Hi Chris,  the two large area Si photocells BPW34 connected to a =
differential amplifier, sound like something I would like to try.  Could =
you help me with a simple schematic of this, even a sketch?   Does the =
amp just use easy to find op amps?

Thanks, Ted
  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Thursday, October 15, 2009 4:13 PM
  Subject: Re: Torsion Seismometer in a jar


  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.=20

  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.=20

      Regards,

      Chris Chapman








Hi Chris,  the two large area Si photocells BPW34 connected to =
a=20
differential amplifier, sound like something I would like to try.  =
Could=20
you help me with a simple schematic of this, even a sketch?   =
Does the=20
amp just use easy to find op amps?
 
Thanks, Ted

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Thursday, October 15, =
2009 4:13=20
  PM
  
Subject: Re: Torsion =
Seismometer in a=20
  jar
  


  

  
In a message dated 15/10/2009 21:27:46 GMT Daylight Time, kevin.mckee@....... =
writes:
  There=20
    seemed to be some concern that the system would perform poorly or be =
very=20
    noisy. There was some discussion over the LED being a cause for =
noise. I=20
    used various incandescent bulbs, LEDs and sensors, and in some =
cases, locked=20
    the pendulum in a stationary position to test whether the sensor was =
picking=20
    up noise from the LED. In fact, I got a flat line even when the =
amplifier=20
    was maxed out--the LED was significantly quieter than other light =
sources.=20
    

  

  
Hi Kevin,
  
 
  
    You seem to be experiencing apparatus =
problems.=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. =
They=20
  are both more noisy than a filament bulb driven from a regulated =
voltage=20
  source. Your flat line response was probably due to low gain. You =
should be=20
  able to pick up microseisms all the time.
  
    You can build a very good detector using =
two=20
  large area Si photocells BPW34 connected to a differential amplifier. =
I get a=20
  noise level of about 14 nano metres. You can't get adequate stability =
with a=20
  single detector or with a LED which is not temperature compensated.=20
  Phototransistors are extremely noisy compared to photodiodes. You can =
get very=20
  fine Nichrome wire which makes a good suspension. Alternatively, =
you can=20
  use a single glass or carbon filament. 
  
 
  
    Regards,
  
 
  
    Chris Chapman