In each PIR there is pyroelectric sensor, which will undergo a change in polarization if it detects a change in thermal radiation, thus producing an electric output, causing the alarm to be triggered or in this instance, the light columns to change.
The level of detection is limited by the window material that is placed in front of the sensor (clear white plastic part at the bottom of the sensor on the above photo). Most off-the-shelf pyroelectric sensors limit the spectral response to all or parts of a range between 0.15 and 20 µm. The radiation of the human body is strongest between 8 and 14 µm. The goal of every filter (window) is to make the sensor immune to unwanted radiation and pass a desired band, both of which are defined by the sensor's target application. Sensing elements usually have built-in impedance converters or pre-amplifiers to lower the number of external components and minimize the costs of the circuitry.
A variety of businesses sell various PIRs. In order to make a decision about which one I will use, I need to consider; price, coverage, the voltage that it operates at, operation temperature and ease of wiring. The majority of the PIR detectors that I will be looking at will probably be suitable, but the main specifications to check are price and the voltage that it operates at. This will prove to be extremely important as I will need to put power through the PIRs, and will also need to plug them into a form of hardware to receive the signals that they will omit.
I was originally planning to use twelve sensors, but since then a lot has changed. The main changes have been from square to cylindrical columns and from a system similar to lasers to PIR detectors. The detection zone of a PIR is far greater than a laser, which is obviously just a single line, consequently I will need far less PIR detectors. I plan to test this before I make a final decision concerning how many I will need but I imagine at least six.
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