Hi Jason,

It looks good. THanks a lot for sharing the specs and the pictures with us.

Dave...

Jason Brady wrote:

> Hi All,
>
> Finished building my first seismometer last weekend.  It's the familiar
> Lehman design based on instructions, ideas and information from PSN and
> other resources on the 'Net.  Several rather grainy pictures can be seen
> at http://home.comcast.net/~jr_brady/index.html (temp URL).  Sorry about
> that; I really must purchase a better digital camera...
>
> The seismometer is built on a 12" x 36" solid aluminum base anchored to
> the concrete floor.  1" square hollow aluminum tubing forms the upright
> apparatus and supports.  The boom is 1/2" square aluminum rod, 30" long,
>
> with a 5 pound lead mass.  The pivot is a ball bearing/polished bolt
> head
> combination.  Guitar wire and turnbuckle forms the suspension.  I
> custom-
> wound the coil using almost 1 pound of #34 magnet wire.  Coil impedance
> is 1.9K ohms.  The magnet is standard Alnico bridge with 35-pound pull.
> Boom stops, component mounting brackets, etc. are formed from 1/2" flat
> aluminum stock 1/8" thick.  Used brass hardware throughout.
>
> No damping yet in place.  I'm fine-tuning the period and centering and
> want to give it some time to settle before damping.  About a 15 second
> period seems to be the maximum obtainable thus far.  Note the DIP socket
> mounted on a small piece of perfboard to the left of the BNC connector
> to
> allow easy insertion of a resistor across the coil output.  I also
> attached
> a copper vane for oil damping if the resistive approach doesn't work.
> Even
> made provision for mounting eddy current damping hardware (the beauty of
> working with soft metals like aluminum--easily cut, drilled, tapped).
>
> I attached the device to Larry Cochrane's Amplifier/Filter and ADC.  The
> undamped sensitivity is amazing!  Gently stepping across the floor a
> good
> distance away sets the boom in motion.  Will be interesting to see how
> it
> responds to true seismic events once properly damped.  I missed the
> recent
> Alaska quake, but being near the Juan de Fuca fault/subduction zone I
> hope
> to record a decent amount of activity.
>
> Any comments or suggestions on the design, construction, etc.?
>
> Thanks,
> Jason Brady   Lynnwood, WA USA
>
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