personal project
build around 30
Doesn't sound like a personal project.
The things you refer to are reasonable, you need to experiment and do some fine tuning to get things to work.
Using a SR04 and coding things to have at least one measurement near by (example 3 feet) within a period of time should work.
Perhaps a warning beep prior to alarm, to warn people.
Note:
Suggested this in a similar situation once before, everyone evolved refused to be monitored as it was too much like big brother was watching.
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