How to get the UID of RFID in Arduino?

Question

I have a RFID-RC522 (MF-RC522) module and I'm using Arduino sketch program. To use this RFID, I downloaded the Arduino MFRC522 library.

And I run the example code of library.

Here is the code.

/*
 * MFRC522 - Library to use ARDUINO RFID MODULE KIT 13.56 MHZ WITH TAGS SPI W AND R BY COOQROBOT.
 * The library file MFRC522.h has a wealth of useful info. Please read it.
 * The functions are documented in MFRC522.cpp.
 *
 * Based on code Dr.Leong   ( WWW.B2CQSHOP.COM )
 * Created by Miguel Balboa (circuitito.com), Jan, 2012.
 * Rewritten by Søren Thing Andersen (access.thing.dk), fall of 2013 (Translation to English, refactored, comments, anti collision, cascade levels.)
 * Released into the public domain.
 *
 * Sample program showing how to read data from a PICC using a MFRC522 reader on the Arduino SPI interface.
 *----------------------------------------------------------------------------- empty_skull 
 * Aggiunti pin per arduino Mega
 * add pin configuration for arduino mega
 * http://mac86project.altervista.org/
 ----------------------------------------------------------------------------- Nicola Coppola
 * Pin layout should be as follows:
 * Signal     Pin              Pin               Pin
 *            Arduino Uno      Arduino Mega      MFRC522 board
 * ------------------------------------------------------------
 * Reset      9                5                 RST
 * SPI SS     10               53                SDA
 * SPI MOSI   11               51                MOSI
 * SPI MISO   12               50                MISO
 * SPI SCK    13               52                SCK
 *
 * The reader can be found on eBay for around 5 dollars. Search for "mf-rc522" on ebay.com. 
 */

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

#define SS_PIN 10
#define RST_PIN 9
MFRC522 mfrc522(SS_PIN, RST_PIN);   // Create MFRC522 instance.

void setup() {
    Serial.begin(9600); // Initialize serial communications with the PC
    SPI.begin();            // Init SPI bus
    mfrc522.PCD_Init(); // Init MFRC522 card
    Serial.println("Scan PICC to see UID and type...");
}

void loop() {
    // Look for new cards
    if ( ! mfrc522.PICC_IsNewCardPresent()) {
        return;
    }

    // Select one of the cards
    if ( ! mfrc522.PICC_ReadCardSerial()) {
        return;
    }

    // Dump debug info about the card. PICC_HaltA() is automatically called.
    mfrc522.PICC_DumpToSerial(&(mfrc522.uid));
}

When I run this code and put one of the card to RFID reader, a lot of information show on Serial monitor of Arduino IDE. Something like this... (I couldn't post the images)

//
Scan PICC to see UID and type...

Card UID : 84 90 6C A7

PICC type : MIFARE 1KB

Sector Block 0 1 2 3 4 5 6 7 ...

15     63   00 00 00 00 ...
//

But What I need is just the Card UID. This case is 84 90 6C A7.

Actually, I have project. I want to switch on LED if i put a certain Card of RFID. To do this, I need to read the Card UID and assign this to some variable on Arduino sketch program.

But I don't know how to get the UID of RFID tag in this case(This library and functions are complicated for me).

If anybody knows how to do this, please help me.

Answer 1

Here is a function which returns the UID.

/**
 * mfrc522.PICC_IsNewCardPresent() should be checked before 
 * @return the card UID
 */
unsigned long getID(){
  if ( ! mfrc522.PICC_ReadCardSerial()) { //Since a PICC placed get Serial and continue
    return -1;
  }
  unsigned long hex_num;
  hex_num =  mfrc522.uid.uidByte[0] << 24;
  hex_num += mfrc522.uid.uidByte[1] << 16;
  hex_num += mfrc522.uid.uidByte[2] <<  8;
  hex_num += mfrc522.uid.uidByte[3];
  mfrc522.PICC_HaltA(); // Stop reading
  return hex_num;
}

Use it like this:

if(mfrc522.PICC_IsNewCardPresent()) {
  unsigned long uid = getID();
  if(uid != -1){
    Serial.print("Card detected, UID: "); Serial.println(uid);
  }
}

Based on the answer by Thomas Matthews in a similar discussion.