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:
<=
FONT=20
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