The product links are on the main article at GunsAmerica Digest.
There are two sides to the subject of perimeter alarms. On the one hand, it is a life saving precaution to warn you if someone is coming, but on the other hand, you are alerting someone that there is something here to steal, and they are too far away for you to kill them before they can go get reinforcements. Please see my other perimeter security articles on the Digest, because there are other things you can do. These .22 cal blank mines are a no brainer.