Basic window detector

Discussion in 'Homework Help' started by vandr, Oct 28, 2011.

  1. vandr

    Thread Starter New Member

    Oct 28, 2011
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    Hello all, this is sort of a continuation of a circuit question asked on this forum a couple years ago. I think I know how to do this but wanted to make sure before I actually make the circuit.

    Here's a general diagram:

    [​IMG]
    EDIT: seems to be broken, here is url: http://forum.allaboutcircuits.com/c...ome.cogeco.ca/~rpaisley4/ComparatorWindow.GIF

    So as you can see I'm trying to make a window detector circuit that makes an LED turn on if the voltage is below or above a certain threshold. In this case, I want the LED to turn on above 2V or below 1V.

    I think I know how to devise the relevant equations:

    VREF1 = R3 / (R1 + R2 + R3) * V+
    VREF2 = (R2 + R3) / (R1 + R2 + R3) * V+

    I'm just unclear on what to make R values. It seems that R2 always comes out as 1/2, which makes sense mathematically (if you set VREF1 = 2, VREF2 = 1) but makes no sense to me in the real world. I've always used resistors with usually at least 100 Ohm values. But maybe I'm wrong. Help?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Do you know V+ ??
    And set the current that flow through R1; R2; R3 in range 10mA....1mA
     
  3. Jony130

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    Feb 17, 2009
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    For example
    R3 = R2 = 1KΩ; and R1 = (V+ - 2V)/1mA =
     
  4. vandr

    Thread Starter New Member

    Oct 28, 2011
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    Yeah I set V+ = 5V. Are you saying I should apply loop rule? I'm unsure how to do that with comparators.

    EDIT: Upon seeing your new post, I see how if V+ is 5, R1 should be 3K for those values. Any particular reason why you chose R3 = R2?
     
    Last edited: Oct 28, 2011
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Simply use the Ohms law.
    The current thats flow through R1; R2; R3 is equal
    I = V+ / ( R1 + R2 + R3)
    So if we set I =1mA we have
    R3 = 1V/1mA =1K
    R2 = (2V - 1V)/1mA = 1K
    R1 = (5V - 2V)/1mA = 3K
    So use R3 = R2 = 1K and R3 = 3K
    Or for example R3 = R2 = 6.8K and R1 = 20K.
    For more accurate voltage setting you need to use the potentiometer.
     
    vandr likes this.
  6. shortbus

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    Sep 30, 2009
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