The transducer circuit can be operated as either a tone encoder or decoder by changing the position of Sl. The operating frequency of that dual-purpose circuit is determined by C3 and R2. Capacitors Cl and C2 are not critical and can be of almost any value between 1 and 5 mf. When the circuit is receiving an on-frequency signal, LEDl lights. Although a two wire piezo transducer with a resonance frequency of 2500 Hz was used in the circuit, any piezo unit should work-as long as the values of C3 and R2 are selected to tune to the transducer`s operating frequency.
Encoder-decoder - schematic

With power on and Sl in the B position, adjust R2 for the loudest tone output. The circuit should be tuned to the resonance frequency of the transducer. In that position, the circuit can be used as an acoustical or tone signal encoder. Next, switch to the A position and aim an on-frequency audible tone toward the transducer; the LED should light, indicating a decoded signal.

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