I'm reading RFID cards' UID with an ESP8266 and trying to send that information to my web server hosted on a Raspberry Pi. I'm currently storing the UID in an int array, but the http.POST(); function takes a string input. I tried adding each UID digit together, but that doesn't work either.

How would you go about sending the uid in the request?

My code:

    Created by TheCircuit

#define SS_PIN 4  //D2
#define RST_PIN 5 //D1

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

#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>
#include <WiFiClient.h>

const char* ssid = "****";
const char* password = "****";
const char* serverName = "****";
int RELAY = 2; //D6

int UIDs[1][4] = {
  {83, 111, 39, 47}

MFRC522 mfrc522(SS_PIN, RST_PIN);   // Create MFRC522 instance.

void setup()
  Serial.begin(9600);   // Initiate a serial communication
  SPI.begin();      // Initiate  SPI bus
  mfrc522.PCD_Init();   // Initiate MFRC522

  pinMode(RELAY, OUTPUT);
  digitalWrite(RELAY, HIGH);

  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
  Serial.print("Connected to WiFi network with IP Address: ");

boolean compareUIDs(int a[4], int b[4]) {
  for (int i = 0; i < 4; i++) {
    if (a[i] != b[i]) return false;

  return true;

void loop()
  // Look for new cards
  if ( ! mfrc522.PICC_IsNewCardPresent())
  // Select one of the cards
  if ( ! mfrc522.PICC_ReadCardSerial())

  // mfrc522.PICC_DumpToSerial(&(mfrc522.uid));

  int UID[4];
  for (int i = 0; i < 4; i++) {
    UID[i] = mfrc522.uid.uidByte[i];

  Serial.print(" ");
  Serial.print(" ");
  Serial.print(" ");

  for (int i = 0; i < sizeof(UIDs); i++) {
    boolean success = compareUIDs(UIDs[i], UID);
    if (success) {
      Serial.println("ACCESS GRANTED");
      digitalWrite(RELAY, LOW);

      if (WiFi.status() == WL_CONNECTED) {
        HTTPClient http;

        // Your Domain name with URL path or IP address with path

        // Specify content-type header
        http.addHeader("Content-Type", "application/json");
        int httpResponseCode = http.POST("UID GOES HERE");

        Serial.print("HTTP Response code: ");

        // Free resources

      digitalWrite(RELAY, HIGH);
  • 1
    You encode it in some string format that the website at the remote end is expecting. Either as a HTTP parameter list, or as a JSON array string, or whatever is needed.
    – Majenko
    Commented May 14, 2021 at 12:28
  • 1
    @Majenko could you give me a link or short explanation to get started on that one? Commented May 14, 2021 at 12:45
  • It looks like you've already decided on (or had imposed on you) JSON. So the simplest method is to use ArduinoJSON (arduinojson.org) to create the JSON string for you.
    – Majenko
    Commented May 14, 2021 at 12:47
  • my StreamLib has CStringBuilder to print to string same way as you print to Serial
    – Juraj
    Commented May 14, 2021 at 13:37
  • you could send it as an integer ... that UID in your code is 536f272f
    – jsotola
    Commented May 14, 2021 at 15:06

1 Answer 1


You could use the (awesome) ArduinoJSON library, but likely overkill for this app. I found it more useful to parse JSON than to construct it.

You could simply print to a buffer and the POST that:

char payload[MAX_PAYLOAD];
sprintf(payload, "{\"uid1\":%d,\"uid2\":%d,\"uid3\":%d,\"uid4\":%d}", UID[0], UID[1], UID[2], UID[3]);


You would #define MAX_PAYLOAD with a value that will be sufficient to hold all the information.

Notice that sprintf allows you to use any formatting options you want (e.g. hexadecimal, etc).

If you wanted to send it as an array:

sprintf(payload, "{\"uid\":[%d,%d,%d,%d]}", UID[0], UID[1], UID[2], UID[3]);

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