Squeeze More Juice Out of a Dead Battery!

You’re walking the dog at night, and your little flashlight, which has been getting dimmer, conks out. Instead of stumbling home in the dark, wouldn't it be great if you could somehow eke a little more energy out of the batteries inside?

That's not as crazy as it sounds. When a battery-powered device stops working, we say the battery is “dead”—but it's not really. It still contains chemical energy, and the voltage isn't zero; it's just not high enough to run a current through the light bulb or LED (or whatever load you have).

But with a little physics chicanery, you can indeed get that light to run longer. Much longer! I'm going to show you how to build a simple electrical circuit, pairing a transformer and a transistor, to tap that residual energy. It's waggishly called a “joule thief” circuit. Get it?

It's not only fun to build and...

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