We found this circuit in a Radio Electronics from February 1966. The transistor is from the time and so are the transformers. The circuit is battery powered. Observe the impedances.
Great sensitivity for the circuit can be obtained with a Darlington amplification step for the...
This 2005 circuit was found in an English publication and can be implemented with virtually any...
This car alarm was found in an American '80s documentation. The circuit automatically activates...