CIRCUITIDEAS

9 0 • F E B R U A R Y 2 0 0 6 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

CMYK

This ultrasonic proximity detec-tor comprising independent,battery-powered transmitter

and receiver sections makes useof a pair of matched ultrasonicpiezoceramic transducers operating ataround 40 kHz each. This circuit canbe used in exhibitions to switch on pre-recorded audio/video messages auto-matically when a visitor evincing in-

S.C. DWIVEDI

EFY LAB

ULTRASONIC PROXIMITYDETECTOR

terest in a product comes near an ex-hibited product.

Fig. 1 shows the transmitter circuit.It comprises CMOS timer IC 7555 (IC1)configured as an astable multivibrator,which may be tuned to the frequencyof the ultrasonic piezoceramictransmitter’s resonant frequency ofaround 40 kHz using preset VR1. Acomplementary pair of transistors T1and T2 is used for driving and buffer-ing the transducer while it draws

spikes of current from IC1 circuit tosustain oscillations and thereby avoidsany damage.

The receiver front-end (referFig. 2) is designed to provide a veryhigh gain for the reflected faintultrasonic frequency signals detectedby the ultrasonic transducer. The am-plifiers built around N1 and N2, re-spectively, provide AC voltage gainof around 80 each. These two stagesshould have a high open-circuit gain,

Fig. 1: Transmitter circuit

Fig. 2: Receiver circuit

Fig. 3: Pinconfigurations oftransistors BC327and BC337

Fig. 4: Installation of transducer pair

wide bandwidth and very low biascurrent apart from being capable ofsingle-supply operation. Quad op-ampLM324 is used here due to its low

cost. For higher efficiency, you mayuse single op-amps such as CA3130or CA3140.

When a visitor pauses before a

product, it signifies his interest.Switching diode D1 followed by a fil-ter comprising capacitor C5 and re-

sistor R10 is used to meetthis requirement. The fil-ter also helps to bypassbrief bursts of ambientnoise in the ultrasonic

range. The third stage comprising N3works as a comparator to provide atriggering pulse when a visitor stopsby. This pulse can be used to trigger

CIRCUITIDEAS

E L E C T R O N I C S F O R Y O U • F E B R U A R Y 2 0 0 6 • 9 1W W W . E F Y M A G . C O M

CMYK

a timer or a monostable, whose out-put may then be used to switch onthe audio/video message concerningthe product for a predetermined pe-riod.

When somebody comes in frontof the ultrasonic piezoceramic trans-

ducer pair, the status LED (LED1)glows because of the signal reflectedfrom the body of the visitor.

The circuit can beassembled on any general-purposePCB. The transmitter and the receivershould be aligned such that the trans-

mitted ultrasonic signal is optimallyreceived by the receiver after reflec-tion. Fig. 3 shows the pin configura-tion of transistors T1 and T2, while Fig.4 shows installation of the ultrasonicpiezoceramic transducer pair operat-ing at around 40 kHz.