I have 130 ft of lakefront property and am wondering if I can set up a laser or something that would run parallel to the shoreline and sound a signal or alarm when someone crosses over it towards the water. I have 2 young kids and this seems like it would be a good safety feature. I know there are laser sensors that do this in the garage, but this would have to stretch much farther obviously. Or if not a laser beam, possibly another idea?

I guess that might be a safety feature. Anyway, I put the battery back in but now the alarm emits small beeps like every two minutes or so. Why?
No no no . . .. I just changed the batteries two weeks ago . .it has something to do with me taking the battery out tonight just to keep it from going off.
I live in a small house. I have five smoke detectors and a carbon monoxide detector all with new batteries. I am not being careless people. And to whomever, I don't need cooking classes, thank you very much. And it's still beeping.
Problem solved . . thank you everyone but especially to ajtdc2006 - you got me looking . . I put the battery in wrong; sorry I didn't know you could put a 9 volt in wrong . . but I appreciate all the feedback. And sorry for your town's loss turnhog.