MQ-2 Gas Sensor with a Raspberry Pi Pico

// MQ-2 Gas Sensor on Raspberry Pi Pico: wiring and a free simulation at https://www.tinkered.ai/components/mq2-gas-sensor/raspberry-pi-pico
// MQ-2 smoke/gas sensor on a Raspberry Pi Pico. AOUT reaches GP28 through a 10 kΩ / 20 kΩ divider
// (the pin sees 2/3 of AOUT). DOUT stays unwired: the module pulls it up to 5 V, so the
// alarm here is a threshold on AOUT, as the module's trimmer sets one for DOUT.

const int MQ2_AOUT = 28;
const float ADC_VREF = 3.3;      // the Pico's ADC reference: its 3.3 V rail
const float ADC_STEPS = 4096.0;  // 12 bits
const float DIVIDER = 1.5;        // (10 kΩ + 20 kΩ) / 20 kΩ: AOUT = pin voltage x 1.5
const float CLEAN_AIR_V = 0.30;   // AOUT in clean air, warmed up (0 %)
const float SMOKE_V = 4.50;       // AOUT in heavy smoke (100 %)
const int ALARM_PERCENT = 30;     // set it from your own readings in clean air and near a gas source

void setup() {
  Serial.begin(9600);
  analogReadResolution(12);                      // arduino-pico defaults to 10 bits
}

void loop() {
  float aout = analogRead(MQ2_AOUT) * ADC_VREF / ADC_STEPS * DIVIDER;
  int gasPercent = round((aout - CLEAN_AIR_V) * 100.0 / (SMOKE_V - CLEAN_AIR_V));
  gasPercent = constrain(gasPercent, 0, 100);
  bool alarm = gasPercent >= ALARM_PERCENT;

  Serial.print("AOUT: ");
  Serial.print(aout, 2);
  Serial.print(" V | Gas level: ");
  Serial.print(gasPercent);
  Serial.print(" % | Alarm: ");
  Serial.println(alarm ? "GAS DETECTED" : "clear");
  delay(500);
}