Joule Thief Flashlight Upgrade




Introduction: Joule Thief Flashlight Upgrade

About: I'm a building maintenance engineer, been at it over forty years, and still learning.............

Well, Here we go......"The Victim", a very small AAA flashlight.

I'd like to thank " unknownuser 2007 " and " 1up" for the details and information about this project, giving me the gumption to try this.......

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Step 1: The " Victim " to Be Upgraded.

This is a small flashlight that came with a cellphone case. It's good point is it came with the belt carry case for the cellphone in it's own pocket. It's bad point(s) are that it eats a AAA cell in less than three hours,  the light output is dismal, and the light pattern stinks. 

Step 2: The Infamous Parts List.

Here is what I used :

(1)  2N3904 Transistor  {1}
(1)  3K 1/8W Resistor    {3}
(1)  .001 uf Capacitor      {2}
(1)  Toroid Transformer Core  ( See Details ) {4}
(1)  White LED
Magnet Wire, Hot Melt Glue, Iron, Optical Visor, Etc.
cussing, throwing stuff, crossed eyes, thundering headaches all optional.

The toroid cores were scavanged from dead CFL's, the other parts can also be scavanged, or picked up locally or on-line for cheap. The values can be changed, depending on whats avalible or on hand. All of this can be tinkered with, adjusting for what works best.

A supplier for LED's that  I've found is " ", darn good prices, good shipping times, usually free shipping.

Step 3: Building the LED Upgrade.

I started with making the toroid transformer. The core is salvaged from a defunct CFL lamp. Other sources can be a dead computer power supply, or other electronics.

I removed the original windings, then re-wound the core with twenty turns for the "primary", forty turns for the "secondary", marking the ends. The easiest way is to fold the magnet wire about two-thirds of the length and marking the longer side, then using the folded wire as a "needle" to wind the first twenty turns together, then winding twenty more turns using only the "marked" longer wire. Depending on the size of the core, You'll need about three or four feet of magnet wire.

I've included the wiring diagram, after some tinkering, the values a good starting point.

Step 4: Building the Upgrade Part 2

Using the toroid as a base,  I attached the transistor,  using a tiny dab of the ever-present hot-melt glue,  then added the resistor, making a small, solid assembly.

The unmarked winding from the top of the toroid is connected to one end of the resistor, then marked as the "positive" lead. The marked winding from the bottom of the toroid is connected to the other side of the resistor, then trimmed short. The unmarked winding from the bottom of the toroid is connected to the base lead of the transistor. The marked winding from the top of the toroid is connected to the collector lead of the transistor, and to the cathode of the LED.

The emitter lead and the anode of the LED are connected together and marked as the "negative" lead.

The capacitor is connected between the "negitive" lead and the junction of the resistor and bottom marked toriod winding that was trimmed short. 

The LED usually has one wire shorter than the other, the longer is the anode. Also, most LEDs have a flat spot at the base, showing the cathode.

What I've done each time so far is to " rough " build the upgrade, for testing and adjustments, as I've found the capacitor's value has some effect on the output, depending on the other parts involved. Usually, different values can be tried, looking for the best output. 

Step 5: Clearing the Way.

Insofar as I needed the room, the lamp, reflector and the front "window" was removed, leaving the front shell empty. The upgrade will simply plug in where the bulb used to go. If it won't light, simply pull out the upgrade, give it a half turn to reverse the polarity, then stick it back in.

Step 6: Sealing & Finishing

To finish this, I used a short stub of heat-shrink tubing to protect the LED, then sealed all of this into the flashlight using the infamous hot-melt glue.

Step 7: The End Results and a Few Variants

After the hot-melt glue cools and sets, trim as needed.

I've added photos, both off and turned on.

The project also lends itself well to other ideas, as well.  I've  adapted this goodie for accent lighting,  using a glass cylinder that used to use an oil candle cartridge,  and re-claimed air fresheners.

It's been a blast, let me know what you think, now to go find some aspirin..............

Step 8: The Aftermath.....

Well,  I've done all I can, for now, after a couple of days at the computer putting this together.

Now, I can share a couple of photos, sorta behind the scenes........

The first I've entitled " Danged, That's Small "

The second is entitled " Need Some Help To See "

The biggest question................................

Where has  that danged asprin wandered off to ?...........

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    5 Discussions


    8 years ago on Step 2

    use a 2n2222 transistor instead of 2n3904, its works 30% better


    Reply 8 years ago on Introduction

    Hi, Max,
    I've heard a few people suggest that, along with the 2N4401, which I'll try at my next opportunity. Thanks for your input.


    10 years ago on Introduction

    Hey, there.........Just an update. I've modded three different flashlights and I all I can say is how rarely the batteries need replacing. That, and how quickly the air freshener version dissappears ! Seems I can't build 'em quick enough.