Adjusting the volume dial on a sound system, sensing your finger position on a touch screen, and knowing when someone’s in the car are just a few examples of where you encounter variable resistors in everyday life. The ability to change resistance means the ability to interact, and that’s why variable resistance devices are found in so many things.
The principles are the same, but there are so many ways to split a volt. Let’s take a look at what goes into rotary pots, rheostats, membrane potentiometers, resistive touchscreens, force sensitive resistors, as well as flex and stretch sensors.
Potentiometer
Potentiometers, or pots, are basically voltage dividers: a method of dividing a given voltage down to a lower level. As the schematic shows, a potentiometer (in grey) has three connection points. The middle connection point is adjustable (denoted by an arrow head) and makes contact with the resistance material inside at a point somewhere along the material’s length.
The voltage between the adjustable point and one of the other points (the ends of the resistance material) is determined by the resistance between those two points. If only two points are connected to then it’s considered a variable resistor or a rheostat.
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In the photo is a pot with a cylindrical shaft that you turn. The plastic volume knob on your sound system is hiding one of these pots. Note the three connection points (terminals), the middle one being the one that’s connected to the adjustable point. The photo shows a new, uninstalled pot. Here’s an example of where I used a pot for volume adjustment on a peanut butter jar amplifier (incidentally this was covered on Hackaday).
How Potentiometer Resistance Changes
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Potentiometers can have a linear range of resistances or a logarithmic range. Linear means that as you turn the shaft, the resistance values you get change linearly. Turn it a quarter of the way and the resistance will change by one-quarter the full range. Turn it half way and the resistance will change by half the full range.
But for a volume knob the volume will sound like it’s changed too fast; that’s due to the way our brain interprets what our ears hear. So for a volume knob it’s best to use a pot whose resistance changes logarithmically as you turn the knob. The graph shows how the volume changes as the knob is turned, or rotated, for both linear and logarithmic. Note that some logarithmic pots are only pseudo logarithmic and are cheaper than true logarithmic ones. These consist of two linear sections with different rates of change and meet at 50% of their rotation. Those are also shown in the graph.
This logarithmic behavior is sometimes accomplished by making the resistive element inside be tapered in shape; its width changes from one end to the other. For this reason pots are often referred to as either linear tapered pots or logarithmic tapered pots.
Another form potentiometers come in are as trimmers, or trimpots. They’re smaller than the above ones and are used on circuit boards. They’re usually adjusted just once (or very rarely) as a way to calibrate an electronic circuit.
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Potentiometers aren’t all rotating components though. They can also be sliders, as shown in the photo of an equalizer on a keyboard. These are prone to dirt getting inside them and interfering with the mechanism, which is the case with the one pictured (I know because it’s mine and some are a little hard to move).
Rheostat
As I said above, when only two terminals of a potentiometer are connected to then it’s often called a rheostat. Rheostats are usually rated for a higher wattage than the types of potentiometers pictured above, and certainly for more watts than a volume control.
To handle the higher wattage they’re usually made by winding a resistance wire on an insulated core and then having a wiper slide over the wire, making light contact wherever it touches the wire. Recall the electrical symbol for a potentiometer from the beginning of the article where three terminals are used. Since you connect to only two terminals of a rheostat the symbol is different; a resistor symbol with an angled arrow (which doesn’t connect) placed through it. Below you can see both the zigzag line IEEE standard version and the rectangle IEC version here.
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Membrane Potentiometer
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A membrane potentiometer consists of a flexible, insulating and often transparent membrane with a resistive strip attached under it. Spaced out below that is a base on top of which is printed a conductive path. When a finger or other object presses down on the flexible membrane with its resistive strip, an electrical connection is made to the conductive path. This results in a voltage at the terminal of the resistive strip. The voltage varies depending on where the connection is made with the conductive path. Note that this is the same circuit as in the first schematic for a potentiometer at the beginning of this article.
The resistance for one from Sparkfun called the SoftPot varies linearly from 100 ohms to 10 kilohms with a 1 watt rating.
If the connection isn’t always present, as when a finger is used to make the connection, then a pull-down resistor should be present (e.g. 100 kilohms). However, with some membrane potentiometers a magnet or a wiper is always putting pressure on the membrane, resulting in a constant connection.
Resistive Touchscreen
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A resistive touchscreen is similar to a membrane potentiometer except that both layers have resistive material on them and that material is transparent. The front membrane is flexible and also transparent so that a finger or stylus can press against it to create a connection. Think about inexpensive PDAs from a decade ago, or some children’s toys still today. The technology is still in use but the smartphone revolution has been built on capacitive touchscreens which don’t require a flexible membrane.
For a 4-wire resistive touchscreen, voltage is first applied across the top layer while a value is read from the bottom layer in order to get the X value. Then voltage is applied across the bottom layer while a value is read from the top layer to get the Y value. All this is done in milliseconds and the screen is constantly being scanned for these values.
The calculations are all done by a supporting controller. Resistive touchscreens are not as responsive as capacitive touchscreens and often require a stylus to get the required accuracy. They are used in lower end smartphones.
Force Sensing Resistor
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Flex And Stretch Sensors
A flex sensor is a resistive material, carbon for example, coated on a flexible membrane. When the sensor is bent this causes the material to stretch, increasing its resistance in a manner correlated to the bend radius. From one datasheet the resistance when flat is 10 kilohms but could be twice that when bent 180 degrees with the two ends pinched together. A popular example of its use is in the fingers of gaming gloves like the original Nintendo Power Glove (here’s one hacked to control a quadcopter). Bending a finger registers as a change in resistance giving an indication of how much the finger is bent.
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A stretch sensor works on the same principle except that you stretch it to increase the resistance. And example is a rubber cord that’s infused with carbon-black, looking much like a bungee cord. Taking one example from Adafruit if you have a 6″ length that’s 2.1 kilohms and stretch it to 10″ long then the resistance increases to 3.5 kilohms. Another example is a conductive yarn made up of steel fibers mixed with polyester and there are others that are in the shape of ribbons or belts.
Resistance Now Minimized
That is a great collection of the manually variable resistors you’ll find in use these days. We may have missed one or two and if so let us and everyone else know in the comments below.
If you want to learn about resistors whose values don’t change check out my previous article on fixed value resistors (well, they do change slightly due to temperature, humidity and other factors be we talk about that too). I have one more article in this series planned: resistors whose values change without human interaction (e.g. a photoresistor’s resistance varies with light). Be on the look out for the final installment.
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