const int LEDred=23;
void setup() {
  Serial.begin(9600); 
  pinMode(LEDred,OUTPUT);
}

void loop() {
  if (Serial.available() > 0) {
    char command = Serial.read(); 
   if (command == 'S') {
     // Turn LED on
      digitalWrite(LEDred, HIGH);
    } 
    else if (command == 'END') {
  
      // Turn LED off
      digitalWrite(LEDred, LOW);
    }
    else if 
      ((command != 'E') || (command != 'S')){      
      Serial.println("try again");
      delay(10);
    } 
  }
}