Analog Input: Using the ADC

How the ESP32's analog-to-digital converter works, its quirks, and how to get steadier readings.

Sensors like light-dependent resistors, potentiometers, and many temperature sensors output a voltage that varies smoothly, not just HIGH or LOW. The analog-to-digital converter (ADC) turns that voltage into a number your code can use.

Reading a value

#define SENSOR_PIN 34

void setup() {
  Serial.begin(115200);
}

void loop() {
  int raw = analogRead(SENSOR_PIN);       // 0-4095
  float volts = raw * (3.3 / 4095.0);
  Serial.println(volts);
  delay(200);
}

The ESP32's ADC reports a 12-bit value by default — a number from 0 to 4095 — representing a voltage between 0 V and roughly the board's reference voltage.

Quirks to know about

• Only certain pins (mostly in the GPIO 32–39 range) connect to the ADC — check your board's pinout before wiring a sensor to an arbitrary pin.
• Readings near the very top and bottom of the range are less linear than the middle; leave some headroom in your circuit rather than driving the pin to the extremes.
• Wi-Fi activity can add noise to ADC readings on some boards. Averaging several samples smooths this out.