The design for the Arduino PLC shield is coming together now.
I’ve removed the “flywheel” diodes from the relays as the ULN2803 already has them. I’ve added a simple +5V power supply and changed the Arduino footprint to the Seeedstudio clone so we get an extra two analog inputs. The standard Adrunio Duemilanove will work as well but you won’t get the extra two analog inputs. I don’t think the TLP521 can sink sufficient current to illuminate the input LED’s so I might added some transistors to do that or just remove the input LED’s completely.
The updated schematic is here: