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

The MQ-135 is a versatile gas sensor 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 response to the concentration of target gases in the air. This makes the MQ-135 an excellent choice for air quality monitoring and environmental safety applications.

• Air quality monitoring systems
• Industrial gas detection
• Smoke and fire detection systems
• Environmental pollution monitoring
• Home and office air purifiers

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 has four pins. Below is the pinout description:

1. Power the Sensor: Connect the VCC pin to a 5V DC power source and the GND pin to the ground.
2. Choose Output Type:
   • Use the AOUT pin for analog readings to measure gas concentration levels.
   • Use the DOUT pin for digital readings, which indicate whether the gas concentration exceeds a preset threshold.
3. Preheat the Sensor: Allow the sensor to preheat for at least 24 hours for accurate and stable readings.
4. Connect to a Microcontroller: For example, connect the AOUT pin to an analog input pin on an Arduino UNO for real-time monitoring.

• Preheating: The sensor requires a long preheating time (≥ 24 hours) for optimal performance.
• Calibration: Calibrate the sensor in a clean air environment to set a baseline for gas concentration.
• Power Supply: Ensure a stable 5V power supply to avoid fluctuations in readings.
• Ventilation: Place the sensor in a well-ventilated area for accurate air quality measurements.

Below is an example of how to use the MQ-135 sensor 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 ground

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 will vary based on the gas concentration. You can map this value to a specific gas concentration using calibration data.
• Use an external potentiometer to adjust the threshold for the digital output (DOUT).

1. Inaccurate Readings:

   • Cause: Insufficient preheating time.
   • Solution: Ensure the sensor is preheated for at least 24 hours before use.

2. No Output from DOUT:

   • Cause: Threshold not properly set.
   • Solution: Adjust the potentiometer on the module to set the desired threshold.

3. Fluctuating Analog Values:

   • Cause: Unstable power supply or environmental interference.
   • Solution: Use a regulated 5V power supply and avoid placing the sensor near strong electromagnetic sources.

4. Sensor Not Responding:

   • Cause: Incorrect wiring or damaged sensor.
   • Solution: Double-check the wiring and ensure the sensor is not physically damaged.

Q1: Can the MQ-135 detect specific gases?
A1: Yes, the MQ-135 can detect gases like ammonia, benzene, alcohol, and smoke. However, it is not selective and may respond to multiple gases simultaneously.

Q2: How do I calibrate the MQ-135 sensor?
A2: Place the sensor in clean air and record the baseline analog value. Use this value as a reference for detecting gas concentrations.

Q3: Can I use the MQ-135 with a 3.3V microcontroller?
A3: The MQ-135 is designed for 5V operation. Use a level shifter or voltage regulator to interface with 3.3V systems.

Q4: How long does the sensor last?
A4: The MQ-135 has a typical lifespan of 2-3 years under normal operating conditions.