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 LEDs
15 diffused green LEDs
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 shield
some male headers to attach the protoshield to the Arduino
I decided to make the tree with five layers and a single white LED on top. This meant I needed to construct 5 six sided stars to support the different layers. I decided on a base 8 cm across which implied that the stars should decrease around 1.3 cm per layer. I then measured out an 8 cm equilateral triangle on a piece of hard wood and drove in three nails at the vertices. I then bent some medium gauge copper wire around the nails to form a triangle. I cut off the excess wire and soldered the two ends together.
I then built a second identical triangle. The two triangles were soldered together to form a six sided star.
Four more stars were made similarly by decreasing the distance between the nails by about 1.3 cm per layer. The smallest layer was pretty tiny and I might try decreasing the amount of change per layer next time.