Originally Posted by Classic Lighting
Why is it that photo cells fail in the "on" position. I've come across 3 this month and they have all been "on".
The default state of a photocell is a closed circuit (lights on). When the photo receptor is energized with light, a current goes to a relay that opens the circuit (lights off).
If the receptor window is dirty or if the sensing circuit has been damaged, then the relay is not triggered and the lights stay on.