#define led 21
#define btn 2

int btnState = 0;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(btn, INPUT_PULLUP);
  pinMode(led, OUTPUT);
}

void loop() {
  btnState = digitalRead(btn);

  if(btnState==LOW){
    digitalWrite(led, HIGH);
  }
  else{
    digitalWrite(led, LOW);
  }
}