Kinda mixed reviews on it but I liked this one:

"Comments about RadioShack Portable Motion Detector Alarm/Chime:

I own two of these motion detectors. I have found them to be ultra reliable. In particular, I have never had a false alarm. False alarms would be a problem for me, since I would have to grab my Glock, spring out of bed and wait in vain for an intruder to appear."