This Arduino voltmeter measures four independent voltages and displays the results on a liquid crystal display (LCD). Each channel can measure up to about 50V DC which is achieved by adding a voltage divider to each channel. Click the above link for more details, a circuit diagram and Arduino sketch code.
How can one measure or monitor voltages greater than 5V using an Arduino board? The easiest way is to use a voltage divider network — which consists of only two resistors as shown in the circuit diagram below.
A full article on the subject is available with more information, code (an Arduino sketch) and some precautions that need to be taken into consideration.