Hi Geoff,
 
You have virtually said it all. Quadrant photo cells are a bit pricey but  
if 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 
displacement 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 
signal  proportional  to displacement but with a phase corresponding to the 
direction. The phase  sensitive rectifier will convert this to a D.C signal 
proportional to  displacement direction and amplitude. Differentiating this 
will give volts/metre  sec. as per the coil and magnet system but free of 
flicker noise. The low  frequency response is then set by the differentiator 
and 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.







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.