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 signal, 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
• 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 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 accurate measurements. This ensures 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 Arduino for further processing.

• Preheating: The sensor requires a preheating period for accurate readings. Avoid using it immediately after powering it on.
• 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.
• Ventilation: Place the sensor in a well-ventilated area for accurate gas detection.

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 A0 pin, VCC to 5V, and GND to GND

const int analogPin = A0;  // Analog pin connected to MQ135 AOUT
int sensorValue = 0;       // Variable to store sensor reading

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

void loop() {
  sensorValue = analogRead(analogPin);  // 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 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 a calibration curve, which depends on the specific gas being measured.

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: Double-check 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 ensure no interfering gases are present.

4. Digital Output Not Triggering:

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

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

Q2: How do I calibrate the MQ135 sensor?
A2: Place the sensor in a clean air environment and record the baseline analog value. Use this value as a reference for detecting gas concentrations.

Q3: Can the MQ135 be used outdoors?
A3: While the MQ135 can operate outdoors, it should be protected from extreme weather conditions and high humidity to ensure accurate readings and longevity.

Q4: What is the lifespan of the MQ135 sensor?
A4: Under normal operating conditions, the sensor has a lifespan of approximately 2 years.