We found this circuit in the 1964 RCA transistors manual. The sensor may be a common LDR and the transistor BC558. The relay has an operating voltage according to the power supply.
This circuit was found in an American publication in 1966. The circuit uses a general-purpose FET that...
This circuit from a Popular Electronics from the 90s uses a common optocoupler and a...
The selected circuit was found in the Texas Instruments optoelectronics manual. RC and C1...