RFID Access Reader

// RFID Access Reader -- 13.56 MHz SM130 in Auto Mode, ASCII output + CR/LF
//
// The module is configured (template 02/AUTO-B-P-ASCII-CRLF) to print each
// card's UID as plain ASCII the moment the card enters the field. So this
// sketch just reads a line of text and compares it -- no binary framing,
// no checksums, nothing to get subtly wrong.
//
// Wiring:  SM130 TX -> D2   SM130 RX -> D3   relay control -> D7
// SM130 UART is 5V tolerant, so no level converter is needed on an Uno.

#include <SoftwareSerial.h>

const int  RFID_RX_PIN = 2;      // Arduino receives  <- SM130 TX
const int  RFID_TX_PIN = 3;      // Arduino transmits -> SM130 RX
const long RFID_BAUD   = 19200;  // stdMifare factory default

const int  RELAY_PIN   = 7;
const unsigned long UNLOCK_MS = 3000;   // how long the strike stays released

SoftwareSerial rfid(RFID_RX_PIN, RFID_TX_PIN);

// Identifiers that may open the door. Read these off your own cards in
// step 4 -- these two are placeholders and will not match anything.
const char *ALLOWED[] = {
  "A1B2C3D4",
  "11223344"
};
const uint8_t ALLOWED_COUNT = sizeof(ALLOWED) / sizeof(ALLOWED[0]);

char    uid[24];
uint8_t idx = 0;

bool isAllowed(const char *candidate) {
  for (uint8_t i = 0; i < ALLOWED_COUNT; i++) {
    if (strcasecmp(candidate, ALLOWED[i]) == 0) return true;
  }
  return false;
}

void unlock() {
  digitalWrite(RELAY_PIN, HIGH);
  delay(UNLOCK_MS);
  digitalWrite(RELAY_PIN, LOW);
}

void handleUid(const char *u) {
  Serial.print(F("card: "));
  Serial.print(u);

  if (isAllowed(u)) {
    Serial.println(F("  -> GRANTED"));
    unlock();
  } else {
    Serial.println(F("  -> denied"));
  }
}

void setup() {
  Serial.begin(9600);      // local monitor over USB
  rfid.begin(RFID_BAUD);   // the reader module

  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, LOW);   // fail secure: door locked on power-up

  Serial.println(F("reader ready -- present a card"));
}

void loop() {
  while (rfid.available()) {
    char c = rfid.read();

    if (c == '\r' || c == '\n') {      // end of one reported card
      if (idx > 0) {
        uid[idx] = '\0';
        handleUid(uid);
        idx = 0;
      }
    } else if (idx < sizeof(uid) - 1) {
      uid[idx++] = c;
    } else {
      idx = 0;   // overlong line: discard rather than overrun the buffer
    }
  }
}