We stock many low cost "Passive IR" Presence / Motion sensors, and they're very popular for detecting motion from humanoids and pets 5 to 20 feet away. (They possibly work on zombies, not guaranteed, depends on how warm they are). The vast majority of PIR sensors use a chunky infrared receiver, a processing chip (sometimes integrated into the receiver) and a lens. However, this makes them kinda chunky. Also most have a digital, or maybe analog signal output - which makes them very simple.