How to Use the RFID RC522: Reading Cards with Arduino and a Card-Operated Door Lock

RFID (Radio Frequency IDentification) is a technology that identifies a card or key fob by radio waves without touching the reader. Transit cards, office access cards, anti-theft tags in shops and pet microchips all use RFID. The RC522 is a cheap reader module working at 13.56 MHz, usually sold with a white card and a blue key fob. It's the basis of many projects, from card-operated door locks to attendance systems.

How does RFID work?

The copper antenna on the reader board creates a 13.56 MHz magnetic field. Inside the card or key fob there's a thin coil and a tiny chip, but no battery. When the card comes close to the field, energy induced in the coil powers the chip, and the chip sends its identity and data back to the reader by changing the field very slightly. That's why cards work for years without ever being charged, but with the RC522 the reading distance is only about 3–5 centimetres.

The cards in kits are usually MIFARE Classic 1K: its 1 KB of memory is split into 16 sectors, each made of 4 blocks of 16 bytes. Every card has a UID (unique identifier) written at the factory, usually 4 bytes long. Most simple projects recognise a card by reading this UID rather than writing data into the card.

RC522 specifications

RC522 wiring (SPI)

The RC522 talks to the board over SPI. The SCK, MOSI and MISO pins must go to each board's fixed SPI pins, while SDA (SS) and RST can go to any digital pin as long as the code matches. You can look up your board's SPI pins in the Pinout tool.

The MFRC522 library and reading a card's UID

In the Library Manager, search for MFRC522 and install the library by GithubCommunity (miguelbalboa). The code below prints the UID of every card you scan to the Serial Monitor in hexadecimal (HEX):

1#include <SPI.h>
2#include <MFRC522.h>
3
4const int SS_PIN = 10;
5const int RST_PIN = 9;
6
7MFRC522 rfid(SS_PIN, RST_PIN);
8
9void setup() {
10  Serial.begin(9600);
11  SPI.begin();
12  rfid.PCD_Init();
13  Serial.println("Kartı okutun / scan a card");
14}
15
16void loop() {
17  if (!rfid.PICC_IsNewCardPresent() || !rfid.PICC_ReadCardSerial()) return;  // kart yok / no card
18
19  Serial.print("UID:");
20  for (byte i = 0; i < rfid.uid.size; i++) {
21    Serial.print(rfid.uid.uidByte[i] < 0x10 ? " 0" : " ");
22    Serial.print(rfid.uid.uidByte[i], HEX);
23  }
24  Serial.println();
25
26  rfid.PICC_HaltA();        // kartı beklemeye al / halt the card
27  rfid.PCD_StopCrypto1();
28}

Checking for an authorised card

By comparing the UID you read with a value you've stored, you can decide whether the card is authorised. The example below checks a single authorised card; for several cards, just make the array two-dimensional and compare in a loop:

1// yukarıdaki tanımlar ve setup() aynen / definitions and setup() as above
2byte allowed[4] = {0xDE, 0xAD, 0xBE, 0xEF};   // kendi kartının UID'si / your card's UID
3
4bool isAllowed() {
5  if (rfid.uid.size != 4) return false;
6  for (byte i = 0; i < 4; i++) {
7    if (rfid.uid.uidByte[i] != allowed[i]) return false;
8  }
9  return true;
10}
11
12void loop() {
13  if (!rfid.PICC_IsNewCardPresent() || !rfid.PICC_ReadCardSerial()) return;
14  Serial.println(isAllowed() ? "İzin verildi / access granted" : "Reddedildi / access denied");
15  rfid.PICC_HaltA();
16  rfid.PCD_StopCrypto1();
17}

A servo-operated card door lock

Let's build a lock that turns a servo 90 degrees to open for 3 seconds when an authorised card is scanned, and gives a short warning beep from a buzzer for an unauthorised card. For servo wiring see What Is a Servo Motor?, and for the warning sound What Is a Buzzer? On the Uno the Servo library uses Timer1 and tone() uses Timer2, so the two work together without trouble.

1#include <SPI.h>
2#include <MFRC522.h>
3#include <Servo.h>
4
5MFRC522 rfid(10, 9);       // SS, RST
6Servo lock;
7const int BUZZER_PIN = 8;
8byte allowed[4] = {0xDE, 0xAD, 0xBE, 0xEF};
9
10void setup() {
11  SPI.begin();
12  rfid.PCD_Init();
13  lock.attach(6);
14  lock.write(0);                          // kilitli / locked
15}
16
17void loop() {
18  if (!rfid.PICC_IsNewCardPresent() || !rfid.PICC_ReadCardSerial()) return;
19  bool ok = rfid.uid.size == 4 && memcmp(rfid.uid.uidByte, allowed, 4) == 0;
20  if (ok) {
21    lock.write(90);                       // kilidi aç / unlock
22    delay(3000);
23    lock.write(0);                        // tekrar kilitle / lock again
24  } else {
25    tone(BUZZER_PIN, 400, 600);           // reddedildi sesi / denied beep
26  }
27  rfid.PICC_HaltA();
28  rfid.PCD_StopCrypto1();
29}

RC522 with the ESP32 and Raspberry Pi Pico

The same MFRC522 library runs on the ESP32; use the VSPI pins and declare the object as MFRC522 rfid(5, 22);. Because the ESP32 runs at 3.3 V it's a perfect match for the RC522, and thanks to Wi-Fi you can log scanned cards to the internet. On the Pico with MicroPython, copy an mfrc522.py file (for example danjperron's library) to the Pico and use it like this:

1from mfrc522 import MFRC522      # mfrc522.py Pico'da olmalı / mfrc522.py must be on the Pico
2import time
3
4reader = MFRC522(spi_id=0, sck=18, miso=16, mosi=19, cs=17, rst=20)
5
6while True:
7    reader.init()
8    stat, tag_type = reader.request(reader.REQIDL)
9    if stat == reader.OK:
10        stat, uid = reader.SelectTagSN()
11        if stat == reader.OK:
12            print("UID:", "".join("{:02X}".format(b) for b in uid))
13    time.sleep_ms(200)

Types of RFID: which cards does the RC522 read?

The RC522 only reads 13.56 MHz HF cards. The 125 kHz cards used at many building entrances (often marked "EM") can't be read by the RC522; they need a 125 kHz reader such as the RDM6300. NFC is a type of 13.56 MHz RFID that phones can read; the RC522 can read the UID of NTAG stickers.

RFID project ideas

• Card-operated door or drawer lock: a lock that opens with an authorised card and warns on an unauthorised one.
• Attendance system: match each card to a name and log the arrival time to an SD card or the internet.
• Welcome display: show "Welcome" and the person's name on an LCD when a card is scanned; for the display see Using an I2C LCD Display.
• Story box: assign a different sound file to each card and make an audio toy for children with a DFPlayer.
• Smart pet feeder: dispense food when the tag on a pet's collar is read.
• Card game: build a quiz or memory game with cards.

Common mistakes and how to fix them

• No card is read and the version shows 0x0: the SPI wires are on the wrong pins; check MOSI and MISO in particular.
• Connecting the module to 5V: the RC522 runs on 3.3 V; 5 V can destroy it.
• Trying to read a 125 kHz card: the RC522 can't see these cards; use a 13.56 MHz card.
• The card is too far away or on a metal surface: bring the card within 1–3 cm; metal surfaces disturb the field.
• The same card is read over and over: PICC_HaltA() isn't called; halt the card after reading it.
• Long wires: SPI signals degrade over long wires; keep them under 20 cm.

RFID answers the question "who are you?" with a single tap, which adds a lot to security and automation projects. In the BlueGrays app you'll find RFID value reading, LCD card reader and servo RFID door lock projects step by step with circuit diagrams and code.