Put the cartridge back together and insert it into the Catgenie.
The Arduino should get power from the Catgenie and boot the program we uploaded to it earlier. It will flash its LEDs several times, and respond to the button press as it did when we were testing it using USB earlier.
The obvious fault with this design is that the Catgenie no longer dispenses any cleaning solution. I will work through the options for this over the next few weeks and will update this Instructable when I've found something that works.
It would be great to see how other people get on with this. If anything needs to be explained in more detail or if I've made any stoopid errors please let me know.
It would also be good to hear from anyone who can get the Catgenie working with plain EEPROM chips, or can get the Arduino to behave as the cartridge. Its a mystery to me why those approaches didn't work out.