# Basic window detector

#### vandr

Joined Oct 28, 2011
2
Hello all, this is sort of a continuation of a circuit question asked on this forum a couple years ago. I think I know how to do this but wanted to make sure before I actually make the circuit.

Here's a general diagram: EDIT: seems to be broken, here is url: http://forum.allaboutcircuits.com/c...ome.cogeco.ca/~rpaisley4/ComparatorWindow.GIF

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.

I think I know how to devise the relevant equations:

VREF1 = R3 / (R1 + R2 + R3) * V+
VREF2 = (R2 + R3) / (R1 + R2 + R3) * V+

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?

#### Jony130

Joined Feb 17, 2009
5,243
Do you know V+ ??
And set the current that flow through R1; R2; R3 in range 10mA....1mA

#### Jony130

Joined Feb 17, 2009
5,243
Do you know V+ ??
And set the current that flow through R1; R2; R3 in range 10mA....1mA
For example
R3 = R2 = 1KΩ; and R1 = (V+ - 2V)/1mA =

#### vandr

Joined Oct 28, 2011
2
Do you know V+ ??
And set the current that flow through R1; R2; R3 in range 10mA....1mA
Yeah I set V+ = 5V. Are you saying I should apply loop rule? I'm unsure how to do that with comparators.

EDIT: Upon seeing your new post, I see how if V+ is 5, R1 should be 3K for those values. Any particular reason why you chose R3 = R2?

Last edited:

#### Jony130

Joined Feb 17, 2009
5,243
Simply use the Ohms law.
The current thats flow through R1; R2; R3 is equal
I = V+ / ( R1 + R2 + R3)
So if we set I =1mA we have
R3 = 1V/1mA =1K
R2 = (2V - 1V)/1mA = 1K
R1 = (5V - 2V)/1mA = 3K
So use R3 = R2 = 1K and R3 = 3K
Or for example R3 = R2 = 6.8K and R1 = 20K.
For more accurate voltage setting you need to use the potentiometer.

• vandr