A great little sensor you can add to your Raspberry Pi projects is a PIR module. These 5V “Passive Infra Red” sensors are available for a few pounds from eBay. They can be powered from 5V and output 3V so can be connected directly to pins on the Pi’s GPIO header without any other components.
The module sets a single output pin high whenever it detects movement within its field of view. It holds this pin High (3.3V) for a minimum period of time. If continuous movement is detected the output pin will stay High. When the time has elapsed and no more movement is detected the output pin returns Low (0V).
I am currently using one in an alarm system and it works great for such a small and cheap device.
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. (read more)
Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.
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