This circuit is from old documentation, but it can be implemented with modern transistors such as 2N2222 for bipolar and BF124 for FET. The crystal determines the operating frequency.
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...