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 air quality indicators. It operates on the principle of resistive change, where the sensor's resistance varies in the presence of target gases. The MQ-135 provides an analog output proportional to the concentration of detected gases, making it ideal for air quality monitoring and environmental sensing applications.

• Air quality monitoring systems
• Indoor air pollution detection
• Industrial gas leakage detection
• Smart home automation for air quality control
• Environmental research and analysis

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

The MQ-135 sensor typically comes with 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 ground.
2. Read the Output:
   • Use the AOUT pin to read the analog signal corresponding to gas concentration.
   • Optionally, use the DOUT pin for a digital 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 stable readings.
4. Connect to a Microcontroller: The analog output can be connected to an ADC (Analog-to-Digital Converter) pin of a microcontroller like the Arduino UNO.

• Preheating: The sensor requires a long preheating time (≥ 24 hours) for accurate and stable readings.
• Calibration: Calibrate the sensor in a clean air environment to establish a baseline reading.
• Power Supply: Ensure a stable 5V power supply to avoid fluctuations in readings.
• Environmental Factors: Avoid exposing the sensor to high humidity or temperatures outside its operating range.

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
const int analogPin = A0; // Define the analog pin connected to AOUT
int sensorValue = 0;      // Variable to store the sensor reading

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

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

  // Add a delay to avoid flooding the Serial Monitor
  delay(1000); // Wait for 1 second before the next reading
}

• The analogRead() function reads the analog voltage from the sensor and converts it to a digital value (0-1023).
• The sensor value can be mapped to gas concentration using calibration data specific to the target gas.

1. Inconsistent Readings:

   • Cause: Insufficient preheating time or unstable power supply.
   • Solution: Ensure the sensor is preheated for at least 24 hours and use a regulated 5V power source.

2. No Output Signal:

   • Cause: Incorrect wiring or damaged sensor.
   • Solution: Verify the connections and ensure the sensor is not physically damaged.

3. High Sensor Values in Clean Air:

   • Cause: Improper calibration or environmental interference.
   • Solution: Calibrate the sensor in a clean air environment and shield it from external interference.

4. Digital Output Not Triggering:

   • Cause: Threshold not set correctly.
   • Solution: Adjust the onboard potentiometer to set the desired threshold level.

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?
A2: Place the sensor in a clean air environment and record the baseline analog output. Use this value to calculate gas concentrations relative to the baseline.

Q3: Can I use the MQ-135 outdoors?
A3: While the MQ-135 can be used outdoors, ensure it is protected from extreme humidity, temperature, and direct exposure to water.

Q4: What is the lifespan of the MQ-135 sensor?
A4: The sensor typically lasts for several years under normal operating conditions, but its sensitivity may degrade over time. Regular calibration is recommended.