photocell / lm317 / bright LED

Discussion in 'The Projects Forum' started by docb, Sep 12, 2010.

  1. docb

    Thread Starter New Member

    Sep 12, 2010
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    I want to use an lm317 to drive a bright emitter type led, but I'd like to have a photocell to dim that same LED.

    Is there some small change in the circuit that will do this?

    If you have some other low parts count way to do this, I'm interested.

    THANKS
     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
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    What is this LED going to do?

    If it's illuminating a scene, then the brighter the ambient light is, the dimmer you'd want to the LED to be (i.e you wouldn't want to run it in daylight).

    If it's an indicator light, then the brighter the ambient light is, the brighter the LED should be, in order to be visible.
     
  3. docb

    Thread Starter New Member

    Sep 12, 2010
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    I'm basically looking for just a switch. Just 'on' or "reduced". (trip alarm type scenario)

    When the PSR gets light, I want the LED to be normal brightness.
    When the PSR gets covered, the LED should dim to a reduced brightness.

    Would be nice to be able to tweak the switching point, and the reduced brightness level.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    A large Cd- S light dependent resistor[ LDR ] or PSR can have a light resistance of around 15Ω. My 1/2 in measured 30Ω. Connect a low value pot in series with LDR and connect to LM 317 as constant current regulator. Adjust pot for proper LED current. Connect a high val. pot[ 1k- 10k] in parallel with LDR to adjust dark brightness.
     
  5. docb

    Thread Starter New Member

    Sep 12, 2010
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    [​IMG]So basically what this drawing I did shows?
    I showed this design on a UK electronic site a few days ago, and was told it was not workable, basically because photocells had too high a resistance...

    [​IMG]
    [​IMG]

    Any suggestions as to a good choice of PSR with low resistance like you seem to have?

    Will the PSR have any issue with too much current going thru it?
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    1 A might a bit much for CDS cell. Found a 1 in. cell but listed at less than 1k- light.or an assortment G14025, 20 pcs at The Electronic Goldmine. Back to drawing board.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    Here's an idea, though you'd need to do some experiments to get the right resistor values. I'm pretty sure the brightness would vary in the right direction as the ambient light changes! You might need one more resistor in series with the photocell, if its resistance is low.

    You could make it adjustable by making R2 and R3 variable resistors instead of fixed.

    As for a threshold, I think that can't be done with a voltage regulator. You'd need to use an op amp or comparator, which would be another very simple design.
     
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  8. docb

    Thread Starter New Member

    Sep 12, 2010
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    If pr1 never goes under about 1k, I don't see how this circuit would do anything.

    Am I missing something?
     
  9. John P

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    Who said that? Bernard talked about a resistance as low as 15 ohms.
     
  10. docb

    Thread Starter New Member

    Sep 12, 2010
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    I can't find any PSRs that go anywhere near that low.
     
  11. John P

    AAC Fanatic!

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    Put a bunch of them in parallel.

    Oh well, if that doesn't appeal, you might try boosting the output with a transistor. It seems viable, but I'm not sure what the resistance in parallel with the transistor base ought to be. Making that resistor variable would essentially give you a setpoint for the LED boost.
     
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  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    What is powersupply voltage?
    What is LED V drop?, Current?
    Where are you located?
     
  13. docb

    Thread Starter New Member

    Sep 12, 2010
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    5v supply
    3.6v or so
    maybe 50-150ma
    Los Angeles

    In this circuit the lm317 is acting as a constant current device, so the desired current is set by that resistor.
     
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