A handy DC to USB assembled circuit that we use to make our Solar USB Chargers. You can use it too. Takes DC power from as low as 2V and bumps it up to the 5V USB needs at around 600mA (0.6A) of current.
Due to popular request, we've now added soldered 6 inch wires to the USB circuit. This should entirely remove the need for soldering in many projects.
This circuit works fine with most simple USB devices, however many modern phones or tablets require a much higher sustained amperage than what this circuit can output. For best results use a lithium or alkaline based battery source.