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Homework Help: Circuit design help

  1. Nov 1, 2008 #1
    This is my first time posting here at PF, so forgive me if i am going against any rules.

    The problem statement, all variables and given/known data
    I haven been given a problem to design a circuit that takes an input ranging from 0 to 5V, and outputs:
    20V (DC) if the input is between 1 and 4V
    nothing for 0 or 5V.

    The attempt at a solution

    From what i have learned from class/research, a Shmidt trigger would be a good approach.
    i designed one that "kind of" works, but doesnt do what i had intended. it does output a 20V DC signal if the input is between 1 and 4V, but if i go above 4V, i still get a moderately large output (~12V).
    below is my circuit and an explanation for my resistor values.


    from what i understand, with a Shmidt trigger,
    Voutput = +V
    V1 > +V*R2/(R1+R2)

    once greater, Voutput would drop to -V.

    i have set the positive and negative supply voltages on my comparator to be 20V and 1mV respectively.
    thus, i was expecting Voutput to go to ~1mV when Vin > 4V.

    any insight/hints would be greatly appreciated.
  2. jcsd
  3. Nov 5, 2008 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    I see 1 problem, and possibly 2, with this design.

    Yes, the output will become ~1 mV when V1 rises above 4 volts. But, at that point, the equation for the reference voltage (+ input of op-amp) will be

    Vref = (1 mV)*R2/(R1+R2)

    What is that new reference value then (hint: it is not 4V anymore).

    You'll need to design things so that the Vref is fixed, not tied to the Vout which can change.

    Another possible issue is that on most op-amps, Vout does not go all the way down to V--, or all the way up to V++. But if this is a class assignment, you can probably assume ideal behavior and not worry about that.
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