Depends upon what kind of electronics
you intend to use.

If you make it two hertz then it is
quite simple to equalize down to
two seconds or so.

If you make it three seconds then
all you need is a band pass filter
of N=4 at the low end of 3.5 to 4 seconds
and the high end being 2 HZ or so.

Then you can get the P waves and some
S but none of the surface stuff
for teleseismic events.

If you want to use prebuilt electronics
you probably should study the
specification before setting your free
period.

it is really much more complex then it seems
at first glance.

regards and happy holidays;
geoff
----- Original Message ----- 
From: "tchannel" 
To: 
Sent: Saturday, December 23, 2006 9:23 PM
Subject: Building vertical spring sensor


Hi Folks,  Merry Christmas and a Happy New Year to all.   I am building a vertical spring sensor, using 1" and 1/2" copper water 
pipe and all the common tees, elbows etc, avail at any Home Building Supply store.   I have seen sensor made from wood, aluminum so 
I thought I would use copper pipe simply because I had some left over pieces.  Interesting to me is that it seems to be working just 
fine.  The unit is about half done, and here are some things I like about it so far.   The shape is basically similar to a AS1, it 
has a "T" for the base, an  "Arm", a vertical column which forms a "L" to the base, a  "zero length" 6" spring, and lots of tees and 
fittings.
1.  If you have some copper tubing, and fittings, the cost is not too bad, because the sensor is so small, about 20" long 9" wide 
and 11" tall.
     I chose the dims to fit inside a fish aquarium, which I will invert and use as a cover for the sensor.  This too was from the 
salvage pile.
2.  I soldered the fitting together, using just a little solder to hold it without big, ugly, globs of solder around the joints, as 
it doesn't  need to hold water.
     It will look better when I polish the copper up.
3.  Some joints I wanted  to rotate.  Not too loose, but loose enough to rotate with your hands.  On these joints, which were too 
loose to hold their position when connected, but not soldered,  I used a punch and created a dimple on the outside of the tube. 
When I inserted the tube into a tee or elbow, it became very tight and would hold it position, until you twisted it to a new 
location.
4.   At this point, the frame is done, the three adjusting leveling feet and the spring.   I still need to build the damper, the 
coil/magnet and mass, but so far so good.

Q.   My question at this point is: What is the target for the period?   The sensor is adjustable, the spring can be tightened, the 
hinge and arm can be lower, etc.  Any adjust I make, changes the period, if I tweak it I can get 4 seconds, but the stable position 
on the arm is small.  If I change the period to 3 seconds, I seem to get a larger area of stability in the arc of the arm.   By 
unstable I mean, If I move the horizontal spring loaded arm up or down a little it will return to center, however,  If I move it too 
far up it will fall toward the spring.    Should I target the longest period I can get, even thought the arm is touchy and could 
fall up or down?  Or should I target less the maximum in order to get a more stable arm?

I can send pictures, of the frame, if anyone is interested...... I am not sure this will be the best design, but it may provide food 
for thought.  Seeing something different can often generate new ideas.
Thanks, Ted 

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