#include <SPI.h> // preinstalled
#include <MFRC522.h> // need to be installed

//MFRC522        Arduino UNO
//----------------------------
//SDA    ─────▶  D10  (Handled internally by the MFRC522 library)
//SCK    ─────▶  D13  (configured automatically by SPI.begin())
//MOSI   ─────▶  D11  (configured automatically by SPI.begin())
//MISO   ─────▶  D12  (configured automatically by SPI.begin())
//IRQ   (not connected)
//GND    ─────▶  GND
//RST    ─────▶  D9   (Handled internally by the MFRC522 library)
//VCC    ─────▶  3.3V

#define SS_PIN 10
#define RST_PIN 9
MFRC522 rfid(SS_PIN, RST_PIN);

void setup() {
  Serial.begin(9600);
  SPI.begin();
  rfid.PCD_Init();
  Serial.println("Scan an RFID card...");
}

void loop() {
  if (!rfid.PICC_IsNewCardPresent()) return;
  if (!rfid.PICC_ReadCardSerial()) return;

  Serial.print("Card UID (decimal): ");

  for (byte i = 0; i < rfid.uid.size; i++) {
    Serial.print(rfid.uid.uidByte[i], DEC);
    Serial.print(" ");
  }
  Serial.println();

  rfid.PICC_HaltA();
  delay(1000);
}