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 very low frequency circuit was found in a 1964 Electronic Design. Transistors may be modern...
Found in an English documentation from 1982, this circuit can be changed to operate in a different...
I found this circuit in an old English magazine from 1979, but it can be easily assembled with...