I have looked everywhere and I can't find the voltage of the GPIO pins. Does anyone know what they are?
The UDOO I/O pins are 3.3V compliant. Higher voltages (like 5V) would damage the board. http://elinux.org/UDOO
Don't forget that voltage isn't your only concern. Make sure you control the current in and out of your GPIO pins. I'm not sure if the UDOO is the same as the Arduino Due (which has the same Sam3x chip) but the Due cannot handle much current in and out of the GPIO pins at all. But then again, these are control boards and they should really only be sending and receiving signals and not driving sensors. Make sure you use some sort of signal conditioning/buffering circuitry for your I/O; for instance, Darlington transistors are great for driving outputs.
