Smart Air Quality & Gas Leakage Monitor Using ESP32, MQ-135, and OLED Display

Smart Air Quality & Gas Leakage Monitor Using ESP32, MQ-135, and OLED Display

For home and workplace safety, real-time detection of hazardous gas leaks and air quality monitoring are crucial. In this project, we construct a small air monitoring node that uses an ESP32 microcontroller-powered 0.96-inch I2C OLED screen to show real-time status updates and identify dangerous airborne gasses using a MQ-135 sensor.

Components Used

• ESP32 Development Board
• MQ-135 Air Sensor Module
• 0.96″ SSD1306 I2C OLED Display (128×64 pixels)
• Breadbaord

Circuit Wiring

ESP32	MQ-135	OLED
VIN	VCC
GND	GND	GND
GPIO 34	AO
GPIO 21		SDA
GPIO 22		SCL

Required Libraries

Adafruit SSD1306

Hardware driver for monochrome SSD1306 OLED displays

Adafruit GFX

Core graphics engine for rendering text, fonts, and shapes

Arduino Code

#include <Adafruit_GFX.h>     // Include the Adafruit GFX library for graphics functions
#include <Adafruit_SSD1306.h> // Include the Adafruit SSD1306 library for the OLED display

// OLED display configuration
#define SCREEN_WIDTH 128      // Define the OLED display width in pixels
#define SCREEN_HEIGHT 64     // Define the OLED display height in pixels
#define OLED_RESET -1        // Define reset pin (-1 if sharing microcontroller reset pin)
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT); // Create display instance

const int Gas34 = 34; // Analog pin connected to MQ-135 AO pin

// Wi-Fi safe analog input pins on ADC1:
// analogRead(39);
// analogRead(34);
// analogRead(35);
// analogRead(32);
// analogRead(33);

int data;  

void setup() {
  Serial.begin(9600);
  pinMode(Gas34, INPUT);
  
  // Initialize OLED display with I2C address 0x3C
  if (!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println(F("SSD1306 allocation failed"));
    while (true); // Loop indefinitely if initialization fails
  }
  
  display.clearDisplay();
  display.setTextSize(2);             // Adjusted size for clean screen layout
  display.setTextColor(SSD1306_WHITE);
  display.setCursor(0, 10);
  display.println(F("Initializing..."));
  display.display();
  delay(2000);
}

void loop() {
  data = analogRead(Gas34);           // Read raw 12-bit ADC value (0 - 4095)
  Serial.print("MQ-135 Value: ");
  Serial.println(data);
  
  // Evaluate gas threshold
  if (data > 300) {
    display.clearDisplay();
    display.setTextSize(2);
    display.setCursor(42, 0);
    display.println(F("GAS"));
    display.setCursor(15, 20);
    display.println(F("has been"));
    display.setCursor(15, 45);
    display.println(F("Detected."));
    display.display();
  } else {
    display.clearDisplay();
    display.setTextSize(2);
    display.setCursor(26, 15);
    display.println(F("No GAS"));
    display.setCursor(15, 45);
    display.println(F("Detected."));
    display.display();
  }

  delay(1000);
}

How the project works

When exposed to airborne pollutants such as smoke, ammonia, benzene, and carbon dioxide, the electrical conductivity of the MQ-135 air quality sensor’s internal Tin Dioxide (SnO2) heating element changes dynamically. The sensor produces a greater analog voltage signal when the concentration of the targeted gasses rises. The ESP32 uses its integrated 12-bit Analog-to-Digital Converter (ADC) on GPIO 34 to sample the signal and provide raw digital values between 0 and 4095.

Every second, the ESP32 examines these analog values within the main execution loop and compares them to a predetermined safety threshold of 300. The ESP32 instructs the SSD1306 OLED screen to display a “No GAS Detected” status message over the I2C bus (GPIO 21 SDA and GPIO 22 SCL) if the reading falls below or remains at the threshold.

The micro-controller provides live telemetry to the Serial Monitor and updates the display with a high-priority “GAS has been Detected” warning if the gas concentration causes the reading to surge above 300.