Pet immune motion detectors disregard the motion caused by animals weighing less than 80 pounds (in many instances). Most security system motion detectors use passive infrared (PIR) motion detector technology to monitor heat changes in a designated area.
With a traditional motion sensor, any motion in the coverage area sets off the sensor. Pets end up triggering false alarms as a result.
Pet immune motion detectors ignore the motion caused by animals and thus reduce the likelihood of a false alarm. Features vary by manufacturer and model, but can include selectable sensitivity for pets up to 40 pounds or those weighing up to 80 pounds and can be set to disregard one large pet, like a full-grown dog or several small pets, like cats.
Where you place the motion sensor can also impact its pet-friendliness. Aiming the sensor toward the top half of the room means any motion near the floor is always out of sight.