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 excites an LED, which will flash at a frequency adjusted by the trimpot. The other...
This circuit, found in a publication from the 1970s, produces a 1 Hz sinusoidal signal....
In this circuit, the output voltage is Vxx plus the zener voltage. The circuit is from the SGS...