MQ-2 (Gas/Smoke) Sensor

The MQ-2 is a gas sensor that detects combustible gases and smoke. It is sensitive to LPG, i-butane, propane, methane, alcohol, hydrogen, and smoke. The module provides both an analog output (proportional to gas concentration) and a digital output (triggered by an adjustable threshold), making it suitable for gas leak detectors, smoke alarms, and air quality monitors.

For this interfacing you need the following components:

• Arduino board (Uno, Nano, Mega, etc.)
• MQ-2 gas/smoke sensor module (with LM393 comparator)
• Breadboard and jumper wires
• USB cable to connect Arduino to your computer

Schematic

Connect the MQ-2 module to the Arduino as follows:

MQ-2 Module           Arduino
-----------           -------
VCC           -->     5V
GND           -->     GND
DO            -->     Digital Pin 2
AO            -->     Analog Pin A0

The module operates at 5V and includes a built-in heater. No external components are required.

Pin Map

Install necessary Library

No external library is required. The MQ-2 uses standard analogRead() and digitalRead() functions built into Arduino.

For precise PPM (parts-per-million) calculations using the MQ-2 datasheet curves, install the MQGas library or MQUnifiedSensor library via the Library Manager. This tutorial uses the simpler direct-read approach.

Code with complete explanation

This sketch reads both the analog and digital outputs of the MQ-2 sensor, estimates gas concentration levels, and prints the values to the Serial Monitor.

const int digitalPin = 2; // DO pin
const int analogPin  = A0; // AO pin

void setup()
{
  Serial.begin(9600);
  pinMode(digitalPin, INPUT);
  Serial.println("MQ-2 Gas/Smoke Sensor Test");
  Serial.println("Warming up sensor...");
  delay(60000); // Allow 60 seconds for sensor warm-up
  Serial.println("Sensor ready");
}

void loop()
{
  int digitalValue = digitalRead(digitalPin); // LOW = gas detected above threshold
  int analogValue  = analogRead(analogPin);   // 0-1023 (higher = more gas)

  // Convert to voltage
  float voltage = analogValue * (5.0 / 1023.0);

  // Estimate gas level description
  String level;
  if (analogValue < 50)
  {
    level = "Clean air";
  }
  else if (analogValue < 200)
  {
    level = "Low gas";
  }
  else if (analogValue < 400)
  {
    level = "Moderate gas";
  }
  else if (analogValue < 600)
  {
    level = "High gas";
  }
  else
  {
    level = "Very high gas - DANGER";
  }

  Serial.print("Digital: ");
  Serial.print(digitalValue);
  Serial.print(" (");

  if (digitalValue == LOW)
  {
    Serial.print("ALARM");
  }
  else
  {
    Serial.print("Normal");
  }

  Serial.print(")  Analog: ");
  Serial.print(analogValue);
  Serial.print("  Voltage: ");
  Serial.print(voltage, 2);
  Serial.print(" V  ");
  Serial.println(level);

  delay(500);
}

Code breakdown

• digitalRead(digitalPin) — reads the DO pin. The LM393 comparator drives this pin LOW when the gas concentration exceeds the threshold set by the module’s potentiometer.
• analogRead(analogPin) — reads the AO pin (0-1023). Higher values indicate higher gas concentration. The voltage output increases as more target gas is detected.
• The 60-second delay() in setup() allows the sensor’s heating element to stabilize. For first-time use, a burn-in period of 24-48 hours is recommended for accurate readings.
• The threshold levels in this example are approximate. For accurate PPM readings, use calibration curves from the MQ-2 datasheet.

Adjusting sensitivity

Turn the potentiometer on the module with a screwdriver to adjust the digital output threshold:

• Clockwise: increases sensitivity (triggers at lower gas concentration)
• Counter-clockwise: decreases sensitivity (triggers at higher gas concentration)

Steps to perform this interfacing

1. Connect the MQ-2 module to the Arduino as shown in the schematic.
2. Copy the code into the Arduino IDE.
3. Select the correct board and port (Tools > Board and Tools > Port).
4. Upload the sketch to the Arduino.
5. Open the Serial Monitor (Tools > Serial Monitor, set baud rate to 9600).
6. Wait 60 seconds for the sensor to warm up.
7. Observe the readings — expose the sensor to a gas source (e.g., lighter gas without flame) and watch the values rise.
8. Adjust the sensitivity potentiometer as needed.

Caution

• The MQ-2 sensor requires a preheat (warm-up) period of at least 60 seconds before readings stabilize. For maximum accuracy, a burn-in of 24-48 hours with continuous power is recommended when the sensor is first used.
• The sensor’s heating element draws significant current (~300 mA at 5V). Do not power it from an Arduino 3.3V pin. Use an external power supply if running other high-current components.
• The MQ-2 is not selective — it responds to multiple gases simultaneously. It cannot distinguish between LPG, methane, alcohol, or smoke.
• Readings drift over time and with temperature and humidity. The analog output is a relative indication, not an absolute PPM measurement unless calibrated.
• Avoid exposing the sensor to very high gas concentrations for extended periods — this can damage the sensing element.
• The sensor’s sensitivity decreases over its lifespan (typically 2-5 years). Periodic recalibration may be necessary.
• Do not apply water or liquids to the sensor surface — the sensing element can be permanently damaged.