Schmitt trigger outputs 12V above 4V instead of shutting off

[danonymous]

Hi!
This is my first time posting here at PF, so forgive me if i am going against any rules.

Homework Statement
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
and
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 doesn't 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
until
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.

 

[Redbelly98]

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.