Best Answer

Most motion detectors work on the principle of the Doppler effect. It's applied to the transmission of an ultrasonic signal and the comparison of the return or echo to what was transmitted. Any difference is caused by Doppler effect. Something was moving toward or away from the transmitter of the detector, and this caused an "apparent" change in the frequency of the echo. And that will set off the alarm. Use the link to the related question (which you will find below) to learn more about the Doppler effect.

Motion detectors used with security systems are passive infra-red (PIR) motion detectors, sensing both motion and heat to detect an intruder. They have a photo-diode that is sensitive to the heat radiation frequencies of the human body, and a special lens that looks at 20 - 30 narrow fields within the protected space. They 'see' the temperature of the background, and any change to this temperature is considered to be a suitable trigger. A person (or animal) moving in the space will move in and out of the fields of view, giving infra-red images to the detector in a 'now you see me, now you don't' pattern.

This generates a series of pulses as the target moves into and out of the fields of view of the PIR lens system. Even if part of your body is at the same temperature as the background, other parts will be at different temperatures, and will probably be detected. (Your head is hottest, your hands are cooler.) Most PIR detectors count the number of times a target appears and disappears from their view, and if it happes (say) 3 times in 5 seconds, they will 'trip' and give an alarm signal. Slow changes are ignored, as are single pulses, which may be environmental changes (air conditioners) or electrical noise (RFI). Because of the way their optical system works, PIR detectors are most sensitive to targets moving ACROSS their field of view. Anything moving directly towards the detector is less likely to be detected, particularly if the rate of movement is low.

To help overcome this problem, and to reduce the likelihood of false alarms from small pets. moving curtains etc, PIRs are increasingly incorporating a second detection system ('Dual Technology detectors') with a microwave based Doppler detector being added to the infra-red detection unit. This system is most sensitive to targets moving TOWARDS the detector, and less sensitive to targets moving slowly across its field of view. By combining the two systems, the detector can be made very difficult to avoid, and by controlling the range and desired target size of the microwave unit, can be made less sensitive to false triggering events.

The answer left out light detectors which grossly replaced the bulky expensive ultrasonic devices. A light sensor is simply a photo diode biased to a point whereby any set disruption of light will trigger an output response

There are many types of motion detection alarms that can be used in and around the home. Most homeowners elect to use a system that integrates the use of infrared technology. These alarm systems emit beams of light. When there is an interruption to the beams, an alarm is activated. In the same respect, there are laser beam motion alarms that are part of many security systems around the nation.

The ultrasonic motion detection alarm is quickly gaining in popularity in home security. This specific type incorporates devices that, if touched, sound an alarm. If you want to ensure that your property is safe, and the safety of those that live in your home, a motion detection alarm can assist you in ensuring security.

User Avatar

Wiki User

โˆ™ 2011-07-21 13:38:27
This answer is:
User Avatar

Add your answer:

Earn +20 pts
Q: How do motion detectors work?
Write your answer...
Related questions

What are examples of motion detectors?

Motion detectors are: infrared detectors, ultrasonic, microwave, radiowave and... dogs.

What are the technology that make use of motion detectors?

technology that makes use of motion detectors

What is the impact motion detectors have on society?

Motion detectors have had a massive impact on today's society. Motion detectors have made life easier for those who are very busy for example.

Can motion detectors be connected to your computer?


Who invented motion detectors?

The first motion detector was invented by Russian Leon Theremin in the year 1920, but the first motion detectors used as a security device were developed in early 1950's by Samuel Bagno. The basis of these first detectors was the Doppler effect and ultrasonic waves application.

What are sonic detectors?

Sonic detectors pick up sound waves and motions When things move, they often create changes in surrounding Read more: How Does a Sonic Motion Detector Work? | not all written by me

What animal get into your home and sit the motion detectors off?


Are cctv securrity cameras also motion detectors?

Some cctv security cameras also have motion detectors. If you are looking for a security camera with this feature, you should check the description before you buy it.

What did people do to protect there homes?

home security, motion detectors, have weapons near

What has the author Julian Kolodko written?

Julian Kolodko has written: 'Motion vision' -- subject(s): Design and construction, Detectors, Measurement, Motion, Motion control devices

What are some possible uuses of sonar on land?

Ultrasonics are used for distance measurements, and as motion detectors.

How does a video entry home security work?

A video entry home security system works through cameras and motion detectors. The video is recorded and easily accessible to the residents of the home.

What are Interior systems made of?

Interior systems usually provide infrared motion detectors located in strategic areas

How thermal detectors work?

They can 'see' infra red radiation or heat

On splinter cell stealth action redefined how do you bypass motion detectors?

you have to find a way around it or sometimes you can just walk through it

What is the ability to do work or cause motion?

Energy is the ability to do work or cause motion

What type of sensor used in fire protection system?

Many different types of sensors:heat detectorssmoke detectorstemperature rise detectorsflame detectorswater flow detectorscarbon monoxide detectorsvolatile organic compound detectorsamong others

What is the science behind motion detectors?

Motion detectors emit a beam (or beams) of Infra-Red light - which is invisible to the naked eye. This is reflected back to the detector by solid objects in the area. So long as the beam returns, the detector remains in a 'ready' state. However, if the beams are disturbed by anyone passing through them - they don't return to the detector - which triggers the alarm.

How do smoke alarm systems work?

There are 2 types of smoke detectors: ionization detectors and photoelectric detectors. Although they use different mechanisms both essential set off an alarm when smoke obscures the line of sight between 2 objects within the detector.

Do crowcon gas detectors work with carbon monoxide?

Crowcon does have gas detectors that will detect carbon monoxide. The detector will send out a signal to warn your household that there is a dangerous amount of carbon monoxide.

Are cctv security cameras also motion detectors?

a cctv security camera can be a motion detector activated camera with this device I found on amazon.CCTV security PIR motion detector security camera 1/3"" SONY420tvl day night .1 LUX DC12

What does a CO2 detector do?

Carbon dioxide detectors are used to identify and to measure carbon dioxide concentration in the atmosphere. These detectors are near infrared detectors or chemical detectors.

How does infrared detectors work?

I believe infrared detectors measure heat being given off by an object. However since everything gives off heat.. the detectors have to be cooled down to very low temperatures themselves using liquid helium croyogen. Emma D

What are the uses of infrared detectors?

There are no uses of infrared detectors!!!!!!!!!!!!!!

How do carbon monoxide detectors actually work?

Carbon monoxide detectors detect the amount of carbon dioxide that is in the air. If it senses too much, it will go off in an alarm to alert you of the carbon monoxide.