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| In a light fitting with a PIR-based motion detector, the PIR sensor is typically mounted on a printed circuit board which also contains the necessary electronics required to interpret the signals from the chip. The sensor is mounted in a location where it can view the area to be monitored. Infrared energy is able to reach the sensor through the opaque window because the plastic used is transparent to infrared radiation. This plastic sheet or bubble prevents the introduction of dust and insects which could obscure the sensor's field of view or fill the light fitting with bugs and dust. A person entering the monitored area is detected when the infrared energy emitted from the intruder's body is focused by a lens or a mirror segment and overlaps a section on the chip which had previously been looking at some much cooler part of the protected area. As the intruder moves, so does the hot spot on the surface of the chip - moving hot spots cause the electronics connected to the chip to de-energize the relay, operating its contacts and illuminating the lamp circuit. |
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