Mq135 Air Quality Sensor

Key Features

• Detects a range of gases, including harmful and flammable ones.
• Analog and digital output for interfacing with microcontrollers.
• Built-in potentiometer for sensitivity adjustment.
• Compact and low-cost module.

Specifications

• Operating Voltage: 3.3V–5V DC
• Operating Current: ~150 mA
• Detectable Gases: Ammonia (NH₃), nitrogen oxides (NOₓ), benzene, CO₂, alcohol, and smoke.
• Output Types:
  • Analog Output: Proportional to gas concentration.
  • Digital Output: HIGH/LOW signal based on the threshold set by the potentiometer.
• Preheat Time: ~20 seconds for stable readings.
• Operating Temperature: -10°C to 50°C

Pinout

1. VCC: Power supply (3.3V or 5V)
2. GND: Ground
3. DO: Digital output
4. AO: Analog output

Working Principle

The MQ135 sensor contains a SnO₂ (tin dioxide) sensing layer that has a low conductivity in clean air. When exposed to certain gases, the conductivity increases, and this change is used to measure gas concentration.

Interfacing with Arduino

Hardware Setup

1. Connect VCC to the Arduino’s 5V pin.
2. Connect GND to the Arduino’s GND pin.
3. Connect AO (Analog Output) to an analog input pin (e.g., A0).
4. (Optional) Connect DO (Digital Output) to a digital input pin for threshold detection.

Sample Code

const int analogPin = A0; // Analog output pin
const int digitalPin = 2; // Digital output pin
int analogValue = 0;

void setup() {
  Serial.begin(9600);
  pinMode(digitalPin, INPUT);
}

void loop() {
  // Read analog value
  analogValue = analogRead(analogPin);
  
  // Print analog value (gas concentration)
  Serial.print("Analog Value: ");
  Serial.println(analogValue);

  // Check digital output
  if (digitalRead(digitalPin) == HIGH) {
    Serial.println("Gas concentration exceeded threshold!");
  }
  
  delay(1000);
}

Calibrating the Sensor

• Allow the sensor to preheat for at least 24 hours before precise calibration.
• Use the built-in potentiometer to adjust the digital threshold value.
• For accurate measurements, compare the analog output with a reference gas analyzer or known concentrations.

Applications

• Indoor air quality monitoring
• Smart homes and IoT systems
• Industrial gas detection
• Pollution monitoring
• Fire and smoke detection