Friday, March 8, 2013

Reusing LEDs

Before I got a hold of the red LEDs I used in the 3x3 LED matrix, I was having a hard time finding a local source for LEDs of any kind. When the holidays rolled around I had a look at some of the Christmas tree lights that were for sale and for a few dollars I bought a string of 100 white lights that look like this:

As you can see from the picture, the head is concave. This type is easy to see from a distance but doesn't look very good on a breadboard. The string of lights this LED came from consists of four seperate strands. The first few LEDs on each strand have resistors soldered to their legs as you can see here. This makes them convenient to plug into a breadboard, even though concave LEDs don't look very good up close.

Later on I found a similar string with frosted convex LEDs. Each of the four strands of 25 LEDs was a different color. I bought several boxes and in the end I paid less than 3 cents per LED! The first few lights on each strand also have resistors soldered to the legs:

One thing I found very interesting about these LEDs is that they grow very brightly. The resistors soldered to the legs are Orange-Black-Red-Gold. This should mean that they are 3000 ohm resistors. In order to get 20mA out of these LEDs I would need around 60 volts. However, when hooked to the 3.6v of my LaunchPad they glow very brightly. By my calculations, if the forward voltage of the LEDs is around 2 volts, the LEDs should be using less than 1mA when the supply voltage is 3.6v. Clearly something isn't as it seems!

Hopefully, I can repurpose some of these LEDs to make a matrix. This should be convenient since they already have wires soldered to their leads. Since each string has four different colors, I don't have enough LEDs to make a large matrix of only one color. My plan now is to combine a red, a green, and a blue LED together to make one RGB pixel. Like other matrixes I saw on the internet, I tried ping pong balls as diffusers. Here is the result using four of the colored LEDs and one of the white ones:

They seem to diffuse very well but ping pong balls here are too expensive to be used for a whole matrix. Once I find a better way of diffusing light, I can move on to soldering the matrix.

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