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.

Thanks in advance.

 

Not so simple for a "n00b"

It would be easiest to use an LM339 quad comparator, with two of them configured as a "window comparator" for the midrange, one for the low voltage range (ON unless the voltage is into the midrange value) and one for if the voltage is over the midrange value.

This Google search will net you many hits:
http://www.google.com/search?hl=en&q=window+comparator+circuit

Note that those circuits using opamps as comparators are not designed well. When a comparator is called for, use a comparator.
Sorry, but I don't have time to help you more at the moment; I'm on my way out of town.

 

Here's basically what you need:
http://www.discovercircuits.com/DJ-Circuits/voltst1.htm

 

This little gadget should work (I didn't tested it). Adjust the two trimmers for proper sequence.

Voltage devider will supply a logic zero to xor gate when below nominal value so led will be off. when 5 volts move to 7 Volts, second devider will trigger both the xor gates turning off the red led and turning on the blu. Same thing for the 12 volts. Zener are there to save the logic gate.

Red led will be on with voltage as low as 1.6 V. If you don't like it, place a third trimmer to adjust also the 5 Volts

Alberto