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Hmm, yes you may be right that in some cases the 7805 output hasn't
dropped sufficiently to represent a digital LOW.

Another way to circumvent this situation might be to define A0 as an
analog input and define a higher threshold for a digital LOW/HIGH. What do
you think?

73 Allard PE1NWL

On Wed, June 14, 2017 18:07, Jerry Gaffke via Groups.Io wrote:
> The code is doing a digitalRead(PTT_SENSE), I'd expect the threshold on
> the nano to be somewhere between 1 and 2 volts
> The problem is that when the +12v TX line into the LM78L05 at U3 goes to
> zero volts, the output of U3 remains up around 5v because there is no
> place for the charge on C150 and C151 to go. ??The pulldown resistor will
> eventually discharge those caps. ??The RC time constant of 0.2uF*20k = 4
> milliseconds, which should be fast enough. ??I'd put that new 10k resistor
> from U3-1 to ground, rather than from A0 to ground, the latter case would
> reduce the maximum voltage into the Nano A0 pin down from 5v to 2.5v.
> ??This also reduces the RC time constant to 2 milliseconds.
> ??
>
> On Wed, Jun 14, 2017 at 08:28 am, Jack Purdum wrote:
>
>>
>> Given your code, Allard, I'm surprised he needs a dropping resistor on
>> that line.
>> ??
>>
>