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.
We found this transistorized radio for the AM band in a Japanese publication from 1960. The...
This small source is suitable for low consumption equipment that uses batteries. The circuit is...
Found in a magazine 73 for radio amateurs, this circuit has two settings, one for the interval and the...