While leafing through the current issue of Electronic Design I read an ad
from National Semiconductor (p. 34-35, 08/09/04 EDN) the might be of
interest to amateurs who are building their own seismometers and need a way
to determing the absolute position of the boom and output a signal
proportional to the boom's displacement. The ad is a design idea for an
application of their LMV2011 precision op amp. they are suggesting that it
be used to control the output of a led and stabilize it. They are doing
this by using a photo diode to look at the LED and through the op amp,
feedback the signal to the LED and stabilize it's output. 

My thought would be to place the occulting vane on the seismometer boom in
the light path and as the boom moves to obscure the light, the amp will
increase the light level to make up. We should get a usable signal across
the resistor that the design has in series with the LED. I do not know if
this will work or if it may be too noisy or temperamental, or some other
"gotchas," but I am going to try and collect the parts and play with it.
You can read the application note if you are interested at:


http://www.national.com/nationaledge/aug04/article.html
George
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