This just popped up today on an Arduino mailing list.

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Every now and then I have some need to crank out a one-off DCC receiver
for some project I’m working on, and I always think, “Sheesh, one of these
days I just need to lay out an Arduino shield that’s got the opto on it
so I can just stack it and a stock Arduino and get the job done.” Then I
go back to whatever I was doing and it never happens…

Well, it never happened until tonight. I’m stuck at O’Hare this week
babysitting a software/hardware beta test and it’s been a pretty quiet
week. So I figured I’d fire up the ol’ EDA tools and finally crank one
out to alleviate boredom tonight. I thought I’d get the group’s feedback
to see if I was missing anything obvious, or there’s any other
features/tweaks would like me to throw in.

Key points:

• While I know many/most of you are working with the Minis, I tend to
  prefer the big ones for doing quick off prototypes. So full size shield
  for me…
• Can route the DCC signal to D2, D3, or D7, which should take care of
  pretty much every available configuration today (Unos, Leos, Megas, Dues,
  etc.) and give you options to work around interrupt conflicts.
• DCC signal is referenced to IOREF, so should work with 3.3V and 5V
  Arduinos
• Onboard I2C jack
• Brings some digital I/O out to a terminal block so you don’t have to
  throw in another shield just to get terminals.
• Capability to self-power off the DCC signal if desired or be isolated,
  depending on jumper settings.

Schematic:
http://ndholmes.com/ard-dccshield/ard-dccshield.pdf

BoM:
http://ndholmes.com/ard-dccshield/ard-dccshield-bom.pdf

Visualization of PCB top:
http://ndholmes.com/ard-dccshield/ard-dccshield-top.png

Full Gerber stackup, if you want more detail:
http://ndholmes.com/ard-dccshield/ard-dccshield-pcb.pdf

GitHub repo, for those who want direct access to the design files:
https://github.com/IowaScaledEngineering/ard-dccshield