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How do street lights turn on automatically at night? - Working of the simple day and night street lamp controller
Some people might have a question on their mind as to how the streetlights turn on automatically when it starts turning dark. There are two ways in which streetlights can work. It depends on what mechanism is actually implemented on the streetlights. Some countries use timers to control the streetlights, while some others use photodetectors to control them.
Timer controlled street lamps usually use an electronic clock that is built into the entire system. The clock turns the street lights on in the evening and off in the morning automatically depending on the timer settings. Some systems have a slightly different timer that varies slightly day by day thereby keeping the lights on for longer periods in the winter months and for shorter periods in the summer.
Now coming to the modern street lights which use photodetectors, a small circuit turns the light on, when the amount of light falls below a particular threshold. Photodetectors are small electric components called photo resistors which are sensitive to light. Cadmium sulfide photo resistor is a commonly used photodetector which is also called CdS cell.
A photo-resistor changes its resistance based on the amount of light that falls on it. When lots of light falls on it, its resistance drops to almost zero, thereby allowing lot of electrons to flow through it, which means it conducts electricity (electricity is flow of electrons) very well. This is due to Photoelectric Effect (When electromagnetic radiation strikes certain materials, usually metallic, electrons are emitted). When no light falls on it, its resistance increases and reaches a maximum, thereby decreasing the flow of electricity considerably.
Now considering a simple circuit, a CdS cell will be connected to a relay (An electromechanical device in which changes in the current flow in one circuit (that flows through the device) are used to open or close electric contacts in a second circuit) directly. The relay circuit is shown in the red square in the diagram. So when lot of light falls on the photodetector, current flows and it energizes the electromagnet while a small amount of light would not. The electricity that flows through the CdS cell, when light hits on it, is so small that it will not be enough to activate the relay. So a transistor amplifier is added to the circuit, to amplify the current flowing through the circuit. The transistor acts like a switch. When light falls on photodetector, it switches the transistor on. This will energise the electromagnet and the light goes off. When it is dark, no light falls on the photodetector and hence the transistor goes off and relay is deactivated making the light go on.
Real street lights may have a bit more advanced circuit, which means along with the CdS cell, transistor and relay; it might have more number of transistors, depending on the size of relay.