Sometimes we travel together with electronic companions. To keep them alive during the trip, we build the ‘Minimal-Charger’. It’s well described and explained on http://fahrradzukunft.de/12/minimal-lader/. Unfortunately only in German. You can ask us for help.
The circuit is very simple and easy to understand. Basically, it consists of a bridge rectifier and a smoothing capacitor. The voltage is limited by a Zener diode to 5.1 V. Therefore the circuit is fairly ineffective (but often sufficient), but very easy to build. The entire system will cost only a few Euros.
We equipped the Charger Tamiya coupling for the Dynamo and a micro USB connection for the devices. To dissipate the heat of the Zener diode, we used piece of a can.