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 is a circuit published in the January 1962 Radio Electronics magazine. The circuit is for...
This diagram was found in the November 1972 issue of Radio Electronics magazine in an article...
This circuit with the CK722 was found in a Raytheon publication from the 60's. The relay is of the...