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.
We found this circuit in 1969 American documentation. FETs admit equivalents, as do transistors. The...
Found in a Popular Electronics from 1992, this circuit was created to give effects to an...
Both the field effect transistor and the integrated circuit of this 1970s circuit are no longer very...