Building a DC voltmeter with Arduino is really a matter of seconds. But for those who is learning microcontrollers and want to build something working this is a great project. Here you have to deal with several areas of electronics. First of all reading analog signal. As voltage cones in directly to analog pin, the range is limited to 5V.
Next thing is driving three 7-segment Led display. It is common anode type where anodes of each digit is driven with transistor key. Program code is also easy to follow and easy to modify if needed. Even if it looks simple project, it can be useful in many areas like checking battery voltage, or probing other things where voltage ranges from 0 to 5V.