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I'm trying to proportion potentiometer value to analog pin value. Like 1024 to 255.

int readVolumePot() {
    int val = analogRead(potVolumePIN);

    Serial.print("VOL");
      Serial.println((val * 255/1024));

      return (val * 255/1024);
  }

As potentiometer turns I got irrelevant values.

VOL3
VOL3
VOL3
VOL17
VOL-16
VOL-16
VOL-21
VOL-1
VOL-10
VOL-3
VOL-4
VOL11
VOL8
VOL13
VOL17
VOL15

In Arduino's website they say

some constant calculations may overflow (e.g. 60 * 1000 will yield a negative result).

Notwithstanding, I cannot find an appropriate way to convert the potentiometer value to analog pin value.

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2
  • 1
    Try using map(). Commented Jul 4, 2016 at 14:44
  • 1
    a more elaborate answer here Commented Feb 28, 2019 at 7:54

1 Answer 1

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5

analogRead() returns a number from 0 to 1023, and you want to convert that to a range from 0 to 255? One easy way would be to simply divide the reading by 4.

On a more general note, given that the numbers will (should!) never be negative, there's a flavour of int called unsigned int. By using that:

unsigned int val = analogRead(...);

you can cover a range from 0 to 65535 instead of int's -32768 to 32767. That will give you a little more head room - but not enough to multiply the maximum value 1023 by 255 first.

There's also another type long. That covers the range from -2 to 2 billion (and a bit), while unsigned long goes from 0 to 4 billion (and a bit) - but that uses a LOT of processing for the poor Arduino to work with.

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1
  • 2
    A nice feature of unsigned ints: if val is unsigned, then val / 4 is optimized by the compiler into a bit shift, much more efficient than the actual division you would get if val was signed. Commented Jul 4, 2016 at 14:56

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