#define GREEN 13
#define YELLOW 12
#define RED 11

#define BUZZER 10

#define THRESHOLD_CONTROL 1400
#define THRESHOLD_CRITICAL 5000

#include <Wire.h> 
#include <LiquidCrystal_I2C.h>
#include <MQ135.h>

LiquidCrystal_I2C lcd(0x27,A4,A5);

MQ135 gasSensor(A0);

long last_beep = 0;

void setup() {
  pinMode(YELLOW, OUTPUT);
  pinMode(GREEN, OUTPUT);
  pinMode(RED, OUTPUT);

  pinMode(BUZZER, OUTPUT);
  lcd.init();                     
  lcd.backlight();
}

void loop() {
  float level = gasSensor.getPPM();
  lcd.clear();
  lcd.print("CO2: ");
  lcd.print(level);

  digitalWrite(YELLOW, LOW);
  digitalWrite(GREEN, LOW);
  digitalWrite(RED, LOW);
  lcd.setCursor(0, 1);

  if(level < THRESHOLD_CONTROL) {
    digitalWrite(GREEN, HIGH);
    lcd.print("OK");
    noTone(BUZZER);
  } else if(level > THRESHOLD_CRITICAL) {
    digitalWrite(RED, HIGH);
    lcd.print("Critical");
    if (millis() - last_beep > 300) {
      tone(BUZZER, 1000, 100);
      last_beep = millis();
    }
  } else {
    digitalWrite(YELLOW, HIGH);
    lcd.print("Control");
    noTone(BUZZER);
    if (millis() - last_beep > 5000) {
      tone(BUZZER, 3000, 1000);
      last_beep = millis();
    }
  }

  delay(200);
}