Precision voltmeter using ADC

How do you plan to solve it?

Use conditions to check on what reference what values are obtaining. If 1023 is obtained durig vref = 1.1, change the reference to DEFAULT (5 V). And if digital values are less than 226 (226 is equivalent digital value of 1.1 V), then set reference as INTERNAL (1.1V).

float vref;
int pot = A0;

void setup(){
  pinMode(pot, INPUT);
  Serial.begin(115200);
  analogReference(INTERNAL); //internal ==> 1.1 V;
  vref = 1.1;
}

void loop(){
  float value = analogRead(pot);
  float voltage = value*vref/1024;
  Serial.println("Digital Value = "+String(value)+" | Voltage = "+String(voltage)+" | V_reference = "+String(vref));
  if(value == 1023 && vref == 1.1){
    analogReference(DEFAULT);
    vref = 5.0;
  }
  if(value < 226 && vref == 5.0){
    analogReference(INTERNAL);
    vref = 1.1;
  }

  delay(500);
}

Digital Value = 409.00 | Voltage = 0.44 | V_reference = 1.10

Digital Value = 618.00 | Voltage = 0.66 | V_reference = 1.10

Digital Value = 777.00 | Voltage = 0.83 | V_reference = 1.10

Digital Value = 1023.00 | Voltage = 1.10 | V_reference = 1.10

Digital Value = 292.00 | Voltage = 1.43 | V_reference = 5.00

Digital Value = 340.00 | Voltage = 1.66 | V_reference = 5.00

Digital Value = 396.00 | Voltage = 1.93 | V_reference = 5.00

Digital Value = 431.00 | Voltage = 2.10 | V_reference = 5.00

Digital Value = 479.00 | Voltage = 2.34 | V_reference = 5.00

Digital Value = 518.00 | Voltage = 2.53 | V_reference = 5.00

Digital Value = 650.00 | Voltage = 3.17 | V_reference = 5.00

Digital Value = 757.00 | Voltage = 3.70 | V_reference = 5.00

Digital Value = 851.00 | Voltage = 4.16 | V_reference = 5.00

Digital Value = 853.00 | Voltage = 4.17 | V_reference = 5.00

Digital Value = 893.00 | Voltage = 4.36 | V_reference = 5.00

Digital Value = 1013.00 | Voltage = 4.95 | V_reference = 5.00

Digital Value = 1019.00 | Voltage = 4.98 | V_reference = 5.00

Digital Value = 1023.00 | Voltage = 5.00 | V_reference = 5.00

Digital Value = 1023.00 | Voltage = 5.00 | V_reference = 5.00

Digital Value = 1023.00 | Voltage = 5.00 | V_reference = 5.00

Digital Value = 1023.00 | Voltage = 5.00 | V_reference = 5.00

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