From: Tangazazen 

Fri, 4 Jan 2013 9:07
Subject: Re: Omnidirectional instrument


Hi Geoff,
=20
You have virtually said it all. Quadrant photo cells are a bit pricey but i=
f you place two of the rectangular surface mounted photo cells next to one =
another and masked them such that each was half covered, then any movement =
of the mask will increase the response of one at the expense of the other. =
If you now shine an LED modulated say with a square wave on them, then the =
outputs will be two square waves whose amplitude corresponds to the displac=
ement but will have the same phase. Now invert one and add it to the other =
so that at one point they will cancel but any displacement will produce a s=
ignal  proportional to displacement but with a phase corresponding to the d=
irection. The phase sensitive rectifier will convert this to a D.C signal p=
roportional to displacement direction and amplitude. Differentiating this w=
ill give volts/metre sec. as per the coil and magnet system but free of fli=
cker noise. The low frequency response is then set by the differentiator an=
d of course the seismometer pendulum period, to which you can now  add one =
of the frequency extension circuits. Though a bit expensive  the AD639 will=
 do all the phase sensitive rectifier business.

=20
Martin

Hi All,=20

    With reference to the above, try checking out the account and circuits =
at=20
http://jclahr.com/science/psn/chapman/photo_detect/index.html
The BPW34 photo diodes are in stock at DigiKey at just $0.87 each.=20
You need to either stabilise the light from an IR LED using another BPW34 a=
nd a precision
reference diode, or use a small tungsten filament bulb with a similar feedb=
ack circuit. The light=20
output of a LED can be expected to decrease by a factor of ~x7 between 0C a=
nd 100C
A LTC1043 will provide a relatively inexpensive quad chopper and an oscilla=
tor.
The AD639 is long out of production and it was a universal trigonometric co=
nverter ???

    Regards,=20

    Chris Chapman

=20
=20







From: Tangazazen <Tangaz=
azen@.......>






Fri, 4 Jan =
2013 9:07




Subject: Re: Omnidirectional instrument






























Hi Geoff,








 








You have virtually said it all. Quadrant photo cells are a bit pricey =
but=20
if you place two of the rectangular surface mounted photo cells next to one=
=20
another and masked them such that each was half covered, then any movement =
of=20
the mask will increase the response of one at the expense of the other. If =
you=20
now shine an LED modulated say with a square wave on them, then the outputs=
 will=20
be two square waves whose amplitude corresponds to the displacement but wil=
l=20
have the same phase. Now invert one and add it to the other so that at one =
point=20
they will cancel but any displacement will produce a signal  proportio=
nal=20
to displacement but with a phase corresponding to the direction. The phase=
=20
sensitive rectifier will convert this to a D.C signal proportional to=20
displacement direction and amplitude. Differentiating this will give volts/=
metre=20
sec. as per the coil and magnet system but free of flicker noise. The low=
=20
frequency response is then set by the differentiator and of course the=20
seismometer pendulum period, to which you can now  add one of the freq=
uency=20
extension circuits. Though a bit expensive  the AD639 will do all the =
phase=20
sensitive rectifier business.













 








<=
font color=3D"#000000" face=3D"Arial" size=3D"2">Martin







Hi All, 







    With reference to the above, try checking out the account and circuits at 



http://jclahr.com/science/psn/chapman/photo_detect/index.html=




The BPW34 photo diodes are in stock at Di=
giKey at just $0.87 each. 



You need to either stabilise the l=
ight from an IR LED using another BPW34 and a precision



reference diode, or use a small tungsten filament bulb wit=
h a similar feedback circuit. The light 

output of a LED can be expected to decrease by a factor of=
 ~x7 between 0C and 100C



A LTC1043 will provide a relatively inexpensive quad chopp=
er and an oscillator.

The AD639 is long out of production and it was a universal=
 trigonometric converter ???



    Regards, 



    Chris Chapman