Opposite photoresistor??

Discussion in 'The Projects Forum' started by Kefka666, Aug 31, 2008.

  1. Kefka666

    Thread Starter Active Member

    Mar 4, 2008
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    I bought a photoresistor at Radio Shack today and noticed something peculiar about it. When exposed to light, the resistance decreased. In the dark, however, the resistance increased dramatically. Isn't this the opposite of how a photoresistor is supposed to function?

    I thought that photoresistors work in the opposite manner: when exposed to light, the resistance should increase, and when in the dark, the resistance should decrease to a very small value.

    Why does my photoresistor operate opposite how normal photoresistors should?
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    You thought wrong...your photoresistors works right. :D

    Ken
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Think of it as a partially photoconductive device.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    Really, it doesn't matter.

    You can use a photoresistor on either side of a voltage divider to drive the base of a transistor to control much greater currents/voltages.

    For a dark detector, you can place the photoresistor on one side of a voltage divider.
    For a light detector, you can place the photoresistor on other side of a voltage divider.

    You can use a 10k resistor or a pot for the other side of the voltage divider.
     
  6. Kefka666

    Thread Starter Active Member

    Mar 4, 2008
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    Here's why I'm confused. I built the circuit in the photo but it still performs the opposite way I'd expect. I've varied R1 from 1K ohm to 51k ohm and observed no significant difference. I can't figure out why the LED still turns on only when the photoresistor is exposed to light. Shouldn't it now turn on only in the dark?
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    You might be better off using the photoresistor between the base and the positive supply. You could then introduce another common emitter stage if you needed the LED to illuminate in the absense of light.

    hgmjr
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    That's odd. :confused:

    Are you sure that's an NPN transistor?
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    Are you absolutely convinced that you have the pinouts correctly identified?

    hgmjr
     
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