This very simple circuit uses a phototransistor and a very sensitive relay (reed type) being used over short distances with a lens system. The circuit is from a 1967 Japanese publication.
This circuit works only with carbon microphones, due its low impedance. The speaker should be good or...
This circuit produces a sinusoidal signal whose frequency can be calculated using the formula next...
This circuit was found in the 1987 Radio Electronics magazine. The components used are integrated...