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.
Maxim MAX16813B 4 Channel LED Driver These Maxim Integrated LED drivers are available from Mouser...
This circuit was found in a factory manual from the 1980s. It can serve as the basis for an...
Found in an American publication from the 1980s, this circuit is designed to regulate the voltage of...