Introduction: Magnetic Induction Bike Lights (Mod)

Picture of Magnetic Induction Bike Lights (Mod)
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

Picture of "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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


Unkful (author)2011-12-17

Hey Nagutron, thanks you for these photos and share it, I'm really glad to see your manual, but I have 2 questions. What is capacitor's value (in F)? and, if you only put one of these, may be posible that only one of the couple of led works right? I think that so the coil makes AC and I think the leds are reverse biased to fully profit the AC. I think may be better put 2 capacitors, one for each led, what do you think?

nagutron (author)Unkful2011-12-17

Sadly, the whole thing's been coated and the details aren't visible. For a more from-scratch approach, see these instructables: and

astral_mage (author)nagutron2014-09-20

10 volt around 2500mfd

safesearchoff (author)2014-05-06

would it be possible to mount the coil & circuit to the spokes, and instead have the magnets be fixed to the fork, in order to wire LEDs to the tire rims/spokes themselves? or would the motion of the wheels mess up the way the magnets moved within the coil itself?

monjnoux (author)2014-02-18

great job! an inspiration for my next mod!

berserk (author)2013-08-07

I have a reellight, and a cheaper Danish (or more likely Chinese) knock-off, love them. I thought about opening them to separate components when I first got mine, but when it didn't want to come apart easily I chickened out. Good for you for going through with it.

Dranem (author)2012-12-18

Why haven't you put all the magnet on the same wheel, and the two coils in the same housing? Like that, each coils is used with four magnets...

F:Pourquoi n'avez vous pas mis tous les aimants sur la même roue et les deux bobines dans le même logement ? Comme ça, chaques bobines est utilisées avec quatre aimants...

Tkdwn (author)Dranem2013-05-04

Like! :P

Ian01 (author)2012-01-15

Step 2, photo 5. What's the tool in the background that looks like a nail clipper?

golddigger1559 (author)2010-11-15

lets say i wanted to make my own induction charging system. where can i go to find the plans for the charger and the induction coils. i know how to make the coils the problem i have is i dont know how to use them. and help would be greatly appreciated.

nagutron (author)golddigger15592010-11-15

I've never built one from scratch, but this guy did and he has some details in his instructable:

I'd love to see photos of whatever you make, when you're done!

nagutron (author)2010-03-14

Note: Reelight now makes lights that do all of this out of the box:

nagutron (author)nagutron2010-03-16

 I also wrote a blog post on the new lights. There are photos and PDF in the post.

onrust (author)2010-03-07

  These light are GREAT!  I had only one complaint.  When I would walk away from my bike they kept flashing.  Every TWEAKER in the area would be drawn to my bike.........they had to go!  
  If this is not a problem in your area,  these are well worth the money.

rpvanpatt (author)2010-02-11

 do you kno about how much volts are created with the coil?

cantsolder (author)2009-09-03

Which solder joint on the circuit is positive and which is negative? I plan to directly wire mine rather than use a jack. Also if thicker wiring was to be used on the coil would there be more windings or fewer windings? Do windings have to follow a certain direction?

nagutron (author)cantsolder2009-09-03

I actually don't know which side was positive. I just did the dumb thing and kept track of which wire went where on the lights, and kept those connected on the proper side through all of the components I added (wiring, audio jack, audio cable, coil.) There's probably a better way...

cantsolder (author)2009-07-30

One reason why I went to this instructable was that my front Reelight 120 failed (the rear is still working great). I heard a ticking noise inside the housing just before it failed. When I opened up the housing the two strands going to the LEDs had completely broken off their solder joints, leaving the two strands attached to the coil. I think the fragile connection between the LED and coil strands broke during use and the rest broke off when I dremeled the housing. Reelight should have put in beefier wires as there is a lot of vibration on a bike wheel as well as jolts from everyday riding. Maybe speaker wires will work.

nagutron (author)cantsolder2009-07-30

Yes, those wires coming off the coil really are extremely thin. I thought that that just made them difficult to hack, which is excusable, but it they're also less durable under normal use, i'd say that's inexcusable.

cantsolder (author)2009-07-28

Having the coil separate from the lights makes more sense; especially for small wheeled bikes. Reelight should consider this and design their nest gen lights to allow remote mounting. Is there a way to do this with electrically conductive glue? I am sensitive to solder fumes and don't handle a soldering tool very well anyway.

funbob (author)2009-07-07

Nice insight into the reelight. Can't see what the values are of the capacitor are from the photos. I have some of those hard drive magnets, I also think they would be ideal for this project.

ampeyro (author)2009-05-03

could this produce more power putting it further to the enter of the wheel? (maybe it's not correctli written, but, i'm not english)

wobbler (author)ampeyro2009-06-08

Actually, you should get more power/voltage by putting the coils further out from the centre as the increased linear speed of passing the magnets past the coils should raise the voltage.

ampeyro (author)wobbler2009-06-09

it's vhat i vas trying to say

james.mcglashan (author)ampeyro2009-05-30

what is it you mean non english may be a little proplom for reading this

james.mcglashan (author)2009-06-09

I want to first put power/charge: usb divices, electric bell, batterys and lights. and how much does 1 magnet make with 1 magnet???

james.mcglashan (author)2009-04-05

is the coil u have used copper if it is then y not just get a lot of it and then make ur own coil that will work in a bigger area for more power and were can i get the magnets like theese and i want to make a betta 1 so i need more to generate enough power to charge a project i am working on...

leed (author)james.mcglashan2009-06-08

A dead harddrive (new or old) will provide perfect magnets for this use. Be careful not to let the magnets stick to each to each other, though, they are quite difficult to get apart.

james.mcglashan (author)leed2009-06-09

i mean the magnets he used there difrent and i have tryed many hard drives and i only have 1 magnet thats from 3 hard drives only 1 so i dont get them that easy

but again i can use my dremal to cut off those bits that are stoping me to use them...

thepelton (author)2009-02-27

Any idea how much voltage is produced by each pass of the magnet past the coil? What ocurred to me is that if it was 3 volts, or a little more, you could power a bright LED with it, and if the voltage exceeded 3 V, you could either add another LED in series, or a resistor.

nagutron (author)thepelton2009-02-28

I actually worry that the voltage will vary greatly with the speed of the wheels turning. I wonder how complicated a voltage limiter would be, or if it would be needed; I know little about electronics.

chuckr44 (author)nagutron2009-03-04

If you stored the power from the inductor in a capacitor before passing it on to the light/LED, wouldn't that effectively even out the power levels to the light?

nagutron (author)chuckr442009-03-04

That's what the commercial Reelights do. You can see the capacitor in the close up shots on step 2. I hope at some point that someone designs a cheap, easy-to-build circuit that we can all use.

thepelton (author)nagutron2009-02-28

Sorry, but same here.

neuraxon77 (author)2009-01-31

Excellent! You may also want to add side reflectors, you disappeared in the night test when you turned side on.

nagutron (author)neuraxon772009-02-02

Yeah, thanks. I've since added some great tires (Specialized Armadillos) that add built-in reflection along each rim. They're great and the most puncture-resistant tires I've ridden, too.

milokp (author)nagutron2009-02-09

I do like my Armadillos, but the sidewall reflector gets covered with road dirt way too quickly. Not to mention I found a sizable chunk of glass sticking out of my rear just last week. I'm disapointed in that. But the instructable; I'd never thought of induction charging bike lights, I might have to do that for side&rear; lights. Front light is a 30 LED shop light with a LiON rechargable that lights up the night quite well. Good job on the rebuild, and lots of pictures to boot!

wobbler (author)2009-02-06

Depending on how often you use the bike in the dark, you could feed this into a simple battery charging circuit (using Schottky diodes to eliminate diode losses) and then switch the battery on when necessary. Alternatively, you could use the battery only when stopped. I did this in the past on a bottle dynamo with a simple voltage comparator on a relay (to stop any voltage drops) and switched in the battery whenever the dynamo voltage dropped. That worked with old style bulbs and kept the lights on at traffic lights etc., so it will certainly work with LEDs. However, I currently use two high capacity (7Ah) D-type NiMh driving a puck based 3W Cree headlight made from a Tescos (UK) torch (extremely bright and cheap at £12.99p and sturdy) and a standard 3V flashing front and rear light.

PKM (author)2009-02-02

Any idea how much power you can usefully get using these things? They are fine for visibility-only "marker" lights but I think a 3W led headlamp might make your wheel "cog" a bit (or just not get enough power). I'd love to have a go at making the induction coils but I suspect it's a bit too time-consuming.

nagutron (author)PKM2009-02-02

I imagine the Reelights draw less power than that. They are definitely more "marker" lights than headlights. I don't really illuminate the road in front of me with them. That's the tradeoff for not feeling any drag when I'm riding. You could definitely draw enough power for a 3W headlight, though. The average human can put out 150W, pedaling on a sustained basis. We can put out maybe twice that, in bursts. That seems more than enough, even if you factor in various losses along the way. The limiting factor might be that you have to increase the size of your coils and your magnets to get more power, and that could get heavy.

bpfh (author)nagutron2009-02-05

Remember that part of that 150 watts is used to propulse you along.... so adding a generator for a twin beam car headlight glued onto the handlebars is a real possibility, but flick the switch, especially when going uphill and you will come to a fairly unexpected stop :)

nagutron (author)bpfh2009-02-05

Yeah, I wonder how much power draw would feel like a drag. It might be noticeable at as little as 10% or less.

goldscott (author)2009-02-05

I've been wanting to make induction bike lights for a few months now, but I just haven't been motivated enough to do it (also lots of other projects have more priority). I like the idea of modifying this commercially available light. Also, sweet fixed.

nagutron (author)goldscott2009-02-05

Thanks. That bike is my baby. I got it cheap, used, and it's been treating me nice for five years.

jolshefsky (author)2009-02-05

I don't mean for this to sound negative, but I think your title should be something like "Modifying Magnetic Induction Bike LIghts". As it stands, the title implies that you're making them.

That said, it's otherwise a solid instructable -- particularly for those people with enough electronics experience to solder wires, but not enough to make their own circuits.


nagutron (author)jolshefsky2009-02-05

I didn't want the title to get too long, but I added (Mod) at the end to make this clearer. I do hope that there's enough information in step 2 for someone to attempt a full build themselves, though. Perhaps that someone will be me, down the line!

ste5442 (author)2009-02-02

Nice idea - you could adapt it to charge your mobile I guess!
It would take a lot of cycling though - I don't think I am up to it :-)

PCBPolice Electronics Forum

TheLucster (author)2009-02-01

I was thinking about making something like this a couple of months ago, I have all the tools except for time! The paper you found looks great, I shall have a look through at some point, thanks!

rachel (author)2009-01-31

Nice! What size capacitor is that, if you remember? How long does it hold charge for (i.e. how long after you stop pedalling do the lights go off)?

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