This circuit is from a 1968 Electronics World. It acts directly on a high voltage alternating current relay. The sensor can be a common LDR.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...