How can I use 5V devices and sensors with a Fio?
From my perspective, it seems the only (reasonable) solution is to use a transistor as an amplifier/switch. I'm presuming you know how a transistor works - basically, a current to the base results in a gain between the collector and the emitter - if you don't understand about transistors, you can find out on Electrical Engineering. This way you can effectively convert your 3.3V signal into a 5V one, so your Fio can interface with the 5V logic.