Capacitive Touch Sensors Demystified
You likely use capacitive sensors every day—you may even be using them right now to read this article. When scrolling through a phone that uses this technology to navigate, you likely don’t give it much thought—except perhaps when you’re wearing gloves and things don’t work correctly. So how does it work?
Capacitors, at their most basic level, store a charge between two parallel plates, and are often used to compensate for power fluctuations. When voltage is applied between a capacitor’s leads, with a resistor in series, it takes a certain amount of time to charge and discharge the capacitor. This time value varies depending on the resistor used and the value of the capacitor, measured in farads, or more likely a tiny fraction of a farad.
What makes this useful as a sensor is that the human body also acts as a capacitor, with a value in the range of 100 picofarads (pF: one trillionth of a farad). If a human touches one of the capacitor leads, he injects himself into the circuit, lengthening the time it takes for both the charging and discharge cycle. If you can measure the change in time of the cycle, this can be used as an input. A simple example of this is shown in this video from GreatScott!, using an Arduino Nano for measurement:
Building on this concept, one could, for example, construct a panel of sensors that act as buttons. Rather than construct each sensor from resistors and capacitors yourself, ready-made detectors are also available, like the one shown in the video based on the TTP223 chip.
Sensors as Touch Panels
Taking this concept further, an array of capacitive sensors can be combined to form a touch panel. One such input method is via surface capacitance, where an input panel is coated with a conductive layer. Each corner of the panel detects a certain capacitance value, which varies depending on where a finger is placed. While functional, this method can’t pick out the placement of multiple fingers, and since the conductor is directly exposed to the elements, it’s susceptible to damage.
Another method is known as projected capacitance. This allows control without directly touching a conductor, and instead detects a finger by changes in an electric field through a flat medium, such as a phone’s screen. This method can be further divided up into self-capacitance and mutual capacitance.
In self-capacitance, sensor pads are surrounded by a ground plane, and detect the additional capacitance added when a finger is near a certain pad.
Mutual capacitance uses two conductive layers that form a conductive grid of rows and columns when stacked with a thin separation. This configuration allows it to pick out a finger’s position through the change in mutual capacitance between the layers at a certain X/Y position. Importantly, mutual capacitance touchscreens can also detect multiple fingers, something that other technologies mentioned here are not able to reliably accomplish.
Applications of Capacitive Touch
The most obvious example of capacitive touch tech use is in smartphones. Billions of these devices now exist, placing a previously inconceivable amount of knowledge at the world’s fingertips. While some may bemoan the lack of a physical keypad, the ability to project a virtual keypad as needed provides much more screen real estate than could otherwise be had. Multi-touch capabilities even allow for manipulation of photos, zooming in on a map, or playing your favorite game using an on-screen control pad. Add to that the fact that that the glowing screen shines clearly though this invisible sensing grid, and you have a technology that is widely used, but seldom truly appreciated.
So what else can capacitive touch be used for? One interesting application is the Makey Makey Go, which allows you to connect everyday objects to keyboard inputs, simply by connecting them to the board with alligator clips. There are tons of fun examples here. The device detects the change in capacitance of an object connected to it with an alligator clip, turning items such as apples, bananas, and even donuts into input devices. If you watched the GreatScott! video embedded earlier, you’ll note that he was trying to do something similar using a lamp base as a switch.
On a more serious note, SawStop table saws use capacitive sensing to detect when a human finger touches a saw blade. If a human finger is detected, it triggers a cartridge that catches the rotating blade, stopping it and throwing it below the table. This happens so fast that what would normally be a grievous injury is turned into a minor scratch. See it in action here.
While capacitive sensing isn’t as easy to understand as flipping a switch, for certain applications it works extremely well. Perhaps you’ll give this type of sensing a try on your next project. If not, hopefully you’ll have a greater appreciation for your phone’s capabilities the next time you enlarge a photo, text a friend, or check various social networks just one more time.
Zach Wendt and Jeremy S. Cook are engineers who cover emerging technology. Zach, with Arrow Electronics, has a background in consumer product development. Jeremy writes for a variety of technical publications. Visit Arrow Electronics to learn more about sensor applications.