wemos-d1-mini-esp8266-rfid-rc522.ino

/**

Useful links
https://wiki.wemos.cc/products:d1:d1_mini
https://cdn-images-1.medium.com/max/1400/1*YKc8KpAfMrlhrOLmNjdRwQ.png (D1 full pinout)
https://github.com/Jorgen-VikingGod/ESP8266-MFRC522
https://github.com/miguelbalboa/rfid

d1 mini rc52 wiring
https://discourse-cdn-sjc1.com/business5/uploads/mydevices/original/2X/e/ecedba79dc05f2c0b02b7fba8b3da2681590a11a.jpg
RST  - D3
MISO - D6
MOSI - D7
SCK  - D5
SDA  - D8
*/

#include "ESP8266WiFi.h"
#include <SPI.h>
#include <MFRC522.h>

constexpr uint8_t RST_PIN =  0;          // Configurable, see typical pin layout above 18
constexpr uint8_t SS_PIN =  15;         // Configurable, see typical pin layout above  16

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

void setup() {
  Serial.begin(74880);   // Initialize serial communications with the PC
  delay(1000);
  Serial.println("Setup");
  
  while (!Serial);    // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
  SPI.begin();      // Init SPI bus
  mfrc522.PCD_Init();   // Init MFRC522
  mfrc522.PCD_DumpVersionToSerial();  // Show details of PCD - MFRC522 Card Reader details
  Serial.println(F("Scan PICC to see UID, SAK, type, and data blocks..."));
  Serial.println("Setup done");
}

void loop() {
//  Serial.println("Loop...");
  
  // Look for new cards
  if ( ! mfrc522.PICC_IsNewCardPresent()) {
    //delay(50);
    return;
  }

  // Select one of the cards
  if ( ! mfrc522.PICC_ReadCardSerial()) {
    //delay(50);
    return;
  }

  // Show some details of the PICC (that is: the tag/card)
  Serial.print(F("Card UID:"));
  dump_byte_array(mfrc522.uid.uidByte, mfrc522.uid.size);
  Serial.println();
  
  // Dump debug info about the card; PICC_HaltA() is automatically called
  //mfrc522.PICC_DumpToSerial(&(mfrc522.uid));

  delay(1000);
}

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

Thank you!! This is the only sketch I could get to work!

Thank you!! This is the only sketch I could get to work!

You welcome :-) I took me time too to find working solution

I'm at a loss... I've tried 3 different boards (ESP32, ESP8266, and D1 MINI 8266) with 2 different IDEs (Arduino and PlatformIO) and a number of different pinouts. I either end up with a boot loop or a communication error such as this:
Firmware Version: 0xFF = (unknown)
WARNING: Communication failure, is the MFRC522 properly connected?
Scan PICC to see UID, SAK, type, and data blocks...
Setup done

I've literally copied and pasted your code into both IDEs and checked and rechecked the pins: Communication Failure.

Any help would be greatly appreciated.

@ShaolinGardener I am seeing a similar things too. Have you solved your issue? :-)

Sorry guys, I'm really not doing these projects anymore and can not help there.
I remember I had issues with cheap ESP boards and MFRC522, so I usually order 2 from different sellers.

Firmware Version: 0xFF = (unknown)
WARNING: Communication failure, is the MFRC522 properly connected?
Scan PICC to see UID, SAK, type, and data blocks...
Setup done

I had the same error message.

-> Solved when I switched pins MISO and MOSI.