Thursday, June 11, 2009

Joule Thief + Another Fridge Magnet

So the main, big project I'm working on now is getting kind of frustrating, and I thought I'd take a break tonight with a few quick projects.

I started with a empty box of chocolates from Trader Joe's. It had an adorable design with a porthole in the middle, and I had to use it somehow. Using a dremel I cut the bottom of the box off, and then I glued on three magnets with super glue. The top fits snugly onto the bottom, so all that was left was to find a picture for my picture frame.

Big Daddy character copyright 2k, of course.

Anyhow, since that project had just about two steps, I needed something else to do. Half an hour on instructables (an amazing site, by the way) reminded me that I had always wanted to build a Joule Thief. It is a remarkably simple project, but I still feel proud of myself.

You basically need an NPN transistor (2N3904), an LED, an old battery, a small toroid magnet, a 1kohm resistor, and thin wire. Optionally, you'll want a AA battery holder and a switch. I already had everything except the toroid.

Of course, the Radioshack store people had no idea what I was talking about when I asked for the toroidal magnet thing, so I found the part number myself. If you have to go to Radioshack and pay their inflated prices, here it is. Note you can probably try to use the 273-108 if you can't find the 273-109 in stock.

The 2 AA battery case leaves plenty of room to house the battery and the innards (in the second battery's place). It is a little black and boxy, but you can decorate it with stickers and such.



  1. I myself am a fan of Big Daddy brand chocolate wedges from the friendly folks at Trader johan's. And I think that such seals should be created for all sorts of things.
    As for the Joule thief...I still don't really get the purpose. Is it just putting mostly dead batteries to use powering a tiny light?

  2. Instructables is so great. You gotta register for the newsletter, you get to see some awesome stuff.

  3. This comment has been removed by the author.

  4. (Oops, typo. reposting comment)

    In response to naudasd:

    The tiny light actually has many uses. I have one such tiny-light-flashlight on my keychain (but it uses not-dead batteries). Having one on your keychain is extremely useful, for reading menus in dark restaurants, looking under the hood of your car, reading parking signs at night, as an electric torch for slow songs at concerts where lighters are not allowed, and so on. Since I have so many old batteries, it makes sense for me to power it with those. Also, you feel good about using energy that would otherwise go to waste.

    Additionally, the joule thief can be used for other reasons as well. Normally an LED requires at least two AA batteries to run. With the joule thief, it only requires one. Though I'm not sure what modifications the circuit needs, you can design one to power many more LEDs or other devices. I'm actually going to make a joule-thief nightlight that automatically turns off/on depending on light conditions.