PSN-L Email List Message
Subject: Re: Torsion Seismometer in a jar
From: ChrisAtUpw@.......
Date: Thu, 15 Oct 2009 18:13:04 EDT
In a message dated 15/10/2009 21:27:46 GMT Daylight Time,
kevin.mckee@....... writes:
There seemed to be some concern that the system would perform poorly or be
very noisy. There was some discussion over the LED being a cause for
noise. I used various incandescent bulbs, LEDs and sensors, and in some cases,
locked the pendulum in a stationary position to test whether the sensor was
picking up noise from the LED. In fact, I got a flat line even when the
amplifier was maxed out--the LED was significantly quieter than other light
sources.
Hi Kevin,
You seem to be experiencing apparatus problems. Ordinary LEDs are much
more noisy than superbright or IR LEDs. Both vary in light output by about
x5 between 0 and 100 C. The is a HUGE variation. They are both more noisy
than a filament bulb driven from a regulated voltage source. Your flat line
response was probably due to low gain. You should be able to pick up
microseisms all the time.
You can build a very good detector using two large area Si photocells
BPW34 connected to a differential amplifier. I get a noise level of about
14 nano metres. You can't get adequate stability with a single detector or
with a LED which is not temperature compensated. Phototransistors are
extremely noisy compared to photodiodes. You can get very fine Nichrome wire
which makes a good suspension. Alternatively, you can use a single glass or
carbon filament.
Regards,
Chris Chapman
In a message dated 15/10/2009 21:27:46 GMT Daylight Time,=20
kevin.mckee@....... writes:
There=20
seemed to be some concern that the system would perform poorly or be ver=
y=20
noisy. There was some discussion over the LED being a cause for noise.=
I used=20
various incandescent bulbs, LEDs and sensors, and in some cases, locked=
the=20
pendulum in a stationary position to test whether the sensor was picking=
up=20
noise from the LED. In fact, I got a flat line even when the amplifier=
was=20
maxed out--the LED was significantly quieter than other light sources.=
=20
Hi Kevin,
You seem to be experiencing apparatus problem=
s.=20
Ordinary LEDs are much more noisy than superbright or IR LEDs. Both vary=
in=20
light output by about x5 between 0 and 100 C. The is a HUGE variation. The=
y are=20
both more noisy than a filament bulb driven from a regulated voltage sourc=
e.=20
Your flat line response was probably due to low gain. You should be able=
to pick=20
up microseisms all the time.
You can build a very good detector using two=
large=20
area Si photocells BPW34 connected to a differential amplifier. I get a no=
ise=20
level of about 14 nano metres. You can't get adequate stability with a sin=
gle=20
detector or with a LED which is not temperature compensated. Phototransist=
ors=20
are extremely noisy compared to photodiodes. You can get very fine Nichrom=
e wire=20
which makes a good suspension. Alternatively, you can use a single gl=
ass or=20
carbon filament.
Regards,
Chris Chapman
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