Basic Joule Thief Flashlight


Picture of Basic Joule Thief Flashlight

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

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The pictures include all the materials and tools needed. The materials needed are : 2n3904(or equivalent) transistor, 10k resistor, toroid coil, a led, and some wire. 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 can salvage the toroid coil, 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

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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

Picture of Creating The Circuit
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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 emitter 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. 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. 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.