Voltage measure range of an arduino?
Without any further configuration, the ADC (Analog Digital Converter) of the Atmega328P (which is build onto the Arduino Uno) uses Vcc as voltage reference (meaning 5V in the case of the Arduino Uno). A voltage of 5V will result in the maximal value (which is 1023, since this is a 10bit ADC). So the voltage range in this case is 0 to 5V.

The Arduino Uno has an extra pin, where you can supply an own reference voltage. But it's limit is tied to the ADCs supply voltage AVcc, which is 5V for the Arduino Uno rev3 (as shown in the schematics of the Uno). The reference voltage should then be in the range of 1V to AVcc. So this still gives you only a range of 0V to 5V or smaller.

When you use a standalone Atmega328P, you can make AVcc a bit higher than the chips supply voltage Vcc, but at maximum Vcc + 0.3V.

You can check all these numbers by looking into the datasheet of the Atmega328P, chapter 28.9 (Electrical Characteristics --> ADC Characteristics).



How to increase this range depends greatly on your requirements. If you simply need a constant higher range, you can simply put a voltage divider in front of the analog input. Or you can use a more complex circuit involving an op-amp (amplifier ciruit, here with amplification factor below 1). You can even build this to be variable.

Should it be, that your wanted voltage range exceeds the 5V greatly, for example, if you want to measure mains voltage (230V for most europe, 110V in USA), you should definitely take great care and never connect this directly to electronic like the Arduino. High voltages (especially at high possible currents) should always be handled with great care.