Magnetic Induction Safety Bike Lights. Nearly Invisible Design, Cheap, Waterproof and Battery Free!




This safety light system blinks as far the wheel spins: faster it spins, faster rate the leds blink. I'ts contactless so here is quite no friction. It uses magnetic induction so it’s battery and maintenance free, waterproof and really clean in design. Perfect for all purposes!
Here in Italy Reelight’s are quite famous, but i preferred to reply the system by myself: is cheaper, custom and more exciting! There are a lot of DIY versions of this kind of safety lights, and some of those were very inspiring for me. I want to share my design for you because it’s even simpler, cleaner and more reliable thanks to the heat shrinking tubes coating.

- MAINTEINANCE FREE, waterproof assembly gives high reliability
- CONTACTLESS, which means quite no friction (check out the video)
- CHEAP, you can take all materials from "scrap"
- SAFE, it lights up as the wheel spins

Will follow only some tips and hints, due the design is really simple and you can get it just from the images. For any doubt leave a comment!

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Step 1: Materials, Tips, Hints


- neodymium magnets (i took two couples from a pair of old 6Gb HDDs)

- coil (i took one from a 12DC, 10A 250V Relay)

- mastic adhesive

- heat shrink tubes, in different diameters

- 5mm red leds, ~3V 20mA

- wire (coaxial stereo wire worked perfect for me)

- wooden icecream sticks

- cable ties


- everything you need to solder

- heat gun (or lighter)


- the center of the design is to use heat shrink tubes to wrap all components, and then fix it to frame with cable ties. Before heating the tubes, i filled those a bit with mastic glue, this way i obtained a watertight sealing (and these rainy days confirm this!)

- note that there is a convenient combination between magnets and coil orientation, try changing the led connection polarity.

- i found out it’s better to put less in parallel. Less leds, less the rotation speed to light them up.


- the coil produces alternate current. If you want to have a smoother blinking use a diode bridge to rectify the current and feed a capacitor placed in parallel with the leds.


- Reelight’s:

- best inspiring DIY project for me:


i’m using this since a week ago, 150km some under the rain, and is working good. During night i prefer to use another blinking tail light powered by batteries and a yellow reflective jacket, so i don’t think at this light as my only tail light, but in some situations it could be. Tail light blink from 4kph, this means it works also pushing the bike walking!

Hope you enjoy this project, and do even better!

Cheers from Italy!


**EDIT (8th dec) ~400km, some under rain and some under 0°C, and still working :) Average speed ~27kph, Max <60kph

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78 Discussions


1 year ago

Im thinking of doing a similar build but plan on mounting neodymium magnets around the rim, flush mounted so they are out of sight, then have a concealed coil, hopefully this will provide enough current to provide USB Charging and lights.using deep vee style rim on my single speed commute bike, so i can charge my phone while being able to use it as gps when needed. plan to make it as concealed as possible and leave people scratching their heads trying to work it out. willl try and write a Instructiple when done and will post a link.

Alex Pop

2 years ago

Hi man,

Wow what a great job, I'm on a way to share a 3D printed kind of reelight light I worked on for several years.

Here is one of the models I have and designed a blinking when stopped :

Are you still with this kind of stuff in your life ? I'd be really willing to build something together around it.
Looking forward hearing from you,


3 years ago


can you please tell me the characteristics of the relay coil?

I would like to know the resistance, volts, diameter of the wire etc.

When I tried to lit up a led like this, it did not work, I could hardly get 15ohms from an 880ohm relay.


3 years ago

Do you think connecting a 1watt power led would lit up ?

1 reply

Reply 3 years ago

As i remember it lights up, but i finally preferred to use some common 5mm leds to have a bigger surface.

For best rusults i suggest you to use a pre-winded coil, there are many more turns and it is more precise.
In this other project i experemented myself how hard is to wind up a coil :)


4 years ago on Introduction

Hi this is the best DIY sor far.. but I cant figure out what is the purpose of the coil can i use solid state relay?

1 reply

Reply 3 years ago

Go check: and You'll find a nice solution to control Your dynamo


3 years ago

Go check: and You'll find a nice solution to control Your dynamo

The main problem with this design is, If you aren't moving the lights aren't flashing. and so if you stop in the middle of the road at night,.. Cars will not see you until its too late. It probably wouldn't be too hard to put little magnets all around the rim and a coil near the rim with a few capacitors and a rectifier and that could go for both wheels,
and wire them both to one 18650 battery charger(with over discharge protection) and from the battery charger wire out to LED's for flashing red lights or even a front headlight.

2 replies

Hi Bullfrog!
I'm sure this design has pros and cons. For me the essential was to have a safety light ongoing. Usually i have a second battery powered tail light.
Anyway as i tested i suppose the electricity generated would not be enough, but if you will have good results please inform me :)
Thanks for the comment!


Reply 4 years ago on Introduction

maybe you could be interested in this other project i'm working on...


It's easy to reduce the parts count because the LEDs are diodes. You may consider removing the rectifier bridge, which would eliminate that voltage drop (about 1.2 volts), making the LEDs brighter. It might be wise to insert a ~120 Ohm resister in series with the LEDs as a current limiter.

2 replies

Reply 4 years ago on Introduction

Thanks for hints. The rectifier bridge serves to allow the insertion of the capacitor.
actually in my project there is no capacitor, as you suggest I preferred to directly connect the LEDs to get the most of the light at each flash.
In this other project I'm testing the use of capacitor and I thought one thing that soon I share, so if there was any flaw in the circuit we could talk about it!


Reply 4 years ago on Introduction

It's true!
From my point of view two magnets are the best compromise in lightness, balance and flashrate, but it's my taste!