#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27,16,2);  // set the LCD address to 0x3F for a 16 chars and 2 line display

int buzzer = 4;
int trigPin = 8;
int echoPin = 9;
int red = 3;
int green = 2;

//define variable
long duration;
int distance;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(buzzer, OUTPUT);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(red, OUTPUT);
  pinMode(green, OUTPUT);

  lcd.init();
  lcd.backlight();
  lcd.setCursor(0,0);
  lcd.print("Reverse system");
  delay(2000);
}

void loop() {
  // put your main code here, to run repeatedly:
  //Clears the trigPin
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  //Set the trigPin on HIGH for 10 micro seconds
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  //Reads the echoPin, returns the sound wave travel time in microseconds
  duration = pulseIn(echoPin, HIGH);
  //Calculate the distance
  distance = duration*0.034/2;

  if (distance <= 50){
    tone(buzzer,1250);
    digitalWrite(red, HIGH);
    digitalWrite(green, LOW);
    lcd.setCursor(0,0);
    lcd.print(" DISTANCE: ");
    lcd.print(distance);
    lcd.print("cm");
    lcd.setCursor(0,1);
    lcd.print("    Warning!   ");
    delay(500);
    noTone(buzzer);
    delay(500);
  }
  else{
    noTone(buzzer);
    digitalWrite(red, LOW);
    digitalWrite(green, HIGH);
    lcd.setCursor(0,0);
    lcd.print(" DISTANCE: ");
    lcd.print(distance);
    lcd.print("cm");
    lcd.setCursor(0,1);
    lcd.print(" Safe distance. ");
  }
}