analogRead は 0～1023 を返す

Code cannot use raw volts directly. The analogRead function measures a pin and hands you back a plain number you can work with.

Every analogRead result lands between 0 and 1023. That is exactly 1024 possible steps, from the lowest voltage to the highest.

The Uno uses a 10-bit converter, and 2 to the power of 10 is 1024. More bits would simply give you finer resolution.

A reading of 0 means the pin sees 0V, the very bottom of the range. Nothing lower can be measured here.

A reading of 1023 means the pin is at the reference voltage, normally 5V on an Uno. That is the top of the scale.

Around 2.5V the value sits near 512, the middle of the range. The mapping from voltage to number is wonderfully linear.

Pass an analog pin name to read it. analogRead(A0) samples pin A0 and returns its current value as an int.

The result fits easily in an int, since 1023 is small. Save it to a variable so you can use it later in your code.

Analog inputs are ready out of the box. You do not call pinMode for them, you simply call analogRead when you want a value.

To see actual voltage, scale the reading. Multiply by 5.0 and divide by 1023 to turn the number into volts.

Tiny electrical noise makes values jiggle by a point or two. That is normal, and averaging a few reads smooths it out.

What does analogRead hand back to you?

You learned analogRead turns a 0V to 5V signal into a number from 0 to 1023. Store it in an int, and scale it back to volts when needed.