/*
  // This code communicates with the MQ135 air quality sensor. The sensor is supposed to preheat for 2 mins before taking readings
  // Once the code runs, it prints out the concentration of detected gases in ppm on a serial monitor and a LCD 20 x 4 screen
  //An alarm system (LED light) is also set to print out messages saying if the air is of a good quality relying on a predefined threshold value
*/
  // digital output value is converted to ppm value using CO2 gas as parameter                                
#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 = 900; // Threshold on cumulative counts for LED switching ON, the value has been optimized with respect to the used sampling time of 10 ms here. (900 digital outputs ≈ 90 dB from "Shall")

 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

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 the threshold for the alarm
   
  Serial.println(sampleBufferValue); // output on the Serial Monitor 
  
   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
  }
}