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.
Found in an EDN from the 80's, this circuit makes use of an operational one of the time. The...
This circuit can either directly supply a piezoelectric transducer (BZ) or a stage with a...
Only for reference this is a circuito f a commercial receiver of the year 1958. The circuit uses...