This is an image of the computer recording "drum" that shows four events recorded on the same day at my station in Denver, CO; two in Mexico and two on the opposite side of the world in Sumatra. The vertical short-period instrument that recorded these events can be made in a home shop.
Smart phone "earthquake" apps that use the built-in screen-tilt accelerometers can only detect gross movements that can be felt. The seismometer in this Instructable can detect ground motion of less than 50 microns/sec. (a human hair is about 100 microns), way below what can be felt or seen. This makes it sensitive enough to detect earthquakes from anywhere in the world greater than magnitude 6.5 and much smaller for closer events. Yet, mechanical and electronic filtering limits local signal noise.
Step 1: Comparison With Professional Instrument
This instrument rivals those of the USGS Mobile Seismic Array if placed in a suitably quiet and environmentally stable location like a basement and you will be able to gather data in the background through a USB port using free software that requires very few cpu resources while performing other tasks on your computer.
Note that like the professional instrument it nicely differentiates between Primary and Secondary body waves as well as the larger surface (L) waves allowing distance to an event and even magnitude to be accurately measured.
Step 2: Seismometer (seismograph) System Components
The seismometer consists of four basic components each of which I will describe in detail. The total cost will be between $300 and $350 U.S.. Free software to display and manipulate data is available online.