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Homework Help: Investigation on a Light Dependant Resistor

  1. Mar 27, 2007 #1
    Special lamps may be purchased to help plants grow indoors. These lamps produce light with wavelengths comparable to natural sunlight. A manufacturer wishes to check these lamps by measuring the out put of the lamp at different wavelengths.

    The above is the scenario given :smile:

    You are required to design a laboratory experiment using a light dependant resistor (LDR) to investigate how the intensity of light emitted by a lamp varies with wavelength.

    The requirement of what i need to do above :smile:

    A diagram of the arrangement of your apparatus, and in your account you should pay particular atttention to

    a. the procedure to be followed
    b. how the wavelength of the light falling on the LDR is determined
    c. how a measure of the intensity can be obtained from the LDR.
    d. the range and precision of any instruments that would be used
    e. the factors that would need to be controlled to ensure that it is a valid test
    f any safety precautions you would take when carrying out the investigation
    g particular features of the design that would ensure the accuarcy and reliability of your results.

    The above is the crucial content that i need to put into my report :smile:
     
  2. jcsd
  3. Mar 29, 2007 #2

    berkeman

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    Staff: Mentor

    Thread moved from HH-Advanced Physics to HH-Engineering.

    SMUDGY, you got a lot of help in a previous thread where you were asking about measuring light with an LDR:

    https://www.physicsforums.com/showthread.php?t=156512

    Tell us what-all you learned from that thread, and what your thoughts are about this assignment.
     
  4. Apr 9, 2007 #3
    That is a different scenario to the one i am doing now. It doesnt provide enough information to me because it talks about a photocell and i am using an LDR and i need to work out the wavelength and intensity of visible light falling on the LDR. That talks about a laser
    the procedure to be followed.
    how the wavelength of the light falling on the LDR is determined.
    how a measure of the intensity can be obtained from the LDR.
    i cannot find any information on these and i need desperate help.
     
  5. Apr 9, 2007 #4
    i have found that you can use diffraction grating to measure the wavelength of light and use the rearranged formula: lamda=sSintheta/n where s is the separation of 2 slits and n is 1 for the first spectrum. But i am unsure how to do a diagram for it and a method of how to carry it out
     
  6. Apr 9, 2007 #5

    berkeman

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    As you say, you will need to use a grating or prism to separate the wavelengths of light. But then you need to have some calibration mechanism, I would think. Either that, or you would have to know the distribution of the intensity of the various wavelengths of light in your light source. What do you know about your light source?
     
  7. Apr 10, 2007 #6
    i am using a vertical filament lamp. how would you measure the distribution of the intensity of the wavelengths of light in the filament lamp.
     
  8. Apr 10, 2007 #7

    berkeman

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    Staff: Mentor

    If it's a filament lamp, then its emission spectrum should follow the black body radiation curve:

    http://en.wikipedia.org/wiki/Black_body_radiation

    So you need to find out what the operating temperature is for your filament, and that will give you an idea of the intensity variation with wavelength. I'm not sure how to find out the temperature for your filament -- if it's a high-quality bulb, maybe it has a datasheet. If not, you could try looking at the datasheets for high quality lab light sources to see if they list typical operating temperatures.
     
  9. Apr 13, 2007 #8
    What equipment do i need to use to do this experiment and how do i set up. Im stuck on this cause i only need to plan it out. I wont actually be doing the experiment
     
  10. Apr 16, 2007 #9
    I also have this assignment. im having difficulty finding how the wavelength of the light falling on the LDR is determined. How exactly can the diffraction grating be used for this?
     
  11. Apr 16, 2007 #10

    berkeman

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    Well, one thing you can do is to shine the light source through the diffraction grating or prism, and mark on a piece of paper where the light falls, what colors are where. Then look up the wavelength of those different colors of light in the rainbow (use wikipedia.org or whatever), and write those on the paper as well. Then when you move your LDR setup over those places on the paper to intercept that color of light, you will have a moderate calibration to the wavelength. Make sense?
     
  12. Apr 17, 2007 #11
    You can add me to the list of people working on this as well :rolleyes:

    For equipment, I have put:

    Power Supply
    Ray Box
    Prism
    Paper
    LDR
    Ammeter

    I've chosen to use the paper method with prism, as I was unsure about using the diffraction grating. In my method, I have got as far as moving the LDR through the different wavelengths. I am stuck on part C of the assignment: "How can light intensity be determined from the LDR?" Anyone help?
     
  13. Apr 17, 2007 #12

    berkeman

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    What can you tell us about LDRs? How do they work? What are the typical measurement circuits that are used with them, and what kind of error terms do you need to consider?
     
  14. Apr 18, 2007 #13
    I know that as the light intensity decreases, the resistance increases, but do not know a forumla for working out an exact intensity. As an error I have put ambient light, and limited display on meter.
     
  15. Apr 18, 2007 #14

    berkeman

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    Staff: Mentor

    I don't know anything about LDRs, but I'd suspect that their resistance goes down as the light intensity increases. Do you have some datasheets for LDRs? What about application notes on how to use them?
     
  16. Apr 18, 2007 #15
    Here are two datasheets of LDR:
    http://www.biltek.tubitak.gov.tr/gelisim/elektronik/dosyalar/25/LDR_NSL19_M51.pdf
    http://www.makingthings.com/resources/datasheets/acc_pht_001.pdf [Broken]

    And here some websites with circuits including LDR:
    http://homepages.westminster.org.uk/electronics/voltage.htm#what [Broken]
    http://www.antonine-education.co.uk/Electronics_AS/Electronics_Module_1/Topic_5/topic_5__resistive_input_transdu.htm [Broken]
    http://www.nexusresearchgroup.com/technical_data/sensors.htm#intro [Broken]
     
    Last edited by a moderator: May 2, 2017
  17. Apr 18, 2007 #16
    thank you for the help so far, and yes, resistance goes down as light intensity increases. I do not have to explain in great detail about the workings of the LDR, as the assignment is on a 500 word limit. My main problem is actually working out what the light intensity is, based on the wavelength and resistance.
     
  18. Apr 21, 2007 #17
  19. Apr 22, 2007 #18
    Can anyone explain the equation? i dont get it.
     
  20. Apr 22, 2007 #19
    Hello

    I seem to be doing the same thing and my first question is that if the graffting method is not used and the black body is used what apparatus should be used and with the formula since i do not really get it as well should it be quoted or its better explained and if better explained could someone help in doing so........please
     
    Last edited: Apr 22, 2007
  21. Apr 22, 2007 #20
    After seeing the formula i don't know what the numbers included represent but its basically (2500/Vout-500)and all that divided by "3.3"
     
  22. Apr 22, 2007 #21
    after speaking to my lecturer about it, he says that getting a figure for the light intensity is far too complex, and even he wouldn't know where to begin. He told me to use the diffraction grating, and put the LDR in a circuit, using a power supply and a ammeter. Use the currrent to represent light intensity, as you will still be able to see the changes... i.e

    as light intensity increases, resistance will decrease, so the current will increase as well.

    Hope that helps people...
     
  23. Apr 23, 2007 #22
    I've also spoken to many different teachers about it and they've all said that its an extremely difficult thing to do at this level of teaching.

    However if it helps, the method that I was suggested would be to use coloured filters which come with a wavelength datasheet, that way you've got your wavelength and can use an ammeter to measure the intensity (in the way that milleniumbug suggested).

    As for discluding ambient light, its almost impossible unless done in a dark room.

    Hope this helps! :cool:
     
  24. Apr 24, 2007 #23
    hey im also doing same thing - and stuck

    hey
    i was considering doing the prism option for my experiment and but i was wondering how i can measure the angle from the prism in just one colour - e.g blue no red and white

    and if there is a equation to do this

    i was also wondering how u connect your LDR into your circuit and what the whole point of using the LDR is for aswell as how you would go about drawing the diagram

    i understand that the LDR is an input sensor which converts light into resistance - so the red light should have the highest resistance as it has the longest wave lengh?

    i was also wondering where u would connect the LDR if it is to measure light does it have to be infront of the light

    i think these are really stupid questions sorry but i would be grateful if you could help me
    thanks
     
  25. Apr 24, 2007 #24
    hye sorry just though of another question - how do u meausre the light intensity by using a mmeter >? sorry for asking so many questions :P
     
  26. Apr 24, 2007 #25
    getting an exact reading of light intensity, is too complex for this level.

    Substitute light intensity for current, as they are both affected by resistance in the same way. As resistance increases, it means that light intensity will be decreasing, and so will the current.
     
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