Turner Cariker of 1st Alarm Abilene Tx (20 years experience) Great question, 1st Alarm’s PIR (pet immune) motion detector will not pick up motion from any pets under 55 lb. If your puppy is going to grow up to be larger than 55 lb. Then we will need to send one of our technician’s to your home and adjust to motion detector or give you other options to secure your home. Any pet under the weigh of 55 lb.. will not trigger the motion detector.