Fun family science project — electric lighting from trash

The Joule Thief is a way of producing enough electricity to run small, but useful, electric lights using cast-off trash like pop-can tabs and "dead" batteries. It's especially handy in the Himalayas, writes inventor and Google Science Fair judge T.H. Culhane. There, electricity is a precious resource. But the components needed to build a Joule Thief are abundant, thanks to climbers and tourists who leave behind all sorts of surprisingly useful litter.

Last week, Culhane joined a G+ hangout sponsored by National Geographic and Girlstart to talk about the value in things we throw away and walk viewers through the construction of their very own Joule Thief. You can watched the video of the event, or read the instructions for building a Joule Thief at Culhane's blog.

The fact that the Joule thief allows one to run a 3V LED from a 1.5 or 1.2 Volt battery would itself be astounding, because it means you only need half the number of batteries to get the same light.

Some of you are thinking "wait, maybe it enables you to use a single 1.5 volt battery to light a 3V LED instead of the usual two, but doesn't it just make that battery last half as long? Great question, but the answer is that the Joule Thief, which works by building up and collapsing a magnetic field around the torus (which acts as an electromagnetic inductor) actually is more efficient than using a battery directly because it PULSES the energy to the LED. You see the lightbulb shining brightly, but in fact it is turning on and off very rapidly as the magnetic field of the inductor builds up and discharges again and again. That means that though the light appears to be on all the time it is actually turning on and off and saving energy because it isn't on all the time.

It turns out that the Joule Thief enables the battery to keep supplying electrons to the light long after the battery is normally considered DEAD. So the battery actually lasts much much longer than a normal battery. I've observed "dead" batteries working down to about 0.5 Volts. Normally a 1.5 V battery is considered dead when it reaches 1.0 volts. But the Joule Thief can "steal" the remaining energy much below that. And that got me thinking -- could I use other sources of between 0.5 and 1.0 Volts to run a 3V LED?

T.H. Culhane's post on The Joule Thief (includes instructions for making a Joule Thief with batteries and alternative electricity sources)

Watch the video at National Geographic Newswatch

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  1. So you can run a 3v LED, at (essentially) half-brightness, from a 1.5v battery.  Useful in a limited extent, I suppose, but I don’t know that there’s a lot of application for this beyond flashlights…

  2. Some of you are thinking “wait, maybe it enables you to use a single 1.5 volt battery to light a 3V LED instead of the usual two, but doesn’t it just make that battery last half as long? Great question, but the answer is that the Joule Thief […] actually is more efficient than using a battery directly because it PULSES the energy to the LED

    Uhhh, so it enables you to use a single 1.5 volt battery to light a 3V LED instead of the usual two, but it makes the light half as bright.

    That is either very carefully make unclear, or the original author wasn’t clear about it.

  3. It’s just a boost converter, like in every single-cell LED flashlight.  It charges a coil, switches it through the load, rinse, repeat.  

    The switching mechanism is fun (induced voltage from a secondary winding into the transistor), but it’s unregulated.  

    1. The switching mechanism is fun, but it’s unregulated.

      Waiting for the Obama Administration to start regulating it…

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