This is a light switch or light activated relay circuit. The relay on when LDR uncoverd and relay off when LDR covered. Adjust VR1 for light sensitive. LED will turn on at the same time with relay.

This light sensor switch circuit allows the ... the input trigger current is 8 mA. Caution! This circuit is powered directly from 220V. Attention will be given for appropriate isolation of the switch ...

Check out the website of Luke Allen, a US Navy officer, where he details the project – Whistle-activated switch – and provides circuit diagrams, C code and explanations, and lots more. “This device ...