HC-05 Bluetooth Module, AT Command is not working

[sterretje]

Maybe

@atgumus44

  if (Serial.available())         
    Serial.write(Serial.read()); 

should be

  if (Serial.available())         
    BTSerial.write(Serial.read()); 

@atgumus44

I didn't use any resistor.

It depends on which Arduino you use. If it's a 5V one (e.g. Uno, Nano, Mega), you're living on the edge and risk damaging the bluetooth module (it might be a slow process).

Your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advice on) your project See About the IDE 1.x category.