This simple circuit was found in a Japanese magazine in 1967. The circuit is listening on a headset and the coil is 50 + 50 turns of wire 28 or thinner on a ferrite rod.
Found in Radio Electronics this circuit uses a fotodiac to drive a triac. The circuit can use...
This circuit is from an old English documentation. It is intended for coupling a microphone to the...
Here is an interesting circuit to be used in physics demonstrations involving energy...