Project Documentation

Arduino-Based Air Quality Monitoring System with Gas Sensors and LCD Display

Circuit Documentation

Summary

This circuit is designed to monitor air quality using gas sensors (MQ-7 and MQ-135), a temperature and humidity sensor (DHT11), and a PIR sensor. The data is displayed on a 16x2 I2C LCD, and visual/auditory alerts are provided using LEDs and a buzzer. The circuit is controlled by an Arduino UNO microcontroller.

Component List

LED: Two Pin (green)
- Description: Green LED
- Pins: Cathode, Anode
Resistor (220 Ohms)
- Description: Resistor with 220 Ohms resistance
- Pins: Pin1, Pin2
Arduino UNO
- Description: Microcontroller board
- Pins: UNUSED, IOREF, Reset, 3.3V, 5V, GND, Vin, A0, A1, A2, A3, A4, A5, SCL, SDA, AREF, D13, D12, D11, D10, D9, D8, D7, D6, D5, D4, D3, D2, D1, D0
LED: Two Pin (red)
- Description: Red LED
- Pins: Cathode, Anode
Buzzer
- Description: Buzzer for auditory alerts
- Pins: PIN, GND
DHT11
- Description: Temperature and humidity sensor
- Pins: DATA, GND, VCC
MQ135
- Description: Gas sensor
- Pins: VCC, GND, A0, D0
MQ-7 Breakout
- Description: Gas sensor
- Pins: VCC, GND, DO, AO
16x2 I2C LCD
- Description: LCD display with I2C interface
- Pins: GND, VCC, SDA, SCL
LED: Two Pin (blue)
- Description: Blue LED
- Pins: Cathode, Anode
PIR sensor
- Description: Passive Infrared sensor
- Pins: VDD, SIG, GND

Wiring Details

LED: Two Pin (green)

Anode connected to D4 on Arduino UNO
Cathode connected to Pin2 of a 220 Ohm Resistor

Resistor (220 Ohms)

Pin1 connected to GND on Arduino UNO
Pin2 connected to Cathode of the green LED

Arduino UNO

5V connected to VCC of MQ-7 Breakout, PIR sensor, and 16x2 I2C LCD
GND connected to GND of DHT11, MQ-7 Breakout, PIR sensor, and 16x2 I2C LCD
D2 connected to DATA of DHT11
D3 connected to Anode of the red LED
D4 connected to Anode of the green LED
D8 connected to PIN of the buzzer
A0 connected to A0 of MQ135
D10 connected to Anode of the blue LED
D6 connected to DO of MQ-7 Breakout
A1 connected to AO of MQ-7 Breakout
D11 connected to SIG of PIR sensor
SCL connected to SCL of 16x2 I2C LCD
SDA connected to SDA of 16x2 I2C LCD

LED: Two Pin (red)

Anode connected to D3 on Arduino UNO
Cathode connected to Pin2 of a 220 Ohm Resistor

Buzzer

PIN connected to D8 on Arduino UNO
GND connected to Pin1 of a 220 Ohm Resistor

DHT11

DATA connected to D2 on Arduino UNO
GND connected to GND on Arduino UNO
VCC connected to 5V on Arduino UNO

MQ135

A0 connected to A0 on Arduino UNO
GND connected to GND on Arduino UNO
VCC connected to 5V on Arduino UNO

MQ-7 Breakout

VCC connected to 5V on Arduino UNO
GND connected to GND on Arduino UNO
DO connected to D6 on Arduino UNO
AO connected to A1 on Arduino UNO

16x2 I2C LCD

GND connected to GND on Arduino UNO
VCC connected to 5V on Arduino UNO
SDA connected to SDA on Arduino UNO
SCL connected to SCL on Arduino UNO

LED: Two Pin (blue)

Anode connected to D10 on Arduino UNO
Cathode connected to Pin2 of a 220 Ohm Resistor

PIR sensor

VDD connected to 5V on Arduino UNO
SIG connected to D11 on Arduino UNO
GND connected to GND on Arduino UNO

Code Documentation

/*
  AnalogReadSerial

  Reads an analog input on pin 0, prints the result to the Serial Monitor.
  Graphical representation is available using Serial Plotter (Tools > Serial Plotter menu).
  Attach the center pin of a potentiometer to pin A0, and the outside pins to +5V and ground.

  This example code is in the public domain.

  https://www.arduino.cc/en/Tutorial/BuiltInExamples/AnalogReadSerial
*/

#include <DHT11.h>
#include <Wire.h> 
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27,16 ,2);
// Define sensor pins
int mq7Pin = A0;  // MQ-7 analog sensor connected to A0
int mq135Pin = A1;  // MQ-135 analog sensor connected to A1
// DHT11 sensor settings

DHT11 dht11(2); // Create an instance of the DHT sensorlcd.begin();

// Define other pins for LEDs and buzzer
int redLED = 3;    
int greenLED = 4;  
int buzzer = 8;     

void setup() {

  lcd.init();                      // initialize the lcd 
  lcd.init();

  lcd.backlight();
  
  // Start serial communication
  Serial.begin(9600);
  delay(2000);  // Wait for 2 seconds
  pinMode(redLED, OUTPUT);
  pinMode(greenLED, OUTPUT);
  pinMode(buzzer, OUTPUT);
}

void loop() {

  // Read gas sensor values (analog)
  int mq7Value = analogRead(mq7Pin);
  int mq135Value = analogRead(mq135Pin);

   // Read temperature and humidity from DHT11
  int temperature = 0;
  int humidity = 0;      // Humidity in percentage
  int result = dht11.readTemperatureHumidity(temperature, humidity);
  if (result == 0) {
        Serial.print("Temperature: ");
        Serial.print(temperature);
        Serial.print(" °C\tHumidity: ");
        Serial.print(humidity);
        Serial.println(" %");
    } else {
        // Print error message based on the error code.
        Serial.println(DHT11::getErrorString(result));
    }
  // Print values to serial for ESP8266
  
  
  // Simulate an alert if gas levels are too high (you can change these threshold values)
  if (mq7Value > 400 || mq135Value > 400) {
    digitalWrite(redLED, HIGH);  // Turn on red LED
    digitalWrite(greenLED, LOW); // Turn off green LED
    digitalWrite(buzzer, HIGH); 
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("Dangerous gases");
    lcd.setCursor(0, 1);
    lcd.print("Detected!"); // Turn on buzzer
  } else {
    digitalWrite(redLED, LOW);   // Turn off red LED
    digitalWrite(greenLED, HIGH);// Turn on green LED
    digitalWrite(buzzer, LOW);
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("Air is clean");   // Turn off buzzer
  }
  Serial.print("MQ7: ");
  Serial.print(mq7Value);
  Serial.print(" | MQ135: ");
  Serial.println(mq135Value);
  digitalWrite(greenLED, HIGH);

  // int dangerousThreshold = 400; // Set a