This circuit was obtained from a Radio Electronics issue from January 1989. The indicator is a ceramic transducer, and the frequency of the emitted sound is given by C3.
This circuit was obtained from a 2005 Linear Technology application manual. The circuit is...
Small time intervals up o some minutes can be achieved with this circuit. Only incandescent lamps...
This circuit is from a 1977 Radio Shack documentation. It produces a modulated sound of good...