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Improving wearables with capacitive sensing

Posted: 06 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:wearable devices? Smart watches? user interface? Capacitive Proximity Sensing? Trackpad?

Nowadays, smart, connected wearable electronics are undeniably the trend. Fitness trackers keep count of how many calories users burn and even monitor sleep patterns. Smart watches let users know when there are new text or social media messages and wireless headsets and headphones detect when they are being worn.

These wearable smart devices make life easier for the user. Wearable devices are usually light and small. Typical characteristics include:
???Small Displays (LED or LCD)
???Limited Space for User Interface
???Need to be Easy to Access
???Have Low Power Requirements
???Need to be Low in Cost

Many wearable devices, however, still have clunky click buttons for a user interface. Some products have only one button to turn on the display and click through many options. Space is the issue on most of these products. They have to be small, but there isn't room for multiple buttons without detracting from the overall aesthetics.

In comparison, a touch interface provides a more intuitive user interface. Wearables with small displays can incorporate a touchscreen for gestures or a swipe interface. Power consumption is lowered by incorporating a capacitive sensor to only turn the device on when it's worn. Touchpads on headphones can be used for gestures to implement an intuitive user interface. Figure 1 illustrates using a trackpad within a headset.

Figure 1: Placing a Trackpad within a headset can add a modern look to the aesthetics.

Capacitive Proximity Sensing can be used to wake the microcontroller from sleep to light up the user interface. There are sensors in the market that can run below 2.5?A.

Device type for different applications

Different kinds of ICs can be used for a wide variety of applications. Three main categories for capacitive sensors are Single Channel Devices, Multi-Channel Devices and Trackpad/Touchscreen Devices.

Single Channel Devices:
???Proximity Wake Up
???Wear Detection
???Single Button Activation

For applications that only need to detect when the wearable device is on the user, a single channel sensor with movement detection is highly recommended. This helps distinguish between when the device is being worn and when it is placed on surface such as a metal table.

Single Channel devices can be used in products such as watches to eliminate single buttons and move them to the watch face.

Figure 2: Use single channel devices for touch-on applications.

Multi-Channel Devices:
???Swipe Activation
???Slider/Scroll Wheel
???Multiple Buttons

Multi-Channel Devices are great for a wide range of uses. A 3 Channel Device can be used for swipe activation and once on, use the three channels as individual buttons. Wearables with displays could use single area zone buttons for simpler designs. Devices such as the IQS333 [a] and the IQS213A [b] are devices suitable for these applications. For a slider or swipe activation design, Figure 3 below illustrates the electrode design.

Figure 3: Self-capacitance electrode design.

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