This lock avoids the pitfalls of traditional tumbler/key locks and RFID locks.

The media has been making much noise about the insecurity of tumbler locks. These are the kind of locks that you'd typically associate with a normal key and will probably find on your own front door. It's been widely publicized that you can use a "bump key" to open pretty much any standard tumbler lock in seconds with little or no training. Not very secure.

The other option is to use an electronic lock on your front door. The current trend would dictate that such a lock would be made with an RFID "key" that would unlock the door whenever you get near it. This is feasible and definitely handy, but today's early generation RFID devices have been shown to be exceptionally weak from a security standpoint. Typically, you can use a device to record the signal emitted from the RFID "key", which can be done without the owner's knowledge since you only have to get withing a few feet and not actually touch the key. This recorded string of information can then be played back to unlock the door just as if the owner were there with the correct key.

A better option, albeit not hands-free and devoid of the cool RFID buzz, is to use a Dallas Semiconductor iButton. The iButton is a small, watch battery sized device that emits a coded string of data, similar to the RFID tags. However, the iButton requires a physical electrical connection to transmit the codes, which eliminates the ability of a person to simply walk near you to swipe your key code.

So, combine the iButton with a solenoid door lock controlled by a microcontroller, and you end-up with a fun and very functional project! This project uses an Atmel microcontroller and a minimal assortment of support components. It features a 7 segment LED display which aids in programming the buttons.

One aspect of programmable locks I've always thought would be handy is that you can enable certain codes/keys only for certain times of day. For example, if you're lucky enough to have a maid or house cleaning service, you could provide them with an iButton key and then program the lock to only authorize that key from 8am to 5pm.

The only problem with electronic locks is how they operate when the power is removed (an outage or the batteries run-out). Some pre-made locks default to "unlocked" when power is removed, which is good in that you can get out of the house if it's on fire. Other models default to "locked" when without power, which is obviously more secure from an intrusion perspective. I suppose that building the lock with a connection to the household power along with a battery backup would make the "default to locked" model the choice I'd choose, but if you build something like this, be sure to consider this situation and how you want to handle it.