Nuts and Volts - December 2010

■ FIGURE 4. Stripboard layout for a simple two-key touchpad.

parameters, but I was in too much of a hurry! When I ran the program without touching the jumper wire, the value of variable b1 that was displayed in the Terminal Window ranged between 13 and 15; as soon as I touched the wire (actually, the insulation – more about that shortly), the value of b1 jumped to 50 or more. In fact, if I squeezed the insulation between two fingers, I could easily raise the value above 200. However, it’s not the pressure of the “squeeze” that it being sensed; it’s the greater surface area of the two squeezing fingers that come in close proximity to the wire.

Before you actually begin your own experiments, I want to mention an important aspect of using the touch sensors on the 18M2, or any of the other M2 processors that will

■ FIGURE 5. The completed two-key touchpad.

be released in the future. The term “touch sensor” is actually a bit of a misnomer, in that the sensor itself (in this case, the jumper wire) is never actually touched. In fact, the documentation in Section 2 of the manual includes the following, somewhat ominous warning: “Never ‘directly touch’ a touch sensor (e.g., a piece of bare wire)! A touch sensor must be electrically isolated from the end user.” So, “proximity sensor” might be a more apt term for what’s involved, but don’t forget that PICAXE processors are used throughout the British public school system which may explain the choice of terminology. In any case, it’s easy to confirm that proximity is what’s being measured. When my little test program is running, if I place my finger close to the insulation on the jumper wire, I can easily increase the value of b1 to 30 or more without actually touching the insulation.

So, as soon as you can get your hands on an 18M2, set up our little test circuit on a breadboard. The circuit is so simple that you don’t even need a schematic, just use the photo shown in Figure 3.

Next, use the Programming Editor (or AXEpad) to type in Touch Test1.bas and download it to your 18M2 circuit. See how your results compare with mine, and what other interesting findings you can discover.

EXPERIMENT 2: CONSTRUCTING AND TESTING A “PROPER” TOUCHPAD

After playing with (oops – I

meant to say experimenting with) my jumper wire sensor, I decided to construct a simple touch pad. Naturally, I used a small piece of stripboard, and to keep things simple I

limited myself to just two “keys” for my first attempt. The stripboard layout that I used is presented in Figure 4. I haven’t included a bottom view because the only two cuts that need to be made on the bottom of the board are clearly visible in Figure 4. (Their function is to simply create a dead zone between the two keys.) The two-pin male header at the top of the stripboard is reverse-mounted so that the keypad can be directly inserted into a breadboard, connecting the two keys to two adjacent touch inputs on the 18M2. The two bare wire jumpers on the bottom of the board each connect seven adjacent traces to one of the two header pins. To solder each jumper to the bottom of the board, just hold it in place with a small spring clamp, solder it to three or four traces, and then remove the clamp and solder it to each of the remaining traces spanned by the jumper.

Figure 5 is a photo of the completed keypad, and Figure 6 shows it inserted into the same breadboard circuit we just used for our jumper wire. The two pins of the keypad are inserted next to pins B.1 and B. 2 of the 18M2 — both of which are touch inputs. I made the paper label on top of the keypad by printing out the stripboard layout in actual size, and trimming it to fit. Before I taped the paper label to the top of the stripboard (using cellophane tape), I covered the bottom of the board with a piece of electrical tape to be sure the board didn’t make electrical contact with any jumpers that might be located beneath it on a breadboard project. The process was simple, and it makes it clear that it will also be easy to create multi-colored graphic labels for a couple of the larger keypads that I already have in mind.