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A throwie is a small LED attached to a coin battery and a rare earth magnet.
In quantity, they are used for creating non-destructive graffiti and light displays.
Singly or in small quantities they are just fun.
You use them by throwing individual LEDs onto (ferromagnetic) metallic objects.
By throwing LEDs onto an object, the object itself acts as a canvas.

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Throwies usually consist of a CR2032 or pair of CR2016s with an LED held across the battery with tape and a strong magnet. Very simple, no current limiting resistor, easy for kids to assemble. Lots of fun.

What are the properties for a good throwie LED?

Good things, possibly in order of importance:

  • Safe (no exploding or burning batteries, LED can withstand being thrown, etc)
  • Bright
  • Cheap
  • Omnidirectional light
  • Different colors
  • Lasts a long time (days great, a few hours minimum)

While I know there is no right answer, I'd like to know the properties to look for and a range of the values that would make a good throwie.

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  • \$\begingroup\$ What is a Throwie? \$\endgroup\$
    – jippie
    Commented Oct 27, 2012 at 22:44
  • \$\begingroup\$ @jippie See en.wikipedia.org/wiki/LED_art#LED_throwies \$\endgroup\$
    – walrii
    Commented Oct 27, 2012 at 22:49
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    \$\begingroup\$ Knowing what a "throwie" is seems important to your question, so you should explain at least the part that is relevant. No, we shouldn't have to go chasing a link to get basic necessary information. You can't expect everyone to know this non-standard word. \$\endgroup\$
    – Olin Lathrop
    Commented Oct 27, 2012 at 23:02
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    \$\begingroup\$ @OlinLathrop Sorry, I thought throwies were well enough known in popular culture that I didn't need to explain. I added an opening paragraph to the question. \$\endgroup\$
    – walrii
    Commented Oct 27, 2012 at 23:18
  • \$\begingroup\$ I agree with @OlinLathrop - I know this has already been addressed (a few years back :-) )- but I think it worth noting in support that it is far better for anything that MIGHT be unclear to older members (eg Olin or me :-) ) to be at least briefly explained AND for a link to be added to allow those interested to read more. In situations like this a photo also seems like a good idea. I'd never heard of 'throwies' - my knowledge base has been increased. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jul 31, 2014 at 11:37

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This product is a consumer liability. Preventing a short and subsequent exposure to rain or water is paramount.

Ignoring this risk outlined in the MSDS of any Lithium Manganese Diodxide battery and blocking any vents with sealant or epoxy is also a bad idea.

There are kits from China for under a dollar per CR2032 10mm diffused LED, but I have no experience with them so I won't recommend any.

Here are some guidelines:

  • Energizer coin cells have approx 10 ohms internal resistance (IR) which rises quickly towards end of life. Panasonic appears to have a much higher IR (~100ohm) perhaps for safety reasons.
  • Diffused LED's would provide best display properties at all angles. 10mm LED's can be bought for <25 cents but compare mcd levels with 5mm LED's.
  • Different colors have a nominal voltage that may exceed the 3V cell voltage which has a large positive temperature swing of +/-0.3V that may also force excess over or under brightness. Close to 3.2V will last long but dimmer. Close to 2.8V will be brighter but last shorter.
  • Expect maybe 50% of rated 225 ~ 250 mAh capacity when overdriving well beyond rated load.
  • Getting the right match of LED's to CR2032 is a matter of luck since there are so many variables uncontrolled.
  • use silver conductive epoxy for a reliable bond of leads on either side of coin cell, sparingly with excess lead length sheared off.
  • Rare earth magnets must be epoxied to case are very brittle so must be coated.
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