PSN-L Email List Message

From: Jason Brady jr_brady@...........
Date: Tue, 29 Jun 2004 16:59:54 -0700

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 (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
combination.  Guitar wire and turnbuckle forms the suspension.  I
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
allow easy insertion of a resistor across the coil output.  I also
a copper vane for oil damping if the resistive approach doesn't work.
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
distance away sets the boom in motion.  Will be interesting to see how
responds to true seismic events once properly damped.  I missed the
Alaska quake, but being near the Juan de Fuca fault/subduction zone I
to record a decent amount of activity.

Any comments or suggestions on the design, construction, etc.?

Jason Brady   Lynnwood, WA USA


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