const int LEDred=23;
int i=1;
void setup() {
  Serial.begin(9600); 
  pinMode(LEDred,OUTPUT);
while(i==1){
  if (Serial.available() > 0) {
    char command = Serial.read(); 
   if (command == 'S') {
     // Turn LED on
      digitalWrite(LEDred, HIGH);
      i=i+1;
    } 
    else {   
      Serial.println("try again");
      delay(10);
    } 
  }
}
}

void loop() {
   digitalWrite(LEDred, HIGH);
   delay(1000);
   digitalWrite(LEDred, LOW);
   delay(1000);
}