Instructables
Picture of Joule thief bicycle light (back)
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How to convert bicycle light to joule thief bicycle light
 
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Step 1:

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Im not going to explain how j.t. circuit works, or how to make one, for those information you have to look for other sources.
I will explain how to use j.t. circuit with bicycle light and convert it to efficient and relatively cheap source of light.

First you need small ph screwdriver to disassemble leds and electronic switch board (circuit).
Electronic switch cant be used with 1,5 volts or less so we are going to dismiss it and use small lever switch.

Cut out plastic peace to make room for the  lever switch . You can use sharp knife, scalpel, sandpaper or dremel with sanding stones, as you can see in pictures i like to use small chisel which ive made from old screwdriver and piece of sandpaper for finishing

Using soldering iron take out leds from the original board.

Step 2:

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Cut a new peace of board for leds, solder them in parallel connection, watch for the polarity!

Step 3:

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Solder the wires from battery terminals to the j.t. and switch, as you can see ive soldered switch directly to the (+) battery terminal because they were very close, for the other side ive used  peace of insulated wire.
Important !!! Before you glue the led board be sure to apply small drop of melted glue to the battery terminal to avoid short circuit.

Step 4:

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Its ready for use!

Step 5:

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Mounted and ready to go!
brankomaster (author) 2 years ago
some basics
http://www.youtube.com/watch?v=0GVLnyTdqkg

excellent blog with plenty of explanations and schematics

http://rustybolt.info/wordpress/

there you can find a lot of different types of construction and explanation about working principles of j.t. circuit.
Of course you can browse Instructables for j.t. informations.
Now about advantages, first main difference is you need only one battery so we can say its economic, whole light is much easier (i usually carry it in my pocket), and from my experience (i cant really prove this) only one cell with j.t. circuit will work longer than original circuit with two cells.
And for the end knowing that probably nobody from people you know has light like that.
can you post some good sources for info on the joule theif? what are the advantages to using a joule thief instead of using batteries like normal?
brankomaster (author) 2 years ago
Here is the link for instructions how to make Joule thief bicycle light (front)
http://www.instructables.com/id/Joule-thief-bicycle-light/