Need help with a side project I am working on. Help with a simple circuit.

I am working on a custom computer case for my buddy as a surprise gift for when he returns home from Iraq. He is a big gamer and I have been building him a gaming computer over the last few months. I am almost finished with the case and now need a way to control the fans. Instead of buying an off the shelf and over priced rheobus I decided to build one that would fit my needs. The simple circuit for the voltage control using a lm317t, 300 ohm resistor, 240 ohm resistor and a 2.2k trim pot was easy. What I am having a problem with is figuring out how to make an RGB led light up a different color at a supplied voltage. IE: red at 3 volts, blue at 7 volts and green at 12 volts. Can anyone help me with the circuit design? I am going to build this on perf board so no need to generate any pcb designs in eagle or anything. A simple schematic will suffice.

Edited 1 time(s). Last edit at 12/28/2008 09:27PM by SCphotog.

I can't draw you a circuit but if you google for LM339 comparator I'm sure you'll find plenty. A comparator "compares" two voltages and "when Vin exceeds Vref, output switches from LOW to HIGH." Which is a quote from a Forrest Mims book from 1985. Comparators are still in use today.

... look for "window-comparator" (e.g. "Fensterdiskriminator" in german) or something similar - it's essential two combined comparators in one IC with two adjustable switching levels so you'll have your desired behaviour: - below first voltage-level <-> between first and second v-level <-> above second v-level ...

Viktor

This should do it: -



It uses a quad comparator LM139. The input voltage is divided by 2 and compared to half reference voltages because the max input voltage is Vs-1.5V.

It makes use of the fact that the forward voltage of a red led is less than blue when the voltage is between 3V and 7V so only the red lights.

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[www.hydraraptor.blogspot.com]

Thanks nophead.

I posted this questions over on 2 other electronics forums I frequent and got vastly complex answers. Yours was by far the simplest and cheapest way to go. I even had one guy suggest that I use an arduino to control a simple tri color led. I knew it was not as hard as they made it seem. I will be sure to post a photo of the finished product when its ready.

Just to be sure I understood your requirements correctly: the circuit will do the following

0-3V all leds off
3-7V red
7-12v blue
>12V green

Your fan supply would have to be more than 12V to ensure the green lights.

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[www.hydraraptor.blogspot.com]

I noticed that. I dont want to run the fans over 12V. Can the Circuit be modified to do the following?

0-3V red

3-7V blue

7-12V Green?

Yes lose the bottom comparator and connect the reference pins on the other three one resistor lower in the chain of resistors.

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[www.hydraraptor.blogspot.com]