Below is a schematic diagram of a joule thief circuit. This is a very interesting circuit, it will drive 3.6 volt led from a single 1.5 volt cell. This joule thief circuit will run on 1.5 volt battery on which your other device are not working and showing low battery indication.
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So this is a good circuit to extract any power left in your batteries and it will produce bright white light and also ideal for torch. The circuit is a simple dc to dc converter or we can also say it voltage booster circuit. Both the transistors are BC547, 100 uH is a fixed coil you can find it easily in the market or online.
