The piezo speaker, MICl, picks up the incoming ultrasonic signal and feeds it to the base of Ql. The two-transistor booster amplifier, Ql and Q2, raises the signal to a level that is sufficient to drive one input of this most unusual mixer circuit. Integrated circuit U2, a quad bilateral switch, functions as an extremely clean balanced-mixe. circuit for the superheterodyne receiver.
Ultrasonic-receiver - schematic

Integrated circuit U1a, lfz of a dual op amp, is connected in a~variablefrequency square-wave oscillator circuit. Resistors R5, R6, and capacitor C4 determine the frequency and tuning range of the oscillator. The oscillator"s square-wave output is fed along two paths. In one path, the output of Ula is input to pins 12 and 13 of U2. In the other path, the signal is fed to the base of Q3, which is configured as an inverter. The inverter outputs a signal that is 180° out-of-phase with the input signal. The inverted output of Q3 is then fed to U2 at pins 5 and 6. There, the two input signals, the ultrasonic input from MICl and the oscillator output, are mixed. The mixing of the ultrasonic input and the square-wave signal produces an audible product that is fed to the input of a differential amplifier, U1b, the second half of the dual op amp, which has a voltage gain of two. The output of U1b at pin 7 is filtered by R19 and C9 to remove the highfrequency content ofthe mixed signal. Only the difference frequency is important; the sum frequency, the incoming ultrasonic signal added to the oscillator frequency, is too high for the human ear to hear. The sum frequency is removed by R19 and C9 to produce a clean output signal to feed power-amplifier U3. Resistor R21 functions as the circuit"s volume control.

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