Photocell (Dusk-to-Dawn Sensor)

A photocell is a light-sensitive switch that turns outdoor lighting on at dusk and off at dawn automatically, with no timer to drift or schedule to maintain. On street lights it usually plugs into a standardised NEMA or Zhaga receptacle on top of the fixture, making it replaceable in seconds and upgradeable later to a smart-city controller using the same socket. Modern photocells switch electronically with a built-in delay to ignore headlights and lightning, and quality units are rated for hundreds of thousands of switching cycles. In solar street lights the function is built in: the panel itself acts as the sensor, with the controller switching the LED on when panel voltage falls at dusk — one reason all-in-one solar fixtures need no commissioning beyond mounting.