Dennis,

> The actual module (the sub-board soldered to the break-out) is 3.3V...

I wonder about that.   The PDF I got here mentions "1.8V low-power
operation, 1.8 to 3.6V I/O".  Its possible that the 3.3v supply voltage is
only needed when the I/O should be 3.3v ...

> But that one on Amazon whose schematic I keep referencing has
> a transistor cascade (shown just below that diode) which seems
> to use UART_TXD (from the module) to the first transistor to pull
> down a 3.3V pull-up feeding the second transistor, and that second
> appears to be used to pull down a 5V pull-up on the "TX_TTL"
> output.

The module I have has only a single three-legger (the other one I
mistakingly mentioned is a two-legger, probably a diode - one side very
close to VCC), and is probably the voltage regulator.    So, no way
something active happens with any of the signal lines.

> So... back to oscilloscope on the TX and ground, and hope the
> device defaults to High

Thats a bit of a problem with the output lines : no way to "wiggle" it a bit
by using breakout board pins and see which HC-05 module "pin" moves with it.
:-\

I thought of something, and attached my scope to the DMV in resistance
measuring mode.  What-do-you-know, only in the two lowest positions (diode
test and 200 ohm) the voltage over the probes is 3v.  As soon as I switch to
2K ohm the voltage drops to 250 mili-volt.  Not enough to hurt anything.
Putting a 1K resistor over it drops the voltage to below 100 mili-volt.

IOW, it looks like we've (or at least I've) been carefull for no particular
reason. :-)

Don't like to have to cut away the transparent heatschrink tube protecting
the module though ...  I do not even have heatshrink tube of that size
laying about, transparent or not.

Regards,
Rudy Wieser

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