RFID reader with an I2C LCD on an Arduino Uno

// An RFID reader that shows each card's ID on an LCD.
//
//   RC522 SDA (SS) -> pin 10   SCK -> pin 13   MOSI -> pin 11   MISO -> pin 12
//   RC522 RST -> pin 9   3.3V -> 3.3V   GND -> GND
//   I2C LCD GND -> GND   VCC -> 5V   SDA -> A4   SCL -> A5
//
// Every card carries a serial number, its UID. The reader talks SPI on pins
// 11, 12 and 13 and the display I2C on A4 and A5, so the two never share a
// wire. The RC522 runs on 3.3 V: never wire it to 5 V.
//
// Click the reader and pick a card to hold to it.

#include <SPI.h>
#include <MFRC522.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int SS_PIN = 10;
const int RST_PIN = 9;

MFRC522 reader(SS_PIN, RST_PIN);
LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  Serial.begin(9600);
  SPI.begin();
  reader.PCD_Init();
  lcd.init();
  lcd.backlight();
  lcd.print("Hold a card");
  Serial.println("Mokxi Uno: RC522 on SS 10 and RST 9, LCD on A4 and A5");
}

void loop() {
  if (!reader.PICC_IsNewCardPresent() || !reader.PICC_ReadCardSerial()) return;
  lcd.clear();
  lcd.print("UID:");
  lcd.setCursor(0, 1);
  Serial.print("Card UID:");
  for (byte i = 0; i < reader.uid.size && i < 5; i++) {
    if (reader.uid.uidByte[i] < 0x10) lcd.print("0");
    lcd.print(reader.uid.uidByte[i], HEX);
    lcd.print(" ");
    Serial.print(reader.uid.uidByte[i] < 0x10 ? " 0" : " ");
    Serial.print(reader.uid.uidByte[i], HEX);
  }
  Serial.println();
  reader.PICC_HaltA();
  reader.PCD_StopCrypto1();
  delay(1000);
}