How Do Automatic Outdoor Lights Work?

Timed Lights

Some outdoor lights work on a timer. There is a small clock built into an individual light or attached to a system of lights which turns them on and off at preset times. This system is often used in parking lots, public parks and other places which require lighting during the night but not during the day. In some timer systems, the lights can actually be set to turn on earlier or later at different times of year to provide a lot of light during the winter and save energy during summer.

Photodetectors

Like timers, photodetectors turn off the lights when it is light outside and turn them on when it gets dark, but they use a different technology. A photodetector detects the flow of light. Most lights use photoresistors, cells which resist the flow of electricity as it becomes darker. The light control circuit runs a small voltage through the resistor. When the voltage drops enough, the circuit determines that it has become dark and switches the light on. Photodetector lights have a few advantages over timers. They automatically adjust to the changing lights at different times of year, eliminating the need to reprogram them as the days shorten and lengthens. They also go on when the sky gets dark and overcast, which timer lights don't do.

Motion Sensing Security Lights

Motion sensing security lights also use a photodetector, but it is one designed to register infrared light. All objects radiate infrared light. The wavelength of the infrared and the amount depends on how hot the object is. Humans radiate infrared light of about 9 to 10 micrometers. These lights have photodetectors aimed at the ground and looking for a burst of infrared in the right range. When someone walks by, the detector turns the light on until either the person leaves or a preset period of time has passed.