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 monitoring stations
• Home automation for air quality control

The MQ135 gas sensor is manufactured by ESP32 with the part ID 002. Below are the key technical details and pin configuration:

The MQ135 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 Analog Output: Connect the AO pin to an analog input pin of a microcontroller (e.g., Arduino UNO) to measure gas concentration.
3. Digital Output (Optional): Use the DO pin for threshold-based detection. Adjust the onboard potentiometer to set the threshold level.
4. Preheat the Sensor: Allow the sensor to preheat for at least 24 hours for accurate readings.

• Preheating: The sensor requires a preheating period of 24 hours for optimal performance.
• Calibration: Calibrate the sensor in a clean air environment to determine the baseline resistance (RS in clean air).
• Power Supply: Ensure a stable 5V power supply to avoid fluctuations in readings.
• Placement: Avoid placing the sensor in environments with high humidity or temperatures beyond its operating range.

Below is an example code snippet to interface the MQ135 with an Arduino UNO:

// Include necessary libraries
const int analogPin = A0; // Connect AO pin of MQ135 to A0 on Arduino
float sensorValue;        // Variable to store sensor reading

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

void loop() {
  sensorValue = analogRead(analogPin); // Read analog value from MQ135
  float voltage = sensorValue * (5.0 / 1023.0); // Convert to voltage
  Serial.print("Sensor Value: ");
  Serial.print(sensorValue);
  Serial.print(" | Voltage: ");
  Serial.println(voltage);
  
  delay(1000); // Wait for 1 second before next reading
}

• The analogRead() function reads the analog output from the MQ135 sensor.
• The voltage conversion formula assumes a 10-bit ADC with a 5V reference.
• Use the serial monitor to observe the sensor readings in real-time.

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 fans or vents.

3. Sensor Not Detecting Gases

   • Cause: Incorrect threshold setting on the potentiometer.
   • Solution: Adjust the potentiometer to set an appropriate threshold level.

4. High Humidity Impacting Performance

   • Cause: Operating in environments with >95% relative humidity.
   • Solution: Use the sensor in controlled environments within its specified humidity range.

Q1: Can the MQ135 detect CO2?
A1: Yes, the MQ135 can detect CO2, but it is more sensitive to ammonia, benzene, and alcohol. For precise CO2 detection, consider a dedicated CO2 sensor.

Q2: How do I calibrate the MQ135 sensor?
A2: Place the sensor in clean air, measure the resistance (RS), and use it as the baseline for gas concentration calculations.

Q3: Can I use the MQ135 with a 3.3V microcontroller?
A3: The MQ135 requires a 5V power supply. Use a level shifter or voltage divider to interface with 3.3V systems.

Q4: How long does the sensor last?
A4: The MQ135 has a typical lifespan of 2-3 years under normal operating conditions.

By following this documentation, users can effectively integrate the MQ135 gas sensor into their projects for reliable air quality monitoring.