This circuit works like a solid-state contactor. We find it in General Electric documentation from 1965. SCRs can be modern equivalents according to the controlled load.
This source of constant current was obtained in an American documentation of 1973. The transistors can be...
When light fails to focus on the sensor, a common photo-transistor, the relay closes its contacts....
This low impedance circuit was found in an Electronic Design from the 90's. Operational amplifiers admit...