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I have to build a simple Joule thief circuit for school. Here it is :(http://upload.wikimedia.org/wikipedia/commons/8/83/Joule_thief.png). The resistance of the resistor is fixed to 1 kOhms, the voltage across the "dead battery" is about 1.5V, the transistor needs about 0.7V to open and finally, the led needs 2.0V to light up. So I have to figure out (not experimentaly) the number of turns around each inductor so that the led has enough voltage across it to light up at best... and that's where I'm stuck with calculations. With all the formulas that I know : Lenz's law, Ohm's law, etc... I feel like the problem is quite hard to solve (but maybe I can get a good approximation). Does anyone have an idea on how to solve my problem?

Thanks in advance,