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Extending battery life with capacitive touch

26 Nov 2015  | Johan Winterbach

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The wearable market is poised to grow rapidly in the next five years. Designers of wearables face unique challenges to provide long battery life and an intuitive user interface, all in a sufficiently small form factor. Capacitive touch enables an elegant user interface, but must also be low power and robust under all conditions. This article will explore how current technology can deliver what designers need.

Typical applications
Capacitive sensors are used in a wide range of wearable applications. Touch buttons replace tactile solutions and wear detection offers an intuitive answer to device activation. Typical applications include Bluetooth headphones and earphones, fitness trackers, smart watches, toys, remote controls, and medical wearables.

Advantages of capacitive sensing
Wearables require low power consumption for long battery life. Designers can utilise capacitive proximity in conjunction with capacitive touch to achieve power consumption of less than 3µA at 1.8V. Designers can also use capacitive wear detection to place the device in an ultra low power state when a user removes the device.

Robust design
Capacitive touch buttons can replace mechanical switches. This allows wearable devices to be completely water- and dustproof.

Intuitive user interfaces
Capacitive sensing can be used for a variety of user interfaces including proximity to turn on a display or backlight, touch for mode selection, movement to detect when a device is being worn, sliders and scroll wheels for mode selection, touch screens for full function touch interface, and trackpad for gestures such as taps and swipes.

Intelligent sensing algorithms
Advanced capacitive sensing algorithms can operate reliably under all conditions. Azoteq's ProxSense device, for example, adapts to the environment and differentiates between intended user actions and noise.

Designers want to differentiate their products with elegant design. ProxSense devices offer a very high SNR and sensitivity that allows the designer more freedom in the selection materials and mechanical design.

Low cost
The costs of capacitive sensor controllers have declined to the point where capacitive sensing can replace mechanical switches and reduce system cost. A single capacitive controller can be used to sense proximity and multiple buttons.

Application example: Bluetooth headset with touch gestures
Bluetooth headsets offer wireless freedom. The user wants to keep their music player in their pocket and still control music selection and volume. Bluetooth headsets typically have the following controls:

 • Play/Pause/Call
 • Volume up
 • Volume down
 • Skip forward
 • Skip backward

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