Do you wonder how does the sun turn street lights on and off?

Before, most street lights are controlled by time switches, which operate a group of lights in nearby streets. The earliest time switches worked by clockwork and had to be wounded up and adjusted every week. Then this was replaced by an electric clock with a rotating dial, containing levers or off at chosen times. They are similar to many time switches on central heating systems.

Nowadays switching times of street lights need not be adjusted anymore. Street lights automatically turn on as soon as darkness falls and turn off as soon as they are no longer needed. This is made possible using the device called Photoelectric control unit or PECU, a light-operated switch. PECUs switch on the supply when the light level falls beneath a given vaue (at dusk) and switch the supply OFF when it rises above another level (at dawn). The ratio between two ight levels is known as the switching ratio.

A photo cell in a PECU device contains a ight-sensitive compound such as cadmium suphide or silicon. Cadmium suphide has an electrical conductivity approximately equal to the level of light. At dawn, light falling on the photocell causes the eletrons to flow from one atom to another, onducting electricity to the switch and turning it off. When darkness fas, the eectrons in the compound become immobile, the current stops, and the lights are turned on. The exact time that the current is switched on and off depends on weather conditions.


Sources:
What are PECUs?www.zodionltd.eu.com/pecus_1.html
How does the sun turn lights on and off?Bato Balani;ISSN 0117-7079 Vol.28 NO.6 SY 2008-2009