In my continuing effort to bring seismology to the masses ;-) I've constructed a sensor using the coil and magnets obtained from an obsolete hard disk drive. The pictures below show the construction.
I'll have to add more details later, but this system did record a recent magnitude 4.4 earthquake in northern New Mexico.
This figure compares a 2 Hz commercial sensor connected to a home-built amplifier and DATAQ DI-154RS AD (top); with a standard AS1 instrument (middle); and the home-built horizontal shown above connected to a home-built amplifier and DATAQ DI-154RS AD (bottom). The hard-disk-drive magnet/coil arrangement appear to provide an order of magnitude less sensitivity than the other two system - but the system is quick to build!
The amplifier is described here.