PSN-L Email List Message

Subject: Re: Torsion Seismometer in a jar
From: ChrisAtUpw@.......
Date: Thu, 15 Oct 2009 21:49:17 EDT


 
In a message dated 16/10/2009, tchannel@............ writes:

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



Hi Ted,
 
    Check out the notes and the circuit at  
_http://jclahr.com/science/psn/chapman/photo_detect/index.html_ 
(http://jclahr.com/science/psn/chapman/photo_detect/index.html) 
    This circuit works fine. You need the INA118 or  similar to get the 
very high CMRR common mode rejection ratio. You may need to  alter filter 
values to suit your application. The photocells drive current into  the opamps, 
so they run at 0V and there is no leakage current. I usually use  a miniature 
filament bulb driven from a stabilised voltage and a lens to  give a 
~parallel light beam. You can use an IR LED if you provide an auxiliary  Si 
photodiode and a feedback amplifier to stabilise the photo output. Otherwise  it 
drifts all over the place. I use two small razor blades for the sides of the  
shutter, painted matt black, but with clean edges. They move just clear of 
the  surface of the photocells.  I measured the noise level at about 14 nano 
 metres. There is inevitably some heat associated with the bulb. I suggest  
placing the bulb in a separate housing on top of the sensor. You under run 
the  bulb voltage supply to give ~infinite life. 
 
    Regards,
 
    Chris Chapman





In a message dated 16/10/2009, tchannel@............ writes:
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=20 the amp just use easy to find op amps?
 
Thanks, Ted
Hi Ted,
 
    Check out the notes and the circuit at  = htt= p://jclahr.com/science/psn/chapman/photo_detect/index.html
    This circuit works fine. You need the INA118= or=20 similar to get the very high CMRR common mode rejection ratio. You may nee= d to=20 alter filter values to suit your application. The photocells drive current= into=20 the opamps, so they run at 0V and there is no leakage current. I usually= use=20 a miniature filament bulb driven from a stabilised voltage and a lens= to=20 give a ~parallel light beam. You can use an IR LED if you provide an auxil= iary=20 Si photodiode and a feedback amplifier to stabilise the photo output. Othe= rwise=20 it drifts all over the place. I use two small razor blades for the sides= of the=20 shutter, painted matt black, but with clean edges. They move just clear of= the=20 surface of the photocells.  I measured the noise level at about 14 na= no=20 metres. There is inevitably some heat associated with the bulb. I suggest= =20 placing the bulb in a separate housing on top of the sensor. You under run= the=20 bulb voltage supply to give ~infinite life.
 
    Regards,
 
    Chris Chapman

[ Top ] [ Back ] [ Home Page ]