Ultrasonic Voice Receiver Circuit
Shown below is my ultrasonic voice receiver circuit. It is simply pointed ( U.T. ) toward the direction where a ultrasonic voice transmitter circuit is in operation. This unit picks up the 40kHz signal and then converts it into audible audio.
Dotted lines through suitch sw 1 and sw 2 indicate that this is a double pole single throw switch. in other words, a single switch is used to turn both the U.T. amplifier and its audio amplifier on at the same time.
Two batteries are used to avoid feedback and instability problems due to the high gain of the U.T. amp.
It is important that the negative side of both amplifiers connect together for a return as shown by the single trace from the negative side of battery # 1 to the negative side of battery # 2.
Specs: lm386 power amp is a standard chip amp with a gain of 20. U.T. amp draws 8 ma (.008 amps) of current at 9 volts. Frequency response is from 30 kHz to 70 kHz / its center frequency @ 40 kHz with 3 db down @ 30 and 70 kHz. Its output signal from the 10 x pot to the input pin 3 of the lm386 chip is a unique 40 kHz saw tooth that is modulated vertically and seen by the chip as am modulation in the voice range.