This circuit, from a 1973 publication, locks the relay when the light that falls on the LDR is cut off. The circuit can be used as a alarm or presence annunciator. The LDR is the large type used in direct high voltage applications and the high voltage relay from 48 V to 110 V. The R2 adjustment pot can be 22k x 5 W of wire. For the 110 V network, the transformer can be eliminated.
