Why do you have to use a voltage divider with HC-05 bluetooth module? (Arduino)
The output of the Arduino is a 0V - 5V logic level. The HC-05 needs a 0V - 3.3 volt logic level. The two resistors divide the 5V high level to create a voltage at the RX pin of $$2k / (2k+1k)* 5.0V$$ or 3.3 volts. The divider must be connected to ground in order to work. The Arduino will be able to read a 3.3v logic level correctly, so nothing needs to be done to the HC-05 output. See here for a discussion of voltage dividers.