Nuts and Volts - February 2011

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