void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("welcome to 1st exp on LED!");
  pinMode(26, OUTPUT);
  pinMode(21, INPUT_PULLUP);
}

void loop() {
  int k=digitalRead(21);
  Serial.println(k);
  if(k==1)
  {
    digitalWrite(26,1);
  }
  else
  {
    digitalWrite(26,0);
    delay(3000);
  }
  Serial.println(k);
}