Using a capacitor as a sensor, this circuit was found in the 1989 Radio Electronics Experimenter magazine. The circuit must be assembled in a shielded box.
Bipolar transistors are types of general use like BC548, while the power MOSFET can be of any type...
This circuit is from a Popular Electronics from the 90's. The audio amplifier admits equivalent. The...
This interesting source, obtained in a 2001 catalog, can vary the output voltage between -16 and...