Measure a voltage.

The challenge

Vary brightness with a potentiometer. Distinguish the input voltage, the number returned by the converter and PWM control.

Equipment: UNO R4, 10 kΩ linear potentiometer, red LED and 1 kΩ resistor. Identify the wiper in the component data sheet: the drawing shows its functions, not a universal pin order.

Wire the circuit

The potentiometer forms a voltage divider; A0 samples the wiper voltage. Check that no position creates a 5V/GND short circuit.

The complete program

// Potentiometre lineaire 10 kohms : extremites 5V/GND, curseur A0.
// LED : D9 -> resistance 1 kohm -> anode ; cathode -> GND.
constexpr uint8_t POT = A0;
constexpr uint8_t LED = 9; // Broche PWM sur UNO R4.
unsigned long dernierAffichage = 0;

void setup() {
  Serial.begin(115200);
  pinMode(LED, OUTPUT);
  analogReadResolution(10); // Valeurs de 0 a 1023, choix explicite.
  analogWriteResolution(8); // Rapport cyclique code de 0 a 255.
}

void loop() {
  const int brut = analogRead(POT);
  const uint8_t commande = (uint32_t(brut) * 255U) / 1023U;
  analogWrite(LED, commande);
  if (millis() - dernierAffichage >= 200) {
    dernierAffichage = millis();
    Serial.print("ADC="); Serial.print(brut);
    Serial.print(" ; PWM="); Serial.print(commande);
    Serial.print(" ; U estimee=");
    Serial.print(brut * (5.0f / 1023.0f), 3);
    Serial.println(" V (reference supposee 5.0 V)");
  }
}

From volts to a number

The code sets 10 bits: 2¹⁰ = 1024 codes, from 0 to 1023. The estimate U ≈ raw × 5 / 1023 assumes a 5.0 V reference. The actual supply and converter characteristics introduce differences.

The UNO R4 also supports 14 bits: more codes alone do not guarantee greater accuracy. Assuming 5.0 V, the theoretical step is 5 / 1024 ≈ 4.88 mV at 10 bits.

uint32_t(brut) widens the calculation before multiplication. The result is mapped to 0…255 for 8-bit PWM. At the midpoint, duty cycle is close to 50%: D9 alternates between 0 and 5 V; it is not a constant 2.5 V voltage. A0's DAC function is not used here.