Human detection - PIR sensor

Hello.
I'm trying to make a circuit that could do human detection. I will use the attatched PIR sensor, that has a circuit in pag. 7, that's the typical Application Circuit (Human detection). Can anybody explain me more or less what the circuit does in stages?
Regards

That circuit is pretty complicated. You can build it if you want, but I would reccomend using a PIR Control Chip. The only control chip I've found is:

https://www.electro-tech-online.com/custompdfs/2005/08/kc778b.pdf

I have used it in my PIR detection circuits, and it works great.

You can but the chip, a PIR Detector, and the fresnel lens all from this site:

**broken link removed**

Their is also a kit for 20 dollars, if your interested in that.

If you're interested, allelectronics.com has a pre-built PIR motion detector for only $11. https://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=PIR-3&type=store

I bought one, but havent got around to using it yet. I fiddled around with it and couldnt figure out how to get an output out of it, but im sure that itll work just fine.