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enter image description here

I need some explain how does comparator work as zero cross detection and integrated amp

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  • \$\begingroup\$ What comparator do you refer to. I see none. \$\endgroup\$
    – Andy aka
    Commented May 1, 2020 at 15:28
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    \$\begingroup\$ yep, there's no comparator, and especially not in the part you circled. It's unclear what you're asking, even if there was a comparator, though. We don't know what you don't understand, so we don't know where to start explaining. \$\endgroup\$ Commented May 1, 2020 at 15:31
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    \$\begingroup\$ Also, and this is an overly broad heuristic, but for schematics pasted on this site, it has a subjective succes rate > 90%: If it's a schematic from "some unnamed website", and if it uses the LM324 with a 5V supply, it's bad, and has been copied over for three decades from magazine to website to website to website by people who don't know what they're doing. Find a better schematic that comes with explanations by someone who understands it. \$\endgroup\$ Commented May 1, 2020 at 15:34
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    \$\begingroup\$ (this circuit really isn't completely correct in its notation, but I can overlook being not well-educated enough to use k for kilo and M for Mega, and not the other capitalization around, although it's hard to imagine an EE who hasn't learned that; what's very problematic is the polarized caps used here where there's absolute no guarantees on the polarity, and the fact that these components aren't named – How could I even discuss "the second 100 kΩ resistor, no the other one"?) \$\endgroup\$ Commented May 1, 2020 at 15:39
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    \$\begingroup\$ Generally speaking, what you are showing there is just what the output label indicates- a pre-amplifier, not a comparator. It's quite high gain and AC-coupled with a particular bandwidth targeted to correspond to the target motion modulated with the Fresnel lens on the pyroelectric sensor. \$\endgroup\$ Commented May 1, 2020 at 15:39

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1) There is no comparator, instead the LM324 is a very slow quad opamp. 2) The 1m resistors are a dead short, they are probably 1M which is 1 million ohms. 3) The total gain is 12195 so a very small signal produces a saturated output. 4) The LM324 is so noisy that with the gain that is so high, I think its output will be its own noise at a saturated output level.

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