In a message dated 16/08/04, ke6pxp@....... writes:

> 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 
> determine 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 stabilise 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 
> stabilise 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:-
> 

Hi George,

       To measure seismometer movements, you usually allow over +/-0.5mm 
total movement (or your seismometer becomes very difficult to set up) and you 
require to measure the arm position to maybe 20 nano metres for 'amateur' use. To 
do this you have to reduce / compensate / design for low drifts and minimum 
noise, both with time and with temperature - you are considering 1 part in 
25,000.
       You need two large area photo diodes (~7sq mm - VTD34?) connected to 
the inverting input of two low noise opamps TLC2201? with a suitable value of 
feedback resistors. You then subtract the opamp outputs with a differential 
opamp eg INA118 and apply bandwidth filtering. This reduces the effects of 
temperature on the photo sensitivity and on the leakage currents. Noise 
considerations in photodiodes require you to use a photo current of the order of 0.5 mA, 
which implies a high intensity light source. The photodiodes should be fixed to 
a common heatsink to minimise temperature variations.
       It is possible to use light from one the high power, metal cased IR 
LEDs, but the photo output of a LED shows an exponential decrease as the 
temperature increases. This makes getting high stability and low noise a little bit 
difficult. Laser diodes tend to be very noisy. (You can buy laser diodes which 
have an internal photo diode to 'stabilise' the output.) An easier approach is 
to use a tungsten filament bulb in a feedback bridge circuit, which 
stabilises the hot filament resistance. You reduce the voltage on the bulb to < 0.8 x 
that rated. This gives an essentially infinite filament life. The dimmer 
filament doesn't effect the sensitivity as much as you would expect, since the 
sensitivity of Si photo cells increases in the near Infra Red. GaAs photocells may 
also be used with superbright visible orange LEDs. 'Ordinary' LEDs tend to be 
quite to very noisy. the superbright ones tend to be quieter.
        You need the dimensions of the photocells to be large compared with 
the wavelength of light, to minimise interference fringe effects. The minimum 
conduction noise in a photodiode is proportional to the square root of the 
photocurrent, so increasing the photocurrent will give a lower overall noise. With 
a significant amount of heat being shone on the photocells + optical shutter, 
the detector needs to be near the top of the seismometer case, maybe in a 
semi isolated light box, to minimise heating effects and air currents. The bulb 
is best mounted outside the main seismometer case, to give adequate cooling by 
direct contact with the housing. You can make quite good windows using 
Microscope slides / cover slips.
       There was a note on PSN some years ago which said that you couldn't 
use optical detectors, but this was a misunderstanding. If you try to use 
interference fringe methods, your resolution will be limited, commonly to a fraction 
of a micron.    

       Regards,

       Chris Chapman
In a message=20=
dated 16/08/04, ke6pxp@....... writes:



While leafing through the c=
urrent 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 buildin=
g their own seismometers and need a way to determine the absolute position o=
f the boom and output a signal proportional to the boom's displacement. The=20=
ad is a design idea for an application of their LMV2011 precision op amp. th=
ey are suggesting that it be used to control the output of a led and stabili=
se it. They are doing this by using a photo diode to look at the LED and thr=
ough the op amp, feedback the signal to the LED and stabilise it's output.=20



My thought would be to place the occulting vane on the seismometer boom=20=
in the light path and as the boom moves to obscure the light, the amp will i=
ncrease 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=20=
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



Hi George,



       To measure seismometer movements, y=
ou usually allow over +/-0.5mm total movement (or your seismometer becomes v=
ery difficult to set up) and you require to measure the arm position to mayb=
e 20 nano metres for 'amateur' use. To do this you have to reduce / compensa=
te / design for low drifts and minimum noise, both with time and with temper=
ature - you are considering 1 part in 25,000.

       You need two large area photo diode=
s (~7sq mm - VTD34?) connected to the inverting input of two low noise opamp=
s TLC2201? with a suitable value of feedback resistors. You then subtract th=
e opamp outputs with a differential opamp eg INA118 and apply bandwidth filt=
ering. This reduces the effects of temperature on the photo sensitivity and=20=
on the leakage currents. Noise considerations in photodiodes require you to=20=
use a photo current of the order of 0.5 mA, which implies a high intensity l=
ight source. The photodiodes should be fixed to a common heatsink to minimis=
e temperature variations.

       It is possible to use light from on=
e the high power, metal cased IR LEDs, but the photo output of a LED shows a=
n exponential decrease as the temperature increases. This makes getting high=
 stability and low noise a little bit difficult. Laser diodes tend to be ver=
y noisy. (You can buy laser diodes which have an internal photo diode to 'st=
abilise' the output.) An easier approach is to use a tungsten filament bulb=20=
in a feedback bridge circuit, which stabilises the hot filament resistance.=20=
You reduce the voltage on the bulb to < 0.8 x that rated. This gives an e=
ssentially infinite filament life. The dimmer filament doesn't effect the se=
nsitivity as much as you would expect, since the sensitivity of Si photo cel=
ls increases in the near Infra Red. GaAs photocells may also be used with su=
perbright visible orange LEDs. 'Ordinary' LEDs tend to be quite to very nois=
y. the superbright ones tend to be quieter.

        You need the dimensions of th=
e photocells to be large compared with the wavelength of light, to minimise=20=
interference fringe effects. The minimum conduction noise in a photodiode is=
 proportional to the square root of the photocurrent, so increasing the phot=
ocurrent will give a lower overall noise. With a significant amount of heat=20=
being shone on the photocells + optical shutter, the detector needs to be ne=
ar the top of the seismometer case, maybe in a semi isolated light box, to m=
inimise heating effects and air currents. The bulb is best mounted outside t=
he main seismometer case, to give adequate cooling by direct contact with th=
e housing. You can make quite good windows using Microscope slides / cover s=
lips.

       There was a note on PSN some years=20=
ago which said that you couldn't use optical detectors, but this was a misun=
derstanding. If you try to use interference fringe methods, your resolution=20=
will be limited, commonly to a fraction of a micron.    



       Regards,



       Chris Chapman