Membrane potentiometers function as voltage dividers, just as mechanical potentiometers do. They are composed of three principal membranes, or foils, and the whole assembly can fit into a total depth of less than a millimeter. An important purpose of the device’s outer layer, or outer membrane, is to keep the layers underneath it—the conductive and resistive layers—free from contaminants and mechanical trauma. All three membranes, though well under a millimeter in depth, can be several inches long, or longer if needed.
Inside a Membrane Potentiometer
The outer membrane lies directly on top of conductive membrane, which is separated from the resistive membrane by an empty space; this space is about the same thickness as each of the three membranes. With the application of pressure on the outer membrane, from something like a fingertip on a tablet computer, the conductive membrane makes contact with the resistive membrane, and an electrical connection is established between them. Thus, the conductive membrane serves the same purpose as the wiper in a mechanical potentiometer.
Figure 1: Looking inside a membrane potentiometer. (Source: Spectra Signal)
As the illustration above shows, not all membrane potentiometers include an outer layer; in some applications, the device exists in a protected environment and doesn’t need this layer. In this version, the conductive membrane is called the collector, and an adhesive bottom is included to fasten the device firmly in place. Of course, this illustration is not drawn to scale. When all four layers sit firmly on top of each other, as they would in a fully manufactured device, the entire thickness is, as stated, less than a millimeter.
Figure 2: A membrane potentiometer in action. (Source: Spectra Signal)
As the picture illustrates, when the user pushes on the conductive membrane, he or she pushes it down through the circuit spacer. The conductive membrane makes contact with one place on the resistive membrane. Both ends of the resistive membrane are connected electrically to the manufactured device of which they are a part, as is the conductive membrane. The resistance measured between the conductive membrane and either end of the resistive membrane depends only on where the conductive membrane makes contact on the resistive membrane.
Two-Dimensional Membrane Potentiometers
A membrane potentiometer, as defined thus far, can only be accessed along a straight line, and can only give back one resistance value at a time. It is also possible to build membrane potentiometers in two dimensions that span the entire surface of touchscreen devices that can return two resistances—one specifying the vertical point of the touch, and the other specifying the horizontal.
The connections to such a device would include, as before, the conductive membrane and the left and right sides of the now-rectangular resistive membrane. However, in addition to these three connections, there would also be two more—one to its top and one to the bottom of the resistive membrane. A microcontroller would alternately measure the resistance of the conductive membrane relative to the top and bottom, and then would measure the resistance of the conductive membrane relative to the left and right. The two measured resistances would define the point on the touchscreen that the user has accessed.
Membrane potentiometers are smaller and cheaper than mechanical potentiometers. The manufacturing process is simple and easily modified, so designers can get exactly what they need, even if it’s for a relatively small run. In addition, they can give manufacturers an inexpensive solution for what might otherwise be difficult to accomplish.