all right this video is about the
resistive multi-touch pad that I put
together that's a very quick demo but
first I want to go over the materials
that are used to make membrane keyboards
keypads and resistive touch pads first
so there's conductive inks that are
placed down many of them contain silver
gold nickel or copper you can guess
which ones cost the most and uh have the
higher performance and then there's
resistive inks uh usually it's carbon
that's the Workhorse it's been used for
at least 30 years and then there's some
other interesting ones like antimon tin
oxide which is semi-transparent and
that's used in displays there's other
non- in type of conductors like indium
tin oxide that's used on transparent
screens so it's a transparent
conductor all right so this material
goes down like regular ink this is some
carbon based
ink resistive ink used to repair circuit
boards it can be silk
screened in uh kind of a standard
process much like making a
t-shirt you can print on multiple sides
of a substrate which could be rigid or
flexible you can have vas that go
through to connect top to
bottom can print on rubber if you want
to make for instance rubber buttons so
that went to this which when you press
that down it would make contact to this
particular pattern which would make a
row column
connection there's also materials that
might be interesting to us like these
high density Foams that are high in
carbon actually um very
low resistivity compared to other foams
and uh there's o Open cell and closed
cell versions of this stuff with
different kind of
compressions and it can also be put on
Rigid substrates like this toy here and
these little button places here have had
carbon silk screened on to Aid in making
the connection when these rubberized
carbon buttons are pushed against it
okay so I think I've covered most
of silk screenable conductors and
resistive materials well wait there's
one last thing so here's a piece that I
cut up and this shows a carbon strip in
a particular
pattern but it also has silver laid on
top of the carbon to lower the
resistance so a connector hooks on over
here and then comes up to the carbon so
you you can do little tricks like that
by
adding silver traces here's another
example of
it here's
some resistive touch pads and it has
silver traces going around so that you
have a uniform resistance across that
and that's going to be important in uh
my next explanation
here so my first experiment and this is
what iect a lot of multi-touch resistive
pads are made like
this you have silver conductors that
make columns that come down that make
contact to
resistive carbon which is silk screened
on very much like this
or like this see this um you can
actually print the stuff very close
together here's an example of some very
fine lines
printed so those are printed in this
pattern for
your
columns and
then layers are added silk screen down
to insulate certain areas so you can put
polymers down so you don't get shorts
between your top layer and
your your lower conductors in places you
don't want them and also you can put
bumps that keep the two
the two pieces separated slightly so
they're not making connection until you
actually push
down so and then you would have a top
layer which would be some kind of
flexible membrane that would
have
row connections and when you push it
would push down make
contact
along these resistive um columns and
then by grounding and then activating
each of the rows or columns at a time
and then sensing the output you can
determine which location is pressed now
if there's sufficient resistance between
each uh Row for instance in this one
here you can determine if there's more
than one press at a time so that's
pretty important that you have enough
resolution in your system to be able to
determine if two
adjacent rows or columns are pressed you
depending on your configuration of
what's resistive or not and so that's
what I tried here I cut up some of
these resistive touch pads that are out
of some toys that I worked on and I did
a
3x4 and that worked really well um it
didn't scale very well because there
were so few pixels and I didn't want to
go through all that so I worked on a
different
scheme which actually found some
literature out out there some folks have
done stuff like this
before which use a substrate that has a
top and a backside or a top an
insulating layer and an under layer that
has different connections so the way
this works is you have columns which are
solid conductors that come
down okay and then on a another layer
whether it's the insulating layer or the
bottom layer you have traces that run
across pop up through and touch pads
that are isolated so these are all
silver there's no resistive material on
there you can buy material that's high
in carbon like this foam that I was
showing you earlier or I used
anti-static bags so these are used to
hold circuit boards to prevent them from
being Zapped by static electricity so
that's what I put together here so now
I'm going to back up so we can take a
look at the
demo so the first thing I'm going to
show is the touch pad that's arranged in
that
configuration of
um you know columns
and and pads that pop up all right here
it is right
here so on top is the resistive plastic
high in carbon I have some trimmer
resistors which I ended up not using so
I have rows and columns configured in
here and up here on the
screen will be
the the result uninterpolated just raw
um values as I press down so the harder
I press down we'll see a value from 0 to
16 I didn't do a lot of resolution on
the pixels
and this has 8 by five pixel elements in
it and they're fed into an analog to
digital converter and then into an fpga
which is generating the VGA
signal I'm kind of getting out of order
here but that's what this is very
simplistic um uh any microcontroller
could do this also I use fpga because I
can work
in that um domain much faster all right
so if I push on here you can see up on
the
screen that
there's pixels lighting up if I lightly
press down yeah there'll be less
intensity I press down hard and spread
my finger across more pixels more of
them light up and brighter now if I take
two fingers and push on
here now there's multiple
touch points I can even do three kind of
the limit of the number of pixels that
I've got okay and behind here is my
absolute brain dead linear interpolated
only in the horizontal
Direction
um uh algorithm it's nothing special I
only did the horizontal location so I
didn't have to have a line store and
this is all being generated on the fly
there's not even a single temporal frame
being stored so if I put my finger on
here we can see I've doubled the
resolution by doing linear interpolation
in the horizontal
Direction
so um one could
imagine you could increase your
Precision of your analog to digital
converter from 8 Bits and do some more
complicated interpolation in both
axis and add some clipping functions to
it and
have um far more resolution all right
well I better cut the short I think I
only have 10 minutes on YouTube so uh
I'll talk to you guys
later