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.
We found this circuit in an English magazine in 1975, but it can be easily assembled, as the...
Found in a 90s Popular Electronics, this circuit uses a 741 to generate frequencies from 0.1 to 30...
This circuit is from an 80's documentation. The circuit makes use of a component of the era, the...