Joule Thief from PC Power Supply
Channel: Geo Schoonmaker
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With a couple strands of wire from a old telephone cord to wind on the toroid core, this project was free to do and actually worked on the first try. It was powered by one of the "dead" batteries that came with my new camera.
Pretty cool project - getting a LED to light from a dead battery. There is a lot of room for improvement with this circuit. Build one yourself and see how much you can tweak it for efficiency.