We found this circuit in a 1957 Raytheon transistor publication. The circuit can be assembled with more modern transistors with minor changes. The handset must be high impedance magnetic.
This circuit shows on the screen of an oscilloscope two signals alternately at a frequency that depends...
This circuit is from a Radio Electronics from August 1992. The transistor used allows the control of...
This circuit measures the strength of audio signals. We found it in an American publication from...