Hi.
I'm new to Arduino.
I would like to build a voltmeter that will light up LEDs.
0,01mV to 0,05mV would light a red LED.
0,05mV to 0,15mV would light a yellow LED.
More than 0,15mv would light a green LED.
Is that possible?
What is the necessary ARDUINO equipment to do so?
Is that a advanced project?
Thank you!
The Arduino part is trivial.
The analogue electronics not quite so.
You will need a bit of clever amplification , those are very lowvoltages -
The resolution of the A/D is 5/1023. You could use the internal 1.1 volt reference , but you still have 1mV resolution , not enough for Your project.
If you can get the voltage range right it is a simple project
Why the low voltages ??
You need an op amp.
A very good op amp !
Luckily, it is 2017 not 1977, and good op amps are cheap and available. 
How are you going to calibrate/validate your system when you are using such low voltages ?
Have you access to something like this: http://uk.farnell.com/keithley/2182a-e/nano-voltmeter-bench-100v-7-5digit/dp/2772524