Keep Cats out of Doggie Door

I'm having a problem. We have a small inside dog that goes in and out through a doggie door to our fenced in back yard. We since adopted 2 cats planned to be outside. They learned our swinging doggie door really quickly. I purchased an electronic doggie door and put a collar on the dog so it will open when he walks up to it. Now they know to wait for him and when they hear the door open they run in right in front of him. Is there any way to keep the cats out and still let the dog go in and out. I'm allergic to cats and don't want a litter box in the house. Please help!!!!

Oh dear sorry to laugh but that tickled me. Clever puss!!