I have another idea that may be easier. I'm not sure of the accuracy but I 
got it from a book so I can only assume it is fairly good. read the page I 
scanned here (red arrow) 
http://www.geocities.com/travis5765/seismo/images/displacement_sensor.png

Although I have not tried it, it seems like it should work. You would have 
to make sure NO excess light fell onto the sensors though.
If somebody gave the design a try, I would be interested in the results.

By the way, I missed the page in scanning that told that the output needs 
only minimal amplification though it varies with the rated output of the 
solar panels and the consistency of the light output.
My idea for a modification would be to use a laser pointer as a light 
source. make sure to have a well regulated power supply for it.

~Travis

>From: Mark Andrews 
>Reply-To: psn-l@..............
>To: PSN-L@..............
>Subject: Equipment: SG vs. Lehman...Hybrid
>Date: Thu, 24 May 2001 22:51:28 -0500
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>From larry_cochrane@.............. Thu May 24 21:04:15 2001
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>
>Dear Everyone,
>
>
>I still have a lot to learn so please forgive my ignorance.
>
>I know that teleseismic events generally dominate in the lower frequency
>range.  Therefore a Lehman seismometer with its longer natural period seems
>like it would be the instrument of choice for detecting teleseismic events.
>  However, everything I read tells me the displacement sensor, which is
>found on the bob of the Shackleford-Gundersen Seismometer can be built to
>detect much weaker events than the velocity sensor found on the end of a
>Lehman.
>
>It seems like the thing to do then is build Lehman with a displacement
>sensor similar to the one found on a Shackleford-Gundersen seismometer in
>place of the magnet and coil on the boom of the Lehman.
>
>I have not heard of anyone doing this.  Is there a reason?  Something I
>have not taken into consideration?
>
>I'm just curious because I'm looking to build my second instrument.  My
>first is a Shackleford-Gundersen which I have been running since the end of
>December.  I've picked up lots of quakes with it since then.  The only big
>one I missed was the great quake in India due to my data logging computer
>re-booting after a minor power blip.  (Just my luck!!!)  Now the computer
>is on a UPS, go figure.
>
>Thanks for any info anyone may have.
>
>Mark Andrews
>Texas A&M Class of '02 (Gig 'em)
>
>
>
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