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
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.