testing a LDR beginner mistake ??

Discussion in 'General Electronics Chat' started by greenJames, May 7, 2013.

  1. greenJames

    Thread Starter Member

    Apr 24, 2013
    68
    3
    Ok I Back from my long weekend away and thought I would test a LDR so I pulled up a tutorial on the net

    this is thew one I have chosen http://www.circuitstoday.com/how-to-test-an-ldr

    But I'm not getting the same results as explained in the Tutorial ?

    I have set my LDR on a bread board with a digital multimeter as I photo

    [​IMG]

    This image is in full sun light and giving me a reading of .138 KΩ and not 500 Ω

    in image 2

    [​IMG]

    My meter reading in the shade is .488 kΩ when it should read 200 km

    what have I done wrong

    James
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Did you try to connect the LDR directlty to the crocodille clamps?
    The wires if the LDR might be to thin for the breadboard.

    Bertus
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    First of all, do you have the exact same LDR as in the tutorial?
    Also, shade≠darkness. It could be that the sun is a little brighter than what the author had at the time, and your shade clearly is not dark, there still is a LOT of incident light hitting the sensor. Try putting it in a sealed box or something and measure again.
     
    Last edited: May 7, 2013
  4. greenJames

    Thread Starter Member

    Apr 24, 2013
    68
    3
    do I have the meter on the right settings ??

    James
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    The LDR is working as its resistance is changing with varying light levels.. Thats all you need to know.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Yes, .488kΩ is the same as 488Ω. Also you can try putting black tape over the sensor or blocking it in some other way to get the dark resistance.
     
  7. greenJames

    Thread Starter Member

    Apr 24, 2013
    68
    3
    Ok I will

    I will try later as I have to go out now

    James
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,370
    Your LDR is working properly. It's resistance has changed from 138Ω in the sun to 488Ω in the shade. Every LDR will respond differently to light.
     
  9. #12

    Expert

    Nov 30, 2010
    16,346
    6,835
    I've played with those, and all mine went to millions of ohms in the dead dark, but they are slow. More than a minute to settle on a dark reading. And they are all a bit different and YOU can't hope to get the exact amount of sunlight or shadiness that somebody else did unless you use the ohm meter readings to tell you when you got it right, not make some level of shade and demand that the variable resistor comply to your level of shadiness with a proper ohm meter reading.

    Brightness on Earth varies by about 100,000 to one between moonlight and sunlight. You can't possibly hit exactly 500 ohms by holding you arm over a photocell.
     
  10. greenJames

    Thread Starter Member

    Apr 24, 2013
    68
    3
    Thank you all

    James
     
  11. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,652
    767
    Have you got only one LDR?

    With more than one in the same protoboard (similar conditions) testing them would be very enlightening.

    My suggestion for the future: get used to have several of each component even expensive chips.
     
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