MQ-2 Gas Sensor with an Arduino Uno

// MQ-2 Gas Sensor on Arduino Uno: wiring and a free simulation at https://www.tinkered.ai/components/mq2-gas-sensor/arduino-uno
// MQ-2 smoke/gas sensor: analog level + active-LOW digital alarm.

const int MQ2_AOUT = A0;   // Arduino: A0-A5 | ESP32: an ADC1 GPIO (e.g. 34)
const int MQ2_DOUT = 2;    // any digital pin

#if defined(ESP32)
const int CLEAN_AIR_RAW = 372;   // 0% gas (clean air) on a 12-bit ESP32 ADC
const int SMOKE_RAW     = 4095;  // 100% gas (heavy smoke): full 12-bit scale
#else
const int CLEAN_AIR_RAW = 61;    // 0% gas (clean air) on a 10-bit Uno ADC
const int SMOKE_RAW     = 921;   // 100% gas (heavy smoke)
#endif

void setup() {
  Serial.begin(9600);
  pinMode(MQ2_DOUT, INPUT);
}

void loop() {
  int raw = analogRead(MQ2_AOUT);
  int gasPercent = round((float)(raw - CLEAN_AIR_RAW) * 100.0 / (SMOKE_RAW - CLEAN_AIR_RAW));
  gasPercent = constrain(gasPercent, 0, 100);

  bool alarm = digitalRead(MQ2_DOUT) == LOW; // active-LOW: LOW = gas detected

  Serial.print("Raw ADC: ");
  Serial.print(raw);
  Serial.print(" | Gas level: ");
  Serial.print(gasPercent);
  Serial.print(" % | Alarm: ");
  Serial.println(alarm ? "GAS DETECTED" : "clear");
  delay(500);
}