The overloading of potentiometers

Creating complex firmware for existing hardware is often complicated by unforseen ( or ignored by the hardware designer ) human-interface issues. One example of this situation comes up when a project requires more settings than there are physical controls for. Modern cameras provide examples of the problem, and also of its solution.

Consider the case of a camera with a touchscreen. Problem solved! The touchscreen provides the capability of having a given location on the screen provide control for many different things, each made available with a different screen display.

Some modern cameras, however, have a few buttons which could be called multipurpose. In one mode, a given button might be mapped to a different function than it is in another mode.

I’m currently working with the OpenEffects Project hardware, which sports 4 potentiometers. I want to experiment with quite a few different effects and explore their interactions with my electric violin. The number of parameters for these functions in total is about 40. I broke up the settings so that each effects module had its own settings mode. Its settings are displayed and can be changed while its mode is active.

Since this design overloads the potentiometers, I had to think long and hard about what should happen when the mode is changed, since the pot can’t move to reflect the ( different ) setting of the ( different ) parameter of the mode’s ( different ) effect module.

I chose to ignore each pot until it is moved, at which time the related effect module setting will ( unfortunately ) jump to the current pot setting.  (Note – consider using a rotary encoder rather than a pot in your designs!)

The need to ignore a pot when the mode changes brings up a *different* gotcha. Reading a voltage several times will often yield different values. This will cause the pot value to be perceived as changed. Rats. So I had to add a guard band around the current setting, considering any reading within the guard band to be unchanged.

I also added an EWMA smoothing algorithm  to minimize the required size of the guard band. The larger the guard band, the more one has to move the pot in order for the change to be noticed. The longer the averaging period, the more sluggishly the reaction to a change occurs and settles.

I  ended up by splitting the difference to balance rotational- and time- sluggishness. But it works!

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