I have a small motion-sensor alarm. I have this model;
http://www.gofastandlight.com/Portable-Motion-Sensing-Perimeter-Alarm-for-Camp-and-Tent/productinfo/E-DBL-MOTION/
Portable Motion Sensing Perimeter Alarm for Camp and Tent
It has about a 60 degree wedge shaped area of detection about 30' out. Anything moves within there it makes a very load noise. It uses a 9 volt battery. It's also really cheap.
It will not trip for leaves, very small animals, etc, but a cat-size critter might. You also have to watch out for wind, if you just sit it on the ground, or on something else without it being held securely. I just put a rock or something leaning on it to hold it steady.
In on of the first campground I stayed at, a National Forest campground, someone came by in the middle of the night and used bolt cutters to steal about a dozen generators while their owners were sleeping inside their rigs. Chains won't stop some thieves.
If you are leaving your campsite, either put then in your rig, or take them with you, as I do. I have two plastic boxes with lids that I keep my two Yamahas in.
I won't stop someone who breaks into your rig, but they would have to try it in broad daylight in my case.