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, benzene, alcohol, and smoke. 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 sensing applications. Manufactured by ESP32 with part ID 001, the MQ135 is widely used in both industrial and hobbyist projects.

• Air quality monitoring systems
• Industrial gas detection
• Smoke and fire detection systems
• Environmental monitoring
• IoT-based smart home applications

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

• Preheat Time: The sensor requires a long preheat time (≥ 24 hours) for accurate readings. Ensure this is accounted for in your project timeline.
• Calibration: Calibrate the sensor in a clean air environment to establish a baseline resistance value.
• Placement: Avoid placing the sensor in areas with high humidity or temperatures outside the specified range.
• 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:

// Include necessary libraries (if any additional libraries are required, install them)
// Define the analog pin connected to the MQ135 sensor
const int MQ135_PIN = A0;

void setup() {
  Serial.begin(9600); // Initialize serial communication at 9600 baud
  pinMode(MQ135_PIN, INPUT); // Set the MQ135 pin as input
}

void loop() {
  int sensorValue = analogRead(MQ135_PIN); // Read the analog value from the sensor
  float voltage = sensorValue * (5.0 / 1023.0); // Convert the analog value to voltage
  
  // Print the sensor value and voltage to the Serial Monitor
  Serial.print("Sensor Value: ");
  Serial.print(sensorValue);
  Serial.print(" | Voltage: ");
  Serial.println(voltage);
  
  delay(1000); // Wait for 1 second before taking the next reading
}

1. Inconsistent Readings:

   • Cause: Insufficient preheat time or unstable power supply.
   • Solution: Ensure the sensor has been preheated for at least 24 hours and use a regulated 5V power source.

2. No Output from DOUT Pin:

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

3. High Sensitivity to Humidity:

   • Cause: The sensor is exposed to high humidity levels.
   • Solution: Place the sensor in a controlled environment with humidity ≤ 95% RH.

4. Sensor Not Responding:

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

Q1: Can the MQ135 detect CO2?
A1: Yes, the MQ135 can detect CO2, but it is not specifically calibrated for precise CO2 measurements. For accurate CO2 detection, consider using a dedicated CO2 sensor.

Q2: How do I calibrate the MQ135?
A2: To calibrate the MQ135, expose it to clean air and measure the baseline resistance. Use this value to calculate gas concentrations in your application.

Q3: Can I use the MQ135 with a 3.3V microcontroller?
A3: The MQ135 requires a 5V power supply for proper operation. If using a 3.3V microcontroller, use a level shifter for the output signals.

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