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

The MQ-138 is a gas sensor designed to detect various gases, particularly volatile organic compounds (VOCs) and other harmful gases. It operates on the principle of resistive change, where the sensor's resistance varies in the presence of target gases. This change is converted into an analog output signal, which can be processed by microcontrollers or other electronic systems.

The MQ-138 is widely used in applications such as:

• Air quality monitoring systems
• Industrial safety systems
• Environmental monitoring
• Smart home devices for detecting harmful gases
• Research and development in gas detection technologies

The MQ-138 sensor is a versatile and reliable component. Below are its key technical details:

The MQ-138 sensor typically comes with four pins. Below is the pin configuration:

1. Power the Sensor: Connect the VCC pin to a 5V DC power supply and the GND pin to the ground.
2. Connect the Output:
   • Use the AOUT pin to read the analog signal. This pin provides a voltage proportional to the gas concentration.
   • Optionally, use the DOUT pin for a digital HIGH/LOW signal if a threshold is set using the onboard potentiometer.
3. Preheat the Sensor: Allow the sensor to preheat for at least 24 hours before taking accurate readings. This ensures the internal heater stabilizes.
4. Read the Output:
   • For analog readings, connect the AOUT pin to an analog input pin of a microcontroller (e.g., Arduino).
   • For digital readings, connect the DOUT pin to a digital input pin.

• Preheating: The MQ-138 requires a long preheating time (≥ 24 hours) for stable and accurate readings.
• Calibration: Calibrate the sensor in a controlled environment with known gas concentrations for accurate results.
• Ventilation: Ensure proper ventilation around the sensor to avoid saturation or false readings.
• Load Resistor: Use an appropriate load resistor (typically 10 kΩ) to achieve the desired sensitivity and output range.
• Avoid Contamination: Keep the sensor away from water, oil, and dust to maintain its performance and longevity.

Below is an example code to interface the MQ-138 with an Arduino UNO and read the analog output:

// Define the analog pin connected to the MQ-138 AOUT pin
const int MQ138_AOUT_PIN = A0;

// Variable to store the sensor reading
int sensorValue = 0;

void setup() {
  // Initialize serial communication for debugging
  Serial.begin(9600);
  Serial.println("MQ-138 Gas Sensor Test");
}

void loop() {
  // Read the analog value from the sensor
  sensorValue = analogRead(MQ138_AOUT_PIN);

  // Convert the analog value to a voltage (assuming 5V reference)
  float voltage = sensorValue * (5.0 / 1023.0);

  // Print the sensor value and voltage to the Serial Monitor
  Serial.print("Sensor Value: ");
  Serial.print(sensorValue);
  Serial.print(" | Voltage: ");
  Serial.print(voltage);
  Serial.println(" V");

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

1. No Output Signal:

   • Cause: Incorrect wiring or power supply.
   • Solution: Double-check the connections and ensure the sensor is powered with 5V DC.

2. Unstable Readings:

   • Cause: Insufficient preheating time or environmental interference.
   • Solution: Allow the sensor to preheat for at least 24 hours and ensure proper ventilation.

3. Low Sensitivity:

   • Cause: Incorrect load resistor value or sensor aging.
   • Solution: Adjust the load resistor or replace the sensor if it has exceeded its lifespan.

4. False Alarms:

   • Cause: Contamination or exposure to non-target gases.
   • Solution: Clean the sensor area and ensure it is used in a controlled environment.

Q1: Can the MQ-138 detect multiple gases simultaneously?
A1: The MQ-138 is sensitive to a range of gases, but it does not differentiate between them. Calibration and additional processing are required to identify specific gases.

Q2: How do I calibrate the MQ-138?
A2: Expose the sensor to a known concentration of the target gas and record the output voltage. Use this data to create a calibration curve.

Q3: Can I use the MQ-138 with a 3.3V system?
A3: The MQ-138 is designed for 5V operation. Use a level shifter or voltage regulator to interface with 3.3V systems.

Q4: How long does the MQ-138 last?
A4: The sensor has a typical lifespan of 2 years under normal operating conditions.