This circuit triggers a relay when a light beam that hits the phototransistor is cut off. The circuit is from Electronics Now and the supply voltage depends on the relay voltage.
This circuit is from a March 1978 Radio Electronics magazine article that showed how to build it....
This circuit was obtained from an article in Radio Electronics magazine in October 1966. It...
In the figure we have a bistable circuit that activates LEDs from touch rings and turns off data...