Contactless Dynamo Powering Bike Safety Lights




Hello everyone!! On this instructable you will see how to build a simple contactless dynamo for powering a bike safety light from a relay coil and some hard drive magnets.

As you will see, the electric circuit is really simple and contains only 3 bright red leds and a 4700uF capacitor. The capacitor is used just to keep one of the leds steady while you are driving. The other two leds will be blinking as the magnets pass by the coil. To have all three leds blinking you can remove the capacitor if you want. If you decide to place some more capacitors in parallel, the steady led should keep on even when you're stopped.

On part two we have more 2 coils installed for powering 5 white bright leds for the front light. The circuit is completely independent from part one's.

For more information about how to improve this kind of system, just look around the site, there are several instructables on the subject. You will find different kinds of generators and other circuit ideas.

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Step 1: The Coil

To avoid spending time building your own coil, just look around for an old relay. I got this board from an old telephone central station.

The second picture shows the coil disassembled from the relay. On the bottom part you may notice that the magnetic circuit is open. In fact that's where the relay's moving parts were attached. I didn't noticed much difference closing the magnetic circuit with a peace of metal, so I just left it as it is. But theorically it should improve efficiency.

For a simple system like this, you should try to keep the coil's resistance between 100 and 200 ohms. This one has 200 ohms. With more than that you will generate more energy but efficiency drops because you also have more losses in the coil.

Step 2: Hard Drive Magnets

Now you will need to find some neodymium hard drive magnets. I used 3 of them on my rear wheel, you could use how many you want as long as you are able to firmly attach them.

Step 3: Coil Tests and Circuit

On the scope you can have a clue of what you're generating with your coil. If you are able to save the waveform, you may import it on a circuit simulator and try to model your project.

Unfortunately I could not get Led3 to have a continuous conduction in simulations although it happened always on the real circuit. Maybe it is because I have not used any inductors on my coil model.

Notice that the circuit is not symmetric due to the coil's generated voltage waveform having it's energy concentrated on positive values. This is all dependent on the magnet construction and the magnetic circuit you used around your coil.

Step 4: Rear Light

Now take a cheap rear led light so that you can install your new system into.

Step 5: Part 2 - Front Light

Now on the second part of this project we add 2 relay coils and a front light, with a circuit completely independent from the first part of the project.

Step 6: The Circuit

The following circuit shows a way of powering 5 bright leds with two coils that are not generating energy simultaneously. If you just connect them in series, one will absorb part of the other's energy. This does not happens here.

I have not used any capacitor since I wanted to have all leds blinking a lot! Anyway the only place you could really place one is in parallel with led3 since it never gets negative voltage. So you will have one steady led and the other 4 blinking...

It seems that the coils' resistances should be kept around 100 or200 ohms but I used two 600 ohms coil and it worked fine.

Step 7: Closing Up

The hard part might be to attach all those coils to you bike. Try to use as less ferromagnetic materials as you can to avoid it to be pulled by the magnets.

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154 Discussions


Question 4 months ago

hey, this is super cool but how would you go about making this super jagged ac charge a dc battery?


2 years ago

serpentine coil, bike whell


4 years ago

I need some help i have this coil & set of magnets. The coil is working fine. I have tested with battery and bulb. But when i am setting up on bycicle its not working.. plz help..

Attaching images

1 reply

Reply 3 years ago

I think your magnets are too weak. They need to be strong neodymium magnets. And they need to be mounted very close to the coil as it turns by

Board Tinkerer

3 years ago

How many volts do you get per 200 ohm relay and a strong HD magnet?


Reply 4 years ago

Not really a problem, the further out the coils are from the centre, the faster the magnets go past the coils and therefore the more voltage and power they generate.


4 years ago

if you rectify the current, use schottky diodes or another low voltage drop diode


how mant volts does this make: 1 relay, 1 magnet, i want to do the relays and magnets so i can hopefuly get enough power to power my little list i want to first put power in 10 very good capastor then through regulators to do theese things, and power 8 usb divices (usb battery charger, mp3/4, mobile, camra and other divices), and power my lights, will this work with how many relays and how many magnets to get the wanted power

2 replies

Watts, James you will never produce enough power from this to power those devices. At best you can run LED's and nothing more to charge a Iphone just for eg you need one watt well thats not true 1.5 watts would be charging not just keeping it running with apps going. I'm just doing a bit of maths and i'd say at best .3 of a watt after losses from the rectifier.

that will depend on the relay and the proximity and speed of the magnet passing. I was able to get 5 volts ac off of one of my coils.


4 years ago on Introduction

I was searching for how to make a dynamo without magnets and google sent me here. :(

Great ible though! :D

I just don't have any old hard drives and I need to make a water turbine.

hey i was wondering if you can please post the bottom of the circuit for the rear light? i am trying to do this project but i really can't figure out the circuit diagram.

2 replies

HI! Hope it's not too late.
Current from coil is alternate, ok for blinking leds (best if connected in parallel; one polarity works better, check it).

To add steady led you need a capacitor. It need a DC current, so you have to add a simple diode bridge rectifier. More leds above the first: best connected in parallel.

Check out my version of this project:



Reply 4 years ago on Introduction

right, new link:


5 years ago on Introduction

looks great and love the design but but does anyone know where i can find the relay because all my old home phones have been thrown away


5 years ago

awesome! my next hack for sure!


6 years ago

Anyone trying this to power revolights?


6 years ago on Introduction

So do you actually need a relay to make this work or Could i just make my own coil by wrapping some copper around a pvc pipe?


6 years ago on Introduction

how can i build the coils, or where can i find it, or by it??