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("reaay on");
}
if(z=='b')
{
  digitalWrite(relay, HIGH);
  Serial.println("relay off");
}
}