Arduino RFID and Processing Connection


I’m currently working on a project where I want to switch the colors of a square in processing through using rfid chips with arduino. So everytime I place a rfid-chip on my rfid-reader, arduino should send the data to processing, and depending on the card-number the color of the square in processing should change.
The connection is working and I cannot switch the colors of the square in processing. I think the problem lays in the processing code.
I use the Arduino Elegoo Uno R3 and the joy-it RFID Reader.
Would love to get some help, how to solve this problem!

Here is the Processing-Code

import processing.serial.*;

Serial myPort;         // The serial port  
String finalRead = ""; // Variable with final data  
String inputRead = ""; // Variable that recieves the data  
int state = 0;         // Variable to store the program state  

void setup() {

  size(800, 800); //Size of the program window
  background(50, 50, 150); //Background colour

  // List all the available serial ports

  // Open the port you are using at the rate you want:
  myPort = new Serial(this, Serial.list()[4], 9600);

  // The rectangle will be placed based on it's own center


void draw() {

  /*When the serial port is available the data from the RFID is 
    loaded into the finalRead variable.*/

  while (myPort.available() > 0)
    //The c variable is updated with the read method

    char c=(char);

      When processing recieves data from Arduinos println command 
      it is recieved with a \r in the beginning
      and a c\ in the end. 
      Therefore we add c to inputRead when 
      we are not encountering \r.

      If we encounter \n we know that the string is done reading. 
      The string is loaded into the finalRead variable and 
      the data is cleared from inputRead.

    if (c=='\n')
      finalRead = inputRead;
      inputRead = ""; 
      println("data: " + finalRead);
    } else if (c!='\r')

   /*Now the program checks if the data is equal to the data
   on a wanted RFID-chip. If thats the case the state of the   
   program changes. Here you need to fill in data from a specific 

  if (finalRead.equals("7320574163")) {
    state = 1;

  /*Another RFID-chip if-statement with another state. You can do this with as many RFID-chips you want.*/

  if (finalRead.equals("21110723902")) {
    state = 2;

  /*If the state is changed to 1, the colour of the center Rectangle 
    is changed to red.*/

  if (state == 1) {

  /*If the state is change to 2, the colour of the center Rectangle is changed to green.*/
  if (state == 2) {

  /*The program now draws a rectangle with a fill colour dependent on the state variable*/
  rect(width/2, height/2, width-100, height-100);

here is the Arduino-Code

 * ----------------------------------------------------------------------
 * Example program showing how to read new NUID from a PICC to serial.
 * ----------------------------------------------------------------------
 * RC522 Interfacing with NodeMCU
 Typical pin layout used:
 * ---------------------------------------------------------------------
 *             MFRC522      Arduino       Arduino   Arduino    Arduino          Arduino
 *             Reader/PCD   Uno/101       Mega      Nano v3    Leonardo/Micro   Pro Micro
 * Signal      Pin          Pin           Pin       Pin        Pin              Pin
 * ----------------------------------------------------------------------
 * RST/Reset   RST          9             5         D9         RESET/ICSP-5     RST
 * SPI SS      SDA(SS)      10            53        D10        10               10
 * SPI MOSI    MOSI         11 / ICSP-4   51        D11        ICSP-4           16
 * SPI MISO    MISO         12 / ICSP-1   50        D12        ICSP-1           14
 * SPI SCK     SCK          13 / ICSP-3   52        D13        ICSP-3           15

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

constexpr uint8_t RST_PIN = 9;     // Configurable, see typical pin layout above
constexpr uint8_t SS_PIN = 10;     // Configurable, see typical pin layout above
MFRC522 rfid(SS_PIN, RST_PIN); // Instance of the class

MFRC522::MIFARE_Key key; 

// Init array that will store new NUID 
byte nuidPICC[4];

void setup() { 
  SPI.begin(); // Init SPI bus
  rfid.PCD_Init(); // Init MFRC522 

  for (byte i = 0; i < 6; i++) {
    key.keyByte[i] = 0xFF;

  Serial.println(F("This code scan the MIFARE Classsic NUID."));
  Serial.print(F("Using the following key:"));
  printHex(key.keyByte, MFRC522::MF_KEY_SIZE);
void loop() {

  // Look for new cards
  if ( ! rfid.PICC_IsNewCardPresent())

  // Verify if the NUID has been readed
  if ( ! rfid.PICC_ReadCardSerial())

  Serial.print(F("PICC type: "));
  MFRC522::PICC_Type piccType = rfid.PICC_GetType(rfid.uid.sak);

  // Check is the PICC of Classic MIFARE type
  if (piccType != MFRC522::PICC_TYPE_MIFARE_MINI &&  
    piccType != MFRC522::PICC_TYPE_MIFARE_1K &&
    piccType != MFRC522::PICC_TYPE_MIFARE_4K) {
    Serial.println(F("Your tag is not of type MIFARE Classic."));

  if (rfid.uid.uidByte[0] != nuidPICC[0] || 
    rfid.uid.uidByte[1] != nuidPICC[1] || 
    rfid.uid.uidByte[2] != nuidPICC[2] || 
    rfid.uid.uidByte[3] != nuidPICC[3] ) {
    Serial.println(F("A new card has been detected."));

    // Store NUID into nuidPICC array
    for (byte i = 0; i < 4; i++) {
      nuidPICC[i] = rfid.uid.uidByte[i];
    Serial.println(F("The NUID tag is:"));
    Serial.print(F("In hex: "));
    printHex(rfid.uid.uidByte, rfid.uid.size);
    Serial.print(F("In dec: "));
    printDec(rfid.uid.uidByte, rfid.uid.size);
  else Serial.println(F("Card read previously."));

  // Halt PICC

  // Stop encryption on PCD

 * Helper routine to dump a byte array as hex values to Serial. 
void printHex(byte *buffer, byte bufferSize) {
  for (byte i = 0; i < bufferSize; i++) {
    Serial.print(buffer[i] < 0x10 ? " 0" : " ");
    Serial.print(buffer[i], HEX);

 * Helper routine to dump a byte array as dec values to Serial.
void printDec(byte *buffer, byte bufferSize) {
  for (byte i = 0; i < bufferSize; i++) {
    Serial.print(buffer[i] < 0x10 ? " 0" : " ");
    Serial.print(buffer[i], DEC);


Your Processing Code is flashing with this simple test code from Arduino:

void setup() 
void loop() 

To get you started:

Most of those Serial.print() statements are intended for the Arduino serial monitor for development; only send the required data to Processing for processing.

Do not use the Arduino serial monitor or plotter when making a serial connection to Processing or other software\devices when using a single COM port.

Edited comment about delay()
I always add a delay(1000) at end of setup() in Processing to allow the Arduino to reset before receiving data. Watch the lights on the Arduino to get familiar with these.
This may not apply to this example.

There are many serial examples in this forum; start simple and build on that.





I added the delay as you requested, but unfortunately I have still the same problem :/.
Would love to hear other tips :smiley:


Did the code example I provided for Arduino work with your Processing code?
You should see output similar to what I have.

The delay() was related to this:

I edited my last post on this.


Yes I can see the number of the rfid chip in the compiler of processing. But the square is not changing the colors. When I scan the rfid chip the color of the square in processing should change depending on the rfid chip number. Any ideas why that is? :smile: #beingveryhelpful

Try background in draw function


When I run the Arduino example I provided and your Processing code the rectangle changes color from black to white as expected and repeats every 1000 ms.

Can you share a screen grab of what you are seeing on the serial monitor of the Arduino IDE when you start Arduino and scan a card?


1 Like

I’m glad it works from your side!
Yeah of course,I can send you a screenshot, here is what I see:

I wish it was that easy, but that’s not working neither

Here is one photo with the processing screen on


Your Processing code is looking for this:

  if (finalRead.equals("7320574163")) {
    state = 1;

You are sending this:


That is why it is not working.

I previously made this suggestion:

You have to decide what to do:

  • Modify your Processing code to look for the String you are sending with the Arduino code you are using as is:
    "data: 49 CD 4A A3"
  • Modify your Arduino code to send a String to match your Processing code.

The data seems to be correct:

The example I provided here sends the correctly formatted String data and Processing is receiving it and it working as expected.

I kept it simple as this is your project.

Give this some more thought.
Try to get it working as is.
Clean it up later.



Thank you so so much, it is finally working!!! :heart_eyes:


I don’t know if your Processing code is using the printed output from that printDec function, but it’s confused. Clearly printHex prints the numbers 0…255 as 2 digit hex values with leading zero where required. printDec is a copy of printHex with HEX changed to DEC, but without rethinking the requirement and logic for leading zeroes. The result is this:

00 01 02 03 04 05 06 07 08 09 010 011 012 013 014 015 16 17 18 19 … 99 100 101 … 255