Motion sensors have been around since the mid-1800s. Astronomers in the 19th century used infrared light to detect the movement of stars and planets, but this was generally its only use. The invention of radar led to modern day motion sensing devices. Samuel Bagno used the principles of radar detection to create the ultrasonic motion alarm.
Microwave sensors work by continuously monitoring the frequency of the microwaves a sensor emits and detecting when the frequency changes. Should an intruder come into the microwave sensor's field, he causes a disruption in the expected frequency and the alarm sounds off. While these type of motion detectors can be hidden behind walls and obstacles, they are also sensitive to electronics and lightning.
Infrared motion detectors emit infrared light, a part of the light spectrum that the human eye cannot see. Infrared sensors work by calibrating the heat of a known object, usually a wall and then comparing that to any thing that moves within it's radius. Infrared detectors are more common and reliable than microwave sensors, but do not cover as much area. The sensor emits a pattern best compared to a hand stretching out.
Sensors that detect vibration can easily be made at home, but store bought ones use a more complex system. The commercially sold "Piezo" method uses a small piece of film that vibrates to create a charge, which then triggers a switch, sounding an alarm. The homemade method uses a small mass on a lever, which activates a switch to an alarm when it vibrates.
Motion detectors are not just a way to keep intruders out, they can keep a child from breaking curfew. In industry, motion detectors can keep track of the number of products that pass through an assembly line. A motion sensor can also be used to shut down dangerous equipment when a person gets too close. They can also turn on lights when a person enters a room.