Question 1: what type of current will be created when passing magnets past a coil? AC or DC?

Question 2: What is the best way of storing the electricity generated with a contactless dynamo?

I'm working on a contactless dynamo project for my bicycle, (similar to this project: http://www.instructables.com/id/Contactless-dynamo-powering-bike-safety-lights/) except my concept includes being able to turn on/off the lights at will instead of them receiving power directly from the coil as shown on the link I just shared. I'm attaching some photos for reference.

I was successfully able to get a simple LED to flicker with 6 small neodymium magnets and one 12V ~80 Ohm coil. From what the internet and some friends have mentioned, I would need to convert the AC being induced by the magnets passing by the coil into a battery for a more complex bike lighting system.

My ideas for the bike lighting system are to power a front-facing headlight, a rear-facing brake light, increase luminescence of rear-facing brakelight when I apply pressure to handbrakes, a blinker system sensitive to turns and toggle-able with hand switches. Similar to how a lighting system would work on a scooter or motorcycle.

Any insights into this would be much appreciated!

Coil attached to the rear fork with 6 magnets taped to the wheel spokes

  • 1
    \$\begingroup\$ Your title mentions charging a battery but your question doesn't explicitly mention a battery. Could you explain what purpose the battery will serve, when the dynamo can power LEDs without one? I think you will get much more help if you can be more specific. For example, what is "a more complex bike lighting system"? Simply more LEDs? Flashing LEDs? \$\endgroup\$
    – user133493
    Jul 5 '17 at 2:40
  • \$\begingroup\$ Ah, sorry for that confusion. I meant battery by "stable power source". I will edit the question now to include more detail. Thanks for your question! \$\endgroup\$ Jul 5 '17 at 19:58

Two to four diodes and a capacitor will get you a long ways, without the complications of battery charging (which is picky to do well, depending on the type/chemistry, some being more difficult than others.) The concept you are after at that level is not too complicated - a rectifier (converts AC to DC) and a filter (smooths out the pulsing DC.) You'll want those anyway for a system with a battery, but with a small LED current and a fairly large capacitor, you may find that your application can be adequately covered with just the capacitor.

You'll also typically get more output if you shift your generator position towards the rim of the wheel, so the magnets go past the coil faster. More magnets will also help, and there's more room for them out there.

Mind you, it's far more convenient with current technology to simply use a rechargeable battery powered light, in practical terms for most riders - you really need an odd set of circumstances to make the "on bike generator" actually make sense in the era of rechargeable lithium-ion batteries and LED lights. But it can be fun to tinker with.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ neare the rim the magnets are faster, but there's also more absent, resulting in shorter taller voltage peaks, avaiable current is proportonal to the strength of the magnet (but not the speed), so moving the magnets is not likely to give much more energy, adding more magnets would help. \$\endgroup\$
    – Jasen
    Jul 5 '17 at 5:23
  • \$\begingroup\$ Thanks so much for these answers! Yes, I also bought a bridge rectifier/diode bridge, so this schematic really helps, thank you. I also considered placing the magnets closer to the rim for the same reason of higher rotational velocity, but they would be further away from the coil since the wheel tapers away from the fork as you get closer to the rim. Could solve that with some sort of arm extension for the coil, though. \$\endgroup\$ Jul 5 '17 at 20:05
  • \$\begingroup\$ also good point on the availability of rechargeable Li batteries, but personally, I can't stand having to remove my lights to recharge my lights all the time. Plus having the added capabilities of responsive brakelights, headlights, blinkers, etc. is nice to have an in-place system that doesn't need to be removed every time you lock up your bike. \$\endgroup\$ Jul 5 '17 at 20:08
  • \$\begingroup\$ Yes, shift the coil to maintain reasonably small spacing. Possibly add another coil. But you may have trouble meeting your power budget even so. A better-designed rechargeable battery system would involve leaving the lights/switches in place and simply popping out the battery itself, choosing a battery/holder that makes that quick and easy, rather than demounting lights. \$\endgroup\$
    – Ecnerwal
    Jul 6 '17 at 3:51

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