SHARPENING YOUR TOOLS OF CREATIVITY
PICAXE PRIMER
■ BY RON HACKETT
EXPERIMENTING WITH
TOUCH-SENSITIVE KEYPADS
In this month’s installment of the Primer, we’re going to explore two different
implementations of a touch-sensitive keypad. However, before we get started, I want to
mention a recent announcement from Revolution Education. Updated versions of the
PICAXE-28X2 and 40X2 processors are now available (as of December ‘ 10). The two new
processors now support both 3V and 5V operation. In other words, the updated
processors can operate anywhere between 2.1V and 5.5V. In addition, both processors
include many additional ADC inputs, as well as support for capacitive touch inputs
similar to those of the 18M2 processor. If you are interested in the details, an updated
datasheet for the X2 parts is available on the Revolution Education website (www.
rev-ed.co.uk/docs/picaxex2.pdf).
CONSTRUCTING A
STRIPBOARD MATRIX
TOUCHPAD
Naturally, my first approach to
constructing a matrix keypad
involved the use of stripboards.
Building the keypad was easier than I
thought it would be, but, as we will
soon see, I haven’t been able to
obtain reliable readings from the
touch inputs. However, I did learn a
considerable amount about touch
inputs in the process, so I think it’s
been a worthwhile endeavor.
would love to hear about it.) If you
would rather not get involved in a
potentially frustrating endeavor, later
on I’ll describe a printed circuit board
(PCB) approach to touch-sensitive
keypads that completely resolves the
issues I had with my stripboard
keypad.
My design for a matrix touchpad
employed two identical stripboards in
a “sandwich” arrangement to
essentially duplicate a standard 4x4
matrix keypad. In other words, I
constructed a two-board sandwich
that had four rows and four columns,
and required eight touch inputs for
interfacing with the 18M2 processor.
Figure 1 shows the LochMaster
layout for the top stripboard in the
sandwich. (A larger version is also
available for downloading from the
N&V website.) As you can see, the
key layout is shown correctly on the
bottom of the board (not the top).
That’s because I used the board
upside-down – I wanted to keep the
copper traces as close to the surface
■ FIGURE 1. Stripboard layout for
a 4x4 matrix touchpad.
14
February 2011