How to Use MQ 138: Examples, Pinouts, and Specs

The MQ-138 is a versatile gas sensor designed to detect a variety of gases, including benzene, alcohol, and smoke. Manufactured by Arduino, this sensor is widely used in air quality monitoring systems and industrial applications. Its high sensitivity and fast response time make it an ideal choice for detecting harmful gases in the environment.

1. Power Supply: Connect the VCC pin to a 5V power supply and the GND pin to the ground.
2. Analog Output: Connect the AOUT pin to an analog input pin on your microcontroller (e.g., Arduino UNO).
3. Digital Output: Connect the DOUT pin to a digital input pin on your microcontroller if you want to use the threshold-based digital output.
4. Heater Connections: Connect H1 and H2 to the heater coil as per the datasheet instructions.

• Preheat Time: Ensure the sensor is preheated for at least 24 hours before initial use to achieve accurate readings.
• Calibration: Calibrate the sensor in a clean air environment to set a baseline for gas detection.
• Placement: Place the sensor in a well-ventilated area to avoid false readings due to stagnant air.
• Power Supply: Use a stable 5V power supply to ensure consistent sensor performance.

// MQ-138 Gas Sensor with Arduino UNO
const int analogPin = A0; // Analog input pin for AOUT
const int digitalPin = 2; // Digital input pin for DOUT
int analogValue = 0;      // Variable to store the analog value
int digitalValue = 0;     // Variable to store the digital value

void setup() {
  Serial.begin(9600);     // Initialize serial communication
  pinMode(digitalPin, INPUT); // Set digital pin as input
}

void loop() {
  analogValue = analogRead(analogPin); // Read the analog value
  digitalValue = digitalRead(digitalPin); // Read the digital value

  // Print the values to the Serial Monitor
  Serial.print("Analog Value: ");
  Serial.print(analogValue);
  Serial.print(" | Digital Value: ");
  Serial.println(digitalValue);

  delay(1000); // Wait for 1 second before the next reading
}

1. Inaccurate Readings: If the sensor provides inaccurate readings, ensure it has been preheated for at least 24 hours and is placed in a well-ventilated area.
2. No Output: If there is no output from the sensor, check the power supply connections and ensure the sensor is receiving 5V.
3. Fluctuating Readings: Fluctuating readings can be caused by unstable power supply or environmental factors. Use a stable power source and place the sensor away from direct airflow.

• Check Connections: Ensure all connections are secure and correct as per the pin configuration.
• Stable Power Supply: Use a regulated power supply to avoid fluctuations in sensor readings.
• Environmental Factors: Place the sensor in a stable environment to avoid false readings due to sudden changes in temperature or humidity.

By following this documentation, users can effectively integrate the MQ-138 gas sensor into their projects, ensuring accurate and reliable gas detection.