Build a Gas Leakage Detector Alarm

LPG cylinders are used in most homes in Bangladesh, so a gas alarm is a genuinely useful project. The MQ-2 sensor detects LPG, smoke and methane, and the Arduino sounds an alarm when the level rises.

1. Wiring

• MQ-2 VCC → 5V, GND → GND, A0 → A0
• Buzzer + → D8, LED → D7, relay IN → D6

2. The Code

const int threshold = 400;  // adjust after testing

void setup() {
  Serial.begin(9600);
  pinMode(8, OUTPUT); pinMode(7, OUTPUT); pinMode(6, OUTPUT);
  digitalWrite(6, HIGH);
  delay(20000);  // MQ-2 needs time to heat up
}

void loop() {
  int gas = analogRead(A0);
  Serial.println(gas);
  bool danger = gas > threshold;
  digitalWrite(7, danger);
  digitalWrite(6, danger ? LOW : HIGH);  // fan on
  if (danger) { tone(8, 2000, 300); }
  delay(500);
}

3. Calibration

Leave the sensor powered for a few minutes in clean air and note the Serial Monitor value. Set threshold about 150 above that. Test carefully with a lighter's gas (unlit) held near the sensor.