The sensor on the right is a 3 sec. vertical with electronic period lengthening and magnetic repulsion. The feedback coil is hidden from view by the repelling magnets. I've been trying to increase the time period of the unit, but as of yet I haven't been able to increase the period much beyond 3 seconds without it starting to oscillate. (natural period of about 1.5 sec.)
To the left is my version of a Lehman sensor with about a 10 second period. Kind-a hidden way in the back of the sensor is a little tub of power steering fluid for the dampening. I used a couple of flywheels from a tape machine for the mass. I plan to replace the guitar string and turnbuckle with a solid rod and a couple of needle valve type adjusters so that it is suspended on the needle points for the pivoting point. The reason for the modification is to try to stop the vertical oscillation around 4.5 hz that I am getting with the present setup. After the modification I plan to turn it more to the west to align it with true north and maybe increase the period as well.
Other modifications to the home-made sensors will include rolling off a little bit more of the high frequencies, as I still am bothered by bus and truck noise from a busy street near me. I find that the street noise is in the 10 hz and up range, and that even the close quakes seem to peter-out above about 8 hz. Currently I am starting to roll off at about 8 hz and am down about 30 db at about 12 hz, so I think I can slide the response down about another 1 or 2 hz.
Lastly way back in the left corner is a plate with three strong motion accelerometer type sensors attached. These are hooked to a Kinemetrics SSA-2 recorder. I also have two of the sensors connected to the SDR recorder through a little preamp that is also mounted on the plate. Not shown is a similar SSA-2 system rigged up in my hanger at the Oakland airport. Each week I down load the data stored in the recorder and compare it with the USGS lists.
Larry Cochrane - www.seismicnet.com/contact.html