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
• 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 four pins. Below is the pin configuration:

1. Power Supply: Connect the VCC pin to a 5V DC power source and the GND pin to ground.
2. Analog Output: Use the AO pin to read the gas concentration as an analog voltage. This pin is typically connected to an ADC (Analog-to-Digital Converter) pin on a microcontroller.
3. Digital Output: The DO pin provides a digital HIGH or LOW signal based on a preset threshold. Adjust the threshold using the onboard potentiometer.
4. Preheat Time: Allow the sensor to preheat for at least 24 hours before taking stable readings.

• Preheat Requirement: The sensor requires a long preheat time (≥ 24 hours) for accurate and stable readings.
• Ventilation: Ensure proper ventilation around the sensor to avoid saturation or false readings.
• Calibration: Calibrate the sensor in a clean air environment to establish a baseline resistance.
• Avoid Overvoltage: Do not exceed the recommended operating voltage to prevent damage to the sensor.
• Humidity Sensitivity: The sensor's performance may degrade in high humidity conditions.

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

// MQ135 Gas Sensor Example Code for Arduino UNO
// This code reads the analog output of the MQ135 sensor and prints the value
// to the Serial Monitor. Ensure the sensor is properly connected to the Arduino.

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

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 (0-5V)
  
  // Print the raw 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 stable 5V DC power source and ensure proper ventilation.

3. Digital Output Always HIGH or LOW:

   • Cause: Incorrect threshold setting.
   • Solution: Adjust the potentiometer on the sensor module to set the desired threshold.

4. Sensor Not Responding:

   • Cause: Damaged sensor or incorrect wiring.
   • Solution: Verify wiring connections and replace the sensor if necessary.

Q1: Can the MQ135 detect CO2?
A1: Yes, the MQ135 can detect CO2, but it is more sensitive to gases like ammonia, benzene, and smoke.

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

Q3: Can I use the MQ135 with a 3.3V microcontroller?
A3: The MQ135 is designed for 5V operation. Use a level shifter or voltage regulator to interface with 3.3V systems.

Q4: What is the lifespan of the MQ135 sensor?
A4: The sensor typically lasts for 2-3 years under normal operating conditions.