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, which applies to configurations with triacs of the TIC series, was found in a...
This configuration can be found elsewhere on this website and section. The circuit is from Popular...
The basic component of this circuit is a general-purpose SCR. The voltage is according to the...