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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dbeller14 days ago

the circuit is basicly a high freq oscillator circiuit around 40 kHz depending on the number of turns on the toroide. if a higher voltage source is used the current thru the transistor will increase making it go pop the 1k resistor is used to bias the transistor and can be increases to reduce current draw also a power transistor can be used for higher current tolerance.

I used a BC547 transistor. Is it OK?

Can we use a metal hexagonal bolt instead of Toroid...?????

You can, but it isn't as efficient as a ferrit bead(toroid).

Can I use a ferrit bar instead of a toroid ?

Hey :) I am making a mini wind turbine and my motor generates about a 1V . With the joule thief I get 1.5V . How to get ~4.5V ? How to do it? Is it possible?
moicarre made it!26 days ago

I made it, and it works great. Thanks so much for the pictures, because they helped a lot. I used a toroid that I had from an old ripped apart CFL (9 turns of thicker gauge wire from a larger toroid from a power supply), and the 2N3904 transistor, with an LED from an optical mouse. I used many different NPN transistors and that was the one with the best results. I did not have either of the two that you suggested (2N4401 or BC337). They would probably give better results, but it works very well as is. I started with a AA battery that measured .8V. It's been on now for 48 hours! I plan on getting a board so I can solder this together for use in another project. Great instructable.

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nfhaines3 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 nfhaines1 month 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 nfhaines3 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.

JRV311 month 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.

JRV311 month 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.

ppanchal11 month ago

Will using it reduce amps?? Please help.

JulienP1 made it!2 months 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.

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I got the same reading as well
larry.rivard made it!2 months 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.
ualbuquerque2 months ago

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

larry.rivard made it!2 months ago
Made a prototype. Now to make it permanent.
clchee made it!3 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

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dONTbEdICK6 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.

ob103ninja10 months ago

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


X3msnake made it!4 months ago

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

mind power5 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 DHeigh6 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.

adarshmodh9 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.

jneave10 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.

ob103ninja10 months ago

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

jeidins1 year 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?
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