In the figure we have a way of receiving ultrasonic signals of a single frequency using a PLL 567. The circuit is from Radio Electronics from November 1985.
Found in the October 1992 Radio Electronics magazine, this oscillator is controlled by switch S1....
This circuit was obtained in a 1976 amateur radio manual. The circuit can be made with a 2N2222...
The component used in this circuit is not very common today as the circuit is from the 80's. It...