Found in a 90s Popular Electronics, this sensor operates by infrared reflection. The components are modern, and the phototransistor can be of almost any type.
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...