This circuit was found in the Electronics Experimenter Handbook from 1991. The sensor is a diode and the circuit has a relay according to the voltage used in the supply.
If you have old tube or transistor amplifiers that hum (60 Hz noise) the circuit shown may be the...
The integrated circuit used in this regulator is unusual, as are the transistors. The circuit is...
Other settings like this can be found on this website. This makes use of old operational...