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.
This circuit was found in the June 1970 Radio Electronics magazine. The power to the load is triac...
This circuit can be used in measurement bridges. We found it in the September 1995 Electronics Now magazine....
Damped oscillators can be used to produce sound from percussion instruments such as the triangle, bell,...