Hand Cranked Flashlight Mod Plus Joule Thief

Introduction: Hand Cranked Flashlight Mod Plus Joule Thief

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I have a couple of hand cranked flashlights laying around and I was never satisfied with their performance.

When I got them fresh out the box they worked great. But when their batteries ran out it was too exhausting to recharge the batteries manually. And after a couple of years, it was almost impossible recharge the batteries because they were too old. Now, a hefty 5 minutes of cranking produces only 20 seconds of light.

I decided to mod one of the flashlights by switching the lithium ion rechargeable battery with a super capacitor.

Surprisingly, the mod was easy and the results were very satisfying. Now, with just a mere 5 seconds of cranking I get 5 minutes of light. A major improvement.

In a further mod I added a joule thief which increased brightness (see step 5)

Step 1: The Back Up Super Capacitor

A super capacitor acts like a battery in computers and electronic devices, and therefore it can serve as a back up power supply.

The neat thing about super capacitors is that they last much longer than rechargeable batteries. They are easier to recharge because they have almost 100% efficient while the batteries lose between 50% to 30% of the energy when recharging.

I am using a 1 Farad super capacitor. But you can use bigger values. The bigger the super capacitor the longer the flashlight works.

Step 2: Replacing the Old Battery

This step was easier than I thought. It took a couple of minutes.

The only thing one has to worry about in this step is to get the positive of the capacitor in the same hole as the positive side of the battery. The same thing with the negative side.

Step 3: Finished

Here you can see the cap in place of the old battery and the flashlight working.

Step 4: Reverse Engineering the Hand Cranked Circuit

I am a curious guy and to make sure my mods work, I reverse engineered the hand cranked circuit. I use the same technique that I used in my Spy Ear Instructable.


The pictures below show the different steps in reverse engineering the circuit.

Step 5: Adding a Joule Thief

This mod is more challenging and requires that the circuit be reverse engineered in order to determine how to replace components.

To add a joule thief the whole circuit between the switch to the LEDs must be modified.

The first thing I noticed when I reversed engineered the circuit is that The LEDs are in parallel. The joule thief is able to run three LEDs in series. I rewired the LEDs by scoring the circuit then soldering jumper wire.

The second major modification, is to remove all the diodes and resistors between the LEDs and switch then adding a standard Joule thief in place.

The result was a brighter flashlight that lasted 1/2 the time as the circuit without the joule thief. Adding more capacitance will increase the run time of the flashlight.

However, it does not bother me cranking the flashlight every few minutes or so. The light is brighter than the factory original.



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22 Discussions

What kind of hand crank flashlight are you using?

how would i go about doing that for one of these ?
aswell as the fact that im modding one to make a hand crank phone charger from this instructable

you should consider replacing the leds with brighter ones.

anything that regulates or converts power. Old computers have tons of capacitors in them. Electronic balasts for lights also have a lot of caps in them.

as long as the plastic insulator lasts. They should last decades if operated within specified parameters.

Where did you get that capacitor? I have several of those lights. The only supercapicitors I've been able to find won't fit in the space available.

1 reply

I got it from one the electronic retailers. You can Google them. I have a supercapacitor that is big and I opened a hole in the case to accommodate it.


I'm going to order a bunch of these suckers. I have a couple of these lights and they suck (and luckily I haven't taken them apart yet for other projects). This might make them useful after all!

5 replies

If you have a Harbor Freight in your area you can pick them up there. I have one of these and this sounds like a good mod. If a super capacitor can outlast the little lithium ion battery, then its worth it.

that is if they are properly charged and discharged. In this application batteries die out very fast.

It can outlast old batteries but not fresh batteries. Batteries can store more charge than super capacitors. But I rather have a light for 5 minutes that I can get by cranking for 5 second, then to hand crack a fresh battery for 2 hours to get 1/2 hour of light. I use a 1 farad supercap, however the bigger the capacitance, the longer the light will last. Capacitors also don't die out like rechargeable batteries. After a couple of years the battery based hand cranked flashlights become useless. This mod just makes the flashlight more convenient.

Is that part under "battery charging circuit" a one-way rectifier without the diode voltage drop? That's pretty cunning. Are capacitors 100% efficient? I thought they were only ever 50% efficient, sure I remember deriving that in A level physics. Still, from an engineering point of view they sound much better than the old batteries. If I can find a place to get them around here there might be a bike light 'Ible in it.