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.
) Other operational amplifiers series, such as the MC34072, MC34074, MC35071, MC35072, MC35074 or...
The transformer in this power supply can have a secondary winding from 1 to 15 A of current....
Found in American documentation from 1969, this circuit switches signals with a FET of the time....