Magic Pie 5, Magic Pie Edge and Smart Pie 5 now available with Bluetooth connectivity for setting and monitoring.(Bluetooth connection requires Android 4.4 onwards)
You can use a regular potentiometer if you place fixed resistors in series with the 5v and ground wires. I would use a 5k pot, but the value is not critical. The fixed resistors for a 5k pot would be about 1.2k each. You want the signal to go from about 1v to 4v with a 5v source.