In a message dated 10/02/2009, tchannel@............ writes:

 Another question about this site.... he  uses a LED and Photo Transistor in 
place of the coil and magnet. I have never  used this. Is one better than the 
other? This approach would be smaller and  lighter.  
_http://tinkeringcaveman.googlepages.com/home_ 
(http://tinkeringcaveman.googlepages.com/home) 



Hi Ted,
 
    He will get very poor performance out of his  proposed LED + 
Phototransistor system. You can get photo detection systems with  resolutions of maybe 10 
nano metres, if you design them correctly. Microseisms  have a quoted 
amplitude range between 0.5 and 15 microns. Multiply by 1000 to  get nano metres.
    The manufacturers of LEDs don't often tell you that  the light output at 
a constant cuttent varies by >x10  for substrate  temperature changes between 
0 and 100 C. Can't think why! The unstabilised  drifts are huge and the photo 
noise is considerable. You can stabilise the  output with another Si photocell 
+ a driver amplifier.
    You need differential large area photocells and an  intense photo emitter 
to give the high photo currents required to get the photo  noise and drift 
down. The current is proportional to the number of electrons in  a sample. The 
photo noise is the square root of this number. You can do it OK  with 
differential Si photocells BPW34 or larger and an under run  tungsten filament bulb. 
Use about 3/4 the rated voltage to give ~infinite  filament life. Maybe put the 
bulb outside the glass jar to keep down the heating  / air convection? There 
is a differential photo amplifier on John's  website.
    Remember that this is a position detector, NOT a  velocity detector.
    If you use a coil + magnet block sensor, you have  to get the output 
current away from the armature. Maybe use the suspension  wires?
 
    You need very small diameter suspension wires to  get the longer periods. 
The Wood-Anderson seismometers used about 0.8 thou OD  Tungsten wire. The 
smallest that I can get off the shelf is 2.4 thou OD. It  is used in laser 
printers. Check used cartridges? You might also be able to use  small diameter 
NiChrome wire. I have some 2 thou OD. What do _www.smallparts.com_ 
(http://www.smallparts.com)  keep? The smallest steel  music wire is about 7 thou diameter - 
much too large. Smaller diameters are  produced, but I don't know of a source. 
Glass filaments tend to be brittle and  don't conduct electric current. S 
glass is often used for fibreglass  construction.
 
    Regards,
 
    Chris Chapman






In a message dated 10/02/2009, tchannel@............ writes:
<=
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  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
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 Another question about this site...=
.. he=20
  uses a LED and Photo Transistor in place of the coil and magnet. I have ne=
ver=20
  used this. Is one better than the other? This approach would be smaller an=
d=20
  lighter.  
  
http://tinkeringcave=
man.googlepages.com/home

Hi Ted,
 
    He will get very poor performance out of his=20
proposed LED + Phototransistor system. You can get photo detection systems w=
ith=20
resolutions of maybe 10 nano metres, if you design them correctly. Microseis=
ms=20
have a quoted amplitude range between 0.5 and 15 microns. Multiply by 1000 t=
o=20
get nano metres.
    The manufacturers of LEDs don't often tell you=20=
that=20
the light output at a constant cuttent varies by >x10  for substrate=
=20
temperature changes between 0 and 100 C. Can't think why! The unstabili=
sed=20
drifts are huge and the photo noise is considerable. You can stabilise the=20
output with another Si photocell + a driver amplifier.
    You need differential large area photocells and=
 an=20
intense photo emitter to give the high photo currents required to get the ph=
oto=20
noise and drift down. The current is proportional to the number of electrons=
 in=20
a sample. The photo noise is the square root of this number. You can do it O=
K=20
with differential Si photocells BPW34 or larger and an under run=20
tungsten filament bulb. Use about 3/4 the rated voltage to give ~infini=
te=20
filament life. Maybe put the bulb outside the glass jar to keep down the hea=
ting=20
/ air convection? There is a differential photo amplifier on John's=20
website.
    Remember that this is a position detector, NOT=20=
a=20
velocity detector.
    If you use a coil + magnet block sensor, you ha=
ve=20
to get the output current away from the armature. Maybe use the suspension=20
wires?
 
    You need very small diameter suspension wires t=
o=20
get the longer periods. The Wood-Anderson seismometers used about 0.8 thou O=
D=20
Tungsten wire. The smallest that I can get off the shelf is 2.4 thou OD=
.. It=20
is used in laser printers. Check used cartridges? You might also be able to=20=
use=20
small diameter NiChrome wire. I have some 2 thou OD. What do www.smallparts.com keep? The smallest=
 steel=20
music wire is about 7 thou diameter - much too large. Smaller diameters are=20
produced, but I don't know of a source. Glass filaments tend to be brittle a=
nd=20
don't conduct electric current. S glass is often used for fibreglass=20
construction.
 
    Regards,
 
    Chris Chapman