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

The MQ-138 is a gas sensor manufactured by Flying Fish, designed to detect various gases, particularly volatile organic compounds (VOCs). It operates on the principle of resistive change when exposed to target gases, providing an analog output proportional to the gas concentration. This sensor is widely used in air quality monitoring systems, industrial safety equipment, and environmental research.

• Air quality monitoring systems
• Industrial gas detection
• Environmental research and analysis
• Smart home devices for VOC detection
• Automotive air quality control systems

The MQ-138 sensor is a versatile and reliable component with the following key specifications:

The MQ-138 module typically comes with a 4-pin interface. Below is the pin configuration:

1. Power the Sensor: Connect the VCC pin to a 5V DC power source and the GND pin to the ground.
2. Connect the Output:
   • Use the AOUT pin to read the analog signal for precise gas concentration measurements.
   • Optionally, use the DOUT pin for a simple HIGH/LOW signal based on a preset threshold.
3. Adjust the Load Resistance: Use the onboard potentiometer to adjust the load resistance (RL) and set the desired sensitivity for the DOUT pin.
4. Preheat the Sensor: Allow the sensor to preheat for at least 24 hours before taking accurate measurements.
5. Read the Output: Use an ADC (Analog-to-Digital Converter) to read the AOUT signal or directly interface the DOUT pin with a microcontroller.

• Preheating: The sensor requires a preheating period of at least 24 hours for stable and accurate readings.
• Ventilation: Ensure proper ventilation around the sensor to avoid saturation and improve response time.
• Calibration: Calibrate the sensor in a controlled environment with known gas concentrations for accurate results.
• Power Supply: Use a stable 5V DC power source to avoid fluctuations in the output signal.
• Avoid Contamination: Keep the sensor away from water, dust, and corrosive gases to maintain its longevity.

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

// MQ-138 Gas Sensor Example Code for Arduino UNO
// This code reads the analog output (AOUT) of the MQ-138 sensor and prints
// the value to the Serial Monitor. Ensure the sensor is preheated before use.

const int MQ138_AOUT_PIN = A0; // Connect AOUT pin of MQ-138 to Arduino A0

void setup() {
  Serial.begin(9600); // Initialize serial communication at 9600 baud
  pinMode(MQ138_AOUT_PIN, INPUT); // Set AOUT pin as input
}

void loop() {
  int sensorValue = analogRead(MQ138_AOUT_PIN); // Read analog value from MQ-138
  float voltage = sensorValue * (5.0 / 1023.0); // Convert ADC value to voltage

  // 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");

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

1. No Output Signal:

   • Cause: Incorrect wiring or insufficient power supply.
   • Solution: Double-check the connections and ensure a stable 5V DC power source.

2. Inconsistent Readings:

   • Cause: Sensor not preheated or exposed to fluctuating environmental conditions.
   • Solution: Allow the sensor to preheat for at least 24 hours and ensure a stable environment.

3. DOUT Pin Always HIGH or LOW:

   • Cause: Incorrect threshold setting on the potentiometer.
   • Solution: Adjust the potentiometer to set the desired sensitivity.

4. Slow Response Time:

   • Cause: Poor ventilation or sensor saturation.
   • Solution: Improve ventilation around the sensor and avoid prolonged exposure to high gas concentrations.

Q1: Can the MQ-138 detect gases other than VOCs?
A1: Yes, the MQ-138 can detect a range of gases, but it is optimized for VOCs. For other gases, calibration may be required.

Q2: How do I calibrate the MQ-138 sensor?
A2: Expose the sensor to a known concentration of the target gas and adjust the load resistance (RL) or use software calibration to map the output voltage to the gas concentration.

Q3: Can I use the MQ-138 with a 3.3V microcontroller?
A3: The MQ-138 requires a 5V power supply for the heater. However, you can use a voltage divider or level shifter to interface the output with a 3.3V microcontroller.

Q4: How long does the MQ-138 sensor last?
A4: The sensor has a typical lifespan of 2-3 years under normal operating conditions. Proper care and maintenance can extend its life.