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Step 5: The 'Impossible' Joule Thief

An entry from acmefixer's blog brings us this strange beast - it's a Joule Thief type circuit that runs on 200mV! That's a fifth of a volt! And all it uses is the plain old MPS2222 silicon transistor!

A few changes allow this to happen: First, the coil must have a Secondary with at least 3 times as many turns as the Primary, and the LED is connected between the Base (-) and the Collector (+) of the transistor. The circuit cannot start at this voltage, but, once given a jump (with a 1v jolt), it is very stable, drawing a measly 7-milliAmps from the supply.

A look at the scope's reading tells us how it works. The top (yellow) trace shows the voltage slowly growing, which increases the drive on the Base, forcing the transistor to conduct harder. This stops when the supply cannot drive the coil any more. The base drive disappears, the field collapses, producing the pulse. This induces a negative pulse on the Base (Blue trace) which is 3-times greater. By placing the LED across this, it has enough power to light, if dimly. The falling edge of the pulse pushes the transistor back into conducting mode and the cycle continues.

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Thank for the great instructable! You mention that your inductors measure to within 2% of eachother. How is this measurement made? Is there such a thing as an inductance meter?
One of the best bargains around is the capacitance &amp; inductance meter from<a href="http://www.aade.com/lcmeter.htm" rel="nofollow"> Almost All Digital Equipment</a>. I've had their IIB meter since it first came out 15 years ago and it's never let me down.<br> <br> Amazingly their prices have not changed in all this time - their 'kit' is still available for under US$100.
It was from this Instructable that I learned of Almost All Digital Equipment. <br>Almost All Digital Equipment also sells a frequency meter. <br>I bought one and am quite happy with it. I use it to measure the frequency of the &quot;Joule Thiefs&quot; I've been working with. These &quot;Joule Thiefs&quot; use only a four legged ic, a tiny inductor (as shown above), a battery and a LED. More info about this at: <br>http://www.instructables.com/id/Joule-Thief-Circuits-crude-to-modern/ <br>I wonder how the circuit I use would stack up against the variations revealed in this Instructable... <br> <br>
Thanks for the info, I ordered a kit yesterday!
Thanks for your comments - I will add these points to the Instructable.<br><br>

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