About this Article
Written by: Tim Furnish
Written on: September 3rd, 2007
Tags: electrical engineering, energy & sustainability
Thumbnail by: Charles Beichman/CalTech
About the Author
In the fall of 2007, Timothy Allen Furnish was a mechanical engineering student at the University of Southern California Viterbi School of Engineering. His interests in engineering include learning new ways to solve mechanical problems and learning about new technology.
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Volume IX Issue IV > Motion Sensors

Motion Sensors and How They Work

Radar Sensors

Radar sensors are much like the sensors used by Bango's ultrasonic alarm system. They work by "radiating electromagnetic energy and detecting the echo returned from reflecting objects" [5]. The electromagnetic energy, which is in wave-form, is sent into the environment by a transmitter. Once the waves reach an object, they reflect off the object and are sent back in the opposite direction; this is known as an echo. A receiver is then used to collect the data from the echo and can assess whether a change has been made to the environment [5] Radar sensors are known as active sensors because they emit energy in order to detect a change [6].
Georg Wiora/Wikimedia Commons
Figure 2: How Motion Sensors Work.

Infrared Sensors

Unlike radar sensors, infrared sensors are passive; this means that instead of having to emit energy to detect changes, they are capable of detecting radiation emitted by other objects, such as thermal energy from human beings [6] (see Fig. 2). Infrared sensors are made from different materials that sense different ranges of infrared wavelengths. When an appropriate wavelength of infrared radiation strikes one of the material's cells, it changes the cell's resistance. By measuring the resistance of the cell, one can measure its infrared radiation [7]. Since many different objects, both living and inert, emit a certain level of infrared radiation, it is convenient for the sensors to detect rapid changes in infrared radiation instead of a particular wavelength [8]. This way, a human being can still be detected through the slow change in atmospheric temperature. When a sensor detects a rapid change, a device is triggered to either start or stop working. One example is a motion sensor porch light, in which the sensor detects a sudden change in infrared radiation and triggers the light to turn on.