If you used a current source to keep a steady
10ma or 20 ma to the LED would that not be
adequate stability for the light output also ?


----- Original Message ----- 
From: "tchannel" 
To: 
Sent: Thursday, October 15, 2009 4:18 PM
Subject: Re: Torsion Seismometer in a jar


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 ----- 
  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.

  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


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