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Help needed in AS physics sensors coursework.

  1. Nov 10, 2006 #1
    Hello everybody,
    This is my first post and I need some help in my AS Advancing physics OCR coursework.
    My coursework is about seeing how well an LDR responds to various frequencies of light (eg 5Hz, 10Hz...). I am looking at responding times. I have got data about extra time taken to respond, and also measured voltages. I need help in knowing what all I can do with this data to make my coursework better!!
    Please can anyone advice me and give me some help.
    Thanks
     
  2. jcsd
  3. Nov 10, 2006 #2

    berkeman

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    Staff: Mentor

    In this context, you are referring to the frequency of the amplitude modulation of the light, not the frequency of the light itself. To vary the frequency of the light, you would need to use different colors, and extend into infrared and ultraviolet frequencies.

    As to your paper, you should be able to get a lot of what you need from the datasheet for the light dependent resistor (LDR). Look for the transient response figures, and also for any frequency response graphs. Your test data should match that information in the datasheet.
     
  4. Nov 10, 2006 #3
    Hello berkeman
    First of all thanks for ur help!!
    My coursework involves using a stroboscope to flash light at different frequencies (5 times a second, 10 times a second... etc) on the LDR. This LDR is then connected to the oscilloscope where I see how long the LDR takes to respond to the flashes of light. This may be the amplitude modulation as you said, but I'm not sure.
    I have also measured voltages from the oscilloscope. But now I don't know what to do with it. How should I analyse my results?? Also, I need to state some uses of knowing the response times for an LDR, which I'm stuck at as well!!
    Any help on these two issues would be great!!
    Thanks
     
  5. Nov 10, 2006 #4

    berkeman

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    Staff: Mentor

    What kind of bias circuit was used with the LDR? I've used photodiodes and phototransistors before as detectors, but never any LDRs. Do you have a link to the datasheet for the device you used?

    With photodiodes, their response time (and hence their bandwidth) varies with the bias circuit. In general, the higher the reverse bias voltage applied, the faster they will run (because of the reduced junction capacitance and other factors). Is there a similar effect with LDRs?

    So assuming the bias circuit was right, what you probably got on the oscilloscope was something like a square wave response to the pulsing (yes, that's 100% AM) light, with RC-like tails on the rise and fall. As you increase the frequency of the modulation, the RC-like rounded parts of the waveform start to dominate, and eventually the size of the waveform diminishes since there is not enough time for the LDR to respond fully to the light. The "3dB" corner frequency is where the amplitude is only making it up to the full possible amplitude minus 3dB. Quiz question -- what is the ratio of two peak voltages that gives you a 3dB difference?
     
  6. Nov 11, 2006 #5
    I do not understand what you mean by a 'bias circuit'. My circuit involves a 5V dc power supply. Then a LDR (or PHOTO RESISTOR) and variable resistor (10k, 100k, and 1M Ohms) connected to it. Two wire across the LDR are connected to a PC scope, and the PC scope connected to a computer (which makes it a oscilloscope). The stroboscope flashes the light on the LDR, and I can then look at the oscillations on the computer screen. I also change the resistance, which changes the voltage readings. Now I have taken 5 sets of readings with each frequency. I have also taken 2 reading with EACH of the three resistances (which is the voltage).
    Now the thing that I want to know is what all I can do with the voltage readings? Also, what are the uses in general of knowing how well an LDR responds?

    ps. If you want then I can email you my 'method' sheet to you that I was given by my teacher.
     
  7. Nov 13, 2006 #6

    berkeman

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    Staff: Mentor

    Do you have a pointer to the datasheet for the LDR? Is the LDR a photodiode (which generates a reverse photocurrent across the PN junction in response to light), or just an amorphous silicon blob that changes its resistance based on light falling on it?
     
  8. Nov 14, 2006 #7
    Hello
    I have given in my coursework to my teacher now, so I can't make any more improvements to it.
    Thanks a lot for your help!!:smile:
     
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