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How do street lights activate automatically at night? – Working of the easy day and night street lamp controller
Updated on April 16, 2013 livingsta moreContact Author Some people might need a question on their mind as to how the streetlights activate automatically when it starts turning dark. There are two ways in which streetlights can work. It is determined by what mechanism is definitely implemented on the streetlights. Some countries use timers to control the streetlights, while some others use photodetectors to regulate them.

Timer controlled street lamps usually use an electronic clock that’s built into the complete 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-to-day thereby keeping the lights on for longer periods within 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 sunshine on, when the quantity 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 can also be called CdS cell.

A photograph-resistor changes its resistance based on the quantity 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 implies it conducts electricity (electricity is flow of electrons) very well. This is because of 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.

A simple circuit showing the photodetector, transistor and relay. Now considering a simple circuit, a CdS cell will likely be connected to a relay (An electromechanical device through 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 within the diagram. So when lot of light falls on the photodetector, current flows and it energizes the electromagnet while a small amount of light wouldn’t. The electricity that flows through the CdS cell, when light hits on it, is so small that it won’t 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 implies along with the CdS cell, transistor and relay; it might have more number of transistors, depending on the dimensions of relay.

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sendingAuthorlivingsta 17 months ago from United Kingdom

Hello Prachi,

Thanks for stopping by. I’m glad that you just found this useful! 🙂

Prachi 18 months ago

very helpful article. I loved this one!!!

Hello Whipoorwill , thanks for stopping by and sharing your experiences.

Hello Deji , thanks. I am glad that you just found this helpful!

Authorlivingsta 20 months ago from United Kingdom

Hello vivek parashar, thank you. Glad that you just found this interesting!

Whipoorwill 21 months ago

Our town also has street lights that switch on and off otherwise dark cloudy and moonless nights can be much more darker

Deji 2 years ago

Good article. Very helpful

vivek parashar 2 years ago

nice i like it……………

Authorlivingsta 5 years ago from United Kingdom

Thanks Mr-Mediocre!

Mr-Mediocre 5 years ago

Ah, yes. Electromagnetic waves – the bane of my existence yet the basic source of the way in which we live. Good article.

Tim 6 years ago

What would you need to connect 2 street lights off off one relay, duplex or triplex??

Authorlivingsta 6 years ago from United Kingdom

Thank you everyone in your encouraging words !!!

esraustun 6 years ago

very nice concise explanation here. bravo

Peter Enmore 7 years ago

Never though I might read a hub about the photodectors in streetlights. Awesome, this makes my day!

patrick 7 years ago

thank this may help me doing my technology project!

Authorlivingsta 7 years ago from United Kingdom

Thanks Kaka for passing by!!!

Kaka 7 years ago

nice hub dahlia

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