Having cats and dogs under one roof may be a problem some times, because they like to sneak around and share each other’s food. So the only solution is to make some kind of a barrier so dog would be kept away from cat’s food.

Setup I based on AVR microcontroller and it uses an infrared trip-wire system that distinguishes dogs from cats. In this case dog is bigger than cats, so it is pretty easier to recognize when dog passes. System reject short interrupts of beam as it supposes to be cat’s tail. Anyway what’s left is to find an annoying sound to play during alarm so dog would hate it and run away. Simple, cheap and effective!