We found this circuit in the September 1971 issue of Radio Electronics magazine. The sensor is an LDR and R1 adjusts the sensitivity. A relay with the appropriate voltage can be used instead of a buzzer.
In this circuit, the current in solenoid X1 can be adjusted and its intensity measured to the...
Found in American documentation from the 90's, this circuit is very current and can be implemented with...
This interesting high-quality amplifier configuration was featured on the cover of the June 1994...