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.
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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

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.
KiranS91 month 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
kedwa306 months ago

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.

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 stonioni9 months ago

right, new link:


stonioni made it!9 months ago

Thanks to your hints i did my version of this light system :)
Check it out: http://www.instructables.com/id/Magnetic-induction-safety-bike-lights-nearly-invis/

Cheers from Italy!

1959536_509137259228823_7357591747919372016_n copy.jpg
stonioni stonioni9 months ago

right link:


whodragon1 year ago

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

monjnoux1 year ago
awesome! my next hack for sure!
albercuba2 years ago
Only a problem with disc brake wheels :(

You have to place the magnets far from the disc.
mkretman2 years ago
Anyone trying this to power revolights?
ranelson932 years ago
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?
pedrogg952 years ago
how can i build the coils, or where can i find it, or by it??
rchanter2 years ago
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.

adhawan2 years ago
I am having trouble finding the coils? Any help on where I can either buy them or get them would be appreciated.
nick222 adhawan2 years ago
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 :-)
gtoal3 years ago
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?
kent19563 years ago
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.zafootz3 years ago
This device is awsome I am learning to make one myself and hope to learn everything there is to know about contactless dynamos =D
SinAmos4 years ago
Simple, yet awesome. I can't wait to try.
jules155 years ago
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
You're not using the DC setting on your meter are you ?
loccci5 years ago
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
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,,,
yonutzky4 years ago
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?

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.
Could you give an example of some other windings you could use? For example would a toroid work? or a transformer?
SuperKuper4 years ago
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)
how long do you think it would take to charge a 200v 10000UF capaciter?
ploomus4 years ago
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
rpvanpatt5 years ago
 ..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 Jameco.com. It's a pretty good site if you want low prices and fast delivery.
iceng4 years ago
It is a nice generator but those magnets are powerful enough to grab street metallic scraps and possibly jam the wheel.
f22archrer4 years ago
can i use the part of motors on which the wiring is done ??? as i fail to find myself a relay coil
joel265 years ago
Doesn't the magnetic field slow the bike down?
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.
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