How does the sun turn city lights on and off?

At dusk and dawn, millions of street lights are turned on and off throughout the world every day  – many of them by the light of the sun its self.

Most 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 wound up and adjusted every week.

Many modern time switches now have an electric clock with a rotating dial, containing levers or tappets, which turn the lights on or off at the chosen times. They are similar to many times switches on air-conditioning systems.

Since the sun rises and sets at different times throughout the year, street lights must also go on and off at different times, so these dials also alter the switching times according to the season of the year.

This is arranged in the time switch by a mechanical device which adjusts the ‘On’ and ‘Off’ tappets month by month to follow the changes in the hours of daylight.

Recently, street lighting engineers have developed a photoelectric control unit called ‘pecu’, which operates a switch in the electrical supply to the lights.

A photocell in the unit contains a light sensitive compound such as cadmium sulphide or silicon. At dawn, light falling on the photocell causes electrons to flow from one atom to another, conducting electricity to the switch and turning it off. When darkness falls, the electrons in the compound became immobile, the current stops, and the lights are turned on. The exact time that the current is switched on and off depends on the weather conditions.

 

Picture Credit : Google