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.

http://www.arduino.cc/playground/Code/PIRsense

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

A version of the aforementioned PIR sensor:

http://www.cutedigi.com/product_info.php?cPath=278_295&products_id=4290&osCsid=d85fe3fae5f141f936abfd1adeae2c7e

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.

http://www.security.honeywell.com/hsc/products/sensor/mo/pi/21577.html

Oh! That looks perfect!