After reading about using an Arduino and a traffic light, I was inspired to do a smaller-and-cheaper version at work for our Buildbot setup.

Lights: 10,000 mcd LEDs from Sparkfun in blue, green and red. $1.50 each, get two of each in case I toast a few. (I also bought a BlinkM RGB light, which at 8,000 mcd is almost as bright and easier to drive.)

Driver circuit: The LEDs are 80mA, 3.0V drop so I needed a current amplifier. It’s been a while, and my EE was nearly gone, but this excellent post walked me through the design. Simple emitter-follower circuit based on the PN2222 transistor does the job nicely:

Assuming a beta of 100, the base resistor works out to 4.7k, and the current limiter to 25 ohms. (I used 33 as being the closest).

Three circuits in a row on the breadboard, driven by digital I/O pins 6-8:

Yep, it works! The LEDs, slightly current-limited by the USB power, are still too bright to look at directly. Quite noticeable.

Next up, the ethernet shield is on order, and I have Python code to poll the buildbot for last-build-status. Fun stuff!