This circuit for audio signals was found in the March 1990 Radio Electronics magazine. The circuit uses a unusual optocoupler that can be replaced with an equivalent one.
This oscillator has its frequency controlled by crystal and its frequency can be between 100 kHz...
This small crystal microphone transmitter was indicated in an 80s spy book. The range depends on...
The circuit shown is from the 1994 Linear Applications Handbook by National Semiconductor. The...