Depending on the motion, they can ignore pets up to 100 lbs. Pet Immune motions will ignore animals up to 40 lbs. A 5 LB Yorkshire Terrier should not set off the motion. Motions are designed to pick bigger heat signatures (usually from large pets and humans). If your small dog (or cat) goes vertical (if they jump on furniture or climb on curtains), they may set it off the motion detector. It is a best practice to put the motions in an area that small pets cannot go vertical.