ELECTRONICS FOR YOU� ❚❚❚❚❚ �JUNE 2001
C I R C U I T I D E A S
ADTMF-based IR transmitter andreceiver pair can be used to realise a proximity detector. The circuit
presented here enables you to detect anyobject capable of reflecting the IR beamand moving in front of the IR LED photo-detector pair up to a distance of about 12cm from it.
The circuit uses the commonly avail-able telephony ICs such as dial-tone gen-erator 91214B/91215B (IC1) and DTMFdecoder CM8870 (IC2) in conjunction withinfrared LED (IR LED1), photodiode D1,and other components as shown in thefigure. A properly regulated 5V DC powersupply is required for operation of the cir-cuit.
The transmitter part is configuredaround dialer IC1. Its row 1 (pin 15) and
column 1 (pin 12) get connected togethervia transistor T2 after a power-on delay(determined by capacitor C1 and resis-tors R1 and R16 in the base circuit of thetransistor) to generate DTMF tone (com-bination of 697 Hz and 1209 Hz) corre-sponding to keypad digit “1” continuously.
LED 2 is used to indicate the tone
output from IC3. This tone output is am-plified by Darlington transistor pair of T3and T4 to drive IR LED1 via variable re-sistor VR1 in series with fixed 10-ohmresistor R14. Thus IR LED1 producestone-modulated IR light. Variable resis-tor VR1 controls the emission level to varythe transmission range. LED 3 indicatesthat transmission is taking place.
A part of modulated IR light signaltransmitted by IR LED1, after reflection
from an object, falls on photodetector di-ode D1. (The photodetector is to beshielded from direct IR light transmis-sion path of IR LED1 by using any opaquepartition so that it receives only the re-flected IR light.) On detection of the sig-nal by photodetector, it is coupled toDTMF decoder IC2 through emitter-fol-lower transistor T1.
When the valid tone pair is detectedby the decoder, its StD pin 15 (shorted toTOE pin 10) goes ‘high’. The detection of
the object in proximity of IR transmitter-receiver combination is indicated byLED1. The active-high logic output pulse(terminated at connector CON1, in thefigure) can be used to switch on/off anydevice (such as a siren via a latch andrelay driver) or it can be used to clock acounter, etc.
This DTMF proximity detector findsapplications in burglar alarms, objectcounter and tachometers, etc.
RUPANJANAK.S. SANKAR
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