PIR Sensor Circuits

  
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Of particular interest to us are 12 Volt DC powered PIR sensors and associated circuits since these can be powered directly from a 12 Volt battery which is in turn charged by wind, solar, or other renewable energy sources. In this article we will look at the ways in which the excellent low current consumption Honeywell IS-215T* 12 Volt DC PIR sensor (available in the REUK Shop ) can be
PIR Sensor Circuits - schematic

used by the renewable energy enthusiast for lighting, security, and other motion detection systems. There are many other home security PIR sensors from other manufacturers which have the same mode of operation as the Honeywell unit, so the following circuits and details will still be applicable with those. When the cover of the PIR Sensor unit is popped off it appears as shown below. A row of terminal strip at the top right of the circuit board is used for making the necessary connections - two connections to provide power to the unit (labelled + and -) and two for the internal relay (labelled C and NC). The `C` is short for Common, and the `NC` for Normally Closed. The most important thing to note about the vast majority of home alarm system PIR sensors is that they output a HIGH signal when motion is NOT being detected, and no signal at all when it IS being detected. Why are they designed this way Because if there was no signal at all when motion was not being detected, a burglar could simply cut the cables leading into the security alarm controller, and then when the signals went high when a PIR detected motion, the controller would not receive those signals and the alarm would not be triggered. With a high signal when motion is not detected, if any cables are cut or if motion is detected no signal will get to the controller and the alarm will be triggered immediately. The `C` and `NC` screw-in terminals on the PIR...



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