Picture of Make a Joule Thief
Yes, it's the infamous Joule Thief, in Instructable form! For those of you who don't know, the Joule Thief is a tiny little circuit that allows you to drive a white or blue LED from voltages as low as 0.5 volts. You think those batteries are dead? Don't throw them out yet! Hook them up to the Joule Thief to squeeze every last drop of energy out of them!

The idea and circuit came from this Make weekend project. Why don't you pay them a visit?
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Step 1: Parts and Tools

Picture of Parts and Tools
For this project you will need very few tools and parts, as you will see in the picture below.
But for those of you who like it in text, here it is:

Helping Hands (Optional)
Soldering Iron
A Blue or White LED (Other colors are fine, too)
2N3904 Transistor or equivalent
1k Resistor (Brown-Black-Red)
Toroid Bead
Thin wire, two colors (magnet wire works)

You can get the toroid and transistor from a dead CFL; the transistor is usually labeled 13002.

Also, if you use a 2N4401 or BC337 transistor, your LED will be brighter because they can handle more amps.
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nfhaines2 months ago

Help me understand the role of the transistor? Is this specific to the joule thief? Running and LED directly from battery source (with resistor) doesn't seem to require the transistor, so I'm curious about what changes with the joule thief to make the transistor a requirement?

DHeigh nfhaines12 days ago

My understanding: LED's require >3V to work. Joule Thiefs use one battery (1.5V or less) by charging the coil and adding its discharge to the battery's charge to power the LED.

The transistor does the switching from charge to discharge. The LED is actually blinking really fast like AC florescent bulbs do, but just too fast to see.

eelco_g nfhaines2 months ago

The LED will only light up if the battery voltage exceeds te forward voltage of the LED. Think of it as a threshold. So connecting a 1.5V battery to an LED with a forward voltage of 3V won't light it up. The Joule thief circuit here pulses the power through the coil, which causes a voltage spike, which is high enough to light up LEDs with a forward voltage much higher than the battery can provide directly. Furthermore, it can keep doing so untill the battery is almost completely drained, in effect 'stealing' every last bit of energy from the battery. It is so good at cranking up the voltage, it can work with batteries that are too discharged for most other devices.

JRV3115 days ago

I used a picture of your Joule Thief circuit in my latest instructable:

I gave you credit and provided a link to this instructable.

I also enjoyed, and benefited greatly from, your instructable on better photography.

My Joule Thief kept the LED on for 68 hours.

JRV3119 days ago

Great project, I made it. I am going to leave it on and see how long the battery lasts. I let you know when it dies.

ppanchal123 days ago

Will using it reduce amps?? Please help.

JulienP1 made it!1 month ago

Made it but have hard time to measure output voltage. My true RMS multimeter show me exactly the same output voltage as the input voltage. But it make sense, as the output is a pulse, the mean of the output may actually be equal to the input. Am I right ? If so I may read my VC850 to find a way to measure peak voltage.

2015-01-11 11.51.30.jpg
I got the same reading as well
larry.rivard made it!1 month ago
madeor on a proto board and in the process I made an improv AA battery holder.
Would a 2N3906B work i don't know about transiters
The 2N3906 is a PNP transistor. The circuit requires a NPN transistor like the 2N3904 or the 2N2222.

I like high voltage, can I use my transformer in place of the toroid?

larry.rivard made it!1 month ago
Made a prototype. Now to make it permanent.
clchee made it!2 months ago

I did not have 2N3904 at my disposal so i used a used 2N2222 instead. The toroid winding part is great, helped me a lot. Thanks !

Check out my project

Step6_01.jpg2014-11-20 10.41.43.jpg
dONTbEdICK5 months ago

What effect does this technique have on a fully charged battery? can it fold up at the transister? Im a total noob so please forgive,

It will just be brighter. I actually took 2 rechargeable AAA batteries that had 1.2v in them and the LED shined really brightly.

ob103ninja9 months ago

What happens with a full 9v battery and 30 turns?


X3msnake made it!3 months ago

Hard to see at first what is connected to what, I used parts of an old psu to build mine :)

mind power4 months ago

Can i use a transistor from an old CFl

colin552 years ago
Here is a Joule Thief with 10 LEDs on a 9v battery
colin55 colin552 years ago
Here is a circuit for 10 LEDs on a 9v battery
DHeigh colin552 years ago
I'm trying to fix a 3 LED, 2 D-cell flashlight, the LEDs don't light.
The circuit they are in has a 3-pin device labelled only "8N 11027" & a 2-pin device w/o any label.
I thought I might rebuild it with a joule thief that would work with 3 white leds off 2 D-cells. Can you help?
mjhearn DHeigh5 months ago

I'm a little late with this post, but built a pretty small (like dime sized) Joule Thief yesterday night, per 1up's instructions/suggestions. Used magnet wire suggestion, as well as the CFL toroid and the 2N4401. I say all that to say: for the LED portion of that circuit, i used a 3 LED lamp from a keychain light. The 3 LED's were VERY bright, powered with only a 'dead' 1.5V button cell (AG13?) and the JT.

adarshmodh8 months ago

hey i was able to make a simple working model bt wt i'm curious about is dat

1) wt effect would we see if change the no of windings ?

2) wt does it do exactly does it amplify the voltage, if yes then can we amplify a voltage of 3.5V to 5V by maybe increasing the no of windings on both sides....

please reply asap

suvinks9 months ago

Hello,When am trying, 1.3v Battery is providing only 1.2v across the LED and hence it is not lit up. Could anyone tell me where I might have went wrong?

Tried with 10, 11 and 12 turns around the toroid. Same result :(

mittu gat2 years ago
Hi my transister is getting rlly hot!! but the led lights up. i hooked up 1.5v battery......

Same here, I would like help with this issue if you have solved it.

I found out that there isn't really much you can do. Unless u want to add a heat sink. But what I did in my new version is that I glued the transistor to the bottom of the LED and this helps reduce the heat.

too much voltage and/or amperage going through. is it the same as the one in the instructable?

if not, you might be interested in getting the same one.

jneave9 months ago

spent an evening with the wrong two wires on the inductor twisted together. Made a hot transistor. Seems to require at least one volt but there we go.

ob103ninja9 months ago

Try cranking it up with 50 - to - 500 turns on both coils and attempting to arc it

jeidins11 months ago
weird thing happening. as I take AA battery at 0.8v it makes it 1.5v but when I take 5v joule thief makes no difference. any know why?
عالعصمي11 months ago
GreeceFallout12 months ago
Wow , well it worked just fine with 10 wrappings around the toroid i managed to make a "dead" battery to light up a Led it couldn't before , pretty simple and useful instructable
P.S. Does it really reduce amperage ?
Cause im thinking of using it as a boost to run a motor
Can we use 2 separate toroids 1 for each wire ?
caruano95 made it!1 year ago

5 white leds, powering on a single AA battery.

chexin511 year ago

I tried a GET5172 ( = NTE123A ) and the LED is the brightest yet. It consumed .1V from the single battery in 24 hours.

2N4401 lights better than a 3904

2N2222A doesn't light at all ? ? ?

chexin511 year ago

The critical part of the winding the toroid instructions are:

Keep winding the two strands through the toroid until you have about 11 turns
We now have two pairs of wires facing opposite directions.
Take one wire from each pair ensuring they are a different color and solder this
"crossover pair" together. This goes to the positive of the battery.

PhilKE3FL1 year ago

None of these transistors can handle "amps" they are all probably in the 200 mA range. A 2N3904 is listed as 200 mA typical EC current I believe. But I'm sure most people will understand what you're saying. The other point is that while the transistor can push 200mA if 200mA went through the LED it would burn up in a very short time. Most of these LEDs are rated for around 20-30mA that's about one tenth of the rated current handling capacity of the transistor.

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