Motion sensors, as their name suggests, detect motion over a limited distance through infrared heat sensors. They can transmit their response to motion, through radio frequency transmitters. These devices are mainly used as home security devices.
Wireless motion sensors are designed to detect rapid motion in a specific area around them. They spot movement through detection of rapid changes in infrared radiation, incident on them.
There are certain crystalline materials that show pyroelectric effect. That is, when heat in the form of infrared radiation is incident on them, they generate a current on their surface. These crystals are very sensitive to small changes in infrared radiation and therefore, the current they generate, changes accordingly. Such crystalline material wafers are used as thermal sensors. Every object, which has a quantity of heat, radiates infrared radiation. The human body emits infrared radiation, which is around a wavelength of 9.4 micrometers. Motion of a human body, therefore, creates changes in the local infrared radiation pattern, in the vicinity of the sensor.
The incident infrared radiation is focused on the sensor by a 'Fresnel lens', which is made up of a material that is transparent to radiation, between 8-15 micrometer range. This frequency range is used specifically so that human movements are detectable. The Fresnel lens is a plano-convex lens, with the convex part diminished or made thinner. The convex part faces the sensor crystal and focuses infrared radiation from a limited angular range on the crystal. Movement within an angular range of 80°, in front of these sensors, can be detected this way.
To summarize, they have a pyroelectric crystal that acts as a sensor, fitted with a Fresnel lens. The change in heat around the sensor generates electrical signals, which are passed through an appropriate electrical circuit, that measures the pulse width and the rapidity of change.
It is important that environmental heat changes and temperature changes due to sunlight, do not trigger the response of the sensor. So, to avoid the triggering by these factors, sensors which monitor ambient temperature and heat changes are installed.
The thing that differentiates a human body movement, from an environmental heat change, is the rapidity of change in radiation pattern. So, the electrical feedback system of the motion sensor is designed such that, only rapid changes in motion, that create rapid changes in electrical signal output, are detected.
Once it detects motion this way, there are transmitters inside the device, which relay radio signals and switch on lights instantly, or create any other kind of programmed response like activating an alarm.
How to Set Up A Wireless Motion Sensor Security System?
The sensor comes as a single unit, which can be coupled with your light devices outside the house, by using a radio frequency base receiver. The base receiver can be connected to many light devices, which can be instantly switched on, as a response to motion detection. You can set up the motion detector in the porch or around the house, where you suspect intrusion.
If any motion is detected during the night, it lights up the porch lights or triggers an alarm. It also has an inbuilt timer, which switches off the lights, if no motion is detected for a prefixed amount of time, after the first instance. The outside casing is made weather-resistant, as well as tamper-proof, with a specially designed screw.
The sensor, coupled with a video surveillance security system, can be an effective deterrent and prevent burglaries and trespassing on property. Such devices are essential for protection of vital assets.
It can also be used indoors, as an electricity saving mechanism, which switches off lights in the room automatically, if there is no movement for a specific period of time. So, it can help in bringing down unnecessary wastage of electricity and reduce your power bills.
With devices like these sensors around, one can rest easy about home security and enjoy a good night's sleep.