The Bluetooth module wants 3.3v logic levels so that is taken into account. The image above shows a buffer chip doing the conversion, but the Fritzing schematic on the post uses a voltage divider.

The HC-05 is one of a series of castellated modules (HC-06, -07, etc). the JY-MCU is a breakout board for whatever version of module happens to be soldered onto it. Report comment Reply ...