A geophone is a device that detects low frequency vibrations, like footsteps or a vehicle passing.
Geophone element (images are CC BY-NC-SA 3.0)
If you live in an area with heavy vehicle traffic, this project may not be for you.
Geophone detector block diagram
The previous figure shows the block diagram of a geophone sensor. What we do is we take the analog signal that comes from the geophone and amplify it so that we can detect footsteps. Then we pass it through a low-pass filter, which removes any high frequencies that might have gotten into the signal. We then pass this analog signal to a detector, which gives us a DC level that corresponds to the amplified level of the signal from the geophone. We compare this DC level to a preset level (think of this as a sensitivity adjustment) and connect the output of the comparator to an I/O pin of the BeagleBone.
Now we can detect footsteps in the vicinity of our home or business.