It's been a while since I stumbled across LED Throwies. I had never been compelled enough to build one, but then I stumbled across RGB blinking LEDs at Adafruit. There must be a dozen instructables on Throwies, but I've done a couple of tweaks to the original project and wanted to share it, so here you go. Blinkies! Under a buck each, and last about a week!

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!

Why add a resistor? The internal resistance of a 3V coin cell is enough to keep the LED from blowing. I always keep a coin cell around to test LEDs before they go in to projects. The question is, how much current are they drawing. Here is probably the biggest revelation for me on this project:
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.

Step 2: Rare Earth

Our magnet is not just going to hold the blinky in place. It is going to hold one of the LED's leg against a metal object to make the current flow. This will act as an on-off switch, which will further extend the length of time the LED will glow.

Place the magnet on the positive side of the battery and tack it in place with hot glue. Not too much now, or it will leave a gap, preventing the lead from making contact.
Curl the end of the LED with the resistor attached. This will give you more area for the glue to hold it.
Hold the battery in place, ensuring it lights, and solder the resistor side to the battery negative side. Make sure the LED stay lit until the glue hardens.

Step 3: Could We Be Done?

All that is left is to align the free LED leg so it doesn't contact the magnet. Test it out against something metal. If you need to, you can put something non-conductive between the leg and the battery until you are ready to use it.

Remember, once the battery is dead, peel the LED off and reuse it while recycling the dead battery safely.

We did this for a library event craft project. See more of my projects at MakersBox.blogspot.com

Your challenge is to find a crafty project to encorporate a blinky in to. How about a card-stock lighthouse or a firefly? Send me a picture and I'll post it here!

Have fun!