Dynamo hub (LED) headlights have a switch on them you turn off the light . and then plug your devise into the 6vAC to 5vDC converter that you also have to Buy.

its not that difficult they make it all simple for maximizing sales .

Pg 22, 23 http://www.bumm.de/fileadmin/user_up...5__English.pdf an example . theres others All wanting a share of the Market for Phone addicts
there's more companies, you can search for them on your Phone.

6v_ 3w is all you got , dont expect a charge rate like plugging into the mains.