Hello,

Thanks for the pointers with the PDF file.=20

Thanks for the tip - I've been playing around with a Texas Instruments =
TSL230 chip - http://www.taosinc.com/pdf/tsl230.pdf  which is a light to =
frequency converter - which if you have enough time between samples may =
possibly be used as a photon counter???.  One of the problems in using =
an inferometer outside of the resolution issue is that the arc of the =
boom would tend to throw the alignment required out of line. =20

I saw really neat web page @ =
http://www.users.globalnet.co.uk/~blootl/trackers/sunflower.htm - some =
folk in the U.K. have been constructing very large parabolic mirror to =
point at the sun,  > 40 KW outputs...   The TSL230 would make for a =
dandy solar alignment sensor.

Here on the tundra of Minnesota, I've just built my second 24 bit ADC =
board - sent it off to my brother in Canada for eval.  I'm using a 30K =
turn coil and a (1,10,100,1000 x ) programmable gain amp - and a stamp =
to control it all - sending 10 hz data to my PC with a VB front end that =
displays my traces in realtime....  I get quite a signal just rotating =
the assemby away from any magnet - just using the earth magnetic feild =
to interact with the coil...  The whole thing sits on a 2 x 3 inch board =
running of a single sided supply.  I steal a +12 v line from the PC to =
run it.

Jim


----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Saturday, November 17, 2001 6:48 PM
  Subject: Re: Publication on the Web


  In a message dated 17/11/2001, jimesler@.......... writes:=20


    Does anyone have any experiences in building an inferometer for =
measuring=20
    displacement in seismographs?=20
    Jim Esler


  Dear Jim Esler,=20

        The problem with using methods which depend on the wavelength of =
light is that you are ultimately limited by this dimension, which is =
about 0.6 micron. When observing seismic signals, you will probably want =
a resolution of better than 1/20 of this. While optical sensors which =
measure to small fractions of a wavelength can be bought, they are =
neither simple nor cheap.=20

        The wavelength limitation may be largely avoided by using a =
fairly intense and  constant light source to illuminate a pair of =
silicon photodiodes through a moving shutter attached to the seismometer =
arm. The difference in the photocurrents is measured, filtered and =
amplified. This 'photon counting' method is also simpler in principle =
than interferometer techniques. Hope that this helps.=20

        Regards,=20

        Chris Chapman            =20







Hello,
 
Thanks for the pointers with the PDF file. =

 
Thanks for the tip - I've been playing around with a =
Texas=20
Instruments TSL230 chip - http://www.taosinc.com/pdf=
/tsl230.pdf =20
which is a light to frequency converter - which if you have enough time =
between=20
samples may possibly be used as a photon counter???.  One of the =
problems=20
in using an inferometer outside of the resolution issue is that the arc =
of the=20
boom would tend to throw the alignment required out of line.  =

 
I saw really neat web page @ =
http://www.users.globalnet.co.uk/~blootl/trackers/sunflower.htm =
-=20
some folk in the U.K. have been constructing very large parabolic mirror =
to=20
point at the sun,  > 40 KW outputs...   The TSL230 would =
make for a=20
dandy solar alignment sensor.
 
Here on the tundra of Minnesota, I've just built my =
second 24=20
bit ADC board - sent it off to my brother in Canada for eval.  I'm =
using a=20
30K turn coil and a (1,10,100,1000 x ) programmable gain amp - and a =
stamp to=20
control it all - sending 10 hz data to my PC with a VB front end that =
displays=20
my traces in realtime....  I get quite a signal just rotating the =
assemby=20
away from any magnet - just using the earth magnetic feild to interact =
with the=20
coil...  The whole thing sits on a 2 x 3 inch board running of a =
single=20
sided supply.  I steal a +12 v line from the PC to run =
it.
 
Jim
 
 
----- Original Message ----- 

  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Saturday, November 17, =
2001 6:48=20
  PM
  
Subject: Re: Publication on the =
Web
  

In a =
message dated=20
  17/11/2001, jimesler@..........=20
  writes: 


  Does anyone have any experiences in building an =
inferometer for=20
    measuring 
displacement in seismographs? 
Jim =
Esler

Dear Jim Esler,=20
  

      The problem with using =
methods=20
  which depend on the wavelength of light is that you are ultimately =
limited by=20
  this dimension, which is about 0.6 micron. When observing seismic =
signals, you=20
  will probably want a resolution of better than 1/20 of this. While =
optical=20
  sensors which measure to small fractions of a wavelength can be =
bought, they=20
  are neither simple nor cheap. =


      The=20
  wavelength limitation may be largely avoided by using a fairly intense =
and=20
   constant light source to illuminate a pair of silicon =
photodiodes=20
  through a moving shutter attached to the seismometer arm. The =
difference in=20
  the photocurrents is measured, filtered and amplified. This 'photon =
counting'=20
  method is also simpler in principle than interferometer techniques. =
Hope that=20
  this helps. 

      Regards,=20
  

      Chris Chapman=20
  =
           =
=20