How do motion detectors that use ultrasonic or radiowaves work?

Question

What do they try and detect? Do they send out a steady stream and look for changes in the amplitude or in the frequency? Or do they send out pulses and look for changes in response time? How sensitive are they? (how big of a change does it take in whatever it's sensing to trigger it?

Answer 1

read here....

http://home.howstuffworks.com/burglar-alarm2.htm

note that there are many types.

Answer 2

Yes. Motion detectors that use ultrasonic, and radio waves work.

Technically radar is a type of radio wave, so those are quite accurate. Most of the automatic door openers use radar. There are many different techniques to do detection and ranging.

Ultrasonic detectors also work. Some of these look at just the time for a signal to return and can be used for things like calculating the distance to a wall or a moving car. More sophisticated ultrasonic devices include ultrasound used for monitoring pregnancies.

Both radio waves, and sonic waves have their advantages for motion detection. It depends on the application.

Sonar is also used underwater to locate submarines and other things.

Most simple devices will use Doppler shifts to detect motion, but more sophisticated devices use things like time of flight, and sophisticated signal processing techniques.

Answer 3

I think its some of both depending on the type used. I know the ones they used on a experiment on Mythbusters sent on a steady pulse and then measured return times. When this specific timing was broken the alarm sounded or the camera took a picture.

Motion detectors are used in alarm systems, high speed traffic cameras, automatic doors, outside lights and lots of stuff really. Sensitivity would depend on how well its made and for what purpose its made for. An outside patio light might not be as sensitive as a bank alarm.

Answer 4

I think its based on the Doppler effect.

The outgoing wave is sent, if it hits a moving object this will cause the reflected wave to be shifted in frequency. I believe this is in pulses in rapid succession.

An onboard computer detects this shift and sounds the alarm.

As long as the object is bigger than the wavelength of the wave it will interfere and thus be detected. Modifying the pulse rate might decrease the sensitivity slightly.

Answer 5

its just reflection.

when a ultrasonic wave is generated it travels a distance and gets reflected by the object.

the time interval is noted.

since u are generating the ultrasonic wave you know its frequency from which you can calculate the velocity.

so now you know the velocity and time taken and hence
you can calculate the distance of the object.
distance=speed x time.

remember : the distance obtained is twice the original distance.
since the time interval is measured for the travel of ultrasonic wave twice.
i. generator to object
ii. object to receiver

isnt this the best answer?

Answer 6

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