This series of blog posts shows how to reverse engineer an electronic door lock (such as are typically available at any hardware store), and modify it so that it can be controlled (via Bluetooth-LE) from an Android phone. 
This all started with a small itch I wanted to scratch, that I got carried away with.
I have a home office where I work out of every day. The only problem with a home office is that home is where the kids live too. Being the kind of guy I am, my office is filled with tons of high-tech, expensive, breakable, potentially dangerous, and age inappropriate toys.
So I bought an electronic door lock at the local hardware store. Nothing fancy, just a locking handle with a simple electronic keypad.
... but I got tired of having to key in the code every time, so I wanted to figure out how to modify to door lock to use Bluetooth (LE) to automagically unlock the door whenever my Android phone was close to the door (none of that NFC nonsense where I actually have to take it out of my pocket -- too much work).
...but first I needed to reverse engineer the door lock.
Part 1 - Reverse Engineering an Electronic Door Lock using an Oscilloscope
... then I needed to control the Door Lock over BT, so I used an Arduino Uno compatible device with BT-LE capabilities (Bluno Beetle) to control the doorlock.
Part 2 - Controlling my Electronic Door Lock using an Arduino
... then I needed to write a somewhat decent Android app to actually lock and unlock the door automagically
Part 3 - Controlling my Door Lock from Android using Bluetooth-LE
 I actually chose a pretty uncommon door lock (since it just happened to be what the store had in stock), but I suspect that a similar technique would work for other door locks as well.
IN FACT, if you buy me another more popular doorlock, that you'd like me to reverse engineer, I'll give it a shot.