int led = 25; 
int aTime = 0; 

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

void loop() {
  int nowTime = millis();

  if (nowTime - aTime >= 1000) { // lon hon 1s
    digitalWrite(led, !digitalRead(led));

    aTime = nowTime; // cap nhat time truoc
  }

}