Typical alarms use PIR (passive infrared), microwave or ultrasound detectors for motion detection. PIR are by far the most common type of detector – they work by detecting changes in infrared emitted by warm bodies. They are cheap, very reliable, and actually quite hard to beat.

Laser break beams are only really seen in films, though simple active infra-red break beams are often used on scaffolding alarms.

The problem with all of these is that they cannot see through objects. A common method of circumventing PIR detectors is to “mask” them – you either cover them  using paint (or another infrared opaque coating) or simply put something like a box in front of them. Higher security systems have “anti-masking” detectors which use an active element to check that their view has not been masked.

It can mean that complex, cluttered, or continually changing spaces need a lot of PIRs to be adequately covered.

Step in a new type of motion detection – tomographic motion detection. This sounds really clever and innovative. You might have heard of tomography from the medical world – CT scan stands for computerised tomography. It means “imaging by cross section”. Xandem have come to the market with a new detector that uses 2.4GHz radio signals to detector motion in a space.

A group of wireless nodes form a mesh of connections, as shown in this image from the patent:

Mesh network

Each one of those lines represents a radio path. The system uses 2.4GHz signals, the same as with WiFi or Bluetooth. These are heavily attenuated by anything containing water – such as the human body. A human body placed in the radio path of any two nodes will reduce the received signal strength (RSS).

By carefully measuring the RSS from each node to each other and doing some clever processing, you should be able to build up an image of what the area usually looks like. Any significant disturbance would signal an alarm. Hence, motion of a human body can be detected.

This would work through walls, shelves, furniture and so on – as long as the signal strength is attenuated too much.

This is clever stuff. Very easy to fit (though you do need power to each node), and probably very hard to beat. It is expensive though.

For those interested, here is a link to the patent:

https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US20120146788.pdf

And I have pulled a picture of the PCB from the FCC report on it:

PCB

The markings on the main IC are not visible, but based on the frequency, size of the package, crystal frequency, crystal connections and antenna connections, this is a TI CC2540 RF SoC – a brother to the CC1110 RF SoC, using an 8051 core connected to a RF transceiver.

Interestingly there is a micro-USB and debugging connector on the board as well!