Issue
114 January 2000
Reach
Out and Touch
Designing
a Resistive Touchscreen
by
Tom Dahlin
It’s
never been so easy and inexpensive to build data-acquisition
and data-monitoring applications. For instance,
Stephen recently developed a set of digital gauges
for his car, and all it took was a Palm, an ATmega163,
and a little forethought.
Start
•
An
Inside Look
• Taking
The 5-Wire Route
• Development
Notes
•
Sources
and PDF
The
use of touchscreens as computer interface devices has
become widespread. I don’t think there are many
readers out there who have not used an ATM machine with
a touchscreen or at least played with one of the Apple
Newton-type personal digital assistants. The popularity
of touchscreens is in a large part due to the simplicity
they bring to user interfaces. They eliminate the need
for a large keyboard and offer the benefit of context
specific menu choices.
I
was recently involved in the design of a machine controller
for an industrial application. We liked the advantages
of a touchscreen interface, and prototyped the system
using an industrial embedded PC, and a commercially
available motion-control board. We purchased an off-the-shelf
industrial LCD display/touchscreen combination and connected
it to the PC’s VGA and COM1 interfaces. We then
developed the application software in VB and got the
machine up and running in short order. Life was good.
The
euphoria was short-lived however. Economic reality set
in, and we realized we couldn’t justify spending
over $2k on the off-the-shelf LCD/touchscreen for the
commercial product. Besides that, the mechanical form
factor was wrong. With a short development cycle ahead
of us, life was looking not so good.
We
quickly realized that we would either have to roll our
own LCD display/touchscreen, or drop back to a character-oriented
LCD/16 key keypad for the user interface. While it is
not practical for a small company to consider making
its own touchscreen (the glass part that is), it is
possible to save both money and package size by purchasing
only the raw touchscreen, using a standard LCD or CRT,
and designing your own controller and package. This
article will show you a few of the ways of designing
the touchscreen-to-computer interface.
Types
of Touchscreens
Not
all touchscreens are created equally. There are two
primary technologies used today—resistive and capacitive
sense. There are others, such as IR scanning, acoustic
wave, and electromagnetic technologies. Although they
all have their merits, resistive- and capacitive-sense
technologies have emerged as favorites thanks to their
low cost and high resolution.
Resistive-sense
technology has the added advantage of being able to
detect a touch from a rubber-gloved finger, something
that is a problem for capacitive type touchscreens.
The other technologies mentioned are used when they
offer a unique advantage, such as the sense-before-touch
feature that IR scanning provides.
For
a discussion of the relative pros and cons of each technology,
you might want to read the application notes written
by David Blass of Sharp [1], and another by VJ Kuroodi
of Tritech [2].
