#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 10, 9, 8, 7);

void setup() {
  lcd.begin(16, 2);
  lcd.print("Alarma activada!");
  pinMode(5, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(3, INPUT);
  digitalWrite(5, LOW);
  digitalWrite(4, HIGH);
}

int alerta = 0;

void loop() {
  if (digitalRead(3)==HIGH){
    if (alerta==0){
      alerta = 1;
      lcd.clear();
      lcd.print("Intruso!");
      digitalWrite(5, HIGH);
      digitalWrite(4, LOW);
    }
  } else {
    if (alerta==1){
      alerta = 0;
      lcd.clear();
      lcd.print("Alarma activada");
      digitalWrite(5, LOW);
      digitalWrite(4, HIGH);
    }
  }
}