char IncomingCommand[10];   // for incoming serial data
int ledPin = 10;   // select the pin for the LED
int val = 6000;	 // variable to store the data upper limit

void setup() {
  pinMode(ledPin,OUTPUT);    // declare the LED's pin as output

  Serial.begin(9600);     // opens serial port, sets data rate to 9600 bps
}

void loop () {
  // send data only when you receive data: 83 84 65 82 84
  if (Serial.readBytesUntil(char(13), IncomingCommand, Serial.available()) > 0) {
    if (IncomingCommand == "start") {
      for(int i=0; i<val; i++) {
        digitalWrite(ledPin,HIGH);
        delay(150);
        digitalWrite(ledPin, LOW);
        delay(150);
      }
    }
  }
}