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 version of a mixer with a transistor appears elsewhere in this section and on this website. The...
This circuit, from a 1995 publication, powers incandescent lamps according to the power grid. The...
This circuit is intended for underwater communications. Of course, the critical point is the...