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 came out in a publication called Electronics Handbook of 1993. The circuit shows how...
Found in documentation from the 1980s, this circuit makes use of general-purpose diodes and...
This circuit memorizes through an LED indicator if your phone was used during your absence. The...