On 2020-07-15, R.Wieser  wrote:
> The thing is that currently I cannot find *any* information on what the
> JY-MCU breakout board does or doesn't do (like does it already have such a
> series resistor in place ?) - or even what its STATE and EN pins are
> supposed to be there for. Which is the whole problem.

https://www.moore-duino.co.uk/product/hc05-hc-05-bluetooth-serial-module-breako
ut-board-jy-mcu/

STATE Mode the device is in           from a photo is seems to imply
                                      this is an output from the device
                                      indicating the state
RXD         Receive
TXD         Transmit
GND         0V
VCC         5V
EN         Disables module when LOW   this is fairly explicit.
                                      Pull low to disable the module



The photo shows RX/TX at 3.3V logic levels. Even it has a safety
resistor putting another one will very probalby be ok - and safe!
Or buy a little dual 3.3V  5V level shifter to use between the
device and the MCU. If you are interfacing to the RPI then all is ok as
they are 3.3V logic. (this is the RPI NG!).

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