Introduction:

A motion detector is a device that contains a motion sensor that detects moving objects and transforms the detection of motion into an electric signal

It is often integrated as a component of a system that automatically performs a task or alerts a user of motion in an area. Motion detectors form a vital component of security, automated lighting control, home control, energy efficiency, and other useful systems

Motion Sensors Types

There are two basic kinds of motion detector sensors, active and passive sensors. They are categorized based on how they detect motion. Active sensors sends out bursts of energy ultrasonic sound waves, basic radar signal or beam light. Ultrasonic sound waves being the most popular of the three. Passive sensors don't send out energy, instead it reads the changes in the energy, using a predetermined baseline.

Active Sensors

Active sensors work by sending out bursts of ultrasonic sound waves, after which the sensor waits for the energy to be reflected back. Good example of an active motion sensor is and automatic door opener. If there is nobody in the immediate area the waves will return in the same patter in which they were released. If someone or something disturbs the pattern, the sensor sends an alarm signal in the event of disturbance in the pattern. The disturbed pattern triggers the sensor and opens the door.

Figure 1. How motion motion sensors work.

Passive Sensros

They are also known as passive infrared sensors (or PIR sensors) because they detect and measure incoming infrared energy. They are sometimes also called pyroelectric detectors. Any body, including animals and humans, will emit infrared energy because it creates heat. This energy emitted by humans and animals depending on the temperature of the body but in humans, it's usually between 9 and 10 micrometers. Most passive infrared sensors can actually detect emissions in the range of 8 to 12 micrometers. They do this with use of a photo detector. The photo detector coverts light in these wavelengths into an electrical current, which is run through a tiny computer housed in the unit. The alarm is triggered when the photo detector detects large or fast variations in the distribution of the emitted infrared energy. Normal movement in humans will naturally create such variations. Smaller variations are ignored by the computer to allow for naturally occurring events in the supervised area, such as the slow rise of heat as the sun rises for the day.