A general question: How do I choose the correct diode? Or probably I have to say it differently: Can a diode be oversized? Of course I'm not talking about a 10K volt diode in a 3v3 Arduino sketch, but what about 50V 1A in this very application?
1N4148/1N914 small signal diodes should work fine, although one of the comments on
the Faludi site talks about using Schottky. Looking again at the webpage, I would rather
use a simple voltage divider to go from 5V down to 3.3V inputs. Then you don't need the
clamping diode.
2.4K series and 4.7K to gnd. You'll notice that this combination has an effective source-
resistance of 2.4K || 4.7K = just 1.6K, which greatly reduces rise-time problems versus
using 10K in Faludi's clamp ckt.