• Author: Toader Ion Andrei
• Email: ion_andrei.toader@stud.acs.upb.ro
• Master: SRIC

This project is an air quality monitoring system built using an ESP32 microcontroller, an MQ-135 gas sensor, and a buzzer for audible alerts. The main goal of the system is to continuously monitor the air quality, detect harmful gas levels (such as NH3, NOx, benzene, CO2, and smoke), and:

• Alert the user in real-time using a buzzer when air quality drops below a safe threshold.

• Upload collected air quality data to a web server where it can be visualized and analyzed over time.

• Logs data to a Firebase database and display it on a web dashboard.

The following schematic illustrates the core components and their connections in the air quality monitoring system:

This schematic includes:

• The ESP32 as the central microcontroller

• The MQ-135 sensor connected to an analog input pin for air quality detection

• A buzzer connected to a digital output pin for alerts

• ESP32 DevKit – Microcontroller with built-in Wi-Fi, used to read sensor data and upload it online.

• MQ-135 Gas Sensor – Measures air quality by detecting harmful gases and pollutants.

• Resistors (Voltage Divider for MQ-135 Analog Output): Two resistors (e.g., 110kΩ and 220kΩ) used to step down the MQ-135 analog output voltage from 5V to a safe 3.3V level for the ESP32 ADC input.

• Buzzer – Emits a sound when the air quality exceeds predefined danger thresholds.

• Wi-Fi Access – To connect the ESP32 to the internet and send data to the server.

ESP32 Development Board

Description: The ESP32 is a low-cost, low-power microcontroller with integrated Wi-Fi and Bluetooth. It is ideal for IoT projects due to its wireless capabilities and sufficient computing power.

Key Features:

• Dual-core Xtensa® 32-bit LX6 microprocessor, up to 240 MHz
• 520 KB SRAM
• Integrated 802.11 b/g/n Wi-Fi
• Bluetooth 4.2 (BLE + classic)
• 12-bit ADC (18 channels), 3.3V logic
• Multiple GPIO pins (capacitive touch, SPI, I2C, UART, PWM)
• USB-powered

Use in Project:

• Collects analog data from the MQ-135 sensor.
• Triggers a buzzer if pollution exceeds a threshold.
• Sends data to Firebase for live monitoring.

Important Pins Used:

• GPIO34 (Analog input from MQ-135)
• GPIO25 (Digital output to buzzer)
• 3.3V (Power for sensor)
• GND

MQ-135 Gas Sensor

Description: The MQ-135 is a chemical gas sensor capable of detecting a wide range of gases, including ammonia, nitrogen oxides, benzene, smoke, and carbon dioxide. It is commonly used in air quality monitoring.

Key Features:

• Operating Voltage: 5V
• Analog and Digital Output
• Detectable Gases: NH₃, NOₓ, alcohol, benzene, smoke, CO₂
• Preheat time: 24–48 hours (best accuracy after initial burn-in)

Use in Project: Provides an analog voltage proportional to air pollution levels. Connected to the ESP32’s analog input (GPIO34) through a voltage divider or level shifter, since it outputs 5V and ESP32 is 3.3V-tolerant.

Pinout:

Active Buzzer Module

Description: An active buzzer is a simple sound-emitting component that generates a tone when voltage is applied. Unlike passive buzzers, it does not require PWM control — just a HIGH/LOW signal.

Key Features:

• Operating Voltage: 3.3V – 5V
• Sound Level: ~85 dB at 10 cm
• Built-in oscillation circuit

Use in Project:

• Connected to a digital GPIO pin (e.g., GPIO25) on the ESP32.
• Activates (beeps) when the air quality exceeds a defined threshold.

Pinout:

Voltage Divider Resistors

Description:

• This divides the sensor’s 5V analog output down to approximately 3.3V, protecting the ESP32 ADC pin from damage.

Use in Project:

• Two resistors (e.g., 110kΩ and 220kΩ) are used in series between the sensor output and ground.
• The junction between the two resistors connects to the ESP32 analog input pin.

Breadboard and Jumper Wires

Description:

• Used for prototyping and temporary connections between components without soldering.

Use in Project:

• All components are connected on a breadboard using jumper wires for testing and layout flexibility.

Power Supply

Description:

• Unordered List ItemThe ESP32 is powered via USB (5V), and its onboard regulator provides 3.3V. The MQ-135 needs 5V to operate correctly.

Use in Project:

• USB cable or battery pack used to power ESP32.
• ESP32 powers other components directly or through regulated output pins.

• The MQ-135 sensor is powered with 5V and continuously measures the gas concentration in the air.
• The sensor’s analog output (AOUT) is connected to one of the ESP32’s analog input pins (e.g., GPIO34), through a voltage divider to ensure safe 0–3.3V levels.
• The ESP32 reads the analog values, processes them, and calculates an Air Quality Index (AQI) approximation based on predefined thresholds.
• If the AQI is above a critical threshold, indicating poor air quality, the buzzer is triggered through a digital output pin (e.g., GPIO25) to warn the user.
• The ESP32 then sends the air quality data via Wi-Fi to Firebase
• The web app displays the data in real-time on a web interface, allowing users to monitor air conditions from any device.

The Air Quality is a web application created to monitor and display air quality data in real time. It retrieves sensor readings such as PPM values from an air quality sensor—and presents them through a clean, user-friendly interface. The application is designed for development and educational use, offering a simple way to test, view, and analyze environmental data either locally or through connection with cloud-based platforms. Its structure is modular and flexible, making it easy to adapt to different use cases and hardware setups.

• Wi-Fi & Firebase Initialization

This part connects to Wi-Fi and initializes Firebase for anonymous sign-in and Realtime Database communication.

#include <WiFi.h>
#include <Firebase_ESP_Client.h>
#include "addons/TokenHelper.h"
#include "addons/RTDBHelper.h"

#define WIFI_SSID "********"
#define WIFI_PASSWORD "**********"
#define API_KEY "********"
#define DATABASE_URL "**********"

FirebaseData fbdo;
FirebaseAuth auth;
FirebaseConfig config;

void setup() {
  Serial.begin(115200);

  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
  while (WiFi.status() != WL_CONNECTED) {
    delay(300);
  }

  config.api_key = API_KEY;
  config.database_url = DATABASE_URL;

  if (Firebase.signUp(&config, &auth, "", "")) {
    Serial.println("Firebase signUp succeeded");
  }

  config.token_status_callback = tokenStatusCallback;
  Firebase.begin(&config, &auth);
  Firebase.reconnectWiFi(true);
}

• Sensor Reading & Conversion

Reads the analog value from the MQ-135 sensor, calculates Rs, and then converts that to a PPM (parts per million) estimate using a logarithmic formula.

#define MQ135_PIN 34
#define RZERO 116404.0
#define RL_VALUE 10000.0
#define PARA 116.6020682
#define PARB -2.769034857

void loop() {
  int adcValue = analogRead(MQ135_PIN);
  float sensor_voltage = adcValue * (3.3 / 4095.0);
  float rs = (3.3 - sensor_voltage) * RL_VALUE / sensor_voltage;
  float ratio = rs / RZERO;
  float ppm = PARA * pow(ratio, PARB);
}

• Buzzer Alert Logic

Activates a buzzer if the air quality degrades beyond a certain threshold (800 PPM in this case).

#define BUZZER_PIN 25

void setup() {
  pinMode(BUZZER_PIN, OUTPUT);
  digitalWrite(BUZZER_PIN, HIGH); // buzzer off
}

void loop() {
  // ...
  if (ppm > 800) {
    digitalWrite(BUZZER_PIN, LOW); // buzzer on
  } else {
    digitalWrite(BUZZER_PIN, HIGH); // buzzer off
  }
}

• Sending Data to Firebase

Uploads the latest calculated PPM value to Firebase Realtime Database under the path /air_quality/ppm.

if (Firebase.RTDB.setFloat(&fbdo, "/air_quality/ppm", ppm)) {
  Serial.println("Data sent to Firebase");
} else {
  Serial.print("Failed to send data: ");
  Serial.println(fbdo.errorReason());
}

}

• https://www.instructables.com/Air-Quality-ESP32-Build-a-Portable-IoT-Sensor/
• https://www.hackster.io/dudelabweb/air-quality-monitoring-made-easy-an-iot-project-with-esp32-ee2777
• https://randomnerdtutorials.com/esp32-firebase-web-app/#install-vs-code
• https://randomnerdtutorials.com/getting-started-with-esp32/
• https://www.elprocus.com/mq135-air-quality-sensor/