Well, they work a little, for what little I paid for them! Harbour fruit's driveway alerts work IF you are not any further away than say 50-60 feet. They have limited ranges despite their advertising of 150 feet! Too bad too...I'd like to BUILD some that have longer ranges, IF I could find the plans...but most are motion only and most of the alert units are triggered by heat (IR) and motion detection. Most I ahve tried asnd othere that have purchased some $200 dollar units say they will not perform well in an area that is wooded. Trees seem to disrupt the signal. I wonder, can one be modified with an external antenna to get better reception from the transmitter? The transmitters work great, it's the lousy receivers that are the problem. I have hear that many "detector's" only are able to sense metals, as from a vehicle....?