I was looking at PIR elements and wondering why almost none have just a single element. They usually have two (sometimes four or more) that are wired anti-parallel or anti-series. If they are AC coupled shouldn't just one be enough to detect motion? And if they are for balancign purposes (as my friend suggested but couldn't justify) I don't see why that's necessary with an AC coupled sensor since any long-term biases just go to zero anyways.
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