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We’ve all been there: You attempt to power up a project and it just won’t start. You grab for your multimeter to examine the voltage of each battery, and there’s always that one culprit that isn’t exactly “dead” but simply doesn’t have enough juice to be viable any longer.

You could recycle it, but what if there was a project that could still squeeze every little ounce of power out of the “dead” battery and also provide you with a useful tool?

Enter the joule thief circuit and the Vampire Flashlight project.

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With just a few common components, combined with a toroid that can be salvaged from junk electronics or purchased new, you can build your own flashlight that squeezes a bit more life from all your exhausted batteries. Even a fresh battery with full-charge won’t light “high flux” LEDs, and yet “dead” batteries will with a little ingenuity. We show you how over on the project page.

Maker Juan Garcia saw the project and decided to first breadboard the circuit to test it:

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He noted that the LED wouldn’t light up until the resistor in the circuit was bypassed, possibly due to the toroid being a slightly different material, or that in combination with the wire used to wind the toroid producing a different resistance value. Either way breadboarding the circuit before building will allow you to test the components you have, and adjust accordingly. In the end Juan’s build looks great:

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And the real-world application? Also great!

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Watch our video for the Vampire Flashlight below to see how fun this project is to build (it should also be noted that we have a lot of fun making these videos, as you’re about to see!). If you mod your own version of this project be sure to send us some images and a story of your experience with this Weekend Project!

Nick Normal

I’m an artist & maker. A lifelong biblioholic, and advocate for all-things geekathon. Home is Long Island City, Queens, which I consider the greatest place on Earth. 5-year former Resident of Flux Factory, co-organizer for World Maker Faire (NYC), and blogger all over the net. Howdy!


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