Introduction: Magnetic Induction Bike Lights (Mod)

About: I'm the former Frontend Engineer for Instructables. Problems with the site? It may have been my fault... Like what you see? Sing my praises!
For staying safe at night, magnetic induction lights are a great way to go. They just bleed off a tiny sliver of your momentum to generate nice, bright light. They're perfectly silent (unlike dynamos) and need no batteries (unlike clip-on bike lights.)

Just to be up-front, this instructable doesn't contain details on how to build induction coils or the lights themselves. I adapted commercially available magnetic induction lights to fit my bike perfectly, opening up the cases so that I could separate the lights from the coils.

A short video is below, showing the initial test of the separated coil, the finished setup, and how it looks at night:

Read on to see how it was done!

Step 1: "Reelight," Its Shortcomings, and Other Options

Reelight is a commercial product by a Danish company. At present, they're the only large manufacturer of these induction-powered bike lights that I know of. I've attached some diagrams from their manual, below, showing how the lights work.

The problem with Reelights, though, is that they are mounted directly on the wheel axles. This makes the actual product small and compact (the coil and light are in one integrated unit) but has a number of shortcomings:

  • The lights are very low to the ground, making them less apparent to drivers
  • The arms can flex a bit as the magnets pass by them, so they have to be adjusted every so often
  • The arms are kind of ugly

Separating the coil and the lights would solve these problems. The could could be mounted directly on the frame, making it smaller and less obtrusive. It would be much more secure, requiring no adjustment. Finally, the lights could then be placed anywhere on the bike, connected by wires.

One last, big drawback of Reelights is their cost. They cost anywhere from $25 for a low-end one to $85 for a fully-featured pair, plus shipping. So I'm also including some links to help people get started with making their own induction coils, circuits, and lights.

Okay, onwards to how my Reelights were set up!

Step 2: Break Open the Reelight

Reelights are glued together with some pretty hard-core adhesive, since they were never meant to be opened. You should probably do a neater job than I did. A dremel with a fine cutting wheel would serve you well; I went at it with a saw and a screwdriver and pried the thing open, which sadly cracked the case in a few places.

This step has a number of close-ups of the internals of the Reelight, for others' reference. These pics are for the red rear light; the white front light was identical except for the color of the plastic cover.

Step 3: Re-Assemble LED Housing, Without Coil

Basically, the lights went back into their original housing. I installed a 1/8" audio plug in that housing and connected to the power inputs so that I'd have a simple way to detach the lights, then the whole thing was sealed back up with a hot glue gun.

I repeated these steps for both of my Reelights.

Step 4: Induction Coil Housing: Back Wheel

I tied the audio cable around the induction coils to protect the delicate leads from any pulling force and soldered them in place. Each of these was now independent of the LEDs in their housings; to connect them, I'd just need to plug them in.

Now, to build the housings for the induction coils. For the rear housing, I wanted an aerodynamic housing that would nestle into the rear triangle of my bike frame, right next to the axle. I cut this from a satellite dish (same one that provided me with the audio cables and jacks. Everything was carefully measured out and sketched onto the satellite dish, and then I cut out the housing and shaped it using a dremel.

The induction coil was then sealed with large amounts of hot glue and mounted into the housing. Hot glue was also used to mount the housing to the frame. Afterwards, I did a quick test to make sure that the wheel was inducting a current in the circuit, and that it indeed lit up the LEDs.

Step 5: Induction Coil Housing: Front Wheel

I didn't take as many pictures of the front wheel housing, but it basically used the metal bracket that came with the Relight, but put on behind the front fork so that it would be more cleanly nestled away. The thing was sealed with hot glue and a short length of inner tube that was zip-tied on.

Step 6: Finishing Touches

Finally, the LEDs were mounted on the bike with short strips of old inner tube. I basically fashioned my own, large rubber bands and attached everything that way. The lights were plugged in via their jacks and the cables were then run alon the frame from each induction coil to its respective light.

There's no noticeable drag while riding, and the lights have been working quietly and reliably for many months, now.