PetSafe SmartDoor Plus Key SmartDoor Plus and Passport Pet Doors Works with the PetSafe SmartDoor Plus and Passport Pet Doors for an additional pet to use the pet door or as a replacement if your pet's key is lost Each key uses RFID technology to send a unique signal to the pet door to operate as programmed for each pet Up to 20 SmartDoor Plus Key accessories can be programmed to the SmartDoor Plus and Passport Pet Doors Water-resistant for adventurous pets Includes 2 split rings to attach key to your pet's collar Keys are the size of a quarter with no batteries needed