Build an RFID Door Lock with Arduino

Tap the right card and the door unlocks; tap a wrong card and it stays locked. This RFID project teaches SPI communication and is a favourite for final-year and school projects.

1. Wiring (RC522 runs on 3.3V!)

• SDA → D10, SCK → D13, MOSI → D11, MISO → D12, RST → D9
• 3.3V → 3.3V (not 5V), GND → GND
• Servo signal → D3, green LED → D4, red LED → D5

2. Find Your Card ID

Install the MFRC522 library, upload the code, open Serial Monitor at 9600 and tap your card. Copy the printed ID into allowed.

#include <SPI.h>
#include <MFRC522.h>
#include <Servo.h>

MFRC522 rfid(10, 9);
Servo lock;
String allowed = "A1 B2 C3 D4";  // replace with your card ID

void setup() {
  Serial.begin(9600);
  SPI.begin(); rfid.PCD_Init();
  lock.attach(3); lock.write(0);
  pinMode(4, OUTPUT); pinMode(5, OUTPUT);
}

void loop() {
  if (!rfid.PICC_IsNewCardPresent() || !rfid.PICC_ReadCardSerial()) return;
  String id = "";
  for (byte i = 0; i < rfid.uid.size; i++) {
    if (rfid.uid.uidByte[i] < 0x10) id += "0";
    id += String(rfid.uid.uidByte[i], HEX);
    if (i < rfid.uid.size - 1) id += " ";
  }
  id.toUpperCase();
  Serial.println(id);
  if (id == allowed) {
    digitalWrite(4, HIGH); lock.write(90);  // unlock
    delay(3000);
    lock.write(0); digitalWrite(4, LOW);    // lock again
  } else {
    digitalWrite(5, HIGH); delay(1000); digitalWrite(5, LOW);
  }
  rfid.PICC_HaltA();
}

Add a DS3231 RTC later to log the time of each entry.