project for youth - need 5v source from cycle dynamo?

[bakerlite]

I'm trying to get some young lads interested in alternative energy/excersice.

With ipods being so popular I thought it would be neat to get them to make a charger they use with their bikes?

What i am thinking is that a usb charger already takes care alot of the problem - however that needs a 5 volts feed

i am also wondering will the fluctuating current as you cycle faster and slower have a negative effect on things?

If anyone has any idea how to simply pull it off i'd love to hear it! i am just brainstorming at the moment

the immediate drwarback is any cycle dynamo i find is a 3volt output.

thanks folks

[stingray_65]

the LM7805 regulator chip.

simple 3 leg IC and requires 2 small caps.

Gives a rock solid regulated 5V source that is safe for sensitive electronics.

problem is it needs about 7V DC in

you can use an old 6V permant magnet DC motor as a generator.

run the output of that into a bridge rectifier then into a large cap as a resivoir

The reason for the bridge is that boys like to see what happens if...

and spinning the generator backwards will cause the polarity to reverse. the bridge  will "flip" the polarity to where it needs to be.

[supro66]

Found this one

$16.95

http://www.amazon.com/Generator-12V-Bicycle-Light-Power/dp/B000OBWMGK

12 volts

[PRR]

Bike dynamos are traditionally _AC_ not DC.

The lamp does not care.

Actually, you can see it in the light (and add another lesson). As the RPM drop low, the lamp flickers. A true DC generator (with commutator or rectifier) would have little or no flicker. (But cost a buck more, for no good reason.)

The bike dynamo must be quasi-regulated for lamp-load. You want it working at walking speed, 3MPH, and at 20MPH. A straight dynamo would make 7 times more voltage at the higher speed. Either 3MPH would be uselessly dim or 20MPH lamp life would be a few seconds. The lamp resistance does not help: it rises at higher voltage. So there must be something else in there. The classic automobile regulator is far too complex. There are other systems, mostly forgotten in the mists of time. Would be an interesting advanced project to dissect a dynamo and find the voltage regulation. (These days, it just might BE a chip.)