MQ-135 Air Quality Sensor

1. Introduction

The MQ-135 is an air quality sensor designed for detecting harmful gases in the air. It's commonly used in indoor air quality monitors, pollution detection systems, and HVAC control applications.

2. Technical Specifications

How It Works

Like other MQ sensors, the MQ-135 uses a tin dioxide sensing element:

1. Heater keeps the sensing element at optimal temperature
2. In clean air, resistance is high (low voltage output)
3. Pollutants reduce resistance (higher voltage output)
4. Different gases have different sensitivity curves

3. Wiring Guide

Figure 1: MQ-135 sensor wiring with Arduino

4. Project: Air Quality Monitor

Basic Air Quality Reading

const int sensorPin = A0;

void setup() {
    Serial.begin(115200);
    Serial.println("MQ-135 Air Quality Sensor");
    Serial.println("Warming up (2 minutes)...");
    delay(120000);  // 2 minute warm-up
    Serial.println("Ready!");
}

void loop() {
    int rawValue = analogRead(sensorPin);
    float voltage = rawValue * (5.0 / 1023.0);

    Serial.print("Raw: ");
    Serial.print(rawValue);
    Serial.print(" | Voltage: ");
    Serial.print(voltage, 2);
    Serial.print("V | Air Quality: ");

    if (rawValue < 150) {
        Serial.println("Excellent");
    } else if (rawValue < 300) {
        Serial.println("Good");
    } else if (rawValue < 500) {
        Serial.println("Moderate");
    } else if (rawValue < 700) {
        Serial.println("Poor");
    } else {
        Serial.println("Hazardous!");
    }

    delay(2000);
}

Air Quality Index (AQI) Approximation

const int sensorPin = A0;

// Baseline value in clean air (calibrate this!)
const int cleanAirValue = 120;

void setup() {
    Serial.begin(115200);
    Serial.println("MQ-135 AQI Monitor");
    Serial.println("Calibrating...");
    delay(60000);
}

void loop() {
    int rawValue = analogRead(sensorPin);
    
    // Calculate a relative pollution index
    float ratio = (float)rawValue / cleanAirValue;
    int aqi = (ratio - 1.0) * 100;
    if (aqi < 0) aqi = 0;
    if (aqi > 500) aqi = 500;

    Serial.print("Raw: ");
    Serial.print(rawValue);
    Serial.print(" | AQI: ");
    Serial.print(aqi);
    Serial.print(" | Category: ");

    if (aqi <= 50) {
        Serial.println("Good (Green)");
    } else if (aqi <= 100) {
        Serial.println("Moderate (Yellow)");
    } else if (aqi <= 150) {
        Serial.println("Unhealthy for Sensitive (Orange)");
    } else if (aqi <= 200) {
        Serial.println("Unhealthy (Red)");
    } else if (aqi <= 300) {
        Serial.println("Very Unhealthy (Purple)");
    } else {
        Serial.println("Hazardous (Maroon)");
    }

    delay(2000);
}

5. MQ-135 vs Other Air Quality Sensors

For precise CO2 measurements, consider dedicated CO2 sensors like MH-Z19 or SCD30.

6. Applications

• Indoor air quality: Office and home monitoring
• HVAC control: Ventilation based on air quality
• Smart home: Trigger air purifier or alert
• Industrial: Workshop fume detection
• Education: Environmental science projects

7. Troubleshooting

High readings in fresh air

• Allow longer burn-in time (24-48 hours)
• Ensure good ventilation during calibration

Readings don't change

• Check 5V power supply
• Verify sensor is warm (heater working)

Very low readings

• Check analog pin connection
• Verify ADC is working with a known voltage

8. Summary

• Detects CO2, ammonia, NOx, alcohol, benzene, smoke
• Requires 5V and long burn-in period (24+ hours)
• Analog output proportional to pollution level
• Calibrate baseline in known clean air
• Not a precision sensor—use for relative measurements