Ultrasonic switch

Receiver. Output from the transducer is amplified by Ql and Q2, and rectified by Dl. Voltage on pin 2 of ICl will go more negative as the input signal increases. ICl is used as a comparator and checks the voltage on pin 2 (i the sound level), to that on pin 3 which is the reference level If pin 2 is at a lower voltage than pin 3 (ie, a signal is present), the output of ICl will be high (about 10 volts) and this will turn on Q3 which will close the relay. The converse occurs if pin 2 is at a higher voltage. The oscillator frequency is determined by the transducer characteristics [(minimum (series resonance) at 39 kHz followed by a maximum (parallel resonance) at 41 kHz
Ultrasonic switch - schematic

Two transistors from a noninverting amplifier and positive feedback is supplied via the transducer, R6 and C3. At the series resonant frequency, this feedback is strong enough to cause oscillation.

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