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.
Found in a 1966 documentation, this “coin finder” uses components from the era operating at 2.1...
This circuit is from a documentation on the MC1539 from Motorola and may, however, be implemented...
The 747 is an old operational amplifier, but the configuration applies to modern operatives. In...