If you’ve used the analogRead() function then you’ve already measured a voltage using the Arduino. But do you ... but will only be as accurate as the regulator supplying it.

Rather than featuring the typical voltage regulator used by Arduino boards (used to allow the board to be powered by a voltage source greater than 5V), a switching regulator allows for less energy ...

All Arduino boards have this pin, and it's connected to the built-in voltage regulator that makes sure your board receives a stable supply. For the Nano, the VIN pin is found on Pin 30 (although ...