int pir = 6;
int rel = 4;

int sw = 0, x;

void setup() {
  pinMode(pir, INPUT);
  pinMode(rel, OUTPUT);

  digitalWrite(rel, LOW);

}

void loop() {
  if (sw == 0) {
    if (digitalRead(pir) == HIGH) {
      sw = 1;
      x = millis();
      digitalWrite(rel, HIGH);
    }
  } else {
    if((millis()-x) > 300) {
      if(digitalRead(pir) == HIGH) {
        x = millis();
      } else {
        digitalWrite(rel, LOW);
        sw = 0;
      }
    }
  }
}