[Simple Optical Switch]

You've explained that when pin2 goes LOW (a diode drop above ground) when the light to the phototransistor is blocked, the output goes HIGH.

I was just wondering if the IC's purpose was to "debounce" the sensor output, i.e., remove transient fluctuations in voltage level.    Now I understand that that is indeed the case.

Thank you for your explaining it to me.

[Simple Optical Switch]

Here's a link to the circuit I was looking at:
http://www.electronics-lab.com/projects/misc/017/index.html

[Simple Optical Switch]

I saw this circuit in the "Miscellaneous Projects" section.
I am wondering what is the purpose of the 555 timer IC?
Is it the same idea as using an IC to debounce the input of a mechanical switch?    Or is it something else?