PetSafe SmartDoor Plus Key SmartDoor Plus and Passport Pet Doors;

The PetSafe SmartDoor Plus Key works with the PetSafe SmartDoor Plus and Passport Pet Doors. They can be used for an additional pet using the door or as a replacement if your pet's SmartDoor Plus Key is lost. Worn on your pet's collar, the key uses radio-frequency identification (RFID) technology by signaling a unique code to the SmartDoor Plus or Passport door to operate as programmed for your pet. The SmartDoor Plus Key is about the size of a quarter and does not require batteries to operate. Each SmartDoor Plus and Passport door can program up to 20 SmartDoor Plus Keys.