Ultrasonic Transmitter is based on a 4011 quad 2-input NAND gate. Two of the gates, IC1a and IC1b, are used as a conventional CMOS Astable circuit whose oscillation frequency is set by capacitor C1, resistors R6 and R7, plus preset VR2, which adjusts the frequency. Gates IC1c and IC1d buffer the outputs from IC1a and IC1b and drive the ultrasonic transmitter transducer, X1, in push-pull mode.
In the Receiver circuit, ultrasonic echo signal is received by transducer X2. It is first amplified by the 2-stage amplifier based on transistors TR1 and TR2. The output from TR2 is rectified by diode D1 and smoothed by capacitor C4. The result is a voltage level at inverting terminal of IC2 which varies between 1·4V and 2·7V when X2 is not receiving ultrasound echoes, but falls to 1·2V when a strong enough echo from an object is detected. The rectified signal goes to op amp comparator IC2's inverting input, pin 2. The signal is compared with a reference voltage set by preset VR3, and applied to IC2's non-inverting input, pin 3. The output at IC2 pin 6 is low when there is no echo, but swings high when an obstacle ahead of the system is detected.