Sunlight-only photosensor?

Discussion in 'The Projects Forum' started by optimus9, Sep 15, 2009.

  1. optimus9

    Thread Starter New Member

    Sep 15, 2009
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    My project is to make an automated curtain system which closes at night & opens at dawn.

    It'll be controlled by a photosensor (photodiode or LDR) & a timer.

    The whole circuit is ok, except that I need a photosensor that only works with sunlight.... so that if someone shines a light on the photosensor at night, it won't be activated.

    Any suggestions? :)
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I think this is generally done by implementing a delay. The sun has to be down or up for a set period of time before the curtains will open or close. Lighting or blocking light to the LDR for shorter periods is ignored.

    Ken
     
  3. optimus9

    Thread Starter New Member

    Sep 15, 2009
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    Thanks! Any advice on how to implement the delay?
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
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    Ken's suggestion is a good one, but I can see it failing if a room's lights are detected by the photosensor. Thus, make sure the sensor only sees light coming from the outside.

    It also can be complicated by other things, depending on where you live. In the winter where I live, we can get a snow covering and an overcast. The city lights are reflected back to earth and, while not quite daylight, it's easy to see down the street. This light level might fool a sensor, so an adjustable threshold would be necessary.

    Since appliance timers are dirt-cheap, I'd recommend using one of those. New they're probably $10 or less and you can probably find one at a second hand store for a buck or two. They usually have a time resolution of 15-30 minutes and you'd have to correct for length of day changes a few times per year, but they're reliable and cheap. I'm making a chicken coop door opener/closer that will use one (instead of a photosensor).
     
  5. ELECTRONERD

    Senior Member

    May 26, 2009
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    I designed a similar circuit for my mailbox. I had a device that would tell me when the mail came, and if I happend to forget the mail that day, when night time came about, the circuit would shut off. It used a photocell. You could use this circuit and then feed it to a microprocessor to figure out how long your curtains are and how fast the motor will be rotating. You can experiment and find out that a certain duration will let the curtains all the way up or all the way down. Now you can insert this duration into the microprocessor and then feed this to a driver for the motor. Thus, day time comes and flips the microcontroller to bring the curtains down which acts like a switch for the driver (turning it on) and turning on the motor. The duration stops and the curtains are all the way down. And, vice versa.
     
  6. optimus9

    Thread Starter New Member

    Sep 15, 2009
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    Thanks for your replies. I'll be implementing the delay as advised by KMoffett. ;)

    To someonesdad:
    Actually I've already implemented the application timer into the circuit, but I wanted a 2nd control element - the photosensor.

    To electronerd:
    I initially thought about a microprocessor, but I'm trying to keep it as simple as possible. :)
     
  7. ELECTRONERD

    Senior Member

    May 26, 2009
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    Well I suppose you could implement a RC circuit for the delay, but the problem is that it wouldn't be very accurate. That's why I suggested the microprocessor.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    You might consider a photodiode that is designed to detect only UV (320 or 280 nm) and has IR blocking. Such devices would be quite insensitive by several orders of magnitude to artificial light (sodium vapor, incandescent, fluorescent). They might show response to high-pressure mercury or mercury halide, but that would require testing. Moreover, such lights are not that common. Digikey lists two products, but they are non-stock and do not have prices shown.

    John
     
  9. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    A more reliable solution is a dawn / dusk clock.
     
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