===== For Carbon Dioxide detection in air =====

This code communicates with the MQ135 air quality sensor with the help of the [[amc2021:groupl:extras:mq-135:start|MQ135.h library]]. 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. 

Detailed explanation is given in the [[amc2021:groupl:video:start|video tutorial]] 

<file c++ CO2Sensor.ino>
                              
#include "MQ135.h" //Main library that contains functions to get the ppm values
#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);       //Define the gas pin in the function mq135 gas sensor as per the mq135 library 

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(); //function to get ppm value based on the MQ135.h library
 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
  }

}

</file>
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