How to Use MQ135: Examples, Pinouts, and Specs

The MQ135 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 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 pollution monitoring
• Home and office air purifiers

The MQ135 sensor is designed for ease of use and integration into various electronic systems. Below are its key technical details:

The MQ135 sensor 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. Read Analog Output: Connect the AOUT pin to an analog input pin of your microcontroller (e.g., Arduino UNO) to measure gas concentration.
3. Digital Output: Optionally, connect the DOUT pin to a digital input pin of your microcontroller to detect gas presence based on the threshold set by the onboard potentiometer.
4. Preheat the Sensor: Allow the sensor to preheat for at least 24 hours for accurate readings.
5. Calibrate the Sensor: Use a known gas concentration to calibrate the sensor for precise measurements.

• 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 false readings.
• Load Resistor: Use a 10 kΩ load resistor (RL) for typical applications. Adjust RL for specific sensitivity requirements.
• Avoid Contamination: Keep the sensor away from water, oil, and dust to maintain its performance.
• Power Supply: Use a stable 5V DC power source to avoid fluctuations in readings.

Below is an example of how to interface the MQ135 with an Arduino UNO to read analog values:

// MQ135 Gas Sensor Example Code
// Connect AOUT to Arduino analog pin A0
// Connect VCC to 5V and GND to GND

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

void setup() {
  Serial.begin(9600);      // Initialize serial communication
  Serial.println("MQ135 Gas Sensor Test");
}

void loop() {
  sensorValue = analogRead(analogPin);  // Read analog value from sensor
  Serial.print("Sensor Value: ");
  Serial.println(sensorValue);          // Print sensor value to Serial Monitor
  delay(1000);                          // Wait 1 second before next reading
}

• The analogRead() function reads the voltage from the AOUT pin and converts it to a digital value (0-1023).
• The sensor value can be mapped to gas concentration using a calibration curve, which depends on the specific gas being measured.

1. No Output or Incorrect Readings

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

2. Fluctuating Readings

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

3. Sensor Saturation

   • Cause: Prolonged exposure to high gas concentrations.
   • Solution: Allow the sensor to recover in clean air for a few hours.

4. Digital Output Not Triggering

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

Q1: Can the MQ135 detect CO2?
Yes, the MQ135 can detect CO2, but it is more sensitive to gases like ammonia, benzene, and smoke. For precise CO2 measurements, a dedicated CO2 sensor is recommended.

Q2: How do I calibrate the MQ135?
Calibration involves exposing the sensor to a known concentration of the target gas and adjusting the readings accordingly. Refer to the sensor's datasheet for detailed calibration instructions.

Q3: Can I use the MQ135 with a 3.3V microcontroller?
The MQ135 is designed for 5V operation. If using a 3.3V microcontroller, a level shifter or voltage divider is required for proper interfacing.

Q4: How long does the MQ135 last?
The sensor has a typical lifespan of 2-3 years under normal operating conditions. Regular maintenance and proper usage can extend its life.

Q5: Is the MQ135 suitable for outdoor use?
The MQ135 is not weatherproof and should be used in controlled environments. For outdoor applications, additional protection is required.