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Homework Help: Proximity sensor with LDR

  1. Dec 30, 2007 #1
    1. The problem statement, all variables and given/known data

    I am just about to start my AS level Advancing physics - Sensors coursework.

    I was wondering whether it would be a good idea to build a proximity sensor using an LDR in a potential divider.

    I would reflect an LED off an object, and move the object at different distances away from the LDR.

    I would take readings of the voltage across the potential divider output with the object at these different positions, and plot/analyse graphs of distance against voltage.

    Would this be a good idea? Or would I get a series of inaccurate and unreliable results?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 30, 2007 #2


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    There are two issues here:
    - the amount reflected back to the LDR will also depend on the particulars of the object (meaning you can't accurately determine distance for an arbitrary object)
    - background noise... there are some simple ways to get around this that I can tell you if you want.

    If you are serious about measuring distance I would suggest ultrasound. It is also possible to do this with light but short distances are very difficult due to the high speed of light.
  4. Dec 30, 2007 #3
    could I get rid of the first issue by using an LED to replace the object?

    Wouldn't doing the experiment in a dark room fix the noise problem?

    If not, how would I do an ultrasound sensor? I'm not really very familiar with them.
    Last edited: Dec 30, 2007
  5. Dec 30, 2007 #4


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    To a certain extent an LED object will "fix" the first problem but you'll need to keep the orientation constant. And yes, a dark room will get rid of most of the noise. But this isn't really a realistic situation is it? Depends what your main goal is I suppose.

    The idea with the ultrasound is to make a sonar... send pulses through the transducer and measure the time delay. It will be more complicated than your LDR system.
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