I have a product called scarecrow that does what you describe and works fairly well as a deer deterrent. It has a sensitivity control. At most sensitive it triggers at about 30 ft. But it also is triggered on windy days by leaves moving. This is not a bad thing except it runs on a 9v battery. The battery runs out faster the more it triggers. The charge lasts about 2 weeks on average. So I got a rechegeable battery. I wish it had a mode where it would go off randomly, maybe every 30-90 seconds. I may try to do this with a fan and some of those dancing man puppets.