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 is from a 1976 documentation. The diodes can be 1N34 or equivalent, and the power...
This circuit is from a high voltage tube source. It does not include the filament voltage that must...
This circuit is from the Motorola Linear Databook of 1990 and can be implemented with other...