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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