A motion sensor activates the light when it detects something moving within its field of view. Photocells work with a resistor that changes its conductivity based on how many photons are hitting it.

There's really not much to the motion sensor. The sensor shoots out an invisible light field that spans 100 degrees horizontally and 100 degrees vertically. Its blind spot is 180-degrees to its sides.

If you can install a conventional outdoor light fixture, you can install a motion sensor light, although aiming and adjusting it takes extra effort. The post How To Install Motion Sensor Lights ...