The Sinewave Revolution is an ingenious device used to power USB devices from a bicycle dynamo. It starts charging at 5.5km/h, reaching full charging rate at 14.4km/h. It will deliver at most 1 Amp of current at 6V. It weighs a tiny 37g, and the dimensions are 54mm x 35mm x 14mm.
It's totally waterproof - the circuit board is encased in resin, and the USB connector is gold plated. That means you can ride without worrying about the weather.
You can easily wire it in parallel with your light, or wire it separately and plug in whichever you need at the time. The device itself is a small box that you can attach somewhere near your handlebars with the included zip-tie.
Designed and made in the USA.