This circuit, based on a 555, was found in a Popular Electronics from the 90s. The circuit has the frequency determined by R1, R2 and C1.
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...