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?
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?
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.
An easy calculator for voltage dividers to use with window comparators; http://www.electronics2000.co.uk/calc/potential-divider-calculator.php