Laser detection security

Has anyone out there played around with trying to detect a person crossing a boundary by using a laser beam? Is this a crazy idea only found in movies? I have a boundary about 50m long - if anyone crosses it, I would like to sound a beeper warning in the house. I don't know much electronics but have hacked around with an arduino to monitor a borehole pump system, so am pretty sure it must be possible.

The simplest solution I can think of is to use a LDR Hook it up to one analog input and calibrate so that it knows the difference between ambient light and a powerful laser beam ;)

well a photodetector at the right wavelength will do the same things as an LDR but slightly better. The issue is lining up the laser and sensor and making sure they don't move. Over a distance of 50m we are only talking a very small movement to make the laser miss the sensor.

Mowcius

Thanks folks, this helps a lot already. I see an LDR detects the beam. Do you have any ideas what I can use to make the beam (is there a standard component for a laser emitter?)

That project is in my list and i will use this one.

http://www.dealextreme.com/details.dx/sku.13378

I don’t know if this laser will work or not