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Issue #213 April 2008
Digital Touch
A Potentiometer with No Moving Parts
by Jeff Bachiochi
Start | Touch Slider IC | Digital Potentiometers | Control and Feedback | Value to Digits | Linear Segmentation | Worth It | Sources & PDF
LINEAR SEGMENTATION
Be careful when you lay out the QT411 linear slider input sensor segments. By placing the segments on the bottom side of the PCB, you can place surface-mount resistors between segments and still get a perfectly smooth surface to touch (top side of the PCB), not to mention the insulation factor of the PCB. For smooth linear interpolation, the segment size should be no larger than the smallest object used to touch the slider. Although I’ve chosen to use a vertical sensing area (0x00 = bottom to 0x7F = top), this can be easily inverted by swapping two connections. Horizontal as well as curved sensing areas can be used in the design. Note that small guard traces on the outside of the segments can be used to limit interference with additional sensors that might be used in larger multi-potentiometer applications, such as audio mixers or graphic equalizers (see Photo 2).
Data conversion (QT411 data to MPC41010 position) does not have to be linear. Conversion algorithms can be used to apply a nonlinear function between the slider and the digital potentiometer’s value. For instance, you can give MPC1010 an audio (logarithmic) taper when used in volume control (see Figure 5). This digital device also comes in a dual-potentiometer version. The MPC42010 has two isolated devices that may be used in applications where a dual-ganged potentiometer is required.
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| Figure 5 — This chart demonstrates how the potentiometer resistance at the wiper might change based on its position. While this is a fixed function of the resistive element on a mechanical potentiometer, it can be changed via software in this application. (Source: Elliott Sound Products, http://sound.westhost.com/pots.htm) |
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