#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.println("BCD => DEZ:");
  pinMode(2, INPUT_PULLUP);
  pinMode(3, INPUT_PULLUP);
  pinMode(4, INPUT_PULLUP);
  pinMode(5, INPUT_PULLUP);
  pinMode(6, INPUT_PULLUP);
  pinMode(7, INPUT_PULLUP);
  pinMode(8, INPUT_PULLUP);
  pinMode(9, INPUT_PULLUP);
}

byte bcd2dez(byte v0, byte v1, byte v2, byte v3)
{
  return (v0 + 2*v1 + 4*v2 + 8*v3);
}

void loop() 
{
  byte e_bcd_1, e_bcd_2, e_bcd_3, e_bcd_4;
  byte e_bcd_5, e_bcd_6, e_bcd_7, e_bcd_8;
  byte einer, zehner, summe;

  e_bcd_1 = !digitalRead(2);
  e_bcd_2 = !digitalRead(3);
  e_bcd_3 = !digitalRead(4);
  e_bcd_4 = !digitalRead(5);
  e_bcd_5 = !digitalRead(6);
  e_bcd_6 = !digitalRead(7);
  e_bcd_7 = !digitalRead(8);
  e_bcd_8 = !digitalRead(9);

  einer  = bcd2dez(e_bcd_1, e_bcd_2, e_bcd_3, e_bcd_4);
  zehner = bcd2dez(e_bcd_5, e_bcd_6, e_bcd_7, e_bcd_8);
  summe  = einer + 10*zehner;

  if (einer > 9 || zehner > 9)
  {
    lcd.setCursor(3, 1);
    lcd.print("FEHLER.");

  } 
  else
  {
    lcd.setCursor(3, 1);
    lcd.print(summe);
    lcd.print("     ");
  }  
  delay(500);
}