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Design capacitive touch sensors the easy way (Part 1)

16 Oct 2014  | Shruti Hanumanthaiah, Subbarao Lanka

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Buttons are incorporated in many end products, including consumer products, home appliances, automotive and industrial systems. Today's technical savvy consumers want products with reliable and sleek user interfaces. They expect even simple products to have attractive designs with buttons that don't wear out and are easy to maintain.

Traditional user interfaces (UI) are designed with mechanical buttons that can be unreliable, bulky, and unattractive. For these reasons and others, capacitive buttons have begun to replace mechanical buttons. Capacitive buttons blend into a product design and never wear out. As a result, user interfaces based on capacitive touch sensing technology are the current trend in UI design.

However, designing with capacitive buttons and achieving a robust implementation are often a laborious task for system engineers. In this article, we will focus on how to design reliable capacitive touch sensors in five easy steps.

Traditional button system
Before we take a look at converting a mechanical button system to a capacitive system design, let's take a quick look at mechanical button systems. Mechanical buttons are constructed with materials such as plastic, wood, or metal. These systems need to be designed with utmost reliability as they are prone to environment factors such as dust and will wear out. The numbers of cycles that a switch can function determines the life of the switch (and often the product itself). To achieve a high life time for a switch, the design cycle needs to be robust and can be time-consuming.

Another factor to consider in design is aesthetics. Many mechanical buttons don't look attractive in and of themselves, and hence extra effort needs to be spent on creating an aesthetically attractive design. Designing a sleek and robust user interface system with mechanical buttons is not easy given how they often extend out of a device. In this regard, capacitive sensor buttons can greatly simplify design.

However, designing a capacitive sensing system is not an easy task. It is much more complicated than designing a mechanical button system because:

1. There are many sensing parameters involved in sensing and ON/OFF decision logic.

2. Capacitive touch sensors are vulnerable to external noise.

Sensing circuitry involves many hardware parameters such as reference voltage, source/sink current, clocks, and so on. Thus, the ON/OFF decision logic of a capacitive switch involves many software parameters such as thresholds. It is critical to set these parameters to their optimal values to achieve the best performance. Selecting the optimal values for these parameters is called tuning. Tuning a capacitive sensing system is a laborious process. Capacitive sensing involves many parameters that result in lots of combinations, making tuning even more time consuming.

Configurable capacitive sensing devices
Configurable capacitive sensing devices avoid the cumbersome process of tuning. These are pre-programmed devices with embedded auto-tuning algorithms. An auto-tuning algorithm tunes all the sensing parameters to their optimum values based on the characteristics of sensor and other sensing components. This eliminates the need for tuning, the biggest hurdle in designing a capacitive sensor.

In addition, configurable capacitive sensing devices often support a wide range of peripherals and capabilities, such as LED drive, buzzer, scan-rate control, and so on. Configuring these devices means enabling/disabling the features required and setting the values for active features based on the application's needs. The MBR family of capacitive sensing devices from Cypress is an example of a configurable device that simplifies designing capacitive sensing systems. In this series, we'll use MBR devices as an example for configurable capacitive sensing devices and explore the design of capacitive sensing systems.

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