An automatic pet door is like a private key for your pet to come and go out of your home whenever it wants. They are more secure than traditional pet doors because they actually close and lock when your dog or cat isn't going in or out, and they're also more energy efficient because they form tight seals around the edges, unlike the non-automatic types.
There are a couple different types of automatic pet doors. Chances are there is one that will fit you and Fido's (or Fluffy's) lifestyle and budget.
Remote Operated Automatic Pet Doors
The remote control is typically mounted on a collar and runs on tiny watch batteries. Like all pet doors, you can find them in just about any size. For example, the one in the illustration above is great for cats and small dogs (and only costs about $95 at Amazon, if you're interested).
They also come in larger sizes, like the one in the photo below; it's built for larger sized dogs, but can also accommodate smaller pets, too (this one is a bit more heavy duty and costs about $325 at Amazon).
The drawback is that your pet will inadvertently open and close the door as it walks by, even if it's not intending to use it. Kind of like when you walk through a public restroom with light sensors on the sinks and they all start spraying at once. It might not seem like a big deal, but it will get on your nerves eventually, especially considering you'll be hearing the built-in motor pull the door up and down.
Timer Operated Pet Doors
These doors open and close at times you select via an electronic timer. You decide what times you want the door to open (and stay open), and when you want it to shut again. These won't open and close at your pet's convenience unlike remote operated ones, but they are very popular for households with so many pets that outfitting them with collars is impractical, and for instances where keeping foreign animals out of the house isn't a big problem.
The downside to timer pet doors is that the door opens and closes at your schedule, not your pet's. If you dog or cat really needs to get outside when the door is shut, it won't make it, at least not without you letting it out the old fashioned way (opening the door).
But the upside is that you won't hear that constant opening and closing of the door motor, which gets annoying really fast, especially if space is limited and your pet walks in and out of range of the sensor even if it's not using the door.
Electronic Sensor Pet Doors
These work similarly to the doors at your local supermarket. An electronic eye senses when your pet is close to the door and automatically opens. When it no longer detects anything, it closes. It's a solid system that's worked for years in human buildings.
This is kind of an "in between" system that combines the good parts of remote control pet doors and the timer ones. It doesn't stay open and closed at hard intervals like a timer door would, but it doesn't require remote controls that can lose battery power and fail, or get very expensive if you're trying to outfit multiple pets.
Where To Buy An Automatic Pet Door
Most pet stores like Petco and Petsmart will carry these in their dog or cat isles, and sometimes you'll even find them at home improvement stores like Home Depot or Lowe's. Or you could shop online at mega retailers like Amazon or find a used one (or even an unused new one) on eBay or Craigslist.
Keep in mind that buying the door is only half of the battle, because you'll need to install it, too. If you're handy, it shouldn't be too much of a problem to cut through your solid core exterior door and properly install one of these pet doors, but if DIY isn't your fancy you'll either need to hire a handy man contractor or even purchase from a store that offers installation (usually at a price, so be prepared).
At the end of the day, an automatic pet door is a very wise decision for any pet owner, once you have one you'll wonder how you ever got by without it.