const int relayPin=2;
int comdata=0;
void setup() 
{
pinMode(relayPin,OUTPUT);
Serial.begin(9600);
while(!Serial);
Serial.println("Please input your command to control the relay:");
}
void loop()
{
if(Serial.available())
{ comdata=Serial.read();
        if(comdata==48)
        {
        digitalWrite(relayPin, HIGH);
        Serial.println("light is off");
        }
        else if(comdata==49)
        {
        digitalWrite(relayPin, LOW);
        Serial.println("light is on");
        }
  }
}