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I use a Wahoo RFLKT+ computer. It has a built in backlight for night riding, but to conserve the battery the light times out after a while (30 second default). Since I have a hub generator, I'd like the light on continuously when riding at night.

During the day the display is reflective, so I'm thinking that an LED shining on the display might be able to light it.

I'm wondering if anybody has done something similar and has suggestions for wiring and placement of the LED. The wiring seems like it would simply require adding an LED in parallel to the headlight/taillight with a currently limiting resistor, I'm not sure sure about how to effectively place the LED.

  • For those of us who aren't familiar with this bike computer it would be handy to have some more details. What's the light for? Is it a bike light built in to a bike computer (I have one of those on the shelf) or is it a backlight to read the computer screen? If the former I suggest adding another light. If the latter (which I suspect you mean) you may find that an extra LED shining on the screen is tricky to get right so that you can read the screen easily, but modifying the computer may be harder. – Chris H Jun 14 '15 at 12:57
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    I would go for a Helmet mounted cycle light - apart from the safety benefit of making you more visible to cars, it gives light not only to see the computer but if you have to fix a tire or breakdown. LED technology gives you 10's of hours between charges – mattnz Jun 14 '15 at 21:32
  • I'd thought about a helmet mounted light but wondered if it would be too bright for close up use on the bike (e.g., reading maps or the computer). Do you have specific lights to suggest? Is the brightness an issue when looking at things on the bike while riding? – dlu Jun 14 '15 at 23:04
  • Many LED lights come with 2 or 3 brightness levels (and flashing modes). As far as lights go have a look at Bike light database or ebay for cheap ones. – mattnz Jun 15 '15 at 0:27
  • Two AAA batteries, a red, yellow or green LED, some wire, a resistor and a plastic tube. Plus some DIY and rubber bands and you will have nice lamp to shine on your computer, – Carel Jun 15 '15 at 12:18
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Some time ago I built something like this myself, in more or less the same way you describe.

The wiring seems like it would simply require adding an LED in parallel to the headlight/taillight with a currently limiting resistor.

Exactly. I bought an LED rated for 6V DC by the manufacturer (I think they have a built-in resistor), so I could skip the external resistor. Adding an external resistor should work too, but means more work and more connections that may fail.

I'm not sure sure about how to effectively place the LED.

Yes, that's the tricky part. I obtained a thin plastic tube (about 1mm diameter), and bent it to form a sort of archway above the computer.

I threaded a flexible wire (naked wire used for fastening, not electrical wire) through the tube. This wire provided stability to the arch, and its ends were wrapped around the handlebar left and right of the computer, to fix the "archway" in place. I cut a small hole in the side of the plastic tube, and threaded the two electrical cables into the tube, one from each sides, so they came out at the hole. Then I soldered the LED to the two cables. Finally I gently pushed the legs of the LED into the hole (one to go left, one right, following their respective cable), so the LED sat in the hole. I finally fixed the arch to the handle bar using the wire ends.

The result looked like this:

   ______________
  / ____     ____ \
 / /     |_|     \ \
 ||      / \     | |
 ||              | |
 ||              | |
 ||  |Computer|  | |
######################
      handle bar
######################

I omitted the the wire and cables due to ASCII art limitations :-).

This worked reasonably well. Electrically, it was fine, but mechanically the setup is rather wobbly, and the wire can break if you bend it too often. Also, getting the angle of the LED right is tricky, because LEDs have a fairly narrow beam.

Still, if you want to give it a shot, it's worth a try. It was not very hard to do, and did not require expensive material (the LED cost less than 1 Euro).

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    That's a nice approach if rather fragile. Note that you can buy LEDs with different (wider) viewing angles, probably including those with built-in resistors if you look hard enough. – Chris H Jun 15 '15 at 13:44
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    That's what I was thinking of in my comment! And you could also power it on a battery as well as on the dynamo. – Carel Jun 15 '15 at 17:59

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