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.
We found this circuit in a European publication from the 1970s. The transistors are made of...
The TAA131 used in this circuit is not very easy to find today, but an equivalent low power audio...
This circuit was suggested by National Semiconductor in its Linear Application Handbook. The...