Not obvious the third pin is required for the capacitive touch keyer.
The dit and dah Nano input pins could also have open-drain outputs,
so drive the pin low and then release it to slowly float back up.
Read from the pin after it is released to see how long it takes
the pullup resistor to get the pin high again.
There should be plenty of web hits on how to do a bidirectional open-drain pin under Arduino.
Likely implemented as a tristate output, the output is enabled when actively driving the pin low.?
