In this circuit of an American documentation of the 90s, when the light is cut, we have the production of a pulsating tone. R3 determines the trigger sensitivity.
Found in an English Maplin magazine in 1983, this circuit has an RIAA equalization. We suggest its...
This circuit was found in a 1979 magazine. We can use any JFET like MPF102 or BF245. The relay is 12 V...
This beat frequency oscillator or BFO was found in an American 1980s documentation. The circuit is...