IEEE Spectrum October, 2012 - 24

hands on

Work IN ProGrESS: the prototype mouse engraver uses
a microcontroller and a laser lashed to a uS $10 mouse [left]
connected to a pc. initial testing with a checkerboard image
[above left] produced almost random marks, but reducing
the load on the pc leads to improvement [above right].
Photos: Paul Wallich

LASER MOUSE
A quick-and-dirty tool for engraving
large images

P

reviously, I've written
about how to build
a low-power laser
engraver from the guts of a
DVD burner [see "Laser Cuts
Paper," IEEE Spectrum, June].
My fully automated setup
was similar in spirit to that
of a commercial engraver,
and while effective for small
areas, this approach quickly
becomes unwieldy if you want
to burn larger areas. You need
longer rails and leadscrews,
bigger stepper motors, more
structure, and more space.
As I don't often require
a high level of precision on
larger areas, one evening I
had an inspiration, and a
new laser mouse was born.
Normally, a "laser mouse"
shines a laser onto a surface
in order to track its position.
24

My laser mouse does
something different: When
I move the mouse across a
surface, it uses its tracking
mechanism to make sure
that an attached engraving
laser is shining when it
should be. The resulting
engraving should look a
little like a stone rubbing.
(Note that even a small area
of charred wood or paper
can produce a remarkable
amount of smoke.)
So how does it work?
The laser mouse requires
a host computer, so I coded
up something ugly but
functional in Lisp on my
trusty basement Ubuntu
Linux box (you can use the
computer and language of
your choice, so long as you
can get access to a USB port

NA * iEEE SpEctrum * OctObEr 2012

and can read mouse-position
information). The Lisp code
converts a digital image into
an array of brightness values;
then it reads "/dev/mouse0"-
Linux's standard software
interface for an attached USB
mouse-to decode incoming
three-byte messages from
the mouse into incremental
x and y movements. These
incremental movements
are continually tallied to
determine absolute position.
Then my program consults
the brightness array, and
if the corresponding value
for that position in the array
is less than a threshold set
when the program is started,
it sends a 1 along a second
USB connection to the
microcontroller driving the
laser. Otherwise it sends a 0.
On the mouse side, I
taped a prototyping board
fitted with a microcontroller
to a US $10 Logitech mouse.
The microcontroller I used is
a Teensy, a controller slightly
shorter and narrower than
my thumb, that's mostly
compatible with the popular
but much bulkier Arduino
microcontroller. The Teensy
listens to its USB connection,

decodes the packets being
sent by my Lisp program,
and, as needed, puts a high
or a low voltage on an output
pin that enables or disables
a leftover laser driver
connected to a 100-milliwatt
red laser I had lying around.
Ideally, I'd like a more
powerful laser to draw
faster, but did I mention
that even a small area of
charred wood or paper can
produce a remarkable amount
of smoke? And even this
100-mW laser draws more
power than the Teensy can
supply, so I have to use an
additional external power
supply for the laser driver.
As a consequence, instead of
powering the Teensy from
the USB connection as I
might normally do, I also
had to connect the Teensy to
the external power supply
to ensure a common ground.
Otherwise I'd have run
the risk of unpredictable
voltages on the connection
between the controller
and the laser driver.
So far I've been using a
checkerboard pattern as my
test image. Initial results
produced more or less
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Table of Contents for the Digital Edition of IEEE Spectrum October, 2012
