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The Sensor
At the heart of every PIR detector is the pyroelectric sensor. Typical sensors use pyroelectric materials, such as tri-glycine sulphate (TGS) or lithium tantalite, which undergo a change in polarization if the temperature of the material is altered (in the case of PIR detectors by thermal radiation). If electrodes are placed on opposite faces of a thin slice of the material to form a capacitor, the change in polarization can be observed as an induced voltage across the slice if the external impedance is comparatively high. The sensor will only produce an electrical output signal when the temperature of the incident radiation changes. A differential sensor uses
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Spectral Sensitivity. The spectral response of pyroelectric sensors is limited to the far IR spectrum—wavelengths longer than the near IR of LEDs and shorter than microwaves. The wavelength of the incident radiation is not a determining factor because pyroelectric sensors have a flat response over a very wide spectral range. The spectral range is limited by the window material used by the manufacturer of the sensor component. 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 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
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Response Characteristics. Pyroelectric sensors have a bandpass type of response (see Figure 2), with the response rising at 20 dB per decade at low frequencies until reaching the peak output at 1 Hz. At higher frequencies the response falls at 20 dB per decade. Each pyroelectric sensor is designed to operate over a particular range of frequencies. Manufacturing techniques can move the average peak output at 1 Hz slightly, shifting the sensor's target response rate (the peak within the passband) so as not to coincide with the noise frequency. Typical outdoor detectors, for example, respond to objects moving between 0.2 and 5 m/s. Note that the electrical response characteristics are unrelated to the spectral sensitivity of the sensor.
The Optics
The optics come in various forms and shapes, and they focus the incoming radiation from the area that the detector covers. The longer the distance a detector covers, the higher the quality of the optics necessary.
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Mirror Optics. Long-range and high-precision devices often use mirrors similar to those found in telescopes that bundle and focus the incident radiation onto the sensor elements. These devices commonly feature a more precise detection zone and allow for detection of objects of up to 150 m/500 ft. away. Detectors using such mirrors need a separate window to protect the device against wind, dust, and other elements. The choice of the window material is important for detectors used in harsh environments to minimize maintenance. In addition, very few materials transmit the desired 8–14 µm radiation, adding to the cost of high-quality detectors.
Additional Detector Features
Common features of detectors include tamper switches and signal processing. Tamper switches are fitted into the housings and have a contact that opens when the detector housing is opened. If no separate tamper line is available, you can connect the contact in series with the detector's normally closed relay contact used to signal the alarm. Some units have logic that can distinguish between recurring patterns of radiation and nonrecurring patterns to minimize the number of false alarms while maintaining high sensitivity.
Special Features of Advanced PIR Detectors
Adaptive Threshold Discrimination. To optimally adjust the sensitivity to the level of background noise, the device constantly averages the background noise and adjusts the threshold levels for the alarm. This reduces the probability of nuisance alarms caused by wind, moving vegetation, or objects that produce a thermal contrast, even though the latter usually produce a thermal contrast weaker than that emitted by a human body.
Advanced Signal Shape Analysis. Precision detectors include logic analysis and signal processing, such as rate of rise and time windows between channels. Detectors equipped with multiple sensors that cover multiple areas independently often feature enhanced signal processing to further reduce the incidence of false alarms and increase sensitivity and reliability.
Internal Heating. Detectors that use mirrors instead of Fresnel lenses need heating elements for the optics to prevent steam buildup, especially if the devices are deployed in areas with high humidity or other rough environmental conditions.
Temperature Compensation. Contrast conditions can vary significantly in the course of the day and year. Temperature compensation maintains the detection and nuisance alarm properties under all possible conditions in an outdoor environment.
PIR Detectors for High Security
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The presence of cameras lets you see if something is happening in the area where an alarm has occurred. For perimeter control, pan-tilt cameras (PTCs) are controlled by PIR detectors to minimize the number of comparatively expensive cameras used and/or to minimize the movement of cameras for no reason, lessening wear and tear on the mechanism that moves the camera.
Manufacturers are combining technologies and features to appeal to the large home security/consumer market. In addition to detectors that combine video and PIR in a single housing, there are devices that offer wireless transmission over distances up to 1000 ft. There are also high-end detectors that can be accessed via common bus protocols, such as RS-485. These allow easy integration into surveillance systems when used with PTCs and provide more detailed information about where and how far from a detector an alarm has occurred.
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