Day&Night with Photocell

Discussion in 'The Projects Forum' started by Return, Jan 5, 2013.

  1. Return

    Thread Starter New Member

    Jan 5, 2013
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    I have a 163 5mm leds in parallel powered with 15v that needs to turn on when dark. What else would be required besides a NPN Transistor and a photocell to do this function?
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    A CdS light sensor is better than a "photocell", and is the common method.

    You don't need much more than a CdS snesor, any small signal NPN transistor and a few resistors, maybe a cap.

    That type of control will slowly light the leds over half an hour as it gets darker. If you want the leds to snap fully on you need another NPN transistor and one more resistor, to add a "schmidt trigger" hysteresis effect.
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    What is the total current drawn by your LEDs? Can you post a schematic?

    I, too, think that an LDR is a good choice, but you may need a MOSFET to handle the current.
     
  4. Return

    Thread Starter New Member

    Jan 5, 2013
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    Thanks for the quick replies, as for the current I have something similar that runs off of 600mAh.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Thanks Tracecom I missed the "163 LEDs" part. I thought it might have been a type of LED or something.

    It might need a decent sized FET or even a relay, depending which the OP is more comfortable with.

    A lot also depends on the PSU voltage and whether it is a battery or PSU etc.
     
  6. Return

    Thread Starter New Member

    Jan 5, 2013
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    [​IMG]

    So I think I got everything right. Drain -> Negative of led, Source - > Negative , and Gate -> ldr and resistor.

    You can see the whole pcb here if you wanted to add any feedback as it is my first pcb. http://f.cl.ly/items/0g3r0X2H430S1Y2K1W3W/pcb.png
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    That looks like it will work, assuming it's a NFET.

    I would use a 500k pot instead of a 100k fixed resistor, as it will let you adjust the light level where the FET starts to turn on.
     
  8. sheldons

    Active Member

    Oct 26, 2011
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    an easier way is to use transistors-arrange two as a schmidt trigger and an extra one for switching your leds....bc547s etc may do,depending on the load you are going to switch...
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It only needs one NPN transistor added to give some hysteresis, the NPN and FET together can make a schmidt trigger effect, see post #2.
     
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