speed of running lights. EFY note. The complete source-code of multi-mode chaser lights in C language has been included along with the executable version of the program in the accompanied CD.

Fuse Status Indicators For Power-supplies M.K. CHANDRA MOULEESWARAN

F

use status indicators are very simple to construct using a few components. These go very nicely

with all sorts of power-supplies and other instruments that use power-supply sections. The logic and the formula, if any, used with each circuit/figure are shown in the corresponding truth tables. Fig. 1 shows the use of a 3-pin bi-colour LED. When the fuse is intact, both red and green parts of the LED are lit and the LED emits a yellow light. With the fuse in blown condition, only the bottom part of the LED gets the supply and therefore only the red part of the LED is lit. The formulae for working out the values of currentlimiting resistors for each colour Table I (refer Fig. 1) LED are shown Indicator Details in Table I. These Fuse status Bias to LED1 Colour of LED1 A1-red anode A2-green anode relationships are Intact Forward Forward Red+green=yellow applicable to the Blown Forward Nil Red circuits of Figs 1 Relationship to evaluate R1 and R2 in Figs 1 and 2: and 2. DCVin-VLED % ILED=R1 or R2 in ohms Fig. 2 emwhere Vin and VLED are in volts, ILED in amperes ploys an addiIn Fig. 2, VLED=VD2+VLED for flasher LED path tional flashing

LED in series with the red part of the bi-colour LED. So the fuse failure is indicated by the flashing of LED as well as

the red part of the bi-colour LED. Fig. 3 shows the use of a bi-colour LED in the AC mains supply circuit. The unique feature of this circuit is that just by altering the resistor values, it can be used in low-voltage AC circuits or DC circuits. The AC is converted into pulsating DC using rectifier diodes before appliELECTRONICS PROJECTS Vol. 22

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Table II (refer Fig. 2) Indicator Details Fuse status Bias to LED1 and LED2 LED2-anode A1-red anode A2 green anode Intact Forward Forward Forward Blown Forward Forward Nil

Colour of LED1 and LED2 Continuous green LED1+ flash red LED1+flash red LED2=green and yellow alternate+flash red Flashing red LED1 and LED2

Table III (refer Fig. 3) Indicator Details Bias to LED1 Colour of LED1 A1-red anode A2-green anode Intact Forward Forward Red+green=yellow at 50Hz flicker Blown Forward Nil Red at 50Hz flicker Note. Approximate DCVin at C of D1 or D2 is 200 volts Table IV (refer Fig. 4) Indicator Details Supply input Polarity of Polarity to LED1 the supply at P1 at P2 DC Forward Positive Negative Reverse Negative Positive AC Alternates Alternates at 50Hz/s at 50Hz/s

Colour of LED1 Red-continuous Green-continuous Alternates between red and green at 50 Hz and appears yellow

cation to the LED sections via currentlimiting resistors. For higher power dissipation in current-limiting resistors, a series combination of resistors can be used. Because of the pulsating voltage, the LEDs would produce a flickering effect. The total series resistance with each LED section may be calculated by dividing 200 volts with the desired LED current (say, 10 mA). Fig. 4 shows the use of a 2-pin bicolour LED. The two LEDs are internally connected in reverse, so they glow both ways of the supply polarity connections and can be easily used for AC circuits as indicators. The correct polarity is indicated by green and the reverse polarity by red. The AC supply is shown by yellow, which, in fact, is due to the turning ‘on’ of both the colours at the AC mains frequency. When the frequency is more than 20 Hz, the two colours combine to produce yellow light. Pin identification of this LED is done usually by using a current-limiting resistor and DC supply only. All the circuits can be effectively altered to suit an individual’s requirement.

A Hierarchical Priority Encoder dr bhaskara rao n.

A

normal priority encoder encodes only the highest-order data line. But in many situations, not only the highest but the second-highest priority information is also needed. The circuit presented here encodes both the highest-priority information as well as the second-highest priority information of an 8-line incoming data. The circuit uses the standard octal priority encoder 74148 that is an 8-line-to-3-line (4-2-1) binary encoder with active-‘low’ data inputs and outputs. The first encoder (IC1) generates the highest-priority value, say, F. The active-‘low’ output (A0, A1, A2) of IC1 is inverted by gates N9 through N11 and fed to a 3-line-to-8-line decoder (74138) that requires active-‘high’ inputs. The decoded outputs are active-‘low’. The decoder identifies the highest-priority data line and that

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data value is cancelled using XNOR gates (N1 through N8) to retain the secondhighest priority value that is generated by the second encoder. To understand the logic, let the incoming data lines be denoted as L0 to L7. Lp is the highest-priority line (active-‘low’) and Lq the second-highest priority line (active-‘low’). Thus Lp=0 and Lq=0. All lines above Lp and also between Lp and Lq (denoted as Lj) are at logic 1. All lines below Lq logic state are irrelevant, i.e. ‘don’t care’. Here p is the highest-priority value and q the second-highest-priority value. (Obviously, q has to be lower than p, and the minimum possible value for p is taken as ‘1’.) Priority encoder IC1 generates binary output F2, F1, F0, which represents the value of p in active-‘low’ format. The complemented F2, F1, and F0 are ap-

plied to 3-line-to-8-line (one out of eight outputs is active-‘low’) decoder 74138. Let the output lines of 74138 be denoted as M0 through M7. Now only one line is active-‘low’ among M0 through M7, and that is Mp (where the value of p is explained as above). Therefore the logic level of line Mp is ‘0’ and that of all other M lines ‘1’. The highest-priority line is cancelled using eight XNOR gates as shown in the figure. Let the output lines from XNOR gates be N0 through N7. Consider inputs Lp and Mp of the corresponding XNOR gate. Since Mp = 0 and also Lp = 0, the output of this XNOR gate is Np = complement of Lp = 1. All other L’s are not changed because the corresponding M’s are all 1’s. Thus data lines N0 through N7 are same as L0 through L7, except that the highest-priority level in L0 through

L7 is cancelled in N0 through N7. The highest-priority level in N0 through N7 is the second-highest priority leftover from L0 through L7, i.e. Nq=0 and Nj=1 for q<j≤7. Now these N lines are applied to priority encoder 2 (IC3) to generate S2, S1, S0, which represent q. Thus the second-highest priority value

is extracted. Through cascading one can recover the third-highest priority, and so on. For example, let L0 through L7 = X X X 0 1 1 0 1. Here the highest ‘0’ line is L6 and the next highest is L3 (X denotes ‘don’t care’). Thus p=6 and q=3. Now the active‘low’ output of the first priority encoder will

be F2 F1 F0 = 0 0 1. The input to 74138 is 1 1 0 and it outputs M0 through M7 = 1 1 1 1 1 1 0 1. Since M6=0, only L6 is complemented by XNOR gates. Thus the outputs of XNORs are N0 through N7 = X X X 0 1 1 1 1. Now N3=0 and the highest priority for ‘N’ is 3. This value is recovered by priority encoder 2 (IC3) as S2 S1 S0 = 1 0 0.

Digital Mains Voltage Indicator pratap chandra sahu

C

ontinuous monitoring of the mains voltage is required in many applications such as manual voltage stabilisers and motor pumps. An analogue voltmeter, though cheap, has many disadvantages as it has moving parts and is sensitive to vibrations. The solidstate voltmeter circuit described here indicates the mains voltage with a resolution that is comparable to that of a general-purpose analogue voltmeter. The status of the mains voltage is available in the form of an LED bar graph. Presets VR1 through VR16 are used

to set the DC voltages corresponding to the 16 voltage levels over the 50-250V range as marked on LED1 through LED16, respectively, in the figure. The LED bar graph is multiplexed from the bottom to the top with the help of ICs CD4067B (16-channel multiplexer) and CD4029B (counter). The counter clocked by NE555 timer-based astable multivibrator generates 4-bit binary address for multiplexer-demultiplexer pair of CD4067B and CD4514B. The voltage from the wipers of presets are multiplexed by CD4067B and the

output from pin 1 of CD4067B is fed to the non-inverting input of comparator A2 (half of op-amp LM358) after being buffered by A1 (the other half of IC2). The unregulated voltage sensed from rectifier output is fed to the inverting input of comparator A2. The output of comparator A2 is low until the sensed voltage is greater than the reference input applied at the noninverting pins of comparator A2 via buffer A1. When the sensed voltage goes below the reference voltage, the output of comparator A2 goes high. The high output ELECTRONICS PROJECTS Vol. 22

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from comparator A2 inhibits the decoder (CD4514) that is used to decode the output of IC4029 and drive the LEDs. This ensures that the LEDs of the bar graph are ‘on’ up to the sensed voltage-level proportional to the mains voltage. The initial adjustment of each of the

presets can be done by feeding a known AC voltage through an auto-transformer and then adjusting the corresponding preset to ensure that only those LEDs that are up to the applied voltage glow. (EFY note. It is advisable to use additional transformer, rectifier, filter, and

regulator arrangements for obtaining a regulated supply for the functioning of the circuit so that performance of the circuit is not affected even when the mains voltage falls as low as 50V or goes as high as 280V. During Lab testing regulated 12-volt supply for circuit operation was used.)

Electronic Dice Vijaya kumar P.

H

ere is a small circuit of an electronic dice to interface with your PC. The circuit simulates a digital dice and uses the parallel-port LPT1 provided on the back of the PC. LPT employs a 25-pin ‘D’ type female connector. ‘C’ language provides a built-in outportb() function to output binary data to

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the hardware port. To understand this, let us consider the following program: #include <dos.h> main() { int i; printf(“Input a decimal number”); scanf (“%d”,&i);

outportb(0x0378,i); }

The above program is used to output the binary equivalent of a decimal number entered via the keyboard. ‘Scanf’ function is used to take the input decimal number from the keyboard. The ‘outportb()’ function directly outputs the

Fig. 1 Truth Table Throw Data pins Logic state D2 D1 D0

State of LEDs A

B

C

D

E

F

G

1

0

0

1

OFF

OFF

OFF

OFF

OFF

OFF

ON

2

0

1

0

ON

OFF

OFF

OFF

OFF

ON

OFF

3

0

1

1

ON

OFF

OFF

OFF

OFF

ON

ON

4

1

0

0

ON

OFF

ON

ON

OFF

ON

OFF

5

1

0

1

ON

OFF

ON

ON

OFF

ON

ON

6

1

1

0

ON

ON

ON

ON

ON

ON

OFF

Fig. 2

Fig. 3

Display

binary equivalent of decimal number to LPT1 (port ID is 0x0378). For example, after you convert the above program into an executable file using Turbo C compiler and run it, the program prompts you to “Input a decimal number”. Suppose you enter 15 , then its binary equivalent 00001111 is output (written) to data pins D0 through D7 (pins 2 through 9) of the 25-pin parallel port. If LEDs are connected to the output pins of the parallel port, along with resistors of 220-ohm in series, they can be directly driven to indicate the binary output number, as the parallel port can directly support the current required for driving the LEDs. The electronic dice program generates a random decimal number that is output through data-output lines D0 through D2 (pins 2 through 4) of the LPT port in the form of a 3-bit binary number. Fig. 1 shows the hardware interface circuit of a BCD-to-decimal converter employing a 7-segment display driver IC 7447, which directly converts the input BCD number into 7-segment display. Fig. 2 shows the circuit simulating the electronic dice with dot pattern display that satisfies the truth table shown. Fig. 3 shows the Karnaugh Map simplification of minterms. When the software program using ‘C’ language is run after compilation, it prompts you to press letter ‘T’ for simulating an action equivalent to throwing of dice by generating/outputting a random number, or press letter ‘X’ to exit the program. The program is given below: #include <stdio.h> #include <dos.h> #include #include #include <stdlib.h> #define PORT 0x0378 /*LPT1 Data Port Address */ /*Use 0x0278 for LPT2 */ void main(void) { int ran; int gd=DETECT,gm,ch,x,y; initgraph(&gd,&gm ,""); /* Initializes graphics mode */ /* Decorating the screen */ x = getmaxx();/*Get maximum screen coordinates x & y*/ y = getmaxy(); setbkcolor(BLUE); rectangle(10,y-10,x-10,10); setcolor(YELLOW); settextstyle(DEFAULT_FONT, ELECTRONICS PROJECTS Vol. 22

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HORIZ_DIR,2); outtextxy(120,20,"*** ELECTRONIC DICE ***"); setcolor(GREEN); outtextxy(x/5,180,"1.Press T to Throw Dice"); outtextxy(x/5,230,"2.Press X to Exit"); /* Actual program */ randomize();/*Initializes random number generator */ do {

ch= getch(); ch= toupper(ch); if(ch=='T') /* If T is Pressed */ { ran=random(6); /* to generate random number between 0&7 */ ran=ran+1; outport(PORT,ran); /* outputs BINARY no. to LPT's Data Port */ } }

while(ch!='X'); /* Exit the program if X is pressed */ outportb(PORT,0); /* Clears LPT's Data Port */ closegraph(); /*Shuts down graphics mode */ printf("By Vijaya Kumar.p"); exit(0); }

(EFY note. Source code has also been included in the accompanied CD.)

Light-Operated Organ pradeep g.

included in the emitter circuit of T1 as

H

ere is a circuit based on a unijunction transistor (UJT) 2N2646 or its equivalent that can be used as a light-operated organ. Wired as a relaxation oscillator, it can oscillate independently without a tank circuit or complicated RC feedback network. A light-dependent resistor (LDR) is

shown in the diagram. When LDR receives light from a light source, such as an electric bulb, a sharp and pleasing audio tone is heard from the speaker. The intensity of light falling on LDR can be varied by waving a hand to and fro between the lamp and the LDR. As a result, the frequency of the output sound changes.

Stereo Tape Head Preamplifier for PC Sound Card t.k. hareendran

H

ere is a stereo tape head preamplifier circuit for your PC sound card that can playback your favourite audio cassette through the PC. Audio signals from this circuit can be directly connected to the stereo-input (line-input) socket of the PC sound card for further processing. The circuit is built around a popular stereo head preamp IC LA3161. Weak electrical signals from the playback heads are fed to pins 1 and 8 of IC1 via DC decoupling capacitors C1 and C6, respectively. Components between pins 2 and 3 and pins 6 and 7 provide adequate equalisation to the signals for a normal tape playback.

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The amplified and equalised signals available at output pins 3 and 6 of IC1 are coupled to the inputs of line amplifier circuit built around transistors T1 (via capacitor C5, potmeter VR1, resistor R8, and capacitor C12) and T2 (via capacitor C10, potmeter VR2, resistor R19, and capacitor C16), respectively. Left and right playback levels can be adjusted by variable resistors VR1 and VR2. The audio signals are finally available at the negative ends of capacitors C13 and C17. The circuit wired around relay driver transistor T3 serves as a simple source selector. This is added deliberately to help the user share the common PC sound card

line-input terminal for operating some other audio device as well. When the preamplifier is in ‘off’ state, switching relay RL1 is off and it allows connection of external signals to the sound card. When the preamplifier is turned ‘on’, the relay is energised by transistor T3 after a short delay determined by the values of resistor R21 and capacitor C20. On energisation, the relay contacts changeover the signals to internal source, i.e. the head preamplifier. After constructing the whole circuit on a veroboard, enclose it in a mini metallic cabinet with level controls and sockets at suitable points. Use a regulated 1A, 12V DC power supply for powering the

whole circuit including the tape deck mechanism. (A 1A, 18V AC secondary transformer with 4700µF, 40V electro-

lytic capacitor and 78M12 regulator is sufficient.) You can use any kind of tape deck

mechanism with this circuit. Use of goodquality playback head and well-screened wires are recommended.

Heart Beat Monitor pradeep g.

H

ere is a simple and low-cost circuit of heart beat monitor using readily available components. It uses the piezo-electric plate of audible piezobuzzers as the sensing device, which can be purchased for around Rs 2 only from component vendors. The sensor is pressed against human body near the heart region. It should make a solid contact with your palm to convert heart beat sound into low-frequency electrical variations. These electrical variations are amplified by transistor T1 that is configured as a common-emitter amplifier. Amplified signals are coupled to transistor T2 for driving the audio power amplifier stage. The speaker reproduces heart beat notes as audible sound. The two BEL188 silicon transistors

used in the power output stage are freely available. In case you use AC188/128 germanium transistors in place of BEL188

silicon transistors, replace 220-ohm resistors with 47-ohm resistors and 680-ohm resistors with 1-kilo-ohm resistors. ELECTRONICS PROJECTS Vol. 22

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Digital Fan Regulator sunil p.b.

T

he circuit presented here can be used to control the speed of fans using induction motor. The speed control is nonlinear, i.e. in steps. The current step number is displayed on a 7-segment display. Speed can be varied over a wide range because the circuit can alter the voltage applied to the fan motor from 130V to 230V RMS in a maximum of seven steps. The triac used in the final stage is fired at different angles to get different voltage outputs by applying short-

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duration current pulses at its gate. For this purpose a UJT relaxation oscillator is used that outputs sawtooth waveform. This waveform is coupled to the gate of the triac through an optocoupler (MOC3011) that has a triac driver output stage. Pedestal voltage control is used for varying the firing angle of the triac. The power supply for the relaxation oscillator is derived from the rectified mains via 10-kilo-ohm, 10W series dropping/limiting resistor R2.

The pedestal voltage is derived from the non-filtered DC through optocoupler 4N33. The conductivity of the Darlington pair transistors inside this optocoupler is varied for getting the pedestal voltage. For this, the positive supply to the LED inside the optocoupler is connected via different values of resistors using a multiplexer (CD4051). The value of resistance selected by the multiplexer depends upon the control input from BCD up-/down-counter CD4510 (IC5), which, in turn, controls forward biasing of the transistor inside optocoupler 4N33. The same BCD outputs from IC5 are also connected to the BCD-to-7-segment decoder to display the step number on a 7-segment display. NAND gates N3 and N4 are configured as an astable multivibrator to produce rectangular clock pulses for IC5, while NAND gates N1 and N2 generate the active-low count enable (CE) input using either of push-to-on switches S1 or S2 for count up or count down operation, respectively, of the BCD counter. Optocoupler 4N33 electrically isolates the high-voltage section and the digital section and thus prevents the user from shock hazard when using switches S1 and S2. BCD-to-7-segment decoder CD4543 is used for driving both common-cathode and common-anode 7-segment displays. If phase input pin 6 is ‘high’ the decoder works as a common-anode de-

coder, and if phase input pin 6 is ‘low’ it acts as a common-cathode decoder. Optocoupler 4N33 may still conduct

slightly even when the display is zero, i.e. pin 13 (X0, at ground level) is switched to output pin 3. To avoid this problem, adjust

preset VR1 as required using a plastichandled screwdriver to get no output at zero reading in the display.

Running Lights and Running Holes vijaya kumar p.

T

his four-channel, two-mode light chaser circuit produces effects of running holes and running lights.

astable multivibrator for generating clock signals for decade counter CD4017 (IC2). The speed of running lights can be varied

using preset VR1. CD4030 (IC3) is a quad XOR gate that can be used both as an inverting and a non-inverting gate by tying

•

Fig. 1

Each effect, i.e. running lights or running holes, is repeated five times. Applications include decorating photographs using LEDs or driving 230V bulbs via triacs. Fig. 1 shows the circuit for driving the bulbs using triacs, while Fig. 2 is a modification of Fig. 1 for driving LEDs without using triacs. In Fig. 1, timer 555 is used as an

12V

Fig. 2

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any one of the XOR gate inputs high and low, respectively (refer the table). For every fourth clock to IC2, its pin 7 goes high, which, in turn, clocks IC4. Since the first five outputs of IC4 are connected together (wired ORed) using

diodes D3 through D7, the output at the tied end remains high for every five clock pulses at IC4. This output is coupled to one of the inputs of all the four XOR gates. When the output of IC4 (Q) goes

high, the outputs of IC2 get inverted and produce the running hole effect. And when the output of IC4 (Q) goes low, XOR gates act as non-inverters and the outputs of IC2 remain as original and produce the running light effect.

A Simple Transistor Tester j. balaji

T

his simple-to-construct circuit is useful for testing both npn and pnp low-power transistors. It comprises a few resistors, LEDs, diodes, and a mains step-down transformer. The 230V mains voltage is stepped down to about 6 volts AC before application to the circuit. The leads of transistor under test are inserted in the test terminals (sockets) marked E, B, and C (for emitter, base, and collector, respectively) appropriately, i.e. the emitter of the transistor is to be inserted in terminal E, the base of the transistor in terminal B, and the collector of the transistor in terminal C. The resistor to be connected in series with the base terminal is selected with the help of a 6-position rotary switch S1 as per base current requirement for

the transistor. Two different coloured (green and red) LEDs are used for indication. Green LED glows if the npn transistor under test is good, otherwise

not. Likewise, when a pnp transistor is tested, the glowing of red LED indicates that the transistor is good and no glowing indicates that the transistor is bad.

12V, 3A Power Supply d. prabakaran

T

172

his circuit provides a 12V regulated power supply with output current up to 3 amperes. It is spe-

ELECTRONICS PROJECTS Vol. 22

cially designed for use with 2m handheld rigs with linear power amplifier and CB portable QRP rigs.

The circuit uses monolithic IC CA3085 voltage regulator in 8-lead TO-5 package. Its salient features include good load and line regulation, output current up to 100 mA (which can be increased to several amperes with additional pass transistors), output short-circuit protection, and lower input voltage. A low power dissipation is achieved by driving external series-pass transistor 2N4241 (T1) from pin 2 of CA3085. Normal output pin 8 is returned to ground via diodes D3 and D4 to ensure error amplification operation in the linear region. Ripple rejection is approximately 50 dB on no load and 35 dB on full load.

A 2x2x2.5cm aluminium heat sink fastened onto a 1.5mm blackened aluminium sheet of 12.5cm2 area on 2N4241 helps the circuit in dissipating heat without exceed-

ing maximum device ratings. CA3085 can dissipate up to 650mW power in free air, without any heat sink. AFCO-make C-05-4 heat sink is suitable

for this IC. An improper heat sink may cause device junction temperature to exceed the limit, resulting in progressive deterioration of the device.

Speller effect sign Display vijaya kumar p.

T

he circuit described here uses lowcost and easily available IC CD4017 to produce a speller type light display. In such displays, each letter of the sign sequentially lights up, one after the other, until all letters are glowing. After a few seconds, the letters switch off and the cycle repeats. This circuit provides a maximum of nine channels and therefore can be used to spell a word or sign having up to nine characters. Timer IC1 (555) is configured in astable mode to produce clock signal for triggering IC2 (CD4017). Speed of switching on the display can be controlled by varying preset VR1. CD4017 is a decade counter having ten outputs, of which one output is high for each clock pulse. However, this produces

running lights effect. To change this sequence to get the speller effect, pnp transistors T1 through T9 are wired as shown in the figure. Nine triacs (triac 1 through triac 9) are used to drive 230V bulbs. (In place of 230V bulbs, miniature lamps connected in series in the form of characters or letters can also be used, provided the voltage drop across the series combination is 230 volts.) When any of the outputs of IC2 goes high, the corresponding transistor connected to the output goes off. When Q0 is high, transistor T1 goes off and its output at the collector goes low. Since the emitter of transistor T2 is connected to the collector of transistor T1, and collector and emitter terminals of transistors T1 through T9 are connected in series, all

transistors next to transistor T1, i.e. transistors T2 through T9, do not get supply and hence all their outputs go low. Next, when Q1 output goes high, transistor T2 goes off. Thus outputs of transistors T2 through T9 remain low. Since Q0 output at this instant is low, transistor T1 is forward biased and its output goes high to light up the first character. Similarly, when Q2 output goes high, Q0 and Q1 outputs are low and therefore outputs of transistors T1 and T2 go high to light up the first and second characters. This process continues until all transistors turn on, making all the characters to light up. The cycle repeats endlessly, producing the speller type light effect.

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DarkRoom Timer d. prabakaran

T

he timer circuit described here provides a pleasant musical tone in your darkroom at 1-second intervals. The circuit takes up very little space and can be easily converted into a metronome. Unijunction transistor (UJT) T1 functioning as a relaxation oscillator triggers the phase-shift audio oscillator circuit

built around transistor T2, turning it on and off. As capacitor C1 is charged through preset VR1 and resistor R1, the emitter voltage of UJT rises toward the supply voltage. When the emitter voltage becomes sufficiently positive, the emitter becomes forward biased and discharges capacitor C1 through the emitter-base 1 (B1) junction and resistor R2. The voltage drop across R2 forward biases transistor T2 and turns it on. As capacitor C1 becomes discharged, the current through resistor R2 drops and transistor T2 is cut off. A tone signal is generated by transistor T2 and R-C coupled phase-shift oscillator. Part of the signal taken from the collector of

transistor T2 is coupled to a small speaker through a transistor-radio type output transformer. The 22-kilo-ohm value of resistor R3 represents a compromise between tone duration and intensity. You can use resistors having a value anywhere between 10 kilo-ohms and 25 kilo-ohms for different durations and intensities of the output signals. Since the unijunction transistor is functioning as the oscillator trigger, changing the values of one or more components in the UJT circuit will change the rate of the tone burst. The tone frequency can be varied by changing the value of any or more of capacitors C2 through C4 and resistors R5 and R6 in the phase-shift network. The primary winding of transformer X1 can be tuned for a slight increase in the output, using capacitor values between 0.05 and 0.25 µF for C5 by trial-and-error method. Tone pulses should begin about ten seconds after the unit is turned on. After a minute or so, adjust preset VR1 for 1-second beats by comparing the timing of the beats with the seconds needle on your wristwatch.

Active Shortwave Antenna praveen shanker

T

he circuit presented here boosts weak shortwave signals so that these can be heard with enhanced clarity over a shortwave receiver. Further, the receiver doesn’t require any physical connection as its placement in the vicinity (within 6 to 7 cm) of the circuit will suffice. The circuit works well over a wide range of supply voltage from 3 volts to 12 volts. Low-noise transistor T1 (BF494 or BF495) is connected as shown in the figure. Resistor R1 gives the DC bias to T1. R1’s value may lie anywhere between l00 kilo-ohms and 22 kilo-ohms; it deter-

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mines the quiescent base-emitter current for transistor T1. Resistor R2 limits the current flowing through transistor T1 and, in conjunction with capacitor C2, determines the operating point for its stable operation. The number of turns in inductor L1 would have to be reduced as operation area shifts towards the upper end of the high-frequency band. A 180µH RFC in series with positive supply rail, along with a bypass capacitor to the ground, is recommended for reducing signal loss in the power supply. The current consumption is well be-

low 10 mA. The transistor works well at maximum supply and so reduction of resistor R1’s value below 22 kilo-ohm is not recommended, as otherwise the transistor may burn off. This circuit works satisfactorily for boosting signals in 13m-49m band.

However, as the frequency increases, its performance deteriorates. The same happens when the frequency decreases below that of the shortwave range. For input use a long wire as the antenna, while the output antenna wire may be limited to about 30 cm.

Note. The circuit is prone to selfoscillations if the aerial (input) wire picks up stray radiations from the power supply wires or from the output. So keep the power supply and output wires well isolated from the input.

Long-Range Target shooter pratap chandra sahu

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racticing target shooting using a real gun is both expensive and risky. Also, it is not possible for

everybody to have a gun. The circuit presented here makes you feel the excitement of shooting a target situated at a

distance of more than 100 metres without any risk or much expenditure. The circuit simply uses a laser pointer (also referred to as laser torch) as the transmitter at the gun end. Laser pointers can reach a maximum of 1 kilometre distance but it is advisable to limit the range within, say, 200 metres. While constructing the gun no change has to be made in the readymade pointer. Just tightly fit the pointer inside the toy gun, so that the triggering switch can activate the press-to-on button of the laser pointer, as shown in Fig. 1. The receiver comprises a counter-cum7-segment display driver IC (CD4033) with a debouncer formed by 555 timer and an LDR sensor at the input. The counter works as a scoreboard and directly shows the number of successful hits. The LDR senses the pointer’s laser beam and activates the monostable multivibrator wired around 555 timer IC. To increase the sensitively of the receiver, the LDR signal is amplified by transistor T1. The timer pulse-width is set at around 100 milliseconds so as to work as a debouncer. The timer output is coupled to IC CD4033. CD4033 is a serial decade countercum-7-segment decoder/driver. With every output pulse from monostable IC1, the count in CD4033 gets incremented by one. Thus the output of IC2 reflects the latest score by a competitor. Pressing reset switch resets the display too. You can increase the size of the display board manyfolds using the additional circuit shown in Fig. 3. This multiplexed board avoids higher power ELECTRONICS PROJECTS Vol. 22

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consumption and is necessary if you are using the module for long-range shooting. For each segment, you can wire up to ten LEDs in parallel. Short the anodes of LEDs of all segments as it is a common-anode type display. (The output from ULN2003 will be active-low.) For proper functioning of the receiver, the LDR should be kept covered in such a way that no external light falls on it. Further, the receiver should be fixed at least two metres from the ground so that the laser beam is accidentally not directed towards anybody’s eyes. The game can be played both in daylight as well as at night. Caution. Never look or stare at the beam source and do not bounce the beam on a mirror.

Power Supply for Walkie-Talkies pradeep g.

H

ere is a simple power supply circuit that can be used for citizen-band and VHF walkie-talkies of power rating up to 10 watts. The circuit uses a step-down transformer, followed by bridge rectifier, filter, regulator, and current booster stages. A pnp power transistor is added to the circuit to increase its current sourcing capabilities. Regulator 7812 can support around 100 mA current. When the current flowing through R1 nears 100mA value, the voltage (>0.65V) across the emitter-base junction makes transistor T1 to conduct and provide a path for additional current. The circuit can source around one ampere of current at 12+1.4 volts=13.4 volts.

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Both the regulator IC and the power transistor must be mounted on heat sinks.

High-Performance Interruption De tector junomon abraham

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he circuit presented here detects interruption in security systems. Its features include no false triggering by external factors (such as sunlight and rain), easy relative positioning of the sensors and alignment of the circuit, high sensitivity, and reliability. The circuit comprises three sections, namely, transmitter, receiver, and power supply. The transmitter generates modulated IR signals and the receiver detects the change in IR intensity. Power supply provides regulated +5V to the transmitter and the receiver. The power supply and the speaker are kept inside the premises while the transmitter and the receiver are placed opposite to each other at the entrance where the detection is needed. Three connections (Vcc, GND, and SPKR) are needed from the power supply/speaker to the receiver section, while only two connections (Vcc and GND) are required to the transmitter. The transmitter is basically an astable multivibrator configured around NE555 (IC3). Its frequency should match the frequency of the detector/sensor module (36 kHz for the module shown in figure)

in the receiver. The transmitter frequency is adjusted by preset VR2. For making the duty cycle less than 50 per cent, diode 1N4148 is connected in the charging path of capacitor C7. The output of astable multivibrator modulates the IR signal emitted from IR LEDs that are used in series to obtain a range of 7 metres (maximum). To increase the range any further, the transmitted power has to be raised by using more number of IR LEDs. In such a case, it is advisable to use another pair of IR LEDs and 33-ohm series resistor in parallel with the existing IR LEDs and resistor R5 across points X and Y. The receiver unit consists of a monostable multivibrator built around NE555 (IC2), a melody generator, and an IR sensor module. The output of the IR sensor module goes high in the standby mode or when there is continuous presence of modulated IR signal. When the IR signal path is blocked, the output of the sensor module still remains high. However, when the block is removed, the output of the sensor module briefly goes low to trigger monostable IC3. This is due to the fact that the sen-

sor module is meant for pulsed operation. Thus interruption of the IR path for a brief period gives rise to pulsed operation of the sensor module. Once monostable IC2 gets triggered, its output goes high and stays in that state for the duration of its pulse width that can be controlled by preset VR1. The high output at pin 3 of the monostable makes the musical IC to function. Voltage divider comprising R2 and R3 reduces the 555 output voltage to a safer value (around 3V) for UM66 operation. The duration of the musical notes is set by preset VR1 as stated earlier. For proper operation of the circuit, use 7.5V to 12V power supply. A battery back-up can be provided so that the circuit works in the case of power failure also. Potmeter VR3 serves as a volume control. The transmitter, receiver, and power supply units should be assembled separately. The transmitter and the receiver should have proper coverings (booster) for protection against rain. The length of the wire used for connecting the IR sensor module and IR LEDs should be minimum. Note. The heart of the circuit is the

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IR sensor module (usually used in VCRs and TVs with remote); the circuit works satisfactorily with various makes of

sensors. The entire circuit can be fixed in the same cabinet if the connection wires to the sensors are smaller than 1.5

metres. The reflection property of IR signals can also be used for small-distance coverage.

Readers’ comments:  This circuit helps me in various ways. I have the following queries regarding it: 1. How can I fit a lamp in place of the speaker or another device? 2. Can I perform more than one func-

tion using this circuit?

circuit! A lamp can be connected easily by using a simple transistor driver circuit at output pin 3 of IC2 (NE555). For larger loads, you may need a relay circuit to connect the load through the contacts of the relay.

Bhavik A. Patel Through e-mail The author, Junomon Abraham, replies: Thank you for your keen interest in this

Digital Relay Tester For RAX and MAX krishna sharma

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his high-speed relay tester is intended for testing 12V DC 2C/O (changeover) and 4C/O PCB-

ELECTRONICS PROJECTS Vol. 22

mounted relays used in RAX (smallcapacity rural automatic exchange) and MAX (main automatic exchange) of C-

DOT origin. It is a reliable tool for testing relays in bulk. For other than 2C/O and 4C/O contact relays, slight modification

in the circuit is required. As soon as the relay is inserted in 28-pin ZIF socket and test pushbutton S2 is pressed, the tester displays ‘pass’ or ‘fail’ on 7-segment display. If the relay coil is open or N/O and N/C contacts are not functioning as they should during operate and release conditions, the tester immediately displays ‘fail’ on 7-segment display. If the relay under test is good, the display shows ‘pass’ on 7-segment display. When the mains supply is connected to the circuit by closing switch S1, 5V DC supply goes to the ICs, transistors (collectors), and common points (poles) of the relay under test, and 12V DC supply goes to one of the terminals of the coil of relay under test. The outputs from four N/C and four N/O contacts are alternately applied to N1 and N2 gates, respectively,

of dual 4-input AND gate IC3 (74LS21). The high output of gate N1 goes to pin 1 of gate N3 of IC4 (7400), while the low output of gate N2 goes to pin 2 of gate N3 of IC4. As a result, the output of gate N3 becomes high and transistor T1 conducts to complete the path for supply to the coil of the relay under test. For a good relay, the output of gate N4 is high before its energisation. After energisation of the relay, the output of gate N3 remain high whereas the output of gate N4 goes low. This signal is inverted by gate N5 to display ‘pass’. The output of gate N5 is further inverted by gate N6 to display ‘fail’. The common segments of ‘pass’ and ‘fail’ characters are illuminated by OR gating via diodes D5 and D6, while exclusive ‘pass’ and ‘fail’ segments are illuminated

directly through resistors R12 and R13 (whichever is high), respectively. For testing 2C/O relays, keep the knob of push switch S3 (wiring of S3 to relay socket is shown separately) pressed to bypass two C/O contacts out of four C/O contacts. The test procedure is summarised below: 1. Switch on the power supply to the tester. 2. Insert the relay to be tested into ZIF socket and lock it. 3. For 4C/O relay leave knob S3 released, and for 2C/O relay keep the knob pressed. 4. Press test switch S2 and observe the display for ‘pass’/ ‘fail’. 5. Unlock ZIF socket and segregate the relay as per the result.

Fastest Finger First Indicator p. rajesh bhat

Qu

iz-type game shows are increasingly becoming popular on television these days. In such games, fastest finger first indicators (FFFIs) are used to test the player’s reaction time. The player’s designated number is displayed with an audio alarm when the player presses his entry button. The circuit presented here determines

as to which of the four contestants first pressed the button and locks out the remaining three entries. Simultaneously, an audio alarm and the correct decimal number display of the corresponding contestant are activated. When a contestant presses his switch, the corresponding output of latch IC2 (7475) changes its logic state from

1 to 0. The combinational circuitry comprising dual 4-input NAND gates of IC3 (7420) locks out subsequent entries by producing the appropriate latch-disable signal. Priority encoder IC4 (74147) encodes the active-low input condition into the corresponding binary coded decimal (BCD) number output. The outputs of IC4 after

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inversion by inverter gates inside hex inverter 74LS04 (IC5) are coupled to BCDto-7-segment decoder/display driver IC6 (7447). The output of IC6 drives commonanode 7-segment LED display (DIS.1, FND507 or LT542). The audio alarm generator comprises clock oscillator IC7 (555), whose output drives a loudspeaker. The oscillator fre-

quency can be varied with the help of preset VR1. Logic 0 state at one of the outputs of IC2 produces logic 1 input condition at pin 4 of IC7, thereby enabling the audio oscillator. IC7 needs +12V DC supply for sufficient alarm level. The remaining circuit operates on regulated +5V DC supply, which is obtained using

IC1 (7805). Once the organiser identifies the contestant who pressed the switch first, he disables the audio alarm and at the same time forces the digital display to ‘0’ by pressing reset pushbutton S5. With a slight modification, this circuit can accommodate more than four contestants.

Readers’ comments:  What modifications are required to accommodate more than four contestants? Suyash Narayan Delhi EFY: To accommodate up to eight contestants, the modified circuit is shown in Fig. 1 below. In this circuit, IC8 (7475) is added to the previous circuit to accommodate switches S5 through S8. Fig. 1: Modification of the fastest finger first circuit for eight contestants In place of IC3 (7420) of the (74LS00) have been used to lock out the previous circuit, IC9 (74LS30) and IC10 subsequent entries by producing the ap-

propriate latch-disable signal. The rest of the circuit remains the same.

Decorative Signboard pratap chandra sahu

T

his eye-catching signboard can be used for special occasions such as birthdays and marriage ceremonies. The characters in the display board are illuminated one by one, each for one second. After the last character is illuminated, the entire board gets illuminated for 4 to 5 seconds. The above two sequences are repeated continuously. Timer 555 (IC1) generates 1Hz pulses, which are applied to decade-counter CD4017B (IC5). The output from pin

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Q9 of IC5 triggers 4- to 5-second (pulse width) monostable multivibrator IC2. The output of IC2 is ANDed in gate A1 with 100Hz stepped down/pulsating DC supply available at the output of the bridge rectifier comprising diodes D1 through D4. The output of AND gate A1 drives second decade counter IC4, whose outputs (Q1 through Q8) are ORed with the corresponding outputs of first counter IC5. (Note. Only eight of the ten outputs of CD4017s have been used.)

Driving characters at 1 Hz ensures that the characters are illuminated one by one for one second each. Similarly, 100Hz signal driving IC4 ensures that the characters are refreshed rapidly for a continuous glow effect due to persistence of vision. AND gate A2 is used to block 1Hz signal reaching the first counter (IC5) while the second counter (IC4) is active, i.e. when the output of IC2 is high. When IC2 output goes low after 4-5 seconds, it enables gate A2 to pass 1Hz clock to the

first counter (IC5) and disables the second counter (IC4) via its reset pin 15. Transistor T1 acts as an inverter. For illuminating more than one message, use two rows of characters wired reverse to each other. This sequence of characters in opposite directions gives a

special effect. The characters can be made by wiring LEDs/torch bulbs (6V, 200mA type) in series/parallel combination or densely painted glass or transparent plastic illuminated by torch bulbs. The bulbs should be placed behind each painted character.

Each of the eight outputs of ULN2803 can sink a maximum of 500 mA at supply voltage of up to 50 volts. Note. The supply for ULN2803 can be a separate one or the same as used for the rest of the circuit. However, ensure the ground reference is same in both the cases.

Condenser Mic Audio Amplifier d. prabakaran

T

he compact, low-cost condenser mic audio amplifier described here provides good-quality audio of 0.5

watts at 4.5 volts. It can be used as part of intercoms, walkie-talkies, low-power transmitters, and packet radio receivers.

Transistors T1 and T2 form the mic preamplifier. Resistor R1 provides the necessary bias for the condenser mic while ELECTRONICS PROJECTS Vol. 22

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preset VR1 functions as gain control for varying its gain. In order to increase the audio power, the low-level audio output from the preamplifier stage is coupled via coupling capacitor C7 to the audio power

amplifier built around BEL1895 IC. BEL1895 is a monolithic audio power amplifier IC designed specifically for sensitive AM radio applications that delivers 1 watt into 4 ohms at 6V power supply

voltage. It exhibits low distortion and noise and operates over 3V-9V supply voltage, which makes it ideal for battery operation. A turn-on pop reduction circuit prevents thud when the power supply is switched on. Coupling capacitor C7 determines lowfrequency response of the amplifier. Capacitor C9 acts as the ripplerejection filter. Capacitor C13 couples the output available at pin 1 to the loudspeaker. R15-C13 combination acts as the damping circuit for output oscillations. Capacitor C12 provides the boot strapping function. This circuit is suitable for low-power HAM radio transmitters to supply the necessary audio power for modulation. With simple modifications it can also be used in intercom circuits.

Smoke Alarm pradeep g.

T

he smoke alarm circuit presented here is based on the readily available photon-coupled interrupter module and timer IC NE555. The pho-

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to interrupter module is used as the smoke detector, while timer 555 is wired in astable configuration as an AF oscillator for sounding alarm via a loudspeaker. In the absence of any smoke, the gap of photo interrupter module is clear and the light from LED falls on the phototransistor through the slot. As a result, the collector of phototransistor is pulled towards ground. This causes reset pin 4 of IC 555 to go low. Accordingly,

the timer is reset and hence the alarm does not sound. However, when smoke is present in the gap of the photo interrupter module, the light beam from LED to the phototransistor is obstructed. As a result, the phototransistor stops conducting and pin 4 (reset) of IC 555 goes high to activate the alarm. Note. The unit must be housed inside an enclosure with holes to allow entry of smoke.

OverLoad Protector With Reset But ton vijay kumar p.

I

n applications like inverters and UPS, the load must not exceed the rated output power since it can cause excess heating of output transformer windings and active driving devices and thereby damage them. The circuit presented here can be used as overload protector for inverters or as an electronic fuse in AC mains supply. The mains supply to the load is routed via the the N/C (normally closed) contacts of relay RL1. In an inverter, the relay contacts could be used as ‘inverter oscillator’ on/off control. Whenever overload occurs, it inhibits inverter oscillator circuit, which, in turn, stops generation of power. Resistor R1 is used as the overload sensing element. When the load exceeds the maximum rated value, it draws current in excess of its rated value. This causes the potential drop across resistor R1 to increase. An optocoupler is used to sense this voltage drop. The optocoupler, in addition, isolates the AC mains part from the rest of the circuit. Zener diode ZD1 across the optocoupler MCT2E, which is connected in reverse prevent inbuilt LED of the optocoupler MCT2E from negative half cycles across R1. Resistor R1 is selected as 1 ohm for 230V, 500 watts (max.) load capacity. When the load just exceeds 500 watts, the current through R1 is approximately 2.1 amperes, producing a potential difference of 2.1 volts across R1. The inbuilt transistor inside the optocoupler senses this voltage and its collector current increases proportionally. When the current reaches the required designed value, voltage drop across resistor-pre-

set combination R3-VR1 also increases. (Note. The power dissipated in 1-ohm resistor for 500W load is just 2.1 watts, which is negligible compared to the maximum power rating of the load. To

old pin that resets the flip-flop output to low state. The circuit can be reset after removing unwanted loads. Note. Since the circuit is very sensitive, fluctuations in AC mains can also

use this circuit for 1kW load, select R1 as 0.5-ohm, 10W.) Overload limiting point can be set by preset VR1. When the potential at wiper of preset VR1 becomes greater than VZ+VBE (where VZ is the breakdown voltage of zener diode ZD1 and VBE the forward voltage drop at the base-emitter junction of transistor T1), it causes forward biasing of transistor T1. This results in the collector of transistor T1 to be pulled down to ground and trigger IC555, which is connected in bistable mode. The output of IC1 forward biases transistor T2 to energise the relay RL1 and causes overload indicating LED1 to glow. Once the output of bistable IC1 goes high, it continues to remain high, unless reset pushbutton S1, which is connected between Vcc and threshold terminal (pin 6) of timer 555, is pressed. On pressing S1, a high pulse is applied to the thresh-

trigger the circuit undesirably. This effect can be eliminated by using 4.7µF bypass capacitor C1 as shown in the figure. Since some equipments like TV draws more current initially for few seconds, this can cause the overload protector to cutoff the supply, which is undesirable. Inserting a small time delay of around 5 seconds by using a capacitor C3 of 220µF can eliminate this. The time delay can be increased by using higher values of C3 and can be decreased by using higher values of C3. If you are using the circuit for loads, which do not draw more current initially, the delay feature may be useless and even be harmful. Therefore disable the feature by simply disconnecting capacitor C3. While adjusting the overload cutoff point disconnect C3, which will otherwise can cause confusion, and reconnect it after final adjustments. ELECTRONICS PROJECTS Vol. 22

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