Amigos, estou em um mato sem cachorro. 

Não sei por que o código não funciona sem o serial monitor... Preciso de ajuda...

O objetivo é que toque um arquivo no MP3 shield quando uma tag RFID for identificada.

Funciona muito bem da primeira vez, mas nas próximas somente se abrir o serial  monitor.

Não sei o porque dele estar fazendo isso. 

Alguém pode me ajudar?

O código é:

#include <SPI.h>
#include <SdFat.h>
#include <SdFatUtil.h>
#include <SFEMP3Shield.h>
SdFat sd;
SFEMP3Shield MP3player;

int RFIDResetPin = 13;

//Register your RFID tags here
char tag1[13] = "6A003E4D5B42";
char tag2[13] = "6A003E8719CA";
char tag3[13] = "01023C013A04";
char tag4[13] = "01023101093A";
char tag5[13] = "01023C0A4376";
char tag6[13] = "01023C000E31";
char tag7[13] = "01023C0A3207";
char tag8[13] = "1A004116317C";
char tag9[13] = "1E009A81F9FC";
char tag10[13] = "1A004162261F";

void setup(){
Serial.begin(9600);

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

//ONLY NEEDED IF CONTROLING THESE PINS - EG. LEDs
pinMode(2, OUTPUT);
pinMode(3, OUTPUT);
pinMode(4, OUTPUT);
pinMode(5, OUTPUT);
pinMode(6, OUTPUT);
pinMode(7, OUTPUT);
pinMode(8, OUTPUT);
pinMode(9, OUTPUT);
pinMode(10, OUTPUT);
pinMode(11, OUTPUT);
}

void loop(){

char tagString[13];
int index = 0;
boolean reading = false;

while(Serial.available()){

int readByte = Serial.read(); //read next available byte

if(readByte == 2) reading = true; //begining of tag
if(readByte == 3) reading = false; //end of tag

if(reading && readByte != 2 && readByte != 10 && readByte != 13){
//store the tag
tagString[index] = readByte;
index ++;
}
}

checkTag(tagString); //Check if it is a match
clearTag(tagString); //Clear the char of all value
resetReader(); //eset the RFID reader
}

void checkTag(char tag[]){
///////////////////////////////////
//Check the read tag against known tags
///////////////////////////////////

if(strlen(tag) == 0) return; //empty, no need to contunue

if(compareTag(tag, tag1)){ // if matched tag1, do this
teste();

}else if(compareTag(tag, tag2)){ //if matched tag2, do this
teste_2();

}else if(compareTag(tag, tag3)){
lightLED(4);

}else if(compareTag(tag, tag4)){
lightLED(5);

}else if(compareTag(tag, tag5)){
lightLED(6);

}else if(compareTag(tag, tag6)){
lightLED(7);

}else if(compareTag(tag, tag7)){
lightLED(8);

}else if(compareTag(tag, tag8)){
lightLED(9);

}else if(compareTag(tag, tag9)){
lightLED(10);

}else if(compareTag(tag, tag10)){
lightLED(11);

}else{
Serial.println(tag); //read out any unknown tag
}

}

void lightLED(int pin){
///////////////////////////////////
//Turn on LED on pin "pin" for 250ms
///////////////////////////////////
Serial.println(pin);

digitalWrite(pin, HIGH);
delay(250);
digitalWrite(pin, LOW);
}

void resetReader(){
///////////////////////////////////
//Reset the RFID reader to read again.
///////////////////////////////////
digitalWrite(RFIDResetPin, LOW);
digitalWrite(RFIDResetPin, HIGH);
delay(150);
}

void clearTag(char one[]){
///////////////////////////////////
//clear the char array by filling with null - ASCII 0
//Will think same tag has been read otherwise
///////////////////////////////////
for(int i = 0; i < strlen(one); i++){
one[i] = 0;
}
}

boolean compareTag(char one[], char two[]){
///////////////////////////////////
//compare two value to see if same,
//strcmp not working 100% so we do this
///////////////////////////////////

if(strlen(one) == 0) return false; //empty

for(int i = 0; i < 12; i++){
if(one[i] != two[i]) return false;
}

return true; //no mismatches
}

void teste () {
Serial.begin(9600);
//start the shield
sd.begin(SD_SEL, SPI_HALF_SPEED);
MP3player.begin();
//start playing track 1
MP3player.playTrack(1);
}
void teste_2 () {
Serial.begin(9600);
//start the shield
sd.begin(SD_SEL, SPI_HALF_SPEED);
MP3player.begin();
//start playing track 1
MP3player.playTrack(2);
}

Desde já agradeço.