Gas leak detector with an MQ-2 on an Arduino Uno

// Gas leak detector: an MQ-2 gas sensor, a traffic light module, an active
// buzzer and a 16x2 I2C LCD on an Arduino Uno.
//
//   A0      the MQ-2 board's AO (its VCC on 5 V, GND on GND)
//   2, 3, 4 the traffic light module's G, Y and R (each LED has its resistor)
//   8       the active buzzer's plus leg, its other leg on GND
//   LCD     SDA -> A4    SCL -> A5    VCC -> 5 V    GND -> GND
//
// The MQ-2 is a heated tin dioxide element over a 1 k load resistor. Gas lets
// it conduct, so AO rises: a higher reading means more gas. A cold element
// reads high too, as if the room were full of gas, so for the first 20
// seconds the yellow light is on and nothing is believed.
//
// Clean air reads about 11 here once it has warmed up, and 1000 ppm of LPG
// settles near 120. The alarm sounds above LIMIT and stops only below CLEAR,
// a little lower, so a reading that hovers on the line does not chatter the
// buzzer on and off. Your own sensor will read differently: run it in clean
// air, note the number, and set LIMIT well above it.
//
// Press Run, wait for the green light, then drag the slider on the MQ-2.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int GAS_PIN = A0;
const int GREEN_PIN = 2;
const int YELLOW_PIN = 3;
const int RED_PIN = 4;
const int BUZZER_PIN = 8;

const unsigned long WARMUP_MS = 20000;  // the heater's settling time
const int LIMIT = 100;                  // alarm at or above this reading
const int CLEAR = 80;                   // and quiet again below this one

LiquidCrystal_I2C lcd(0x27, 16, 2);
bool alarm = false;

void lights(bool green, bool yellow, bool red) {
  digitalWrite(GREEN_PIN, green ? HIGH : LOW);
  digitalWrite(YELLOW_PIN, yellow ? HIGH : LOW);
  digitalWrite(RED_PIN, red ? HIGH : LOW);
}

void setup() {
  pinMode(GREEN_PIN, OUTPUT);
  pinMode(YELLOW_PIN, OUTPUT);
  pinMode(RED_PIN, OUTPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Gas leak detector");
  lcd.init();
  lcd.backlight();
}

void loop() {
  int gas = analogRead(GAS_PIN);
  unsigned long now = millis();

  lcd.setCursor(0, 0);
  if (now < WARMUP_MS) {
    lights(false, true, false);
    digitalWrite(BUZZER_PIN, LOW);
    lcd.print("Warming up  ");
    lcd.print((WARMUP_MS - now) / 1000 + 1);
    lcd.print("s ");
    Serial.print("warming up  ");
  } else {
    if (gas >= LIMIT) alarm = true;
    if (gas < CLEAR) alarm = false;
    if (alarm) {
      lights(false, false, true);
      // Beep: on for one reading, off for the next.
      digitalWrite(BUZZER_PIN, digitalRead(BUZZER_PIN) == HIGH ? LOW : HIGH);
      lcd.print("GAS LEAK!       ");
      Serial.print("GAS LEAK!  ");
    } else {
      lights(true, false, false);
      digitalWrite(BUZZER_PIN, LOW);
      lcd.print("Air is clean    ");
      Serial.print("clean  ");
    }
  }

  lcd.setCursor(0, 1);
  lcd.print("Gas level: ");
  lcd.print(gas);
  lcd.print("   ");
  Serial.print("gas level ");
  Serial.println(gas);
  delay(250);
}