We found this circuit with JFETs in a Radio Electronics magazine from June 1969. The circuit has its assembly described in a complete article. Equivalent transistors can be used.
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...