Electronics, Lighting, & Gadgets - USB charger powered by dynamo hub

I have been searching for a solution to run small USB chargable devices powered by the dynamo hub of my Surly big dummy bicycle without additional batteries. I was not willing to design a circuit myself and I do not like to blow up the connected device due to unsufficient voltage protection. There are several converters available for charging USB devices as like music players, phones, PDAs or navigation devices on the bicycle. I got me the bike charger from a German distributor and installed it today. In my blog (http://bla.obda.net/wordpress/?p=155) there are some pictures of initial cabling. I still need to attach the PNA to the handlebar (still waiting for some parts) to be able to complete the installation. Basically it appeared to work, when connecting the charger and spinning the wheel. The device should be virtually sufficient to power any USB charged device according to USB spec (5V DC constantly, 500mA at max, even at low speed). The box has got 50g approximately and is quite small, but must not be exposed to rain or humidity. According to manufacturer it will not heat up due to the digital design of the circuit. I am a novice with electronics, so I cannot tell any details, but I will be using it for some bicycle touring within the next time.

This is the circuit I designed and use.
http://i143.photobucket.com/albums/r154/n4zou/DynamoUSB-1.jpg
As dynamo voltage output approaches 5 volts the impedance of the Ni-MH batteries becomes so high the dynamo is limited to a maximum of 5.2 volts. This is well within USB voltage requirements and sufficient to recharge the batteries. The batteries also absorb excess current as recharging current protecting the USB device. Batteries must be solder tab type with no possibility of poor or open connections. Open or poor connections would allow unregulated voltage and current which could damage the device plugged into the USB port. Any standard dynamo headlight can be used in place of the duel LED headlight shown in the circuit. In places such as tunnels where you would need your headlight during the day you may switch from the USB circuit to the headlight circuit allowing the batteries to continue to provide limited power to the USB device. It's not possible to completely recharge most USB devices without help from the dynamo. The batteries may be rapid recharged by disconnecting any USB devices from the USB connector. Most AA Ni-MH batteries which are completely discharged may be fully recharged by the dynamo in about 2.5 hours. Your cycle computer trip time can be used to time battery charging. Simply reset it and when it shows 2.5 hours your batteries should be fully recharged. It will automatically start and stop timing as you start and stop moving allowing you take breaks during this time.

There was a sequence of articles in a big German computer magazine where they designed and implemented a circuit as like yours. A nice project, I think. For me, I did not want the extra weight of an additional batterie pack, since the devices I would like to use already have internal batteries that just need to be recharged to stay operational. I may even switch off the connected device every now and then if required for recharging. I wanted a minimal solution to run some navigation device the whole day or week or whatever without using transformer power supply. For sure, the bike charger will not allow operating devices without internal batteries, while your circuit could be used as a power supply in itself, I guess.

There's another design on this forum: http://yacf.co.uk/forum/index.php?topic=6672.0