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Issue #213 April 2008
Digital Touch
A Potentiometer with No Moving Parts
by Jeff Bachiochi
Tired of replacing your mechanical parts? Jeff has the solution: eliminate them. His design provides user input and a digital potentiometer for resistance or voltage output.
Start | Touch Slider IC | Digital Potentiometers | Control and Feedback | Value to Digits | Linear Segmentation | Worth It | Sources & PDF
Living in a material world has its benefits and drawbacks. You can either choose to become wrapped up in possessions with a possibility of becoming a King Midas or you can use technology to improve your life and those who share this neighborhood we call Earth. When I see the imbalance of opportunity in the world, I feel guilty to be living in the U.S. However, this will be true only if I squander my good fortune without any attempt to improve life for others, either directly or indirectly.
At times, I wonder if some advances in technology are really improvements. Is an electronic switch that replaces a mechanical one an improvement? Is medicine that brings relief, along with side effects, an improvement? How about a spray that protects a fruit tree from disease, but sterilizes the bee population? We can’t always predict how technology will affect future generations. That’s why there are strict controls on DDT, asbestos, and lead-based paint.
Mechanical wear is viewed as a weak link in designs. Anytime you have two materials that rub against one another, you will have wear. We go to great lengths to reduce this by using bearings or material coatings. When an electrical connection is necessary between materials, as in potentiometers, any wear will affect the device. How many stereo systems have been junked because the audio controls became scratchy? At high volumes (where I like to listen), this is like listening to fingernails drag across a chalkboard.
Digital potentiometers aren’t new, but their use hasn’t greatly impacted the number of mechanical potentiometers used in today’s products. That may be due to the fact that they require additional hardware. This can be as simple as mechanical switches that directly step the digital potentiometer’s wiper up or down or a microcontroller and another user input device to support the up/down control.
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| Photo 1—By keeping all of the components on the solder side of the PCB, the top side remains free of parts that would otherwise interfere with a nice graphic label. |
This month, I will show you how to create a potentiometer with no moving parts (see Photo 1). That’s right: no mechanical parts to wear out or break. The design features a Quantum Research Group (QRG) QT411-ISSG QSlide touch slider IC, which provides user input and a digital potentiometer for resistance or voltage output. It also features a small microcontroller to convert data and display position.
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