const char start_char = '$';
const char stop_char = ';';

String serial_buff = "";
byte enable_rec = 0;


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

void loop(){
  if(Serial.available()){
    char c = Serial.read();

    if(c == start_char){
      if(enable_rec == 0) enable_rec = 1;
      else if(enable_rec == 1) serial_buff = "";   //missing stop char
    }
    else if(c == stop_char){
      process_rec_data();   //use received command
      enable_rec = 0;   //disable receiving
      serial_buff = "";   //clear buffer
    }
    else{
      serial_buff += c;
    }
  }
}



void process_rec_data(){
  Serial.println("Received command:>" + serial_buff + "<");
  if(serial_buff == "on"){
    //turn x on
  }
  else if(serial_buff == "off"){
    //turn x off
  }
}

void sendCommand(String command){
  Serial.print(start_char + command + stop_char);
}