A sense of humor needs to be restored to these proceedings

This was intended in a joking fashion. No serious person would suggest that
any reader of this list is anything other than hardworking and clever.
From griping comes invention, as it is an intermediate state after
necessity.

In spite of the request to forgo handwaving, that's where I'll begin:
The light source is the easiest part, but there are some subtlies: the
light should be nearly collimated. The simplest way to achieve this is to
use a LED with a narrow beam angle. The standard T1 3/4 lamps are 5 mm in
diameter and have a beam a bit smaller than that. There are some larger
diameters which could be handy.

Detectors are another issue. You can use 2 or 4 detectors. I prefer the 4
detector approach, since the electronics then don't have to be as stable.
There are some 4 detector packages, for example the UDT SPOT-4DMI and the
FIL-S4DG, these have detector sizes of 0.5 mm square and 1.3 mm square,
respectively.
It is desirable to have the detectors cover as little area as possible, so
that the system will be less sensitive to angular misalignment.

The enterprising builder might remove the quadrant detector from a CD
player. Personally I don't like fussing with surface mount packages.

Now on to getting patterns printed on plastic. Forgive me if this is too
elementary, but there may be some out there who don't know this trick. The
trick is to use the Linotronic (type) film printer at a printing service.
These print on mylar film very high contrast stable patterns. The machines
have resolutions from between 1200 and 3600 dpi.
They are used by Mac zealots, and only understand file formats that are
popular in the graphic arts world. My advise is to not try to get them to
understand any of your file formats, rather to find out what Mac's like and
use that. The programs they typically use are Pagemaker and Photoshot. So
if you can make a file in one of those, you're all set. They also read eps
files, but I've found that these give poor results- maybe you'll have better
results than me. AutoCAD can output eps files, but the linotronic printers
don't interpret them accurately.
I output files in dxf format and then make a Pagemaker document. This works
quite well.
You might want to consider how the machines native dot spacing can alias at
the spacing you want to use. Tilting the pattern at an angle to the machine
axes can reduce the effect of the aliasing.

If a few people are interested in making these, let me know and I'll work up
a parts list and a graphics file.

Mount the LED so it illuminates the detectors. A pattern drawn with line
patterns stepped in phase 90� is aligned to the detector. Op amps are used
to amplify the signal from the detector, using a transimpedance
configuration. The signals from the 0� and 180� are differenced with another
op amp, as are the 90� and 270� signals. The outputs from the differential
amplifiers are fed to triggers that go to 1 when the output is positive and
0 whent the output is negative.

The people on this list can surely think of ways to use the precise patterns
that the linotronic machines can generate. You can make scales, half-tone
patterns, and my favorite, the business card with microscopic features on
it.

Finally, let me add my voice to those who think that a CD with RT Linux and
EMC on it made as simple as possible to install is a good idea. Like, put
in the disk, get a instant machine controller. Is this possible?

Elliot Burke

>
>>Rather than gripe about HP not building what you want, build one yourself.
>
>Try to moderate your stridency, eh? Nobody was "griping" that HP
>wouldn't build what hobbyists desire. In fact, I alluded to the fact
>that they didn't HAVE the resolutions directly that we most wanted, by
>humorously observing that their Product Marketing Mgr. didn't seem to be
>too concerned with losing the "hobbyist market". This was OBVIOUSLY a
>joke and a recognition what we're small-fry in this market, not a
>"gripe".
>