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.

But the idea persists, and from time to time we see them here at Hackaday. [Gord Payne] has one with an elegant sensor solution, it detects finger movement using a light dependent resistor.

Recently an ad on YouTube about an acoustic light sensor/switch module piqued my interest. Out of curiosity, I contacted the online vendor and bought a pair. Named MR-SK50A, the module is less popular ...

Witnessing its success, over 2,000 LDR devices have been set up in 600 gram panchayats of Hanamkonda, Warangal, Mahabubabad, Jogulamba Gadwal and Sangareddy districts.