Replace a SmartKey™ or add more pets to your PetSafe® Electronic SmartDoor™. The SmartKey™ fits most collar ID tags and uses RFID technology to send a signal that locks and unlocks the door flap when it senses your pet. You can add up to 5 pets to your SmartDoor™. Package includes 1 SmartKey™ and 1 battery (RFA-67). PetSafe® brand is here to help you and your pet live happy together®.