MQ-135 Air Quality Detector Using Arduino | Cirkit Designer Simulation Project

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 MQ-135 Air Quality Detector using Arduino Uno in Cirkit Designer

 Introduction

The MQ-135 Air Quality Monitoring System is a smart environmental project that detects harmful gases and monitors indoor air pollution levels in real time. This project is designed and simulated using Cirkit Designer, making it perfect for:

  • Beginners

  • School & college students

  • Science fair projects

  • IoT learners

  • Electronics hobbyists

It helps you understand how gas sensors work and how microcontrollers process environmental data.

 About the MQ-135 Gas Sensor

The MQ-135 sensor detects multiple harmful gases such as:

  • Ammonia (NH₃)

  • Nitrogen Oxides (NOx)

  • Alcohol vapors

  • Benzene

  • Smoke

  • Carbon Dioxide (CO₂)

It provides:

  • Analog Output (AOUT) → Measures gas concentration level

  • Digital Output (DOUT) → Triggers when gas exceeds a threshold (set via onboard potentiometer)

 Components Required

  • Arduino Uno

  • MQ-135 Gas Sensor Module

  • Red LED

  • Green LED

  • 220Ω Resistors

  • Jumper wires

 Circuit Connections

MQ-135 PinArduino Connection
VCC5V
GNDGND
AOUTA0
DOUTPin 2

LEDs:

LEDArduino Pin
Red LEDPin 9
Green LEDPin 10

  • LED anode → Arduino pin via 220Ω resistor

  • LED cathode → GND

 How the Code Works

1️⃣ Analog Reading

int raw = analogRead(PIN_AOUT);

Reads gas concentration value (0–1023).

2️⃣ Voltage Conversion

float volts = raw * (5.0 / 1023.0);

Converts analog reading to voltage.

3️⃣ Digital Threshold Detection

int doLevel = digitalRead(PIN_DOUT);

  • LOW → Gas level above threshold (Danger)

  • HIGH → Safe

4️⃣ Alert System

  • Red LED ON → Dangerous air quality

  • Green LED ON → Safe air quality

5️⃣ Serial Monitor Output

Displays:

  • Raw sensor value

  • Voltage

  • Safety status

 Example Serial Output

AOUT=350 (1.711 V)  DOUT=HIGH (SAFE)
AOUT=720 (3.520 V)  DOUT=LOW (DANGER)

 Key Features

✔ Real-time air quality monitoring
✔ Analog gas detection
✔ Digital threshold-based alert system
✔ LED safety indicator
✔ Simulation-friendly design
✔ Beginner-friendly Arduino code

 Learning Concepts Covered

  • Analog input using analogRead()

  • Digital input detection

  • Voltage calculation

  • Environmental sensing

  • Embedded system alerts

  • Serial communication debugging

 Applications

  • Indoor air pollution monitoring

  • Smart home automation

  • Industrial safety monitoring

  • IoT environmental systems

  • School science exhibitions

  • Smart classroom air monitoring

 Why Use Cirkit Designer?

  • No physical hardware required

  • Safe simulation of gas sensors

  • Real-time debugging

  • Easy wiring visualization

  • Ideal for beginners

 Important Notes

  • MQ-135 requires preheating time in real hardware for accurate readings.

  • Sensor values are relative — calibration is required for precise ppm measurement.

  • In simulation, readings are simplified for learning purposes.

 Possible Enhancements

You can upgrade this project by adding:

  •  Buzzer for sound alert

  •  16x2 LCD display for air quality value

  •  ESP8266 / IoT cloud monitoring

  •  Air Quality Index (AQI) calculation

  •  Mobile app notifications

 Conclusion

The MQ-135 Air Quality Detector using Arduino Uno is a practical and educational project that teaches:

  • Gas sensing technology

  • Pollution detection systems

  • Analog signal processing

  • Microcontroller programming

This project is ideal for students, beginners, and IoT enthusiasts who want to explore environmental monitoring systems in a simple and interactive way.

Code:
const int PIN_AOUT = A0;
const int PIN_DOUT = 2;
const int LED_RED = 9;
const int LED_GREEN = 10;

void setup() {
  pinMode(PIN_DOUT, INPUT); // DOUT is active-low on LM393 modules
  pinMode(LED_RED, OUTPUT);
  pinMode(LED_GREEN, OUTPUT);
  digitalWrite(LED_RED, LOW);
  digitalWrite(LED_GREEN, LOW);
  Serial.begin(115200);
}

void loop() {
  int raw = analogRead(PIN_AOUT);
  float volts = raw * (5.0f / 1023.0f);

  int doLevel = digitalRead(PIN_DOUT);      // LOW = danger (above threshold), HIGH = safe
  bool danger = (doLevel == LOW);

  digitalWrite(LED_RED, danger ? HIGH : LOW);
  digitalWrite(LED_GREEN, danger ? LOW : HIGH);

  Serial.print("AOUT=");
  Serial.print(raw);
  Serial.print(" (");
  Serial.print(volts, 3);
  Serial.print(" V)  DOUT=");
  Serial.println(doLevel == LOW ? "LOW (DANGER)" : "HIGH (SAFE)");

  delay(100);
}

Using Cirkit Designer simulation, users can:

  • Design the complete circuit online

  • Connect MQ-135 sensor to Arduino virtually

  • Test air quality readings in real time

  • Debug code efficiently

  • Understand gas sensor working principles

 Key Features:

  • Real-time air quality monitoring

  • Analog gas concentration detection

  • Arduino-based alert system

  • Beginner-friendly simulation project

  • Low-cost environmental monitoring solution

This Arduino MQ-135 air quality monitoring project helps students understand gas sensing technology, pollution detection systems, and microcontroller programming in a simple and practical way.

  

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