Monday, April 13, 2009

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.

What are PECUs?
How does the sun turn lights on and off?Bato Balani;ISSN 0117-7079 Vol.28 NO.6 SY 2008-2009


  1. Thanks for publishing this interesting article,
    because i have studied basic electrical engineering ur blog post looks to very be useful for me . I have visited many blogs but ur one is different in many ways.
    So thanks for writing on a topic of my interest.

  2. Jong you have a nice blog. Its very interesting. I have something for you at my blog. I want to pass the tag/award to you. Pls visit

  3. Not the ancient lamp outside our house. It's on all day and off all night. Even when they came and reset it it still reverted back a week or so later.

  4. Hei great blog you have. Interesting writing.

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    Take CARe

  5. I saw a lot of good article here,..thank for sharing!

  6. Hi,
    It's really interesting!
    Thanks for sharing!

    Have a nice day!

  7. Hei nice blog jong... Lot's great information... I invite you to my english blog,

  8. Hi Jong,

    Nice Blog, you just started this year in blogspot. how did you reach pr3 that fast?

    I'm still noob in blogging, hope you don't mind sharing some tips...


    Jhong Medina

  9. That was a major improvement on our street lights engineering, very cost effective and efficient. Saves a lot of resources and never failed when we need lights when it's getting dark. even in daytime that there's a typhoon. thanks for sharing mate!

  10. Hi Jong,

    Thanks for visiting my blog.

    Hmm.. yout blog seems to be a very informative and helpful blog.

    Thanks for sharing your knowledge with us.



  11. Very informative website Jong!!! I liked very much..

  12. Hi Jong, thanks for visiting my blog.
    I like your blog very much because it is very simple in desine and with log of interesting information.

  13. very educative. I knew it had to be something like this but never really knew the details.