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.
This is the circuit of a turntable from the 1970s. We found it in a repair article in the December...
This circuit divides the grid 60 Hz frequency to obtain 20 Hz to drive mechanical watches used at...
This small audio amplifier uses an uncommon integrated circuit today. The circuit is from a...