This high sensitivity circuit was found in the July 1992 Radio Electronics magazine. The circuit drives a buzzer, but a relay can be used to activate an external load.
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...