Basic Motion Detection

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.

PIR sensor will detect something moving.

You could use the standard IR transmitter/receiver pairs across the doors.

A version of the aforementioned PIR sensor:

Use a standard alarm PIR sensor.

They have a relay with C/NO/NC that you wire into the Arduino.

Something like this one should be able to be had on the cheap. You'll need a separate power supply, but they are built specifically to detect people.

Oh! That looks perfect!