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Window detector circuit

  1. Oct 28, 2011 #1
    1. The problem statement, all variables and given/known data

    I think I know how to do this but wanted to make sure before I actually make the circuit.

    Here's a general diagram:
    [PLAIN]http://forum.allaboutcircuits.com/image_cache/httphome.cogeco.ca~rpaisley4ComparatorWindow.GIF [Broken]

    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.

    2. Relevant equations

    I think I know how to devise the relevant equations:

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

    3. The attempt at a solution

    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?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 28, 2011 #2


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    Staff: Mentor

    What will be the output voltage when one of the comparators wants to drive it high while the other wants to drive it low? Which one wins? How can you prevent the fight?
  4. Oct 29, 2011 #3


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    Staff: Mentor

    He's probably using open collector comparators. That would be the traditional way to do it.

    @dylandrop -- I'd suggest adding hysteresis around the comparators. Comparator circuits without explicit hysteresis feedback generally oscillate/buzz near the switching points.
    Last edited by a moderator: May 5, 2017
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