Last Halloween, radiologist George Poulos decided to make his yard extra scary. His plan: Every time a pedestrian passed by, a projector would shine a ghostly animation on a plexiglass pane to make it look like a real ghoul had appeared. It was too dark to rely on a traditional motion sensor, so Poulos used a $10 passive infrared (PIR) one. Every object warmer than absolute zero emits infrared rays, and as one source of radiation passes in front of another, a PIR sensor detects the difference. DIYers can use PIR sensors day or night to automate security cameras, open doors for pets, help trigger Nerf sentry guns—and terrify trick-or-treaters.