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Finding unknown resistance

  1. Nov 8, 2012 #1
    If I have a circuit similar to this: http://i.imgur.com/JaUGb.png

    I am reading in voltage as an input from Vin, when switch is closed, and am wanting to find out the unknown resistance R. Similar to dmm...what would be the best approach? Reading the voltage, calculate current to find R?
  2. jcsd
  3. Nov 8, 2012 #2


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    Staff: Mentor

    Use a PIC uC with an on-chip ADC...
  4. Nov 9, 2012 #3


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    Science Advisor

    The voltage at Vin will be proportional to the resistance R.

    So you can say Vin = R / (R + 1000) * 5 volts

    Once you measure the value of Vin, you just put it in the above formula and work out the value of R.

    For example, if the value of Vin was 3 volts, you would move the 5 volts and the (R + 1000) to the left of the equation and say 3 volts / 5 volts = 0.6

    So, 0.6 * ( R +1000) = R
    so 600 ohms = R - 0.6R = 0.4 R

    R = 1500 ohms

    Now check, 1500 / 2500 * 5 volts = 3 volts
    So, the answer is right.
  5. Nov 9, 2012 #4
    thanks vk, going to give it a try with my module.
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