Two color LEDs

Two color LEDs are LEDs that can glow more than one color. Their body color are usually translucent white. They can have two or three pins. Here we'll focus on the three pin variety - the two pin variety requires a different, more complicated driver circuit to work right.

The three pin unit also contains two distinct LED chips in it. One will glow one color, and the other, some secondary color. Usually one is green and one is red, but a orange+green one has been seen before. If both LEDs are turned on, the color mixing you'd expect will occur, and a orange/yellow color is emitted. Both chips share one pin, the 'common' pin.

PC/Computer LED Use Theory

LEDs are used to display status. They can be your hard disk light, power, turbo (if you have an old one), and many many other purposes. You'd have hundreds of possible LED stats, but reduced to a select few that are important. Reduced to so few, that only a handful is on the front of your CPU case.
So, why not use these multicolor LEDs? For instance, my case HDD LED glows red when IDE is being accessed, and green when SCSI is.

LED Drivers

How are LEDs turned on and off? There are transistors of some sort switching the LED ON and off. It could be any kind, bipolar or metal oxide field effect. This transistor is connected into series with the LED to switch it 'on' and 'off'. To properly turn an LED on and off rapidly, a common emitter/common drain circuit is used. So a PNP BJT transistor could be used to "turn power on" or a NPN to "turn ground on" in layman's terms. Not getting into device physics, it's usually the "NPN" or "N-channel" device that's used to power LEDs because of their ability to conduct current in minimal amount of area (due to electron mobility). So, *most* LED circuits are powerred this way - power to a resistor to the LED to the collector of an N-channel MOSFET or NPN transistor. There could still be a p-channel or PNP transistor there but it won't be used.

The problem

Now, we mentioned that it could be designed either way, we could have the situation where the two circuits are driven differently, so there would be hard to hook up a shared terminal device. There is another issue - most 3 pin LEDs are invariably common cathode devices! So if the circuit is using NPN transistors to power the circuit, you're out of luck.
There is a good scenerio, where both circuits use complimentary drivers and reference to ground. This case you can just use the 2-color LED directly.
This, however is rarely the case! :( And who knows how your output circuit is designed. So, the following circuit will totally eliminate any guesswork involving output drivers and reference points.

Just hook it up (You need power) and go! That's it!

Parts list:
Circuit board
Two 4N35/4N36/4N37/H11A1 optoisolators - to emulate the LEDs
two 4.7K ohm resistors
two 820 ohm resistors
two 2N3906 transistors
one dual color 3-pin LED

Theory of operation: The LEDs in the optoisolators now light up when the output is being accessed. The circuit will then use that output to light the LEDs.

With a common anode device, the two transistors can be omitted, however, you'll get better brightness with this circuit as the saturation current of the 2N3906 transistors is higher than of the optoisolator transistors.

Copyright (c) 2004 blc

Permission is granted to resell this circuit as long as you put credit where it's due.

2004May02-Found error in schematic, fixed (Transistor Collector/Emitters drawn incorrectly. I wish I had a good sch capture program, ooimpress is all I got).