Intro: The Red/Green LED Guide
The Bi-color LED is a handy little component that allows two colors (red and green) in a single LED while only having two pins (cathode and anode). The color of the LED depends on the polarity of the connection only allowing one color at a time. Similar LED's that provide two or even three colors usually have three or four pins allowing for a wide range of mixed colors. One would think having two pins is a disadvantage; not so. This LED can easily be applied to a circuit to visually indicate polarity direction. Or in my case it can save me an extra i/o pin on a forthcoming arduino project.
Step 1: Straight Out of the Package
The first thing you'll notice is that this looks like any other common LED, but with a white diffused lens. It has a short negative lead (cathode), and a long positive lead (anode). On the inside there are two LED's in parallel to each other, one forward and one reverse. The package included with the LED from radio shack provides a similar looking schematic to the one I provided above.
Using circuitlab.com I made this simple schematic. I actually used 3 86R resistors in the demo video so don't worry too much about getting the resistor values exact (Resistor's Here). Just try not to go any lower than that.
In this design you will see exactly how the bi-color LED operates. When the button is open the LED shine one color, either red or green. When the button is closed the other color will light up. So if the LED was green when the button was open, it will change to red when the button is closed. Simple enough.
Step 3: Using BI-Color LED's in Your Projects
The whole reason I purchased a bi-color LED was to be used in a forthcoming Arduino project. The output pin on the arduino (when used in conjunction with a Relay) will act as our push button in the previous step. The idea is to have the led glowing red until certain perimeters are met. Upon meeting a certain criteria the output pin will go HIGH causing the LED to glow green.