user31741: I am using this code.. But everytime after one rfid tag is read by the reader, I have to manually press the reset button on the arduino before scanning the next tag. I want to do this operation after every 10 or 20 seconds from one tag scanning without manually pressing the reset button. What changes I have to make to this code??
/*
* source: http://www.electroschematics.com/11301/arduino-rfid-reader-rc522-access-control-system/
* Read a card using a mfrc522 reader on your SPI interface
* Pin layout should be as follows (on Arduino Uno):
* MOSI: Pin 11 / ICSP-4
* MISO: Pin 12 / ICSP-1
* SCK: Pin 13 / ISCP-3
* SS: Pin 10
* RST: Pin 9
*/

#include <SPI.h>
#include <RFID.h>
#include <Wire.h> 
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C address----------------------------------------------

#define SS_PIN 10
#define RST_PIN 9

RFID rfid(SS_PIN,RST_PIN);

int startAlarm = false;
int resetAlarm = 2;
int relay = 6;
int alarm = 8;

int serNum[5];

int cards[][5] = {
  {00,162,227,135,198} // card 1

};

bool access = false;

void setup()
{
  // Setup size of LCD 20 characters and 2 lines  
  lcd.begin(20,2);    //------------------------------------------------------
  // Back light on
  lcd.backlight();  //---------------------------------------------------------

  lcd.setCursor(0,0);         //----------------------------------------------
  lcd.print("SCAN your ID");   //------------------------------------------------

    Serial.begin(9600);
    SPI.begin();
    rfid.init();
    pinMode(resetAlarm, INPUT);
    pinMode(relay, OUTPUT);
    pinMode(alarm, OUTPUT);
    digitalWrite(relay, HIGH); // or LOW if you have a regular relay------------------------------(ALREADY CHANGED)
    attachInterrupt(0, reset_alarm, LOW);
}

void loop()
{

    if(rfid.isCard()){

        if(rfid.readCardSerial()){
            Serial.print(rfid.serNum[0]);
            Serial.print(",");
            Serial.print(rfid.serNum[1]);
            Serial.print(",");
            Serial.print(rfid.serNum[2]);
            Serial.print(",");
            Serial.print(rfid.serNum[3]);
            Serial.print(",");
            Serial.print(rfid.serNum[4]);
            Serial.println("");

            for(int x = 0; x < sizeof(cards); x++){
              for(int i = 0; i < sizeof(rfid.serNum); i++ ){
                  if(rfid.serNum[i] != cards[x][i]) {
                      access = false;
                      break;
                  } else {
                      access = true;
                  }
              }
              if(access) break;
            }

        }

       if(access){
          Serial.println("Welcome!"); 
          startAlarm = false;
          digitalWrite(relay, LOW); // HIGH with regular relay (I have already changed the code)  //------------------------------   
          lcd.setCursor(0,0);   //----------------------------------------------------------------------------------------------
          lcd.print("Access Granted");  //-------------------------------------------------------------------------------------------
          delay(3000);  
          lcd.clear();
          lcd.setCursor(0,0);         //----------------------------------------------
          lcd.print("SCAN your ID");   //------------------------------------------------
             //---------------------------------------------------------------
       } else {
           Serial.println("Not allowed!"); 
           startAlarm = true; 
           digitalWrite(relay, HIGH);  // LOW with regular relay (I have already changed the code)----------------------------------
           lcd.setCursor(0,0);         //------------------------------------------------------------------------------------
           lcd.print("Invalid Access ");        //------------------------------------------------------------------------------------
           delay(3000);  
           lcd.clear();                            //--------------------------------------------
           lcd.setCursor(0,0);         //----------------------------------------------
           lcd.print("SCAN your ID");   //------------------------------------------------

       }        
    }

    if(startAlarm) {
       digitalWrite(alarm, HIGH); 
    } else {
      digitalWrite(alarm, LOW); 
    }

    rfid.halt();

}

void reset_alarm(){
    startAlarm = false;
}


user31106: I would think the rfid library would be able to read multiple cards in a row... however I'm not familiar with it.
Here is an answer from a similar question about how to initiate a hardware reset with software using the watchdog timer - https://arduino.stackexchange.com/a/1478/31106 :

The SoftReset library makes it easy. Although it is not difficult to
  implement directly. Shown below..
#include <avr/wdt.h>
...
setup() {

  ...
  MCUSR = 0;  // clear out any flags of prior resets.
  ...

then when desired...
...
wdt_enable(WDTO_15MS); // turn on the WatchDog and don't stroke it.
for(;;) { 
  // do nothing and wait for the eventual...
} 
...



user10394: This may not be your only issue, but I think you are using sizeof() wrongly.  This 'function' returns the number of bytes occupied by a variable or type OR the number of bytes occupied by an array. 
So
1 == sizeof(char)
6 == sizeof ("Hello")  // Because of the trailing null
2 == sizeof(int)
int cards[][5] = {
                  {00,162,227,135,198} // card 1
                 };
10 == sizeof (cards)

So you line that says for(int x = 0; x < sizeof(cards); x++){ is happening 10 times, not once.
Personally I would never use sizeof to measure the size of an array, but if you have to then you need to do this:
size = sizeof (cards) / sizeof (int)


31741: <p>I am using this code.. But everytime after one rfid tag is read by the reader, I have to manually press the reset button on the arduino before scanning the next tag. I want to do this operation after every 10 or 20 seconds from one tag scanning without manually pressing the reset button. What changes I have to make to this code??</p>

<pre><code>/*
* source: http://www.electroschematics.com/11301/arduino-rfid-reader-rc522-access-control-system/
* Read a card using a mfrc522 reader on your SPI interface
* Pin layout should be as follows (on Arduino Uno):
* MOSI: Pin 11 / ICSP-4
* MISO: Pin 12 / ICSP-1
* SCK: Pin 13 / ISCP-3
* SS: Pin 10
* RST: Pin 9
*/

#include &lt;SPI.h&gt;
#include &lt;RFID.h&gt;
#include &lt;Wire.h&gt; 
#include &lt;LiquidCrystal_I2C.h&gt;

LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C address----------------------------------------------

#define SS_PIN 10
#define RST_PIN 9

RFID rfid(SS_PIN,RST_PIN);

int startAlarm = false;
int resetAlarm = 2;
int relay = 6;
int alarm = 8;

int serNum[5];

int cards[][5] = {
  {00,162,227,135,198} // card 1

};

bool access = false;

void setup()
{
  // Setup size of LCD 20 characters and 2 lines  
  lcd.begin(20,2);    //------------------------------------------------------
  // Back light on
  lcd.backlight();  //---------------------------------------------------------

  lcd.setCursor(0,0);         //----------------------------------------------
  lcd.print("SCAN your ID");   //------------------------------------------------

    Serial.begin(9600);
    SPI.begin();
    rfid.init();
    pinMode(resetAlarm, INPUT);
    pinMode(relay, OUTPUT);
    pinMode(alarm, OUTPUT);
    digitalWrite(relay, HIGH); // or LOW if you have a regular relay------------------------------(ALREADY CHANGED)
    attachInterrupt(0, reset_alarm, LOW);
}

void loop()
{

    if(rfid.isCard()){

        if(rfid.readCardSerial()){
            Serial.print(rfid.serNum[0]);
            Serial.print(",");
            Serial.print(rfid.serNum[1]);
            Serial.print(",");
            Serial.print(rfid.serNum[2]);
            Serial.print(",");
            Serial.print(rfid.serNum[3]);
            Serial.print(",");
            Serial.print(rfid.serNum[4]);
            Serial.println("");

            for(int x = 0; x &lt; sizeof(cards); x++){
              for(int i = 0; i &lt; sizeof(rfid.serNum); i++ ){
                  if(rfid.serNum[i] != cards[x][i]) {
                      access = false;
                      break;
                  } else {
                      access = true;
                  }
              }
              if(access) break;
            }

        }

       if(access){
          Serial.println("Welcome!"); 
          startAlarm = false;
          digitalWrite(relay, LOW); // HIGH with regular relay (I have already changed the code)  //------------------------------   
          lcd.setCursor(0,0);   //----------------------------------------------------------------------------------------------
          lcd.print("Access Granted");  //-------------------------------------------------------------------------------------------
          delay(3000);  
          lcd.clear();
          lcd.setCursor(0,0);         //----------------------------------------------
          lcd.print("SCAN your ID");   //------------------------------------------------
             //---------------------------------------------------------------
       } else {
           Serial.println("Not allowed!"); 
           startAlarm = true; 
           digitalWrite(relay, HIGH);  // LOW with regular relay (I have already changed the code)----------------------------------
           lcd.setCursor(0,0);         //------------------------------------------------------------------------------------
           lcd.print("Invalid Access ");        //------------------------------------------------------------------------------------
           delay(3000);  
           lcd.clear();                            //--------------------------------------------
           lcd.setCursor(0,0);         //----------------------------------------------
           lcd.print("SCAN your ID");   //------------------------------------------------

       }        
    }

    if(startAlarm) {
       digitalWrite(alarm, HIGH); 
    } else {
      digitalWrite(alarm, LOW); 
    }

    rfid.halt();

}

void reset_alarm(){
    startAlarm = false;
}
</code></pre>
 31106: <p>I would think the rfid library would be able to read multiple cards in a row... however I'm not familiar with it.</p>

<p>Here is an answer from a similar question about how to initiate a hardware reset with software using the watchdog timer - <a href="https://arduino.stackexchange.com/a/1478/31106">https://arduino.stackexchange.com/a/1478/31106</a> :</p>

<blockquote>
  <p>The SoftReset library makes it easy. Although it is not difficult to
  implement directly. Shown below..</p>

<pre><code>#include &lt;avr/wdt.h&gt;
...
setup() {

  ...
  MCUSR = 0;  // clear out any flags of prior resets.
  ...
</code></pre>
  
  <p>then when desired...</p>

<pre><code>...
wdt_enable(WDTO_15MS); // turn on the WatchDog and don't stroke it.
for(;;) { 
  // do nothing and wait for the eventual...
} 
...
</code></pre>
</blockquote>
 10394: <p>This may not be your only issue, but I think you are using <code>sizeof()</code> wrongly.  This 'function' returns the number of bytes occupied by a variable or type OR the number of bytes occupied by an array. </p>

<p>So</p>

<pre><code>1 == sizeof(char)
6 == sizeof ("Hello")  // Because of the trailing null
2 == sizeof(int)
int cards[][5] = {
                  {00,162,227,135,198} // card 1
                 };
10 == sizeof (cards)
</code></pre>

<p>So you line that says <code>for(int x = 0; x &lt; sizeof(cards); x++){</code> is happening 10 times, not once.</p>

<p>Personally I would never use sizeof to measure the size of an array, but if you have to then you need to do this:</p>

<pre><code>size = sizeof (cards) / sizeof (int)
</code></pre>
