🛠 Lire une tension avec l'ADC A0 et un diviseur de tension

Objectives

Understand the main hardware pitfall of the ESP8266EX: its single analog-to-digital converter (pin A0, 10 bits) only measures 0 to 1.0 V, not 0 to 3.3 V like an Arduino. To measure higher voltages, an external voltage divider is required. Be aware of board differences: NodeMCU boards already integrate a divider (ratio approximately 5/1.1) on A0, whereas a bare ESP-12E module has none — the code and wiring are not the same. The actual voltage is calculated from the raw analogRead value and the divider ratio.

Hardware

• 1 bare ESP8266-ESP-12E module (or check if your board is a NodeMCU with an integrated divider)
• 1 resistor R1 = 20 kΩ and 1 resistor R2 = 10 kΩ (×3 divider)
• 1 voltage source to measure between 0 and 3.0 V (e.g., a 10 kΩ potentiometer wired as a divider on the 3.3 V rail)
• Stable 3.3 V power supply, breadboard, wires

Complete code

// Diviseur externe R1=20k (entre Vin et A0) et R2=10k (entre A0 et GND) -> rapport 3
const float RAPPORT = 3.0f;          // (R1 + R2) / R2
const float FULL_SCALE = 1.0f;       // A0 nu : pleine échelle = 1,0 V (pas 3,3 V !)

void setup {
  Serial.begin(115200);
  Serial.println("Lecture ADC A0 (0-1,0 V sur module nu)");
}

void loop {
  int raw = analogRead(A0);                    // 10 bits : 0..1023
  float vA0 = raw / 1023.0f * FULL_SCALE;      // tension sur la broche A0
  float vIn = vA0 * RAPPORT;                   // tension réelle mesurée
  Serial.printf("raw=%4d  A0=%.3f V  Vin=%.3f V\n", raw, vA0, vIn);
  delay(500);
  yield;
}

Expected verification

Turn the potentiometer: the raw value should sweep approximately 0 to 1023, and Vin should follow the voltage measured with a multimeter (±0.1 V, the ESP8266 ADC is mediocre). With a ×3 divider, Vin ≈ 3× the voltage actually present on A0. On a bare module, inject 0 V directly onto A0: raw should drop to 0 or 1.

Common errors

• Believing that A0 accepts 3.3 V on a bare module: above approximately 1.0 V, the measurement is clipped and the pin may be stressed out of range; the divider is mandatory.
• Adding an external divider on a NodeMCU that already has an internal one: the total ratio is incorrect (×3 × 4.5 ≈ ×13) and all measurements are wrong.
• Forgetting to connect the ground of the measured source to the module's ground: without a common reference, the reading is inconsistent.
• Using A0 while Wi-Fi is transmitting: the ADC shares RF circuits, measurements become noisy; perform the reading just before deep-sleep or filter by averaging.
• Expecting clockwork precision: the ADC is highly non-linear at the beginning and end of the range, accept ±50 mV of error.