Mark wrote
Rod Speed wrote
Dave Liquorice wrote
Brian Gaff wrote

1: The glass on the door is set up as a touch screen
sensor and then the taps operate the door bell.

I wonder if a burglar alarm window sensor(*) could be used
to detect such rapping and sound a remote bell or WHY?

Yep, but it makes more sense to use an external PIR instead
because that will go off even if they knock on the wood of
the door, or the door has no glass at all.

And every time a cat passes the door.

Nope, not with a pet immune PIR.

(*) Vibration rather than break glass.

They arent as common. external PIRs are a lot more common.