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Joule Thief Flashlight

This simple flashlight can suck every available bit of power remaining out of 'dead' batteries.

When your AA battery no longer has enough power to run your electric tooth-brush or power the tv remote, pop it into this flashlight to get some more use out of it before chucking it in the recycle bin.

The circuit is described in detail on this wikipedia article, and is a direct copy of the schematic shown.

The backstory

If you just want to build it, feel free to skip ahead.

This little kit came about through a series of fortunate events at the Tokyo HackerSpace. Some time back, Natsuki Tanaka of Okayama MakerSpace (and long time THS friend) mentioned an UNBELIEVABLE listing on Yahoo Auctions Japan. The auction was for "15-20 small to medium boxes filled with electronic components. Buy must arrange pickup. We will not ship packages." What was shocking was the fact that the starting bid stood at 5000 yen (about $50 USD at the time). Being on the south end of Japan, Natsuki could not arrange pick up himself, but suggested that THS should place a bid. In utter disbelief at what I saw in the photos, I arranged a bid through one of our members. Quite shockingly, we actually WON the bid, at 5000 yen!

A week later, we were driving out with a delivery truck to pick up what I had assumed was 20 small boxes. What I saw was... overwhelming. For the first time in 7 years living in Japan, "small" meant large, and "Medium" meant the American equivilant of "Super Sized!" These boxes were HUGE. It ended up taking us THREE trips in a delivery van to get it all moved. Check out our photos on Facebook for more details.

This is about half of the auction boxes, at the pickup site. More boxes were behind me, as well as waiting outside on the patio.

Fast forward about a year and a half. We had been wracking our brains thinking of stuff to do with all these parts. In the midst of that, we moved the hackerspace to a new location. In that time, new members joined us and began contributing to the space, as well as organizing us. Ed, one of those new members, offered to start cataloging all of our components. He had also mentioned wanting to whip up a Joule Thief. One day he grabbed one of the many clamp on cable choke ferrite cores, dissassembled it, and began winding a coil for his Joule Thief. The following week he brougt in his finished project.

Around the same time, I had been going through components and identifying things that may be better off in the recycle bin than on our shelves. I came across some power supply common mode chokes. By some, I mean, a bag of at least 50. Knowing the street value of such an item, I was very resistant to the idea of putting them in the 'Vortex of Doom,' but I also knew no one in the space was going to be building up 50 plus power systems any time soon. What to do.

I took a few home and begin playing around with them. What could the be used for that might be something we could make a kit out of? I tried using them as a current transducer. Not really optimal (I never did get it to work right, but not giving up on the idea just yet). I suddenly remembered Ed winding his coil and realized that the dual mode choke is essentially the same configuration. The only real difference is that Joule Thief coils are usually wound parallel to each other around a round coil. The choke is physically different configuration, but amounts to the same thing. I tried wiring up a Joule Thief with one, and other than needing to swap two wires, It fired right up!

The Joule Thief Flashlight kit is the final results of that unintentional collaboration.

Let's build it!

To start, let's first verify the parts. It is a small list of components:

  • 1 yellow circuit board
  • 1 AA size single battery holder
  • 1 MuRata 1522C or similar common mode choke coil (The big boxy thing with 4 wire pins. These parts are out of production, but any similar product will do in a pinch)
  • 1 small switch
  • 1 resistor (1K ohms)
  • 1 NPN transistor, Toshiba 2SC1815 or similar
  • 2 white LEDs

It is pretty obvious where each component goes, since no two are alike, and can only physically fit in one location. So rather than a step by step narration, I will just give a photo assembly guide. Some things to be cautious of:

  • At some point, we will run out of choke coils, and future kits will require you to wind your own. At which time, additional instructions will be appended to the bottom of this post. If you have such a kit, please look below for more information.
  • If you source your own parts, any NPN transistor will work. However, the 2SC1815 has an unusual pin order. So, be sure to check your transistors carefully. You may need to twist it around to get it the pins in the correct holes. If you use the transistor specified in the parts list, you will have no problems.
  • The board has additional connectors. Near the switch, we have a connection for an external battery, in case you would like to use a cell other than AA. The left pin is positive, and the right (closest to the switch) is negative.
  • Between the LEDs there is an additional output, in case you would like to wire up some other form of load. The positive output can be seen with traces going to the LEDs and the coil, on the top side of the board. Keep in mind that a Joule Thief does not produce much current, so your load must be relatively small. Also, remember that, as per the wiki entry, it is an unpredictible pulse output, rather than a constant current. You will need to add capacitors and regulation compatible with your external load.

Eagle files are available on github.

Shout outs are:

Akiba, Taylan, Richard of THS, Clive Mitchell (the creater of the Joule Thief), Mitch Altman (if you don't know this guy, google him!), Natsuki Tanaka, Okayama MakerSpace, Eric Hellwig (my brother) of ApertureControls, Brian Deyo of Peace Corps Swaziland, and the Swazi Computer Society.


Step 1: Solder in the resistor


Step 2: add the transistor


Step 3: The switch

Bend the large tabs to hold it in place while you solder, and give a real solid phyical mounting.


Step 4: Bend the LED legs

Notice the direction they are bent. Take notice of the short leg. Verify the flat spot on the silk screen of the circuit board.


Step 5: Solder the LEDs


Step 6: The battery case

Notice the + and - markings on the circuit board. The spring of the case is close to the lanyard strap hole.


Step 7: The all important coil


It's finished! Now chuck in an old battery and flip the switch!


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