Steve Hoefer's secret knock detector is little more than a piezo speaker, a tiny gear reduction motor, an Arduino and some PVC pipe—basically a bunch of stuff he had lying around the lab. It works by keeping track of the time between a pattern of successive knocks. It's probably not the most secure way to lock a door, but Hoefer notes that there is a lot of room for improvement on this design. Besides making it small enough to actually fit in a doorknob, you could program it to accept a variety of knock patterns, including different knocks at different times of the day. Hit the link for instructions on how to build one yourself. [Grathio via Make]

Apparently, the problem with that was that the knob had too much surface area for the sensor—but I don't see why the entire doorknob needs to be utilized. It would be even more secure if only touching a specific section would grant access. If you want to try it yourself, the complete instructions are available on the project page. [Washington.edu via Hack-a-Day]

However, the features of the new line of Z-Wave door locks are not all positive. Apparently, the locks will not have companion keyfobs or other handheld RF devices for unlocking the door. The reason behind this move is that Schlage wants users to grow accustomed to using devices like their cellphone—but the bottom line is that will probably take an extra step or two to complete whereas waving a keyfob can be more convenient in certain situations—like when your arms are full. At this point, no pricing or release date information has been made available, but expect to pay more than the $150 price tag of their standard electronic lock. [CEpro]