----- Original Message -----
From: meredith lamb 

>
> One might take a peek at Allan Coleman's web site, in relation
> to the topic of opto-electronics, where he uses a small
> Radio Shack incandescent bulb and acouple small photodiodes
> in his VBB horizontal seismometer.  This looks alot more
> feasible to myself as a non-high-tech amateur.

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).  The
value of the feedback resistor defines the gain.  The sensors part 980-0150
(VTD34) from Allied are about 1/8 inch square, and work well.  At $1.59 each
they are hard to beat.  A major advantage of silicon dells is that they have
very nearly the same sensitivity from unit to unit.  Place the cells in a
hole on one side of a block, the LED on the other, and have the moving
element enter a hole from the third side.
The system is linear for about 1/8 inch of motion.  If larger motion is
desired, the cells can be larger.  A cover may be required for the sensor
area if the complete seismometer is not covered to avoid changes in overall
light level.

The moving element has a projection which has the same width as the
diminsion of one cell and is placed to intercept half of each cell.  Due to
the fact that both cells are illuminated by the same source, small changes
in illumination cancel out.  The system is capable of detecting nanometer
motions with low noise.

George Harris
gjharris@..............
707-751-1936


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