Circuit Design Help for 0 to 5V Input and DC Output of 20V

[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.

 

[piercas]

Hello! Welcome to PF. Your approach with a Shmidt trigger is a good start, but there are a few things you could improve upon to get the desired output. First, the comparator you are using may not have a rail-to-rail output, meaning it cannot output a voltage equal to the supply voltage. You may need to use a different comparator that has a rail-to-rail output. Additionally, the resistor values you have chosen may not be appropriate for the desired output. You may need to use a voltage divider to scale down the input voltage before it reaches the comparator. Finally, you may want to consider using a transistor as a switch to turn on the 20V DC output when the input voltage is within the desired range. I hope this helps and good luck with your circuit design!