🌍 Smart Air Quality Monitoring & Alert System
Arduino UNO • MQ-135 • LED Indicators • Buzzer • START / STOP / RESET{=html}
A simple, low-cost educational prototype for demonstrating how changes in surrounding air can be detected and converted into visual and audio alerts.
📌 Overview
The Smart Air Quality Monitoring & Alert System is an Arduino UNO based embedded-system project that uses an MQ-135 gas sensor to detect changes in the surrounding air.
The system reads the sensor's analog output and classifies the reading into three demonstration conditions:
Condition Indicator Buzzer
🟢 Good Green LED One short beep 🔵 Moderate Blue LED Short beep every 4 seconds 🔴 Poor Red LED Repeating alarm
The project also includes three push buttons:
⏹️ STOP --- stops monitoring and turns outputs OFF
🔄 RESET --- returns the system to its initial stopped state
After the program is uploaded, the project can run without a laptop using a USB power bank.
Important: This is an educational prototype. The MQ-135 provides a raw sensor response and does not directly measure official AQI. The thresholds used here are demonstration thresholds based on the observed sensor baseline.
🎯 Objectives
Demonstrate basic environmental air monitoring.
Learn how an analog gas sensor interfaces with Arduino.
Convert sensor readings into simple condition levels.
Provide both visual and audio alerts.
Build a portable system that can run from a power bank.
Demonstrate how air conditions can vary between different environments.
Create a simple project suitable for science exhibitions and beginner IoT/embedded-systems learning.
🧰 Components
Component Quantity Purpose
Arduino UNO SMD 1 Main controller MQ-135 sensor 1 Detects changes in surrounding air Green LED 1 Good-condition indication Blue LED 1 Moderate-condition indication Red LED 1 Poor-condition indication LED resistors 3 Limit LED current SEW 1201 buzzer 1 Audio indication / alarm Push buttons 3 START, STOP and RESET Breadboard 1 Circuit assembly Jumper wires As required Connections USB power bank 1 Portable power
🔌 Circuit Connections
MQ-135
MQ-135 Arduino UNO
VCC 5V GND GND AO A0 DO Not connected
LEDs
LED Arduino Pin
🟢 Green D8 🔵 Blue D9 🔴 Red D10
Each LED is connected through its resistor to protect the LED.
Buzzer
Buzzer Arduino
-
** D11**
− GND
Push Buttons
Button Arduino Pin Other Side
The buttons use Arduino's internal INPUT_PULLUP, so external pull-up resistors are not required.
⚙️ How It Works
🌬️ SURROUNDING AIR
│
▼
┌───────────┐
│ MQ-135 │
│ SENSOR │
└─────┬─────┘
│ Analog signal
▼
┌───────────┐
│ ARDUINO │
│ UNO │
└─────┬─────┘
│
┌─────────┼─────────┐
▼ ▼ ▼
🟢 GOOD 🔵 MODERATE 🔴 POOR
│ │ │
Green LED Blue LED Red LED
│ │ │
One beep Beep/4 sec Alarm
Operating sequence
Power the Arduino using USB.
The system starts in the stopped state.
Press START.
The MQ-135 reading is continuously checked.
Arduino compares the reading with the demonstration thresholds.
The appropriate LED is activated.
The buzzer provides the corresponding audio indication.
Press STOP to turn the system OFF.
Press RESET to return to the initial stopped state.
📊 Demonstration Thresholds
During initial testing, the MQ-135 produced a stable indoor reading of approximately 69--71, with a baseline of about 70.
The current demonstration logic is:
Raw MQ-135 Reading Demonstration Condition LED Buzzer
`≤ 80` 🟢 Good Green One short beep when entering Good
`81–94` 🔵 Moderate Blue Beep every 4 seconds
`≥ 95` 🔴 Poor Red Repeating alarm
These values are not official AQI limits.
The MQ-135 is a broad gas-response sensor. Its raw readings can vary with:
Sensor warm-up time
Temperature
Humidity
Sensor condition
Different gases
Environment and ventilation
For a certified air-quality system, calibrated sensors and appropriate measurement standards would be required.
🎛️ Controls
Starts the monitoring process.
⏹️ STOP
Immediately stops monitoring and turns:
Green LED OFF
Blue LED OFF
Red LED OFF
Buzzer OFF
🔄 RESET
Returns the system to its initial stopped condition so the demonstration can be started again.
🎥 Project Demonstration
The following images show the three operating conditions of the physical prototype.
🟢 Good Air Condition
Green LED ON + one short beep
🔵 Moderate Air Condition
Blue LED ON + beep every 4 seconds
🔴 Poor Air Condition
Red LED ON + repeating alarm
🎬 Complete Working Demonstration
The GIF demonstrates the working prototype and its different LED/buzzer responses.
🌫️ Smoke Demonstration
A small amount of agarbatti (incense-stick) smoke can be used as a controlled classroom demonstration to show that the MQ-135 responds to changes in surrounding air.
Demonstration
Normal room air ↓ MQ-135 baseline ↓ 🟢 Green indication ↓ Small amount of incense smoke ↓ Sensor response increases ↓ 🔵 / 🔴 indication
Use only a small amount of smoke, keep the flame and hot incense away from the electronics, and ensure good ventilation.
This demonstrates sensor response to a change in air, not a certified AQI measurement.
🏠🏫🌳🏭 Environmental Demonstration Model
The project can be combined with a small model containing:
🏠 House
🏫 School
🌳 Garden
🏭 Factory
The sensor can be moved between these areas to explain why air conditions may differ.
Example discussion
🏠 House: cooking fumes, cleaning products and ventilation can affect indoor air.
🏫 School: dust, people and nearby traffic can influence the surrounding environment.
🌳 Garden: open ventilation and vegetation provide a useful comparison area.
🏭 Factory: industrial activity can create a situation where air monitoring becomes important.
These are demonstration scenarios; actual air quality depends on the specific environment.
🔋 Standalone Operation
The finished project does not require a laptop during demonstration.
🔋 USB POWER BANK
│
▼
┌──────────────┐
│ ARDUINO UNO │
└───────┬──────┘
│
┌────────┼─────────┐
▼ ▼ ▼
MQ-135 LEDs Buzzer
The laptop is only needed for:
Uploading the Arduino program
Editing the code
Testing through Serial Monitor
🧪 Testing
All major components were tested individually before combining the final system.
Component / Function Test Result
Arduino UNO ✅ Tested Green LED ✅ Tested Blue LED ✅ Tested Red LED ✅ Tested START button ✅ Tested STOP button ✅ Tested RESET button ✅ Tested SEW 1201 buzzer ✅ Tested MQ-135 analog output ✅ Tested Serial Monitor ✅ Tested Standalone power-bank operation ✅ Supported
Observed MQ-135 baseline
Typical initial indoor readings during testing:
69 70 70 71 70 70 71 70
Approximate baseline:
~70 raw ADC units
🌟 Applications
This educational prototype can demonstrate the concept of air monitoring in:
🏠 Homes
🏫 Schools
🌳 Parks and gardens
🚗 Roadside / traffic areas
🏭 Industrial areas
🧪 Science exhibitions
👨🎓 Arduino and IoT learning environments
Real industrial and health applications require calibrated and appropriate air-quality instruments.
✅ Advantages
Low-cost prototype
Simple circuit
Beginner-friendly
No display required
Visual + audio feedback
Portable
Power-bank compatible
Easy START/STOP/RESET control
Useful for science exhibitions
Can be expanded into an IoT system
MQ-135 does not directly provide official AQI.
Raw readings require calibration for quantitative measurement.
The sensor responds to multiple gases.
Temperature and humidity can influence readings.
Sensor warm-up affects the response.
Demonstration thresholds are not regulatory limits.
The prototype should not be used as a certified safety or medical device.
🚀 Future Improvements
Possible upgrades include:
📟 OLED/LCD display
📱 Mobile application
🌐 Wi-Fi / IoT monitoring
📊 Web dashboard
☁️ Cloud data storage
📈 Historical graphs
🌡️ Temperature and humidity monitoring
🔬 Calibrated PM2.5 / PM10 sensor
📍 GPS-based pollution mapping
🔔 Remote alerts
🔋 Rechargeable battery and enclosure
💻 Technologies Used
Arduino UNO
Arduino C/C++
MQ-135 Gas Sensor
Analog Sensor Reading
Digital Input/Output
LED Indicators
Buzzer
Push Buttons
Embedded Systems
📜 License
This project is created for educational and demonstration purposes.
You are welcome to study, modify, and extend the project for learning and non-commercial educational use.
⭐ Conclusion
The Smart Air Quality Monitoring & Alert System demonstrates a complete embedded-system workflow:
Sense → Process → Decide → Indicate → Alert
A simple MQ-135 sensor provides the input, Arduino UNO processes the reading, LEDs communicate the condition, and the buzzer provides an audio warning.
The project provides a practical starting point for learning Arduino, sensors, embedded programming, environmental monitoring, and IoT concepts.
Manoj Ramesh
Repository: Smart-Air-Quality-Monitor
create a practical environmental-monitoring prototype.
The project connects an everyday environmental problem with fundamental embedded-system concepts:
Sense → Process → Decide → Indicate → Alert
It provides a strong foundation for future development into a connected IoT-based environmental monitoring system.