I don't understand the connection between the Inverter output and the 5V line.
( I guess it's a drawing problem only, and should go to Vcc pin of your 74HC14 )
If I understand analogReference() - Arduino Reference properly, there's an internal resistor, if you use AREF as a voltage source, which would affect your potentiometer reading.
Rather connect Vcc (ATMEGA Pin7) to the high end of your pot, if you need the full 0..5V range. (?)
I'm afraid that USB power is not enough to provide in my application
USB Power (max 500mA) is by far sufficient, IMHO.