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 change is converted into an analog output, making it suitable for air quality monitoring and environmental sensing applications.

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

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

The MQ135 sensor typically comes with a 4-pin or 6-pin interface. Below is the pin configuration for the 4-pin version:

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 voltage output, which varies with gas concentration.
   • Optionally, use the DOUT pin for a digital high/low 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 accurate readings. This stabilizes the internal heating element.
4. Connect to a Microcontroller: The MQ135 can be interfaced with microcontrollers like Arduino for real-time monitoring and data processing.

• Calibration: The sensor requires calibration to accurately detect specific gas concentrations. Use a known gas concentration for reference.
• Preheat Time: Ensure the sensor is preheated for the recommended duration to achieve stable readings.
• Ventilation: Place the sensor in a well-ventilated area to avoid saturation and ensure accurate measurements.
• Avoid Contaminants: Keep the sensor away from water, oil, and other contaminants that may damage it.
• Power Supply: Use a stable 5V 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 with Arduino UNO
// This code reads the analog output of the MQ135 sensor and prints the value
// to the Serial Monitor. Ensure the sensor is connected to the correct pins.

const int analogPin = A0; // Connect AOUT pin of MQ135 to A0 on Arduino

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

void loop() {
  int sensorValue = analogRead(analogPin); // Read the analog value from MQ135
  float voltage = sensorValue * (5.0 / 1023.0); // Convert to voltage (0-5V)
  
  // Print the sensor value and voltage to the Serial Monitor
  Serial.print("Sensor Value: ");
  Serial.print(sensorValue);
  Serial.print(" | Voltage: ");
  Serial.print(voltage);
  Serial.println(" V");
  
  delay(1000); // Wait for 1 second before the next reading
}

1. No Output or Incorrect Readings:

   • Cause: Insufficient preheat time.
   • Solution: Allow the sensor to preheat for at least 24 hours before use.

2. Fluctuating Readings:

   • Cause: Unstable power supply or environmental interference.
   • Solution: Use a regulated 5V power source and place the sensor in a stable environment.

3. Sensor Not Responding:

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

4. High Analog Values in Clean Air:

   • Cause: Sensor requires calibration.
   • Solution: Calibrate the sensor using a known gas concentration.

Q1: Can the MQ135 detect multiple gases simultaneously?
A1: The MQ135 provides a combined response to multiple gases. It cannot differentiate between specific gases without additional calibration and processing.

Q2: How do I set the threshold for the digital output (DOUT)?
A2: Use the onboard potentiometer to adjust the threshold level. Turn it clockwise or counterclockwise to increase or decrease sensitivity.

Q3: Can the MQ135 be used outdoors?
A3: While the MQ135 can operate outdoors, it should be protected from extreme weather conditions, such as rain or high humidity, to prevent damage.

Q4: How often should the sensor be replaced?
A4: Under normal conditions, the sensor has a lifespan of approximately 2 years. Replace it if readings become unreliable.