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Light dpendent resistor

  1. Dec 16, 2006 #1
    Hi frnds

    Iam working on an investigatory project wherein iam studying the variation of resistance of an LDR with intensity of radiation and frequency.
    Regarding the frequency dependance iam having a doubt as the results tht Ive got arent consistent.
    Could anyone guide me..... whether resistance of an LDR depend upon frequency?
    If so how is the variation
    For this I took cellophene papers of diff colours over a light source and placed the LDR below it and measured the R with multimeter for diff colours.

  2. jcsd
  3. Dec 16, 2006 #2


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    What are the rsults you've got for the frequency dependence? What do you anticipate it should be, and why?

    Do you know how a standard semiconductor LDR works (i.e., the principle behinds its operation)?
  4. Dec 17, 2006 #3
    the results taken for 4 colours are as folows:

    BLUE:3090 ohm
    GREEN:1480 ohm
    RED:1030 ohm ...one of these reading is causing a problem in setting a trend.But As far as my knowledge is concerned I think the resistance of an LDR should fall with increasing frequency of incident radiation,but surprisingly the obsvn is highly contradicting.Moreover the LDRs have highest sensitivity towards Green light...but even this doesnt reflect in the readings...but iam pretty sure abt the accuracy of the readings.
    But I do not have enough ref material, i isnt in our syllabus and moreover the internet has less info.
    Iam in a fix ...pls help
    Last edited: Dec 17, 2006
  5. Dec 17, 2006 #4


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    Hmm...those results are a bit unusual. For one thing, I would have expected a larger resistance for red and yellow, with a typical (CdS) LDR. The reason for that, is that most LDRs are usually direct badgap semiconductors with a bandgap of about 2.5eV, which is roughly equivalent in energy to 500nm wavelength light. This is the minimum energy needed to make a drop in resistance. Pure red and yellow light have larger wavelengths than 500nm (and hence insufficient energy), so would not be expected to have any noticeable effect on resistance. But if the LDR you are using is different, it may have a smaller bandgap and hence respond to all wavelengths of visible light.

    There are, however, some experimental considerations that are important. For instance, how do you know that the light coming out through the different color filters has the same intensity? What is your light source? If the spectrum of the source is non-uniform (eg: if the light looks yellow) in the visible region, that would make for different intensities at different frequencies.

    Do you know what kind of LDR this is? Do you have a link to the specification sheet, or do you know the make and model?
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