#include <SoftwareSerial.h>
SoftwareSerial serial_relay(0, 1); //tx, rx

const int relay = 3;
 
void setup() {
  serial_relay.begin(9600);
  pinMode(relay, OUTPUT); //relay = output
}
 
void loop(){
  digitalWrite(relay, HIGH); // relay on
  delay(5000); // wait 5 sec.
  digitalWrite(relay, LOW); // relay off
  delay(5000); // wait 5 sec. 
}