What you need:
- CR2032 battery. Get them at Digikey for about $0.25 each. I wouldn't trust the ebay ones.
- Diffused 5mm Slow or Fast Fade Flashing RGB LED from Adafruit. $4.95 / 10 pack ($0.50 each!)
- 330 ohm resistor.
- Rare earth magnet. I go to ebay for these. I used 10x1.5mm, but about any type should do..
- Soldering iron and solder.
- Hot glue gun.
Fast fade on the left, slow on the right.
Step 1: Resist!
The lower the LED voltage, the higher the current it will draw! I know you EEs are ready to hand out a dope-slap, but hey, there it is. When I tested a plain old yellow 2V LED with it was pulling more than 40 mA, way more than healthy! A higher voltage LED like blue or white would only pull 10 mA. Still, at 10mA, how long is a 225 mAH battery going to last? Yes, about a day. We still need a resistor.
- Wrap one end of the 330-ohm resistor around the LED's short lead, solder it and clip it.