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After trying unsuccessfully several commercial bike flashlights, I decided to make me one that cover my needs:

- Lightweight and very small size.
- Rechargeable to prevent the continued waste of AA batteries.
- Cheap!!!
- Powerful and long-term for long night cycling routes.

Due to my limited knowledge of electronics, I assume that the circuit can be improved, so any help comment  or suggestion with this, will be welcome ;)

Step 1: Materials Needed

- Old Cellular phone with working battery and charger (as charging station).
 - Rf switch cable TV adapter from an old video game console or some similar size plastic case.
- Hard disk or some electronic device with contact pins.
- A small switch
- 10 white ultra bright leds with prototyping board for welding circuits.
- Solder
- Bicycle clamp from a broken bike light
- Wire

Step 2: Lights and Case

- Place the leds in parallel on the prototyping plate as shown in the picture.

| - +_|_+ - |
| - +_!_+ - |
| - +_!_+ - |
| - +_!_+ - |
| - +_!_+ - |
|__---|--__|
          |       |
         +       -

- Open the RF adapter and remove the inside circuit / wires to get the plastic box empty .
- Take measurements of the led board and cut / sand the plastic box until led board fits perfectly into.
- Unscrew the clamp from the old bike light.
- Mark in the plastic box where to screw the clamp and make the screw holes.
- Screw the clamp to the box ensuring it is firmly attached.

Step 3: The Battery Holder

- You must find a good way to attach the battery that allows you to easily replace to charge it.
- I have chosen a pin connector from an old hard drive.
- Unscrew the pin connector from the hard drive and remove some pins until you have only the positive and negative pins and a pair more for holding the battery.
- Sand the pin board until it fits into the plastic case.
- Solder wires to + and - respectively.
- Solder the positive wire from the pin board to the positive leg of the led board.
- Solder the negative wire from the pin board to the switch and from the switch to the negative leg of the led board.

Step 4: Finishing...

- Put the battery inside the plastic box making all components fits well and the polarities are correct.
 - At this point, maybe you will need to adjust the position and size of the inside components, so sand if you need before gluing the pin board!!!
- Turn on the switch and check if pin board makes good contact to the battery...
- If it works correctly, it's time to glue the pin board to the plastic case ;)
- Close the plastic case and plug the bike clamp...

Ready to go!!! ;)

When the battery runs out after SEVERAL hours of use, all you have to do is remove the battery and place it on the phone to recharge again...
How about a Mentos box?
Nice idea to use a telephone battery but its quiet dangerous. Because you use it on a bike, it could get wet and short circuit so you have to make sure no water gets inside. And without some current limithing its not very safe in use. But al together its a good thought because of the high capacity of the battery you have a long lasting light.
I've been reading about the dangers of using this type of batteries, but due to my limited knowledge of electronics I have not been able to improve the prototype. I hope, at least, this design can serve as inspiration to other users with knowledge of electronics to make a better and more secure device. Thank you all for your comments.
If you put a glase fuse of 250mA in series with the feeding wire of LEDs you could at least prevent a shortcircuit to cause damage to the battery. 10 led x 20mA = 200mA nearest fuse above is 250mA
Great idea! I had a similar instructable planned. Guess I'll have to come up with something different now.
That battery appears to be a lithium-ion or lithium-polymer battery, since that is what most phones use and have used for some time now. even though they are labelled 3.7v or 3.6v the full charge voltage is 4.2v. This is a bit too high for the leds and will probably cause them to burn out eventually. Also this type of battery should never be discharged below 3v or they will quickly lose the ability to be recharged.
You need some kind of current limiting resistor or those LEDs are going to start burning out, i dont know how many volts your battery puts out so i cant recommend a resistor size. Other than that it is a pretty clean design
Thanks for the help!!! Battery outs 3,7 Volts.

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