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 an American publication on transmitters. The components are current, and the...
With a 120 ohms resistor this circuit can also be used as a shield for 3.3 V output voltages. The...
This circuit is from Radio Electronics from March 1977. The integrated circuit is from that time...