Setting the threshold at which the circuit switches the output is adjusted by a variable trimpot acting as a voltage divider with the LDR.

It uses MOC3021. Front end of the circuit is powered by the 5volt dc supply available from the usb port of the PC. When circuit is powered, the light sensor LDR (R2) resistance is low if there is ...

The low-powered device consists of a PIC micro-controller, an LDR (light dependent resistor) for light sensing and a low current relay which switches between the two modes.