int LED = 7;
int btn = 5;

void setup() {
  // put your setup code here, to run once:
   pinMode(LED, OUTPUT);
   pinMode(btn, INPUT_PULLUP);
   digitalWrite(LED, LOW);
}

void loop() {
  // put your main code here, to run repeatedly:
   if(digitalRead(btn) == LOW){
    digitalWrite(LED, HIGH);
    delay(3000);
    digitalWrite(LED, LOW);
    delay(500);
    digitalWrite(LED, HIGH);
    delay(3000);
    digitalWrite(LED, LOW);
   }
}