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Movement detectors

Movement detector PIR

Whilst working at HK security for a few weeks prior to Chubb, I had installed several passive infrared detectors, (PIR’s). 

These consisted of an infrared sensor located behind a Fresnel lens, the lens focussed patches of the floor and walls opposite the detector onto the sensor. The sensor would read the room temperature which may fluctuate slowly, when a person walked into the room, their movement would block one or more of the patches being measured causing a rise of the sensor to body temperature, and then back to room, this could be measured and trigger an alarm.

When I arrived at Chubb, they did not use PIR’s, at this time the jury was out for the insurance companies as to whether PIR detectors were suitable devices to detect intruders in favour of active devices such as microwave and ultrasonic detectors. 

These emitted a signal, either microwave radiation or ultrasonic audio which was reflected from the environment to the detector and received by either a microwave antenna or an ultrasonic microphone. If the frequency returning to the detector was unchanged from that being emitted, the detector would be in an all clear status, when a person entered the area, as they walked the reflected signal would shift in frequency, this was an established scientific principle called Doppler Shift, such a change would trigger an alarm event.

Like most things, there were issues. Ultrasonic detectors used more power the higher the frequency chosen for the design, and more power to attain a greater range hence most were set to around 16.5 kilohertz and just 8 metres range, dogs could definitely hear these, and some humans could just about detect their transmission, so they were not liked by teenagers who had the most sensitive hearing at high frequencies. I had always been able to hear the line output transformers whistle from colour TVs which was 15.6 Kilohertz so I was susceptible to the issue and would not have wanted one in my home.

Ultrasonic detectors did have one benefit over microwaves, they remained enclosed in the room in which they were installed. 

Microwaves however could travel straight through glass, plasterboard, wood, and plastic. Great care had to be taken when installing these units to avoid false alarm hazards such as people in a room within the main building with glass, wood, or plasterboard walls. Water moving in plastic pipes. Detection of larger targets such as lorries and vans on a road outside visible through windows etc.

The MESL microwave unit would be installed to cover large areas, typically 100 X 70 metres within mills and factories. It could take half a day to install a detector, and a week to resolve the environment issues, chicken wire was the answer to ensuring the detector did not see targets through walls/glass. It was necessary to install the wire over windows, water pipes, plasterboard walls etc. to keep the signal within the area of the building required.

Eventually PIR detectors stole the crown and were authorised for use by insurance risk underwriters, it was the development of the dual detector which gave these their overall win. Instead of a single detector which would see a series of increases and decreases in temperature as a person walked in the room, the dual element detectors would see a rise on one element, then a rise in the other element at the same time the first went back to room temperature, then the second element would return to room temperature. Typically, two of these signature changes within a time frame were required for an alarm.