Maybe the best thing about this DIY project is its name: the Joule Thief. It’s a tiny LED flashlight that runs off dead batteries, eking out the last drops of voltage from a dead AA and throwing them into the lamp in spurts of current that are enough to make it light up.
Unless you’re an electronics geek, you won’t have all the parts you need laying around the house, but they’re easy to buy on Amazon (like this toroidal transformer core). Popular Science, which published the project tutorial, estimates a cost of around $30, plus the cost of the dead batteries, which is of course $0, as you have them all in a little baggie in your home’s recycling corner.
Making the circuit requires a little soldering. In layman’s terms, here’s how the circuit works: It pulls the tiny amount of voltage left in the in the cell into a transformer, creating a magnetic field, as transformers do. Then the voltage is cut, and the magnetic field pushes itself into the LED in the form of a current strong enough to light it up.
Think of it as trying to catapult a melon off one side of a see-saw by dropping a ball bearing on the other end. If you drop that ball bearing from the height of a passenger jet, then it’ll make the melon move (more likely the ball bearing will just drill through the wooden teeter-totter, but you get my point). The Joule Thief circuit does this, and it does it in pulses so fast that the LED appears to be constantly lit.
I like three things about this project. First, it looks like great fun, the kind that takes enough time to be satisfying, but not enough that you’ll abandon it half-way through. Second, it’s genuinely useful. Who wouldn’t want a little flashlight that runs of otherwise useless batteries?
And third, it’s clean. Reuse is always better than recycling, and here you get to do both.