This code communicates with the MQ135 air quality sensor. The sensor is supposed to preheat for 24 hours before taking readings. Once the code runs, it prints out the concentration of detected gases in ppm on a serial monitor and the results are displayed on an LCD screen An alarm system (LED light) is also set to glow if the CO<sub>2<\sub> values cross a threshold value of 1000ppm.

                              
#include "MQ135.h" 
#include <Wire.h> 
#include <LiquidCrystal_I2C.h>  //Header file for LCD

LiquidCrystal_I2C lcd(0x27,16,2);//set the LCD address to x27 for a 16 chars and 2 line display

#define led          9                       //led on pin 9
const int gas_pin = A0;                      //analog feed from MQ135
MQ135 gasSensor = MQ135(gas_pin);

void setup(){

  lcd.init();                      // initialize the lcd 
  lcd.begin(16,2);                  // consider 16 chars + 2 lines lcd
  lcd.backlight();                 // illuminate to produce visible reading
  lcd.clear();                     // clear lcd
  lcd.setCursor(4,0);              //set cursor of lcd to 1st row and 5th column
  lcd.print("Group L");          // print as a sentence on lcd

  pinMode(gas_pin,INPUT);     //MQ135 analog feed set for input
  pinMode(led,OUTPUT);        //led set for output
  
  Serial.begin(9600);        //serial comms for debugging
}

void loop(){
 float ppm = gasSensor.getPPM();
 Serial.println(ppm);            // print ppm on serial monitor
 delay(1000);                    
   lcd.clear();                  // clear lcd
   lcd.setCursor(0,0);          // set cursor of lcd to 1st row and 1st column
   lcd.print("Air Quality: ");  // print as a sentence on lcd
   lcd.print(ppm);             // print value of MQ135
 if(ppm>999){                  //if co2 ppm > 1000
    digitalWrite(led,HIGH);    //turn on led
    lcd.setCursor(2,1);        // set cursor of lcd to 2nd row and 3rd column
    lcd.print("AQ Level BAD");  //print as a sentence on lcd
   }
 else{              
    digitalWrite(led,LOW);         //turn off led
    lcd.setCursor(1,1);            // set cursor of lcd to 2nd row and 2nd column
    lcd.print ("AQ Level Good");   // print as a sentence on lcd
  }

}

/* This code is meant to monitor the sound intensity using LM393 sensor connected to Arduino UNO board.
//The used sensor has only a digital output. Therefore, the number of times the sensor detects a sound is summed up over a sampling time called "SAMPLE_TIME". 
//Then the sum called "sampleBufferValue" is printed on a Serial Monitor (laptop), and visualized with the Serial Plotter. 
// The code allows to communicate  with a LED in order to provide a visual alarm if the "sampleBufferValue" surpasses a preset Threshold  "Threshold" */

// 0 means silence and 1 means noise

 const int OUT_PIN = 12;          // The OUTPUT of the sound sensor is connected to the digital pin D12 of the Arduino 
 const int SAMPLE_TIME = 10;      // The sampling time in milliseconds,  can be set differently if required
 const int Threshold = 90;       // Threshold on decibel value for LED switching ON, the value has been optimized with respect to the used sampling time (900 cumulative digital counts ≈ 90 dB from "Schall")

 unsigned long millisCurrent;     // current time
 unsigned long millisLast = 0;    //previous time
 unsigned long millisElapsed = 0; // difference between current time and previous time (time interval)

 int sampleBufferValue = 0;       // initiate the sum of digital outputs over the sampling time
 int led = 8;                    // LED on pin 4 of Arduino
 int dB = 0;                     //initiate sound intensity dB value

void setup() {

  Serial.begin(9600);           //Arduino starts serial communication with baud rate 9600
  pinMode(led,OUTPUT);          // the LED is connected as output for alarm purpose
 
}

void loop() { 

  millisCurrent = millis();                    //the current time is assigned to the dedicated variable
  millisElapsed = millisCurrent - millisLast; //the elapsed time is updated 
  
  if(digitalRead(OUT_PIN) == HIGH){          //HIGH means noise
   
  sampleBufferValue++;                       //increments the sum variable by 1
  }
   if (millisElapsed > SAMPLE_TIME) {       //if the elapsed time surpasses the sampling time, print the sampleBufferValue and test threshold for alarm
   
   dB = 0.0666 *(sampleBufferValue) + 30.223; //linear regression to calculate the decibel value based of the rough calibration of the sensor response 
  Serial.println(dB);                        // print decibel values on the Serial Monitor 
  Serial.print("dB");                        // print sound unit decibel
  
  
   if (sampleBufferValue > Threshold) {     // test if the threshold is surpassed 
  
    digitalWrite(led, HIGH);                //blink LED 2 ms ON and 1 ms OFF
    delay(2);
    digitalWrite(led, LOW);
    delay(1);
  }
  
  digitalWrite(led, LOW);                  // the LED is turned off to be ready for the next sample
  sampleBufferValue = 0;                   // re-initialization of the  sampleBufferValue variable for the new sampling time
  millisLast = millisCurrent;              // update the previous time to be the start for the next sample
  }
}