Let's Make Robots!

Joule Thief

There is not much to document in this project. I built a Joule Thief....who didn't ;-)

Mine is powered by a LR44 button cell (1.5V) and it light up a white LED which usually needs a minimum of 3.3 voltes to light up.

I just built it to test how long the LED can light with this one battery (it might be interesting for throwies). 

Will update the blog when the battery is completely depleted.

Power on time: January 14, 2012, 3:30PM

DIY coil, double side tape to attach the battery.

Transistor soldered directly on the battery to make the whole thing a bit stable.

I am not posting the schematics here since everything is already available in the famous internet but if you want to know why this is working then I recommend this http://en.wikipedia.org/wiki/Joule_thief

Test failed. After 3 hours the battery was depleted. I don't know why it happens but have to check the windings on the ferrite core...maybe have to put more on...

The circuit is working good. It lit up a white LED with a tiny solar panel by 0.45 volts but failed in the long run with the button cell.

UPDATE Jan. 30:

Strange things happend. After the button cell died i attached the JT to a almost full AA battery and it was working for a whole week with full brightness. Then the LED faded away to a weak glow until it went off. This was just before I left for my holidays on Jan. 21. Now I am back home and the JT was still lying dead in my book shelf. Last night I saw it lit up again...wondering what happend. Then after about one minute the LED faded away, went off and after another 5 minutes it lit again. The brightness to the beginnuing was about 50%. 

It seems that battery is recovering again and again but how long it will last? More to discover ...


Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Just saw this thread and decided its quite interesting ..... and was very happy to see it didnt require many parts as i dont have much .... luckly i remembered salvaging a ferrite toroid used for winding the wires around ... also while looking up other sites on this on the net i discovered a good place to find these are in power supplies  .... lucky me just happens to have a old scrap one ( discovered this after i finished making my joule thief) ...  which contains a few nice biggish ones .... as the one i used to make my joule thief is very small and could only take 7 windings ..... wasnt sure it was going to be enough.

in mine i used a photo diode (think its called that) it requires 3v to get it to light up ..... so connected it to a 1.5v battrie and as expected it didnt light up .... then connected it too a 3v and it lights up. Then rigged up the joule thief and 1.5v battrie and a lump of tin foil in the shape of a battrie to use the battrie holder and vola ... diode lights up. Hope to mess around with this now and see what can be done with this circuit .... any suggestions ?????

was thinking of rigging it to a solar light and see can it run longer with this connected as i have a little solar light circuit .... all i have to do is connect this up to it and it shouldnt be too difficult to do.

Joule Thief



Well rigged it up to the solar circuit unit ...... after i had charged the battrie throught the solar panels for a couple of hours .... it ran for a couple of hours then started to flash on and off every few seconds ( around 5 sec ). disconnected the joule circuit and connected bulb directly to battrie but it wouldnt power it so reconnected it to joule circuit and it continues flashing ..... being running like this for about 6hrs now.

just tried the photo diode that requires 3v to run and that is flashing but at a much quicker rate ..... less than a second between flashes ... but not at full brightness.

have it back connected to the solar bulb and will see how much longer it lasts ......

Cool that somebody jump on this boat :-)

I am not sure why my experiment failed but maybe it's the coil. With more windings it should be more power efficient. 

My plan is to combine this circuit with a proper coil with a solar panel to charge batteries. Right now my solar charger charges a 4.5V Li ion battery during the day and the battery lights up a LED during the night. With the Joule Thief I should be able to use a solar panel with a lower voltage output to charge the 6V battery.

But in the end all comes to the current. I tried to run a small motor with the JT but it did not spin...ideas are very welcome especially the relation of number of windings for primary and seondary coil (yes, they can be different as well different size of wire)

After the failed test with the button cell I changed to an AA battery...almost unused. Its running 24 hours per day since January 14, 2012, 7:30PM and still no sign of decreasing the brightness.


Ok i have a question .... these seem to be used to get more out of a battrie but also to power something that requires a higher voltage eg using a 1.5v to power a 3v+ led ..... 

i have a solar bulb running from a 1.2v rechargable battrie .... so i charge battrie with the solar panel and when theres not enough light the battrie kicks in and lights the bulb .... so i connect this to the circuit .... eg i have the 1.2 v rechargable battrie as the power sourse and i connect the led to the transistors legs and connect the core wrapped in wire to the positive side of battrie .... the transistor (base) and the transistor (collector) .....  now is my solar bulb getting extra volts and is this bad for it ..... i can disconnect the core wrapped in wire from the resistor and transistor base and bulb still works (as there is a complet circuit) but is the jewl thief still working now as all the connections arent made ... 

Sorry to reply you so late. Can you send a circuit diagram of your circuit? I might be lost in your explanation ;-)