I’m looking to detect if something is in my store, I don’t care how far away they are or what have you, just is someone there. I can’t use magnetic reed switches on the door because sometimes it is propped open.
I was thinking maybe I could use ultrasonic sensors, they seem like the thing, but I’d rather not spend lots on this ($5-$10 is fine). I searched for them on the forum, but others have seemed to want to know the distance, or used pre-made devices which cost more than I’d like to spend.
Any quick/dirty way to detect if something is “there” that wasn’t there before? Even infrared motion detection is fine.