I have to do a project building DC voltmeter, ammeter and AC voltmeter. It has restricted spec which is really hard to understand for me as a beginner

I answered this SE EE question just now.

My answer

You have to solve two tasks:

• to coarsen the sensitivity (to enlarge the range) of the available 1 mA ammeter to 5 mA
• to convert the available 1 mA ammeter into a 20 V voltmeter

You can solve the first task using the "current divider" device by connecting a resistor ("shunt") in parallel with the ammeter.

You can solve the second task using the "voltage divider" device by including a ("ballast") resistor in series with the ammeter.

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BTW this technique of converting an ammeter to a voltmeter was used in the 19th and maybe until the middle of the 20th century because back then they didn't have voltmeters, only ammeters.

Now we have very good voltmeters (ADCs) and so we use the reverse technique to convert a voltmeter into an ammeter - by connecting a resistor ("current-to-voltage converter") in parallel to the voltmeter.

See also my two stories in Circuit Idea wikibook:

Passive Voltage-to-Current Converter

Passive Current-to-Voltage Converter