The reason for the 2 cells is this. The circuit is connected so the two 
cells basically short each other out.
Here is a basic diagram I will use for the following examples.
http://www.geocities.com/travis5765/seismo/images/photocell_sensor.png

There are 3 examples. Figure 1 with the light spot centered between cells 1 
and 2, figure 2 with the spot on cell 1, and figure 3 with the spot on cell 
2.

Because the cells are shorted, if they both have the same light intensity, 
such as the case in figure 1, the signal at A and B is NULL. no signal. In 
figure 2 and 3, the signal is the difference of cell 1 and 2. Lets say for 
this example that signal B is connected to the + input of the recorder and 
that signal A is connected to the - input. with this connection, cell 1 
outputs a positive phase signal and cell 2 a negative phase signal. We can 
also say for this example that each cell outputs 5 volts with the light 
source.

So in figure 2, the light spot is on cell 1 and not at all on cell 2. so 
cell 1 outputs the full 5 volts. so we figure +5v + -0v = +5v. In figure 2, 
the spot is fully on cell 2 and not at all on cell 1. +0v + -5v = -5v.

anywhere in between the two cells can be calculated the same way. Say for 
instance, the spot is positioned in such a way that cell 1 outputs 3 volts 
and cell 2 outputs -2 volts. then we have +3v + -2v = +1v.

The signal output can be calculated to tell almost exactly where the light 
spot falls on the 2 cells.

Using one cell as you mentioned can be done. you would only have a single 
phase output though. The system would have to be setup so that the spot fell 
on the place needed to output a centered signal (5 volts at full spot would 
need it to be positioned so that 2.5 volts were outputted. So like I said, 
it can be done but it is a little more work to calibrate.

I hope I didn't cause a lot of confusion (my explanations seem to do that 
sometimes). if you need me to clarify something, let me know.

~Travis



>From: "meredith lamb" 
>Reply-To: psn-l@..............
>To: psn-l@..............
>Subject: Re: Simple photoelectrics
>Date: Sun, 27 May 2001 16:21:04 -0700
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>From larry_cochrane@.............. Sun May 27 15:32:16 2001
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><001201c0e6f5$5e4aa4a0$7efdb3d1@george>
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>Is there really....any technical advantage to using two side by
>side photocells (misc variety in a differential bridge), as opposed
>to using just one alone?  I'd think one cell would be simplier
>mechanically but I'am not sure if it would react the same
>circuit wise or as well (?) as two cells.
>
>Its kind of relivant, as I'am on a large bright LED and single
>CDS (photoresistor) sensor project at the moment...which could
>be changed over time.
>
>Meredith Lamb
>
>George Harris wrote:
>
> >
> > The very simple and quiet photosensor system can be made from a bright,
> > large LED and
> > two small silicon solor cells.  The cells are placed side by side and
> > connected in parallel
> > opposing across the + and - inputs of a quiet opamp. (I use OP 77's).
>
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