#define RELAY_PIN 25  // Connect to IN pin of relay

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
}

void loop() {
  digitalWrite(RELAY_PIN, HIGH); // Turn ON relay
  delay(1000);                   // 1 second
  digitalWrite(RELAY_PIN, LOW);  // Turn OFF relay
  delay(1000);                   // 1 second
}