Joule Thief - micro miniature UK design

Joule Thief - micro miniature UK design

I first came across the "Joule Thief" in 1999 when Z. Kaparnik (from Swindon in England) published an article in an Electronics Magazine that I bought. The article was entitled "One Volt LED" because the circuit was designed to drive an LED from less than a volt. In fact it works with a "dead" battery cell as low as 0.75 volts!

Although almost any transistor can be used, one with a very low VCE saturation voltage is best.

The transformer is made by taking one metre of 0.15mm diameter (38 AWG) enamelled copper wire and folding it in half. Feed it twenty times through a ferrite bead and you are ready to build the circuit, which requires just one transistor and one resistor. A complete beginner can assemble this in half an hour!

The result is a circuit that can drive from one to six LEDs from a 1.5 volt battery cell. You can even use a tiny watch battery to make a micro-miniature torch if you wish.

The circuit is extremely efficient because it fires pulses of current through the LED(s) at a high frequency that the eye can't detect. The LED appears to be lit with a steady, bright light, although it's actually flashing on and off very rapidly. Thus, you get a very bright light with minimal power consumption, giving high efficiency and a long battery life.

The video shows a demonstration using a prototype board but the actual components are tiny enough to fit into a key fob or a lipstick or similar housing.

Kit available from www.SatCure.co.uk

I'm not a professional movie maker so no apologies for the crap recording. I have lent out my camcorder and had to make do with a "bridge camera" without remote control. A lot of the time it wasn't even pointing at my hands so I've edited out the worst. I hope what's left is still of interest and not too irritating. Yes, that's a Northern accent. (At least there's no music!)
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