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The sensors have a built in sensor element,amplifiers and a comparator in a three-stage architectural structure . The output from the sensor element has an extremely weak signal in passive infrared scheme and the signal has to be amplified and then converted to an ON/OFF signal with a comparator circuit before output. When a body with temperature different to ambient of the surrounding, moves into the detection field of the sensor,the sensor responds with a small AC signal.The signal is a small voltage around a DC signal that varies,it is therefore compulsory to cancel the DC part of the signal and amplify the AC part. Because the signal is affected by interference from the environment ,a noise filter is necessary.
Figure 3: Schematic of the first architectural stage
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