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For a pet project of mine, I like to generate power from a rotational motion. (similar to bicycle tire turn). I want to build the system without a battery where as the tire turns I generate enough electricity to turn an LED. It is ok if the LED is dim in slower speeds, my application will only cover the high speed cases.

What is the best approach to this?

I do not want a magnetic coupled system, this should be one piece.

update The device is for bicycle, it is an active reflector. I wanted to use the rotation of the bicycle and light an led from the generated energy. T I didn't want a two point solution where I put a magnet somewhere and use that to generate energy. Just use the rotational motion. Some piezo material that would accumulate energy and use to pulse a led for a few msec every second.

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  • \$\begingroup\$ If you are trying to build a POV device on a wheel then you will probably find this discussion interesting: electronics.stackexchange.com/questions/20720/… \$\endgroup\$
    – avra
    Mar 14 '12 at 12:03
  • \$\begingroup\$ It sounds like you're still in the concept phase, and perhaps an electronic design question is premature. I don't think we can answer the question as you have written it. You'll need to tell us what kind of mechanical coupling you want to use, or at least narrow it down enough that we can suggest compatible electronic solutions. We don't have enough information yet. F/ex, you don't seem to want a non-contact solution like a coil and rotating magnets, but tire-dynamos create too much tire wear. You'll need to pick something. \$\endgroup\$
    – JRobert
    Mar 15 '12 at 14:20
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    \$\begingroup\$ ... Another suggestion to think about (only a little bit tongue-in-cheek): a tiny wind dynamo? It's non-contact and all one piece. You'll still need to store & regulate the output for the LED, though. \$\endgroup\$
    – JRobert
    Mar 15 '12 at 14:21
  • \$\begingroup\$ This is a reflector that you attach to spokes I thought about a reflector with a small moving part inside for generating some electricity. Some sort of ball going up and down with motion. However I was hoping I could find some solution that doesn't have a moving part. Hence the question. \$\endgroup\$
    – Ktc
    Mar 15 '12 at 16:41
  • \$\begingroup\$ Wind dynamo is interesting. However this reflector would be attached to the spokes so not sure I can get same direction wind. \$\endgroup\$
    – Ktc
    Mar 15 '12 at 16:42
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I don't understand the "one piece" requirement, but what you need is a motor.

Electric motors have the great power of being capable of converting mechanical (rotational) energy in electricity and vice versa, and with a quite high efficiency.

But if you need to take the energy from a "linear" movement, like from the surface of a tire, you need a gear which takes the rotation from the movement of the tire, like a dinamo. But consider that the energy that you are generating (plus some losses) is taken from the wheel, so you are making an opposing force in your system.

Consider that this kind of harvesting is not the LED's best friend, as it gives a variable voltage and current, and the LED wants them to be quite regular, especially for the limiting circuit specification. That's probably one of the reasons because of many bycicle lights with dinamo use incandescent bulbs.

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  • \$\begingroup\$ I don't tink that works at least I. At least I cannot contemplate how. \$\endgroup\$
    – Ktc
    Mar 14 '12 at 23:43
  • \$\begingroup\$ @Ktc at least you should say why, because you didn't specified the problem very well, and the bycicle dinamo has been used for about a century \$\endgroup\$
    – clabacchio
    Mar 15 '12 at 7:05
  • \$\begingroup\$ Dynamo is not what I want. Does it matter? I like make a flashing led reflector. Dynamo also tears the tires. \$\endgroup\$
    – Ktc
    Mar 15 '12 at 13:02
  • \$\begingroup\$ @Ktc you should at least specify where do you want to take the energy from, if not the tire: the wheel rim? The pedals? \$\endgroup\$
    – clabacchio
    Mar 15 '12 at 13:14
  • \$\begingroup\$ Ok. English is my second language. I am referring to reflectors you attach to the spokes of a tire. There reflectors are micro mirrors that reflect the light back to where they come from. I like to enhance them to generate energy from the rotational motion of the tire. I don't want two piece solution where installation is a problem. I like to build self contained device and I was looking for pure electronic method of gathering energy without mechanical pieces. Sounds like not a common practice. \$\endgroup\$
    – Ktc
    Mar 15 '12 at 16:47
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What you seem to be missing here is that fundamentally, if you want to harvest energy from motion, something needs to be moving with respect to something else. A machine that harvests motion ALWAYS requires at least two parts that move differently. It can be a collar around a shaft, or a weight moving back and forth on ball bearings, but something needs to move.

If this isn't enough, you'll need to tap into some other form of energy: radiation, heat, chemical, etc. Those can be made into 'one' piece (e.g. a solar cell that charges a battery).

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You gave this additional information in the comments above:

This is a reflector that you attach to spokes I thought about a reflector with a small moving part inside for generating some electricity. Some sort of ball going up and down with motion. However I was hoping I could find some solution that doesn't have a moving part. Hence the question.

I see multiple solutions here, but everything involving genrating energy through motion seems to be very difficult / doesn't make so much sense:

  1. You could use spring-seated power magnet and a coil, that can be mounted to the spokes (catching bumps, other vibrations and changes in wheel acceleration => changing centrifugal force). But, I assume that the major energy would be created by vibrations and bumps this way. Some energy could be generated by the change in speed, when the rider pushes the pedals forward. I would assume this mechanic is prune to wear and tear (e.g. through the hard vibrations by bumps on the street).
  2. A possible modification of the last solution could be somewhat bigger weight that is pushed outside by centrifugal force (higher speed) and back by a strong (!) spring (lower speed, no motion) as before. BUT, the weight can only slowly move and drives a wheel (e.g. by some "worm drive" or other mechanic). So the wheel spins for some time while the weight moves to the outside (the more the weight can move, the better). During this time a normal generator/motor produces energy that must be stored somehow. But, when continuously driving the weight will finally reach the limit and the generator will stop to produce energy until the bicycle stops and the weight gets pushed backwards by the spring. So the produced energy must be enough for e.g. the whole night / ride; or you need to support some special riding profile (e.g. within a city with traffic lights where the biker must stop).
  3. I would prefer a two part solution, having a strong magnet fixed at the fork and a coild within the reflector part. That would work quite well. But you said, you dont want such a solution. The magnet could be mounted to the fork easily with a elastic band without the need of tools. There exist some cyclometers on the market that are mounted to the bike in this way.
  4. For a blinking reflector that is only active at night, it might be a valid solution to use a solar cell. Which is of course not generating energy from the motion, but from light. However, this should work. In that case you would need some energy storage that can store enough energy for the night time and a simple motion sensor (e.g. detecting the centrifugal force) that switches the device on for some time. This solution seems rather expensive (especially: solar cell + energy storage)
  5. I would also investigate, if your blinking reflector LED (when only active at night when actually riding the bike) could be powered by a 3V coin cell for some years...
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