char option = ' ' ; 

void setup() {
  // put your setup code here, to run once:
  Serial.end();
  Serial.begin(115200);
  pinMode(12, OUTPUT);
  pinMode(23, OUTPUT);
}

void loop() {
  option =Serial.read();
  Serial.println(option);

  if(option == 'a'){
    digitalWrite(12, HIGH);
    delay(500);
    digitalWrite(12, LOW);
    delay(500); 
  }
else if(option == 'b'){
    digitalWrite(23, HIGH);
    delay(500);
    digitalWrite(23, LOW);
    delay(500); 
  }
 else{
    digitalWrite(12, HIGH);
    digitalWrite(23, HIGH);
    delay(500);
    digitalWrite(12, LOW); 
    digitalWrite(23, LOW);
    delay(500);
 } 

}