Instructables
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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.
 
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Step 1: Resist!

Picture of Resist!
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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.