Why bother with isolation? DC coupled will be fine.
Use a voltage divider and do the de-bounce/filtering in software.
A simple signal diode from input to Vcc will protect your uP as the Vcc is low
imperance.
--+--- +5v
l
l
-------
/ \
/__\ 1N4148
l
l
+---------+ l
12v source >-----l 13k l---+------------> input
input +---------+ l to uP
l
l
+-----+
l l
l 10k l
l l
+-----+
l
l
l
--+----- GND
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