Touch-sensitive Linear Potentiometer from SpectraSymbol (aka “SoftPot”)
Initial report by Michael J. Horan, 01 November 2005


Like a typical potentiometer, the SoftPot is a 3-terminal (power, ground and ‘data’) resistor. Unlike a typical potentiometer which uses charged metal and a sliding wiper, the SoftPot separates a silver shunt and layer of carbon with padded material allowing for touch to trigger a reading rather than a mechanical motion.

The SoftPot acts as a momentary contact device, triggered by anything from a finger to a mechanical device. SpectraSymbol describes the SoftPot as “a conductive plastic membrane potentiometer. Polymeric carbon ink is deposited onto plastics for the static resistive element. This substrate is held in abeyance from a continuous contact wiper using a thin .005" spacer. Some applications close this circuit continuously by using a spring-loaded wiper. When pressure is applied to the wiper, contact is made between top and bottom elements and subsequent analog voltage is read from this wiper.”



The benefit of the SoftPot over its traditional sibling is immediately noticeable. It is far lighter and thinner than a traditional pot, reducing the weight and size of any design you create. There is no mechanical part that can deteriorate over time, like with a traditional slide potentiometer. The SoftPot is touch-sensitive, eliminating the need for a linear slide motion to capture position feedback data. The SoftPot is also fully customizable; resistivity, linearity, part-to-part tolerance and life cycle expectancy can be specified. The SoftPot is available in a range from 0.25” – 72”!

Make sure you know which pin is power, ground and data. The 3 varieties of SoftPots I received from SpectraSymbol used different pin configurations. Accidentally running power through the data pin resulted in its melting...



Simple Rotary Potentiometer


More Images
PIC Code for The LightTouch

LightTouch uses two linear potentiometers, in series, mounted under the front plexi panel to light up under a user's hand. To get around the problem of multiple readings when the panel is touched, we affixed rubber 'nubs' to the back of the plexi which act as contact points on the sensors. This allows the user to touch anywhere on the plexi panel and receive an illumination under their hand. This was an easy solution to a fairly complicated problem...

The Magical, Miracle Box

The Magical, Miracle Box uses a single SoftPot to control both audio and video playback from a relatively simple design. I placed the SoftPot on a bed of foam, as I found this not only more pleasing to touch, but it made the sensor slightly more receptive (and it is incredibly sensitive). Because the normal state of the sensor is 0, I made sure to elimate any reading below 1 (this prevents my Max patch from resetting the video/audio track every time a finger is taken away from the sensor. This application also revealed that the SoftPot is not a linear sensor, it is algorithmic. After attempting to use several code fixes, I realized that a linear reading from 1-100 was given over the first 80% of the sensor, while the remaining 20% gave inconsistent and wild readings. Rather than solving the problem, I covered it up! I limited the access point of the sensor to the consistent 80%.