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This instructable demonstrates how you can modify inexpensive LED flashlights for use on a bike. The range of flashlights I've seen for under $5 USD include the straight on/ off variety and another that is set up to blink on and off at a set rate, usually by multiple button presses. They are typically powered by three AA cells in a compact battery holder that slides out the back of the flashlight after unscrewing the cap which also contains the switch.

Three goals of the build are:

  1. Use inexpensive flashlights as bike lights for visibility
  2. Power the lights with a rechargeable battery pack
  3. Make both lights blink together and at the same rate

The build involves two assemblies that are detailed in the next steps:

  1. Modified flashlights
  2. Power/ control pack

Parts I used are:

  • 2 small LED flashlights
  • 2 small rubber grommets
  • 1 meter of 2-conductor 18ga insulated electrical wire
  • 2 barrel jacks
  • ABS plastic project enclosure
  • 1 small IC PC board (Radio Shack Model: 276-159 Catalog #: 2760159)
  • 8 pin IC socket
  • any 555 timer IC
  • DPST rocker switch
  • resistors & capacitors
  • panel-mount barrel sockets
  • rechargeable battery
  • battery holder
  • hook-up wire

Step 1: Modify a Flashlight

Begin by removing the back cap as you would to remove the batteries.

Most of the lights I have bought came with batteries already installed and may have a piece of insulating plastic film that isolates the battery holder's negative contact from the contact on the cap. You want to start with a known working flashlight so remove the film if it's there and test the flashlight to confirm it works.

Remove the three-cell battery pack and then remove the batteries from the holder, reserving the batteries for another use. For this project we'll use the battery holder but not the batteries.

Step 2: Run Power Wire Into Flashlight

At approximately the mid-point of the battery holder, drill a hole through one side of the flashlight body. This is aluminum and should drill quite easily with wood working tools.

Use a rubber grommet here to prevent the wire from being damaged by the sharp edge of the hole. Be sure to select a hole size appropriate for the grommet that will fit inside it. Try it on a scrap first before sacrificing a good flashlight body.

Once you have drilled the hole, remove any sharp edges with fine sandpaper or nylon scrubbing pad, then install the grommet in the hole.

Now you are ready to insert the wire from outside to inside, extending the wire through the open back of the flashlight body.

Step 3: Solder Wires to Battery Holder

Being careful to correctly identify the positive and negative ends of your empty battery holder, orient the holder so that negative end is facing to the back of the flashlight. Usually you will find an arrow embossed on the outside of the battery holder. This always faces forward, toward the LEDs.

To simplify connections to the battery holder I removed all but two of the contacts/ springs that used to contact the batteries.

Solder the positive power wire to the metal tab inside the holder opposite the positive contact.

Next do the same for the negative end.

In my build I soldered a connector plug on the other end of the power cord so that the power/ controller unit could be removed separately from the lights. This is optional. you could hard wire the lights to the power if you want.

Now insert the wired battery holder back into the flashlight body being careful to tuck the wire into the void left by the batteries. Replace the end cap and the flashlight part is complete.

Repeat the above steps for the second flashlight. Now we're ready to build the new power supply and controller unit.

Step 4: Power Supply/ Controller Build

Consult Instructables for instructions to build an "Astable 555
circuit" (see link). This is the core of the controller which will pulse power from the battery to both flashlights. 555 Timer Cicuits

You might want to tinker with resister and capacitor values to achieve your desired flash rate. One of my favorite tools I use to determine the best combination is to use a 555 simulator program on my PC such as 555 Calculator

In the future I will add a socket so that I can charge the battery without opening the enclosure for convenience. Also swapping out the 9 volt NiCad batterry for a Lipo would be another big improvement.

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