Electronics 2000

Hi there, I'm pretty new to electronics again, having not played for some 20 years (And even then badly) so I'm rather rusty on a couple of points. I know what it is I want to make, but I just need some help around the various bits.

I want to make an LED light that is powered by a magnet attached to the wheel of my bike. So I need an electro magnet (wire wrapped round a ferrite core) attached to a 4 diode AC-DC bridge, a big capacitor, a big resistor and my LEDs. As the neodymium magnet passes the ferrite core it magnetises and demagnetises it, inducing an AC current, that goes through the bridge charges the capacitor, that's slowly released through the resistor and LED, so far, theory sound

I vaguely remember the RC constant, meaning big capacitor and big resistor means LED lit for a longer time, assuming I can put enough juice into the cap. I also remember more turns on the coil means more induced EMF per flux density.

Can anyone help me put numbers to things?

I think I want to put the magnets as far out on the wheel circumference as I can, since induction is proportional to rate of change. I think I probably want a few magnets arrange N up, S up, N up, S up, so as they pass the core, they will switch polarity faster causing a bigger change in magnetic flux.

My tyres will probably handle a ferrite rod core of about 40mm in length, and probably 10mm in diameter to match the neodymium magnets I have. So, first set of questions here. What happens if I use a 5mm core is there any benefit?? Should I wrap the core in thicker wire once or can I use thinner wire and wrap over and over (like in a transformer)? What thickness wire would be best? Will more layered turns add any interference that I would care about? I think I remember back in the day making an electro magnet and it didn't care with more turns giving a better magnet, but then it was powered from power source, not a magnet.

Diode bridge, nothing special here I don't think.

LED's are pretty forgiving about voltage so I don't think I need any converters, but then how would I calculate the Vin off the inductor, I guess I need to hook it up to a meter and find that out as it'll be pretty dependant on magnets and turns? Capacitors seem to be a lot more complicated than I remember, having a voltage rating - do I need to pay attention to this. Can I just put in a 1.5F cap and a 100Ko resistor to give me 150 seconds of juice. As a secondary project I was thinking of being able to give this a "start" by charging the cap from a 9v battery for a couple seconds, or could I do it off of an AA battery at 1.5v?

I think that's all that strikes me, is there anything else I should bear in mind? I have already googled a few things but happy to be guided further.

Thanks in advance.
Nigel.

Hi, this is like a wind generator...the voltage will vary wildly...if you don't drop the voltage it will go sky high.
If the cap does no blow it will kill you if you touch it.
There are so many possibilities, distance between magnets, number of turns, how fast you peddle etc it is not possible to put figures to it.
May be try making a wind generator and do the sums from that or spin the magnets with a motor of known frequency.