Proto G @ instructables.com has build an Arduino based digital voltmeter that is able to measure up to 30V and it’s using an OLED LCD display.
In this instructable, I’ll show you how to make a simple arduino voltmeter. It can measure voltages in excess of 30VDC depending on how you configure it. The smaller the voltage range you want to measure, the more accurate that your meter will be. This project is going to be apart of a gauss meter that I am making to measure magnetic fields strength as well as magnetic field polarity
Arduino Voltmeter Prototype – [Link]