The USB-based door lock

Author: Gareth Halfacree
Published: 23rd October 2008

The USB door lock might look crufty, but the next time you forget your keys but still have your iPod you'll wish you'd built one.

If you're looking for a clever way of keeping your kid sister out of your room, how about a deadbolt that unlocks when you insert a specific USB key?

Hack a Day are currently showing off the work of the Makers Local 256, who have built a mechanical deadbolt system that relies on a certain USB device being inserted before the lock is released. Where it gets clever is that the system will work with any USB device – similar systems rely on storing a 'key file' on a USB-presented mass storage device. Basically, you can unlock your door with a mouse, a keyboard, an MP3 player – anything with a USB interface.

The way the system works is for a host PC to read the unique serial number of a given USB device. Every USB object has one, and – in theory, at least – it's unique to that individual device and is never duplicated, even amongst devices of the same model. The PC then compares this unique serial number to those stored in a flat-file database – if it matches, an unlock instruction is sent to a USB-connected microcontroller, which triggers a servo to withdraw the bolt and allow entry.

If that all sounds a bit confusing, there's a neat video demonstrating the concept of the system and showing the unit in action.

While it's not exactly the prettiest hack in existence – being built into a plastic box the team jokingly refers to as “transparent aluminium” – it's certainly a neat trick, and speaking as an individual who is constantly losing his keys but always has a USB pendrive about his person I can certainly see a practical use for the system.

If you're tempted to implement your own version, the site has instructions and diagrams along with the source code you'll need both to read the USB ID and to interface with the Freeduino microcontroller. The parts for the project can be purchased for around $60 (£37) – obviously discounting the PC needed to power the system.

Fancy a unique locking system for your pad, or would you prefer something with more of a biometrics flavour – or perhaps good old keys are enough for you? Share your thoughts over in the forums.