Well there are a lot of MCUs (microcontroller units) now that has an ADC (analog to digital converter) function. It needs a analog voltage input within its range say 0-5V and say with a 10bit resolution. So from 0-5V the reading that it'll give ranges from 0 to 2^10 -1 or (0-1023). Now if you want to measure higher voltage you gotta use op-amps to design an amplifier with a fractional gain in which the maximum voltage you wanna measure will fall down to the maximum value that the MCU can read which in this case 5V. I'll show you an example.

Example: You want a voltmeter that can measure voltages that ranges from 0-30V.

As I was saying you need a gain amplfier with a fractional gain since the MCU can only read up to 5V. The gain should be,

30A=5 (A is for gain)

A=1/6

Now given that you already did this now and you're actually measuring a voltage and you have a reading,

ADC reading=544

Actual voltage= 30 x (ADC reading/1023)

so we have

Actual voltage= 30 x (544/1023)

Actual voltage= 15.95V

MCUs also have a display function if you wanna display you're reading in an LCD or in a 7-segment display or even in a computer. ADC functions is not only limited in measuring voltages, in can also measure variety of signals (temperature, pressure, speed, etc.), all you gotta do is to translate these signals into voltages wherein the MCU can read. It's quite of fun actually, creating circuitries and calibrating. ;).

Hope this helps.

That is a very complicate device compare to the analogy voltmeter that is made of one resistor and uA-meter.



Digital voltmeter now simplify into one big IC,power supply and display unit.

Need an ADC(analog to digital converter), a front end to convert the test signal to the proper level for the ADC, some converter/driver and display to display the ADC output as a human-readable number.

You can get Kits in spare from Radio Shook for various digital meter better purchase & assemble you will get better Idea.It content Digital Meter Ic & discreet COMPONENTS with PCB &LCD/LED Display Board.You want Back light also possible to get.Best of luck.