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@lae
I looked into converting some vintage dynamo lights to LED operation, but like Brucy suggested, it's quite a complicated process (well, for an electronics novice like me) - the main problem being that bottle dynamos are actually not dynamos at all, but alternators. So you'll need a bridge rectifier to convert it to DC and then a capacitor to smooth out the current and then you'll have to do some fiddling to get the right sort of charging voltage. I can't remember how much power you can get out of a bottle dynamo; it might not be enough to run a phone that's in constant use.
Bottle dynamos add a noticeable amount of resistance, by the way. Even a cheap hub dynamo is a better proposition.