- #1

- 3

- 0

## Homework Statement

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.

## Homework Equations

I think I know how to devise the relevant equations:

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

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

## 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: