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 high current circuit using a MOSFET and an operational one was found in a documentation from...
The frequency of the signal that this circuit passes through depends on the voltage set in RV1....
This trigger uses two switching transistors that can still be found with some ease. The circuit is from...