Sensing when someone enters a room - small z-wave?

I’m looking for a small, inconspicuous sensor that will detect when someone enters a room…which I can use to trigger some action. I could probably do this with a beam of light across the door (although I haven’t looked to see what’s available…but I’d rather have a proximity sensor of some sort.

Any thoughts?

Visible:

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Invisible but needs an external sensor (I am in the proces of d1 mini and tasmota mqtt etc…):