Welcome to my first Instructable. I'd appreciate any constructive criticism you care to leave in the comments, but please be gentle.
Having recently built a 4x4x4 LED cube, I thought it might be interesting to apply the same techniques to building an LED Christmas tree. I used a single white LED as the 'star' and then a mix of red and green LEDs forming the sides. Initially I thought to make the tree star shaped in cross-section, but eventually realized that a six sided star would be easier as the supports could be made out of two identical triangles.
I used an Arduino to control the light patterns with a sketch loosely based on some sketches for a 3x3x3 LED cube.
What you are going to need:
1 medium white LED
15 diffused red
15 diffused green
6 resistors (values depend on the LEDs chosen, I used three 10 ohm and three 1k ohm)
some medium gauge copper wire
some fine gauge copper wire
an Arduino, I used a Freeduino
an Arduino compatable prototype
some male headers to attach the protoshield to the Arduino