Simple Photoresistor Circuit

Discussion in 'The Projects Forum' started by gwpmike, Oct 18, 2015.

  1. gwpmike

    Thread Starter New Member

    Oct 18, 2015
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    I want to create a simple circuit that is essentially the opposite of a dawn/dusk circuit.

    A dawn/dusk circuit would allow a light to turn on (or close the circuit) when it is dark outside, and would turn the light off (open the circuit) when it is light outside. I want to do the opposite.

    I want a LED to be able to turn on ONLY when it is light outside. I want the LED to not turn on when it is dark outside.

    In other words, close the circuit when the photoresistor detects light, and open the circuit when the photoresistor does not detect light. Please see the attached diagram.

    [​IMG]
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    There are quite a few out there of varying complexity; google "light activated switch schematic".
     
  3. GS3

    Senior Member

    Sep 21, 2007
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    Last edited: Oct 18, 2015
  4. ISB123

    Well-Known Member

    May 21, 2014
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    You are going to need to play with VR1 value since we have no idea of whats the resistance of LDR under certain Lux.

    Here:
    1.png
     
    Last edited: Oct 18, 2015
  5. GS3

    Senior Member

    Sep 21, 2007
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    I like to have hysteresis to prevent instability. If it is just an LED for play it does not matter but if it is a relay controlling something it shortens the life of the device and is annoying.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    In the post #4 schematic, add a large value resistor from the comparator + input to its output.

    ak
     
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  7. ISB123

    Well-Known Member

    May 21, 2014
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    Yes,add positive feedback resistor to introduce some hysteresis.
     
  8. GS3

    Senior Member

    Sep 21, 2007
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    Yup.
     
  9. GS3

    Senior Member

    Sep 21, 2007
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  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Here's a hand trace of an outdoor light fitting that shuts off in daylight:-

    The photodiode looks very like the IR PD in the front panel of a 90s CTV, but apparently it worked just fine for sun light.

    The basic circuit is a dual op amp with a voltage follower stage to handle the high resistance of the PD, the second stage is a Schmitt trigger for clean switching.

    The mains aspect of the circuit can be disregarded, it ends up requiring 9V DC for its supply.
     
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