We found this circuit in an English publication from the 60's. The transistors are of germanium of the time and the headphones must be high impedance magnetic.
The presence of a low resistance between the terminals of the sensor causes this modulated...
This circuit was found in a 1970 IEEE publication. The circuit functions as a voltage controlled...
I found this circuit in a 1992 Electronics Experimenter Handbook. The circuit can be easily...