How do motion detectors work?

Answer 1

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.