This circuit is from an article that teaches how to assemble it in the magazine Electronics Now from July 1993. It uses a liquid crystal display module from the time with 3 and a half digits.
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...