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Electronic circuit, LED turns on if Vin is greater than or less than a value

  1. Oct 20, 2011 #1
    1. The problem statement, all variables and given/known data

    Construct a circuit that turns on if it the voltage provided is greater than 3 volts, or less than 2. MUST USE THESE CRITERIA: i<100mA, only one PS. Fewer than ten components.

    2. Relevant equations

    None that I know of, but we've covered resistors, caps, inductors, and op amps thus far.

    3. The attempt at a solution

    I'm pretty lost on this one. We've studied op amps, so I would know how to do this if this involved having one voltage source be greater than another, that is, using a comparator. Where I'm unsure is where you compare Vin to two different voltages. The problem description depicts 2V, 3V, and Vin going into an undefined element (our circuit) and then out to an LED. Unsure if this means that the 2V and 3V are PS's.

    Sorry for the vagueness on this one. I'm also kind of unsure, since the problem is poorly worded.
  2. jcsd
  3. Oct 20, 2011 #2


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    Staff: Mentor

    My first instinct would be to do it with two comparators. Try sketching up that circuit, and see how many components it takes. Assume that the 2V and 3V inputs are available, so you don't need to make them with resistor dividers off of the power supply. You should include hysteresis around the comparators in a practical circuit, but you may not need that if this problem is dealing with more simple ideal circuits.

    Show us what you come up with for a 2-comparator circuit so we can discuss it.
  4. Oct 20, 2011 #3
    I'm assuming it'd be something like the attached (sorry for the crappy MS paint job)

    Attached Files:

  5. Oct 20, 2011 #4


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    Staff: Mentor

    That's a pretty simplified version, and it's missing something on the outputs. Use Google Images to look at various examples of Window Comparators....
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