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.
This circuit is from a Modern Electronics from April 1990. The circuit can be assembled with BC558...
We find this circuit in a Popular Electronics of the 80's. The frequency range depends on C1 and...
This circuit was obtained in an old Argentine publication, using germanium transistors and...