Everything for Electronics
Posted in: Developing Perspectives (October 2008)

The Man-Machine Interface — Getting It Right

By Bryan Bergeron

Technologically and visually, the guitar is stunning. The frame of the guitar is a hardened exoskeleton over a spruce body about half the weight and thickness of a Strat or Les Paul. Electronically, it has a traditional magnetic pickup combined with a not-so-typical piezoelectric pickup. With this dual pickup design, I can create a tone that simulates an airy acoustic (using all piezoelectric pickup), a throaty Les Paul (using all magnetic pickup), or something in between. Furthermore, not only is the tone great, but it plays like butter.

After a few hours of playing my flame red mojo, it started to rattle. The culprit? A loose set screw in the aluminum knob on the magnetic pickup potentiometer. I loosened the set screw and examined the potentiometer shaft. I expected a smooth, solid shaft, but what I found was a deeply slotted knurled shaft. No wonder the knob had worked its way lose.

My first thought was that this must have been a fluke or accident at the factory. After all, we’re talking about a $3,200 guitar. I imagined someone in the parts department at Parker running out of solid shaft pots and running over to RadioShack to buy out their supply of potentiometers — any potentiometers that were of the appropriate resistance. However, I checked the two other potentiometers on the guitar. I didn’t have to remove the knobs to see if they were of the same slotted design — the wobble of the knobs as I turned them told me the knobs and potentiometer shafts were mismatched. In my book, the only thing worse than using smooth knobs with set screws on knurled potentiometer shafts is using a knob with a set screw on a shaft — solid or knurled — that’s too small in diameter. It’s virtually impossible to align a smooth knob on a knurled shaft without the knob tilting off-center. The smaller the shaft relative to the knob’s interior, the worse the knob’s wobble.

I briefly considered keeping the metal knobs and replacing the potentiometers with solid shaft versions from Digi-Key, but didn’t want to void my warranty. Instead, I replaced the aluminum knobs with a set of Gibson push-on plastic knobs. They fit snugly and lightened the guitar by an ounce or two.

The point to the story isn’t that you should avoid a Parker, but that you should pay attention to the man-machine interface in your project designs. In the case of a potentiometer, for example, it isn’t enough to get the resistance, wattage, linearity, rotational life, and element composition right. You also have to think about where to best place the potentiometer on a front panel, and what kind of knob to use. It may sound trivial, but a poor quality interface leads the user to suspect poor workmanship inside. When I discovered the mismatched potentiometers and knobs, I felt as though I had purchased a little red sports car, only to have the door handle fall off after a few miles. I began to wonder what other shortcuts had been taken with the wiring inside the guitar body cavity, away from prying eyes.

Designing a good man-machine (user) interface is part art, part science, and part common sense. You might be surprised at how many electronically ingenious projects submitted for publication in Nuts & Volts never make it to print because the author hadn’t taken the time to think about user interface. Labeling a front panel with masking tape and magic marker and adding a dozen LEDs to boot suggests the underlying circuit is little more than a hack. By the way, if you routinely mount toggle switches with knurled nuts on aluminum front panels, a simple way to avoid scratching the front panel or stripping the nuts is to use a plastic GC Nutwrench. The $18 tool is available from Stewart-MacDonald, [url=http://www.stewmac.com]http://www.stewmac.com[/url]. Tesco [url=http://www.tesco.com]http://www.tesco.com[/url] sells a similar, all metal unit for the same price, but I haven’t worked with it. Consider these and similar tools when you’re putting the finishing touch on your next creation. NV