The use of motion detectors goes back to ancient societies that developed agriculture. Modern motion detection of people and things can be traced back to the early decades of the 20th century, with many of the same principles still in use today.

Uses

Motion detectors have many more uses than the James Bond-like security systems we see in movies. Today's motion detectors can prevent serious accidents with industrial equipment by sensing when a person's arm gets too close to machinery. In everyday life, the average person uses a motion detector when they walk into a store with automatic doors. (See Resource 2)

Pre-1800s

The detection of motion finds its roots in astronomy, which goes back thousands of years. Early farmers looked to the heavens and used the movement of stars to determine when to plant crops and when to harvest them. (See Reference 1) The first motion detection system--radar--was pioneered by Heinrich Hertz. Hertz studied the properties of waves and found that waves could bounce off of objects and had different speeds. (See Reference 2)

World War II

World War II provided a perfect environment for the growth of motion detection technology: decades of study on the properties of waves and the need to track air and naval vessels. By the 1940s, radar technology was sufficiently advanced that the military could detect attacks in advance and guide aircraft. The ubiquitous use of radar would lead to other uses for motion detectors after the war. (See Reference 2)

First "True" Motion Detector

The first motion detector that acted as a burglar alarm was invented in the early 1950s by Samuel Bagno. Bagno applied the fundamentals of radar to ultrasonic waves, a frequency that humans cannot hear, to detect a thief or a fire. Bagno's motion detector also made use of the Doppler effect, the difference in the frequency of waves of a moving object, like a train sounding louder as it gets closer. (See Reference 3)

Modern Motion Detectors

Today's motion sensors work on some of the same basic principles that Samuel Bagno's motion detector employed. Microwave and infrared sensors still detect motion by distortions in the frequencies they emit. However, new motion detectors like microwave sensors can now be placed behind bookshelves and other barriers while still covering a wide radius. (See Resource 1)