PSN-L Email List Message

Subject: Re: Torsion Seismometer in a jar
From: "tchannel" tchannel@............
Date: Thu, 15 Oct 2009 17:18:47 -0600


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
 
    

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