This circuit was found in documentation for radio amateurs of the 1980s. The circuit is intended to produce a tone for modulation in telegraphic transmissions.
The simple transistor test shown in the figure can be powered by a 9V battery. The test is for...
This circuit is from an American publication from 1970. It uses a coal microphone, which is...
The maximum current of this circuit is 250 mA, its purpose being to replace 9 V batteries. Diode D1...