Introduction: Contactless Dynamo Powering Bike Safety Lights

Picture of 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.

Step 1: The Coil

Picture of 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

Picture of 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

Picture of 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

Picture of Rear Light

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

Step 5: Part 2 - Front Light

Picture of 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

Picture of 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

Picture of 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.


IlieP (author)2016-10-14

serpentine coil, bike whell

matthewtweedie (author)2016-09-14


KiranS9 (author)2015-07-12

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

bobyong808 (author)KiranS92016-04-30

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 (author)2015-10-23

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

albercuba (author)2013-07-14

Only a problem with disc brake wheels :(

You have to place the magnets far from the disc.

wobbler (author)albercuba2015-10-02

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.

baecker03 (author)2015-09-07

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

james.mcglashan (author)2009-06-09

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

Shoota75 (author)james.mcglashan2015-07-21

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.

kedwa30 (author)2015-02-23

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.

generalsnooky47 (author)2014-05-17

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.

stonioni (author)generalsnooky472014-12-03

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:


stonioni (author)stonioni2014-12-03

right, new link:

stonioni made it! (author)2014-12-03

Thanks to your hints i did my version of this light system :)
Check it out:

Cheers from Italy!

stonioni (author)stonioni2014-12-03

right link:

whodragon (author)2014-05-06

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

monjnoux (author)2014-02-18

awesome! my next hack for sure!

mkretman (author)2013-04-25

Anyone trying this to power revolights?

ranelson93 (author)2013-02-01

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?

pedrogg95 (author)2013-01-20

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

rchanter (author)2013-01-19

Hi I'm using the theory from this to make a version for myself just trying to sort a few bits out, when mounted like this the magnets rotate for mine I am rotating the coil past a magnet, will I get a better output if I pass the coil between two magnets? assuming the poles are aligned to increase the magnetic field.


adhawan (author)2012-11-04

I am having trouble finding the coils? Any help on where I can either buy them or get them would be appreciated.

nick222 (author)adhawan2012-11-08

I got them in a junk yard, pulled the relays from an old car harness... I did not carry my multimeter with me so I went looking for the heavier ones, they end it up measuring 80-90 ohms, but give up to 15V positive spikes and ~6V negative spikes as measured in the oscilloscope... The magnets I got are not as large as the ones pictured here, so I placed 4 of them in the rear wheel, I got these from my factory, they sell the damaged HD's to a recycling company, so I just got permission to remove the magnets before they go to recycling bin :-)

gtoal (author)2012-05-23

Has anyone done this in reverse? Put the magnets on the bike and the generator on the wheel, to power spoke lights or a POV (persistence of vision) display?

kent1956 (author)2012-03-26

Metal Drilling 101.

-Buy more expensive carbon drill bits.
-Use a cutting oil, and be liberal with it. It will help to cool the bit and keep it from melting and help to clean out the metal bits from the hole.
Cutting oil can be bought in small 8 oz. bottles
-Use a low speed, high speed causes the bit to make small metal bits and can lubricate the drill bit, causing it to ride above the metal you are trying to cut through.
-Drill bits are actually cutting tools, look at one, they normally have two cutting flukes (surfaces). You will know you are using the correct speed when you get a nice ribbon of metal, rather than a lot of small chunks.
-Keep good hard force on the drill, if you don't it will again make small chunks and just float on them above your metal surface.
-For very hard metal, It becomes necessary to start with a smaller pilot hole, and enlarging it drill size by size to your desired hole size.
I don't think these magnets have a very hard metal strap, they are not made for really hard abuse as other metal parts are, so this should not be necessary.


I have used the FreeLights and am not impressed with them. They are way too dim. But the idea is good. The paper from the NM students is good information. I like the way you have made use of salvage parts.

I would consider ways to make this all waterproof, as I commute come rain or shine. PVC tubes with caps could be used to house the relays, with a small hole on the bottom side for the wires to come out, sealed with some silicone.

Thanks for sharing this, over all a great idea, and a great presentation here.

The.zafootz (author)2012-01-08

This device is awsome I am learning to make one myself and hope to learn everything there is to know about contactless dynamos =D

SinAmos (author)2011-07-26

Simple, yet awesome. I can't wait to try.

jules15 (author)2010-05-14

i did this and i was only getting .01 volts around there. I used a 5 volt relay. i don't know what to do :p

scraptopower (author)jules152011-07-13

You're not using the DC setting on your meter are you ?

loccci (author)2010-07-05

hi, i dont understand where to put the capacitor if i want one led or more to stay on constant. do you have to charge the capacitor intitially. why is the only place you could put it parallel with led3? why does it not get negative voltage? why arent all of the leds in parallel? led2 and led4 seem to be in series, why is this? sorry i just dont get it. thanks

scraptopower (author)loccci2011-07-13

I don't think it is possible to add capacitor smoothing to this circuit without adding a bridge rectifier (use schottky diodes for efficiency).

He has wired the circuit in that way because the waveform is not mirrored, so there's less load when the coil - wire is more positive. At least, that's what I think,,,

yonutzky (author)2011-01-13

I have only one screw hole in my magnets. I tried to drill another hole in the panel the magnet sits on, with an ordinary drill, but it only dented the metal. It won't go through. Any suggestions?


bricabracwizard (author)yonutzky2011-07-12

Use a proper drill for metal, they are a bit more expensive but they cut through metal like its cheese. Another thing is keep the drill cool while drilling they will stay sharper longer.

Something I've found that helps is to drill the hole and then use a hammer and a sturdy roofing nail to drive the rest of the way through. However, this works best with smaller diameter holes, so if you intend to use a larger bolt to hold it in place, that might not be the best method.
Best of luck.

snowluck2345 (author)2011-05-18

Could you give an example of some other windings you could use? For example would a toroid work? or a transformer?

SuperKuper (author)2011-04-13

Where could I get my hands on some of these coils? Also are they sealed/waterproofed at all, and if not can the bike be open to the elements? (looks like there's a plastic coating on it)

golddigger1559 (author)2011-04-06

how long do you think it would take to charge a 200v 10000UF capaciter?

ploomus (author)2011-03-25

Really nice project, i'm thinking about making my own but maybe i also want to use it for charging my Ipod. Well i'm not sure will it be powerful enough but i could find it out. I don't want usual dynamo because it feels boring and i love building myself =P

rpvanpatt (author)2010-02-11

 ..i'm new to all this so dont mind the question... could it be possible to put a capacitor in front of all the leds?  and maybe with a switch, so that you could turn them all on and off with out any blinking?  thanks
It's by the same author as this instructable. The circuit diagram is similar enough to what is happening here for you to get some idea as to what's going on. If you need a place to get capacitors/LED's, try It's a pretty good site if you want low prices and fast delivery.

iceng (author)2011-02-04

It is a nice generator but those magnets are powerful enough to grab street metallic scraps and possibly jam the wheel.

f22archrer (author)2010-11-29

can i use the part of motors on which the wiring is done ??? as i fail to find myself a relay coil

joel26 (author)2009-10-22

Doesn't the magnetic field slow the bike down?

kikiclint (author)joel262010-08-18

it will slightly, but generally only the amount by which it is giving you power. Unless you have wierd stuff happening with eddy currents or ferromagnetic materials pulling on it.

xsmurf (author)kikiclint2010-08-22

"only the amount by which it is giving you power" To put that in perspective, 3 leds at roughly 3V and 25mA so 3x0.025x3= 0.225W (no loss I'm aware) of power consumed. Considering a cyclist might put out 300W of power (total ball park, I'm aware), that would be a reduction of power of 0.075% way way under anything noticeable to the rider.

midnightsky (author)2010-03-04

 coils aka relays you can find anywhere like auto zone to walmart lol homedepot  most gate openers whill have relays in them find an gate or a door company they will have tons of old gate sys boards andold relays that i am sure they will give up for free or a cheap price hehehe

rpvanpatt (author)midnightsky2010-03-23

how bout in any other typical household electronics, can you find one in anything else?

kikiclint (author)rpvanpatt2010-08-15

All electronics that have power switching are going to have relays. If you look for dead microwaves, they will have some in them. Other places I have found them are in large power filters for computers. 12 volt relays work good if you get the magnets to pass close to them.

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