Arduino and HC-05 hardware serial issues I have a software sending bitmap data to an Arduino Pro 5V 16MHz 328P that drives an LED display (sketch). This works using the USB-Serial adapter for the Arduino Pro with baud rates up to 500000 Baud. Now I wanted to switch to Bluetooth...
I tried connecting a 5V HC-05 module (set to 230400 Baud 8N1) to the hardware RX/TX pins on the Arduino Pro (Pins 0/1) and connect the PC to the Bluetooth serial port wirelessly (e.g. /dev/rfcomm0 on Linux). My problem is that I can receive data on the PC side using a terminal (the 'Ada' string is received), but nothing is displayed on the LED display. I have tried connecting an oscilloscope to the RX/TX pins on the Arduino and Data is arriving from the HC-05 module TX pin to the Arduino RX pin.
When the USB-Serial adapter is connected to the same pins (0/1 instead of the regular pin header) it still works as before, so there should be no connection issues.
What I tried was adding a 5kOhm resistor from the Arduino RX line to ground. (I found this hint somewhere here), but no dice...
I'm not sure what's wrong, because communication IS working over the USB-Serial adapter. Any ideas on what to try?!
A simple 10k/20k voltage divider is enough to bring the 5V arduino logic-level voltage down to ~3.3 for the HC-05.  It worked for me anyway.

The default baud rate for HC-05 is 9600 baud.  So unless you've changed this with AT- commands you will run into issues.  Although I read that some boards default to 38400 - best check your documentation.  All the cheap HC-05 boards I've tested default to 9600.