How to Use MQ-135: Examples, Pinouts, and Specs

The MQ-135 is a versatile gas sensor manufactured by Flying Fish, designed to detect a wide range of gases, including ammonia (NH₃), benzene (C₆H₆), alcohol, smoke, 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 makes it an ideal choice for air quality monitoring and environmental safety applications.

• Air quality monitoring systems
• Industrial gas detection
• Smoke and fire detection systems
• Environmental monitoring
• Smart home automation for air quality control

The MQ-135 sensor is designed for ease of use and integration into various electronic systems. Below are its key technical details:

The MQ-135 sensor module typically comes with a 4-pin interface. Below is the pinout description:

1. Power the Sensor: Connect the VCC pin to a 5V DC power source and the GND pin to ground.
2. Choose Output Type:
   • Use the AOUT pin for analog readings to measure gas concentration levels.
   • Use the DOUT pin for digital output, which triggers when the gas concentration exceeds a preset threshold. Adjust the threshold using the onboard potentiometer.
3. Preheat the Sensor: Allow the sensor to preheat for at least 24 hours for accurate and stable readings.
4. Connect to a Microcontroller: Interface the sensor with a microcontroller (e.g., Arduino UNO) to process the output signals.

• Preheating: The sensor requires a preheating period of at least 24 hours for optimal performance.
• Ventilation: Ensure proper ventilation around the sensor to avoid saturation and ensure accurate readings.
• Calibration: Calibrate the sensor in a clean air environment to establish a baseline resistance value.
• Power Supply: Use a stable 5V DC power source to avoid fluctuations in readings.

Below is an example of how to interface the MQ-135 with an Arduino UNO to read analog values:

// MQ-135 Gas Sensor Example Code
// Connect AOUT to Arduino analog pin A0
// Connect VCC to 5V and GND to GND

const int mq135Pin = A0; // Analog pin connected to AOUT
int sensorValue = 0;     // Variable to store sensor reading

void setup() {
  Serial.begin(9600); // Initialize serial communication at 9600 baud
  Serial.println("MQ-135 Gas Sensor Test");
}

void loop() {
  sensorValue = analogRead(mq135Pin); // Read analog value from sensor
  Serial.print("Sensor Value: ");
  Serial.println(sensorValue); // Print the sensor value to the Serial Monitor

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

• The sensorValue represents the raw analog output from the sensor. You can map this value to gas concentration levels using calibration data.
• Use the Serial Monitor in the Arduino IDE to view the sensor readings in real time.

1. No Output or Incorrect Readings:

   • Ensure the sensor is properly powered (5V DC).
   • Verify all connections are secure and correct.
   • Allow sufficient preheating time (≥ 24 hours).

2. Fluctuating Readings:

   • Check for stable power supply.
   • Ensure the sensor is placed in a well-ventilated area.
   • Avoid exposing the sensor to extreme humidity or temperature conditions.

3. Digital Output Not Triggering:

   • Adjust the potentiometer to set the desired threshold level.
   • Verify the DOUT pin connection to the microcontroller.

Q: Can the MQ-135 detect CO₂?
A: While the MQ-135 is sensitive to a variety of gases, it is not specifically calibrated for CO₂ detection. For precise CO₂ measurements, consider using a dedicated CO₂ sensor.

Q: How do I calibrate the MQ-135?
A: Place the sensor in a clean air environment and record the baseline resistance value. Use this value to calculate gas concentrations based on the sensor's datasheet.

Q: Can I use the MQ-135 with a 3.3V system?
A: The MQ-135 is designed for 5V operation. If using a 3.3V system, you may need a level shifter or voltage regulator to ensure proper operation.

Q: How long does the sensor last?
A: The MQ-135 has a typical lifespan of 2-3 years under normal operating conditions. Regular calibration and proper usage can extend its lifespan.

By following this documentation, you can effectively integrate the MQ-135 gas sensor into your projects for reliable air quality monitoring and gas detection.