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.
This circuit was found in an 80's General Electric documentation. The circuit uses uncommon...
This circuit was found in Electronics Today International of the 80's. The circuit uses still...
Using transistors common at the time (1960s) this amplifier with transformers in the output stage...