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.
In this circuit from a 1991 publication, we can use the 2N2646 unijunction transistor and the...
In the figure we have the way to use the transistors of an old-paired pair as a trigger circuit. The...
This static switch or electronic contactor was found in a 1968 Motorola publication. The triac...