Here’s a complete example of how to trigger a relay using RFID-RC522 and Arduino Nano when a specific RFID card is detected.

Hardware Needed

• Arduino Nano
• RFID-RC522 module (MFRC522)
• Relay module (5V)
• Jumper wires
• RFID card/tag

Wiring Diagram

RFID-RC522 → Arduino Nano (SPI)

| RC522 Pin | Arduino Nano Pin    |
| --------- | ------------------- |
| SDA (SS)  | D10                 |
| SCK       | D13                 |
| MOSI      | D11                 |
| MISO      | D12                 |
| IRQ       | Not connected       |
| GND       | GND                 |
| RST       | D9                  |
| 3.3V      | 3.3V ⚠️ (IMPORTANT) |

Important:

Relay → Arduino Nano

Install Library

In Arduino IDE:

• Sketch → Include Library → Manage Libraries
• Search: MFRC522
• Install: MFRC522 by Miguel Balboa

Library: MFRC522

Arduino Code (Trigger Relay When Card Matches)

#include 
#include 

#define SS_PIN 10
#define RST_PIN 9
#define RELAY_PIN 3

MFRC522 mfrc522(SS_PIN, RST_PIN);

// Replace with your card UID
byte authorizedUID[4] = {0xDE, 0xAD, 0xBE, 0xEF};

void setup() {
  Serial.begin(9600);
  SPI.begin();
  mfrc522.PCD_Init();

  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, LOW);

  Serial.println("Scan your RFID card...");
}

void loop() {

  // Look for new card
  if (!mfrc522.PICC_IsNewCardPresent()) {
    return;
  }

  // Select card
  if (!mfrc522.PICC_ReadCardSerial()) {
    return;
  }

  Serial.print("UID tag: ");
  
  bool authorized = true;

  for (byte i = 0; i < 4; i++) {
    Serial.print(mfrc522.uid.uidByte[i], HEX);
    Serial.print(" ");
    
    if (mfrc522.uid.uidByte[i] != authorizedUID[i]) {
      authorized = false;
    }
  }

  Serial.println();

  if (authorized) {
    Serial.println("Access Granted!");
    
    digitalWrite(RELAY_PIN, HIGH);  // Relay ON
    delay(3000);                    // 3 seconds
    digitalWrite(RELAY_PIN, LOW);   // Relay OFF
  } else {
    Serial.println("Access Denied!");
  }

  mfrc522.PICC_HaltA();
}

How to Get Your Card UID

Upload this simple code first:

#include 
#include 

#define SS_PIN 10
#define RST_PIN 9

MFRC522 mfrc522(SS_PIN, RST_PIN);

void setup() {
  Serial.begin(9600);
  SPI.begin();
  mfrc522.PCD_Init();
  Serial.println("Scan card...");
}

void loop() {
  if (!mfrc522.PICC_IsNewCardPresent()) return;
  if (!mfrc522.PICC_ReadCardSerial()) return;

  Serial.print("UID: ");
  for (byte i = 0; i < mfrc522.uid.size; i++) {
    Serial.print(mfrc522.uid.uidByte[i], HEX);
    Serial.print(" ");
  }
  Serial.println();
}

Open Serial Monitor, scan card, then replace:

byte authorizedUID[4] = {0xXX, 0xXX, 0xXX, 0xXX};

How It Works

• RC522 reads RFID card UID
• Arduino compares UID with authorized UID
• If match → relay turns ON
• If not → relay stays OFF