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 safety devices
• Home automation for air quality control

1. Power the Sensor: Connect the VCC pin to a 5V DC power source and the GND pin to ground.
2. Choose Output Type:
   • Use the AOUT pin for analog readings to measure gas concentration levels.
   • Use the DOUT pin for digital output, which provides a high or low signal based on the threshold set by 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: For example, connect the AOUT pin to an analog input pin on an Arduino UNO for precise gas concentration measurements.

• Preheating: The MQ135 requires a preheating period of at least 24 hours for accurate and stable readings.
• Calibration: Calibrate the sensor in a clean air environment to establish a baseline resistance value.
• Ventilation: Ensure proper ventilation around the sensor to avoid saturation or false readings.
• Power Supply: Use a stable 5V DC power source to avoid fluctuations in sensor performance.

// MQ135 Gas Sensor Example Code for Arduino UNO
// This code reads the analog output (AOUT) of the MQ135 sensor and prints
// the gas concentration value to the Serial Monitor.

const int mq135Pin = A0; // Connect AOUT pin of MQ135 to Arduino A0

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

void loop() {
  int sensorValue = analogRead(mq135Pin); // Read analog value from MQ135
  float voltage = sensorValue * (5.0 / 1023.0); // Convert to voltage
  Serial.print("Sensor Value: ");
  Serial.print(sensorValue); // Print raw sensor value
  Serial.print(" | Voltage: ");
  Serial.print(voltage); // Print voltage value
  Serial.println(" V");
  
  delay(1000); // Wait for 1 second before next reading
}

1. Unstable Readings:

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

2. No Output from DOUT Pin:

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

3. Incorrect Analog Readings:

   • Cause: Power supply fluctuations or improper calibration.
   • Solution: Use a stable 5V DC power source and calibrate the sensor in clean air.

4. Sensor Not Responding:

   • Cause: Faulty wiring or damaged sensor.
   • Solution: Check all connections and replace the sensor if necessary.

Q1: Can the MQ135 detect specific gases individually?
A1: No, the MQ135 is a general-purpose gas sensor and cannot differentiate between specific gases. It provides a combined response to all detectable gases in its range.

Q2: How do I calibrate the MQ135 sensor?
A2: Place the sensor in a clean air environment and record the baseline resistance value. Use this value to calculate gas concentrations in your application.

Q3: Can the MQ135 be used outdoors?
A3: The MQ135 is not designed for outdoor use as it is sensitive to humidity and temperature variations. Use it in controlled indoor environments for best results.

Q4: What is the lifespan of the MQ135 sensor?
A4: The MQ135 has a typical lifespan of 2-3 years under normal operating conditions.