Cat Door Bell
Pet owners know well how tiring it is to constantly have to be getting up to let the pet in and then back out again. And this is particularly vexing with a cat who might want to make that trip in and out of the house dozens of times each day. That is why a lot of homeowners create one or more cat doors in inconspicuous places so the kitty can get used to coming and going as she pleases. And once the household cat gets used to that kind of freedom, the days of her "nagging" you to open the back door to let her in or back out again are over.
Another alternative to a door that the cat can use at will is to create a cat version of a doorbell. Once the cat gets the idea that she can press that button and get someone's attention that can be a big help for her to be able to summon help on demand. Now, when you first hear about doorbells for the cat, it seems absurd that a domestic feline would be able to operate a machine like that. But domestic animals are amazingly smart and they learn quickly what works and what doesn't work. So once the little cat comes to understand that pressing that doorbell well gain her entrance, she will come to rely on it the way we do on many of our common household appliances.
The doorbell option takes care of one of the biggest problems with cat doors and that is the fact that if you have a cat sized door with open access to the outdoors, any creature of that size can also wander into the house. And there are many clever creatures such as raccoons, possums and squirrels that not only can learn how to work a cat door but have plenty of motivation to spend the night in your kitchen. Many pet doors are little more than a square opening with plastic or thick leather canvas flaps that the cat can push through easily. So just as easily as Kitty can get through them, so can any industrious wildlife that becomes aware of the opportunity.
The doorbell concept relies on the higher intelligence of the household feline. But the problem that a cat door addresses is not solved by a doorbell because you still have to run to that door each time the cat rings the alarm that she wants to be let in or put out. This can be especially aggravating in the middle of the night because none of us like being roused from a deep sleep just to take care of the cat.
A combination approach is a good answer. Many cat door designs allow the cat to activate an electronic lock and allows the door to swing open just long enough for the pet to pass through. Instead of soft flaps, the door uses panels that swing open on hinges and then swing shut locked until the button is pressed again. Using this design, there is no real "bell" to disturb you. The relay that opens the door replaces the bell, which turns the control of the door back over to kitty. But because of higher technology, the door is not as accessible to woodland creatures to operate. This is a good combination of technologies that solves the problem of letting the cat in and out without adding more work on you.