Infrared Obstacle Schematic Help

I found a schematic online, however I'm not 100% sure if I understand it. I was hoping someone could verify what I have so far.

Here is the schematic:

Here is my understanding of it so far:

Thanks for any help!

That 500R is not a resistor but a pot. The bit you don't understand is the wiper (middle connection) of the pot not a component.

I updated the diagram in the first post, other than the trimpot was everything else correct?

I got the schematic here. I was wondering if there was any way of substituting the 500 ohm trimpot with a 1k or 10k trimpot without messing up the whole detector? This was the newest schematic, it's about half way down the page.