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Design capacitive touch sensors the easy way (Part 3)

16 Feb 2015  | Shruti Hanumanthaiah, Subbarao Lanka

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Click Part 1 and Part 2 to access previous instalments.

In Part 2, we covered the layout aspects required when replacing mechanical buttons with capacitive sensing buttons as well as an example smartphone application. In Part 3, we look at more example applications and how MBR devices can be configured to enable the specialised features that these applications require.

Step 3: Create the configuration for your design
Touch Buttons in Home Appliances: Touch buttons are used in such home appliances as induction cooktops to replace mechanical buttons. The user interfaces (UI) for home appliances are expected to be liquid tolerant. It is common for a user to touch the buttons of a cooktop with wet hands. Similarly, water or food may fall on the cook top UI panel. Despite the presence of any kind of liquid, the buttons are expected to function normally. This means the buttons must not register false touches or buttons must not stop responding to touches even in the presence of liquids. Hence, liquid tolerance is a key requirement for capacitive sensing buttons in many applications. Secondly, the most intuitive feedback for a button touch to a user is audio feedback. Hence audio feedback feature is also a key requirement.

The most suitable MBR device for home appliances applications is one with SOIC packaging. This is easy to solder and hence is preferred to QFN packages for home appliances applications. Consider an appliance that requires 8 buttons. Hence 8 buttons are chosen in the MBR device. Shield is a feature supported by the special purpose pin that can support any one of the multiple features based on the configuration. The shield feature must be enabled and the layout must be designed appropriately to make the design liquid tolerant against droplets of liquids. There is another feature called 'Guard sensor'. This feature must be enabled to prevent false triggering of the buttons if a huge stream of water or any other liquid flows on the UI panel or a puddle covers the entire UI. In such cases, Shield cannot prevent false triggering. Guard sensor prevents false touches in these extreme cases. Another feather supported on a special purpose pin is Buzzer. This feature can be enabled for audio feedback. The frequency of the buzzer drive signal and hence the pitch of the audio feedback can be set based on the application need. The duration of the audio feedback is also configurable.

Figure 7: Block diagram of capacitive sensing system in cooktop home appliance application.

Figure 8: Cooktop user interface.

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