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/

> A datasheet for the chip itself says 3.3V VCC
> http://www.electronicaestudio.com/docs/istd016A.pdf

:-) I found the same PDF.   And if the HC-05 module would be all that I had
(not on a break-out board) my problems, other than how to attach wires to
it, would not exist.

The whole thing is that I do not know what the JY-MCU breakout-board the
HC-05 module is placed ontop of does for me.    Having discovered that
multiple *unspecified* versions of that breakout-board circulate doesn't
help either. :-(

On my JY-MCU break-out board I can see 6 (assumed to be) resistors, one LED,
one three-legged component that I assume is the voltage regulator, and
another three-legger which I haven't got the foggiest about (perhaps a
transistor or FET driving the LED ?).

> This page
>
https://maker.pro/custom/tutorial/hc-05-bluetooth-transceiver-module-datasheet-
highlights
> states all are 3.3V except for VCC. And if the unit has on-board
> regulator, there is typically some losses involved, so one needs 4+V
> to get 3.3V out.

Have you looked at that breakout-board image just above it ?    It doesn't
seem to have /any/ components on the break-out board itself ...    Either
all of them are on the other, bottom side or this shown module isn't quite
representative.

> """
> This pin is used to toggle between Data Mode (set low) and AT command
> mode (set high). By default it is in Data mode
> """
> Though the tutorial link states that EN pulled LOW during power-up is
> needed to enter command mode???

:-D Yep, I found similar.    Which one of the two should I believe ?

Ofcourse, its easy enough to test for my particular module (just send an
"AT",CR,LF and see what comes back), but that should /not/ be needed. :-(

(its possible that the above-mentioned three-legger is a transistor or FET
allowing the EN pin signal to rise above 3.3v - but inverting that signal in
the process)

And I see we are bumping against the same inconclusive information. :-)
No matter, thanks for the response.

Regards,
Rudy Wieser

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