PIR (motion) sensor
PIR sensors are used to detect motion from pets/humanoids from about 20 feet away (possibly works on zombies, not guaranteed). This one has an adjustable delay before firing (approx 0.3-18 seconds), adjustable sensitivity and we include a 1 foot (30 cm) cable with a socket so you can easily reposition the sensor or mount it using the two drills on either side
Runs on 5V-16V power (if you need to run it off of 3V you can do that by bypassing the regulator, but that means doing a bit of soldering). Digital signal output is 3.3V high/low. Sensing range is about 7 meters (120 degree cone)
For a full tutorial with wiring diagrams, code examples and project ideas, Adafruits: PIR sensor tutorial page!