Basic Joule Thief Flashlight




Introduction: Basic Joule Thief Flashlight

About: Hello, my name is Andrew, I have recently decided to start posting Instructables, I do projects all the time and I would love to get in on some of these contests. Programming, Circuitry, and Robotics are my...

Hello welcome to my second instructable!!! Today I'm going to show you how to build a very simple joule thief flashlight. If you haven't seen my other instructable, check it out for an easier way to power this than I'm going to show you! I know this has been posted/done before, but I just figured I would post with my own twist added.

My goal here is to build this joule thief with all salvaged parts(except the LED maybe), and have it power my high power white led with a battery most people would think were dead. I will explain where I get the parts and how to build this simple but amazing gadget.

A joule thief is a self-oscillating voltage booster that is small, low-cost, and easy to build. It can use nearly all of the energy in a single-cell battery, way far below the voltage where other circuits consider the battery fully discharged (or "dead"). It was named a "Joule Thief" because the circuit is stealing energy or "joules" from the source. I think it's a very clever name.

The circuit uses the self-oscillating properties of the blocking oscillator, to form an unregulated voltage boost converter. The output voltage is increased at the expense of higher current draw on the input.

If you are any more interested in HOW this circuit actually functions, I would recommend checking out the Wikipedia article for joule thief's, I got most of my information from them, they have very well explained details, better than I could explain. I'm just a guy working in his garage, not a rocket scientist... Yet. =P

Step 1: Parts / Tools

The pictures include all the materials and tools needed. The materials needed are : 2n3904(or equivalent) transistor, 10k resistor, toroid bead, a led, and some spare wire(the gauge depends on the size of the toroid bead, I used 22 gauge). I have gotten some feedback saying that I'm using the wrong transistor, but the 3904, 2222 and 4401 will all work, I have made the circuit work with any of the 3, and I'm sure there are many others that would work as well. You may need a breadboard as well if you don't already have one, but that's only if you want to pre-build the circuit before you solder it.

You can salvage the toroid bead, transistor, wire, and the resistor, from a old PC power supply, that's where I got my parts from. I also included a picture of the high brightness led I purchased to make this circuit really shine, but I used the recycled green led for my instructions.

I didn't get any pictures on the salvage mission through the power supply, and I currently don't have any more scrap ones, so I can't help much with the salvaging. Just remember, anytime you open something with high voltage (such as a power supply) use extreme caution!! They can deliver a lethal zap, even after sitting for a few weeks.

I put heat shrink on my finished product because I think it looks much cleaner than having loose contacts, and it makes it easier to fit in my flash light enclosure. I will explain further into my process in the next step.

For tools, all you need is a soldering iron, some solder, wire cutters/strippers, and a lighter for the heat shrink!

Let's begin building!!!

Step 2: Creating the Coil

I understand there has been much confusion on how the coil is actually wound, so I will spend more time explaining that in this step. So first things first, gather the toroid you want to use(I found the larger the easier, but if you go smaller, you can adjust the wire gauge to make it work). I'm going to use some 22 gauge wire, I got one of each color(picture 1), I ended up using a larger toroid for ease of winding, but when I want to put this circuit in an enclosure, I use a smaller toroid.

I always use extra wire, I would rather have more than less, so collect about a foot long wire in two different colors, and twist the ends (picture 2). Then put the wire through the hole of the toroid, and begin winding until you can't wind any more, or hit around 10 winds (picture 3, 4). I have found you need at least 7 or 8 windings, contrary to what other guides say, I have made it work my way, I always had issues winding 10+ around the smaller toroid, so I just stuck with what worked. Once we have completed the coil, I get one wire from each side, and solder them together, it makes it easier to keep track of once you start creating the circuit. (picture 5, 6) Once that's complete, you have your toroid coil complete, and we can move on to the next step of creating the circuit and putting this coil to use.

Step 3: Creating the Circuit

This is the fun part when all the preparation comes together to become a project! I would recommend first bread-boarding the circuit to ensure that it works before soldering it all together. If you are more advanced of a tinkerer and don't need help building the circuit, just check out the drawn schematic and skip to the next step. If you would like the assistance building the circuit on the bread-board, proceed on.

Get your transistor, led, and resistor together, and place them in the breadboard as shown (picture 2, 3), then place the toroid coil built in the step before this one as shown(picture 4), then put the jumper wire from the collector pin on the transistor to the ground(picture 4). Add power and you got light! Simple as that! Now that you are done you can add power from an old battery and the led should light up(picture 5). If it doesn't light up then I would double check the circuit, if you think you got that right, I would try to rewind the toroid, sometimes they like to be pickey with how they are wound.

For the power source, you could use a AA, AAA, C, D, or any other battery, no matter how full or "empty". I actually use my galvanic battery(penny battery) to power most of mine. I have assembled about ten of these, hooked them up to a high MCD led, and my penny battery. They are PERFECT for emergency flashlights. I just need a 3d printer to make a fancier case than the cardboard I use, haha. =P

Step 4: Solder and Heat Shrink Together

Now we solder together the circuit we built on the breadboard and heat shrink it all together. You should have a pretty good understanding of the circuit after building it once, so this time it should be much easier to build.

Take the transistor and solder the led and resistor as shown in picture one, the positive lead of the led should be on the collector of the transistor and the negative lead to emitter, the resistor hooks from one of the single leads on the toroid to the base of the transistor(picture 2). The other single lead from the toroid coil goes to the positive/collector pin. Then solder a wire to the emitter/negative led connection to hook up to the negative side of the battery when you want to power it, that is your ground wire. The two wires you soldered into one will be hooked up to your positive

So you should end up with a jumble of soldered together parts and wires, so it's time to heat shrink it together to make it look better.(picture 3)

If you are unsure which is the emitter/collector on the transistor, a quick Google search for the data sheet for that type of transistor will give you the answer.

Step 5: Expand!!

After the circuit is all complete, now it's time to use the imagination. You can put it in a enclosure and make a simple flashlight, or you can try one of the other uses I have found for this circuit below. (I'm going to leave some slots open because I know there are many other uses for this circuit, I just need to figure them out. =D)

-I have used it to charge an empty Ni-Cd/Ni-Mh battery with two half full salvaged alkaline batteries. I just de-soldered the led and put those leads to a single battery holder with the rechargeable battery.


Step 6: Vote for Me!!!

I appreciate all the input I have gotten in the comments (positive or negative), I'm doing the best I can to update this instructable and make it as good and educational as it deserves to be, I didn't realize it would be so difficult to edit/update once it has been published, so lesson learned for the next instructable, it will be close to flawless before I post it.

If you enjoyed this instructable please vote for me. =). I would LOVE to be able to make custom projects with a 3d printer!! Even a T-Shirt would be AWESOME!!



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27 Discussions

Please reply as soon as possible and if possible than to my email

Update : i also need to glow 3 led of 3.5V will it work on output of 15-19 mV of peltier tile after going through joule thief ckt.

Hi i am using peltier tile as a input supply so will it charge a 22 F capacitor and will work correctly and please tell through a video how to make torrid easily i got confused please help .

Pretty neat! Check out my diy for a wiring time saver. I bet it would help you, and feel free to vote?

Hello. Interesting idea. Can you be more specific on your instruction. Too many wires to connect to each other. You need to show how they are connected. Not everyone is expert in electricity. Why dont you redo this and explain each step assuming that most of people who read this do not know much about the subject, but what you show might just create next big inventor. It seems very easy thing to build if one can figure out how to connect the wires, Take the criticism as constructive directions.Thanks and I am looking forward to see your redone instructions so I can build one myself.

No offense meant I was merely stating the one I built stated that the coils had to be wound in opposite direction. You started of by using one long piece of wire than folding it in the middle this was the center tap than you wound 8 turns one way and 6 the other way.

My biggest problem was finding a ferrite core torrid as it said that a iron core would not work.

Glad to see you got it to work though. I know when I finally succeeded I wanted to go out and celebrate. I was about to give up on the whole idea.

The winding isn't that specific. Of all the times I have made this circuit I have always done it the same way, and it works. You may have a point with the dots on the schematic, and I bet it works both ways, but I figured out how to do things my own way, and it works just fine.

You guys could be more positive, some of these comments went from constructive criticism to telling me I'm doing it wrong, when I know I'm not... But I appreciate all the feedback. :)

I think this is very cool. I've heard of these things, but never seen one. Thanks.

And as for winding in the opposite direction: I think all that does is reverse the direction of the current... The magnetic field being built and then collapsing doesn't care which direction the wires are oriented- the current just follows the right-hand-rule

When I made this circuit I found out that the coils have to be wound in opposite directions If you look at the picture and notice the dots on the Torrid that is what it means. No where in your instructions do you state this. In everything I have read about a joule thief this is critical as this is what makes the whole thing work. The coils is how you are able to light a led which takes close to 3 volts to light work using a 1.2 volt or a 1.5 volt battery.

In reality this device turns off and on rapidly the human eye will never see it as the oscillation is way to fast. As for the transistor any general purpose NPN will work here some better than others.I chose one that works with high oscillation

I made a similar circuit except my torrid had 8 turns one way and 6 turns the other way I used magnet wire from a old transformer and it works like a champ. I had tried all kinds of combinations but no luck .

Other than that the circuits are the same..

That's really cool my instructable got sent out in an email!!! I appreciate the constructive critisism, I wasn't expecting this to blow up like it did so I was going to make it better over time, but I got the day off today so I'm spending the day taking a bunch of pictures and updating the info, so bear with me for a while. :). I will make sure I pay attention to all your tips, this is still my second instructable so I'm still getting used to teaching. :)

Not kicking butt, just constructive criticism at least on my part. As I had said, because this is so simple, and has blown up (I received it in an email), many newbies are likely to try to it, even kids, with proper supervision because of the iron :). Want to give the best chance of success.

Good start. But I currently have ZERO clue what a Joule thief is, or why it's beneficial here. Name implies “thieving energy,” but from who, why, where? Pictures don't match instructions. The thieved schematic ;) doesn't match instructions, transistor call-out is different. Breadboard doesn't match instructions, I don't see where hookup wire was added for the breadboard, and which wires go to what pins on the transistor. No need to explain the inner workings of a transistor, but a simple description, along with a which wires go where explanation based on a top down look and shape for orientation is all that is needed. Plus a description of what to hook each LED leg to.

This one is super simple = attracting many to make it. You want to make sure they have best chance at succeeding, and it's pretty easy for even for us electric circuit “dabblers” to get things mixed up. Wouldn't want to kill a first timers interest because this doesn't work simply because the LED is backwards.

how long works this flashlight ? longer that without joulethief ??


Sorry some of you feel the instructions are inadequate. I will re-do and explain some more today. I wasn't expecting this instructable to blow up like it did, lol.

Check back for the edited version in a couple hours. I got to remember even if it seems basic to me I should explain it, I have learned that from your comments, so thank you for the honesty. I hope you enjoyed what did make sense.

You didn't give the size of the wire, and the number of turns.

This is all over the internet, even on youtube. Your instructions assume that everyone knows how to wire a transistor. A good instructable should show the proper wiring of all the parts. This is just a bad copycat. The only thing properly done is the wiring of the toroid. NEED SERIOUS WORK!

would there be any mods to do if i added more led's, sorry very new at this stuff