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,
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
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