int relay = 13;
void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(relay, OUTPUT);
  digitalWrite(relay, HIGH);
}
void loop() {
  // put your main code here, to run repeatedly:
  int z = Serial.read();
  if (z == 'a')
  {
    digitalWrite(relay, LOW);
    Serial.println("RELAY ON");
  }
  if (z == 'b')
  {
    digitalWrite(relay, HIGH);
    Serial.println("RELAY OFF");
  }
}