On 2020-07-15, R.Wieser  wrote:
> Dennis,
>
>> If so, the board is optimized for 5V signals (though that page
>> says 4-6V for VCC).
>
> I would think so too, but I've found descriptions using different, but
> rather similar break-out boards, with some of those descriptions
> specifically mentioning 3.3v for the KEY pin, and using a resistor divider
> to drop the 5v signal to a 3v one for the RXD pin.
>
> But the below tutorial directly connects the KEY and RXD input pins to the
> Arduinos output pins (supposedly being driven at 5v).
>
>
https://www.instructables.com/id/How-to-Set-AT-Command-Mode-for-HC-05-Bluetooth
-Mod/
>

Some 3.3v logic interfaces are 5v tolerant. You need to see the
device datasheet for that. Often a resistor (I've seen 1Kohm mentioned)
directly in the signal line is enough to prevent damage. AFAIK it uses
the, often, inbuilt powerline protection diodes - but I'm sure a more
comtetant HW guy could spell chapter and verse.

I think there was a discussion on this NG earlier this year about
interfacing 5v to the Pi - utilising a 1K resistor for protection - or
maybe it was anoth NG or Forum.

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