Many of the tutorials I've seen for using the HC05 with Arduinos use a resistor based potential divider to reduce the logic level at it's RX pin, but not it's TX pin, stating that HC05's Tx level is high enough for the Arduino.
Great!
I'm using a Sparkfun logic level shifter and currently have two unused H/L pairs.
Is there any reason not to use these respectively for the Tx/Rx?
HIGH from the HC05 will be ~3.3V so within bounds to be seen as HIGH by a 5V device sharing common GND and within pins supported voltage. That’s why you don’t need to boost that signal (but boosting it to 5V won’t hurt).
The opposite is an issue, sending a 5V HIGH into a pin that can only accept ~3.3V will destroy it. That’s why you need to adjust Arduino Tx ➜ HC05 Rx
Some HC05 modules have the necessary hardware to handle 5V signals though - so you might want to double check what your module requires
A logic level shifter will do as well as the resistor divider, you can use that if voltage adaptation is required
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