Capacitive Touch Sensing Layout Guidelines --- > Part 2

Buttons

Introduction

Similarly to the mechanical buttons they intend to replace, touch buttons provide ON/OFF information i.e. respectively button touched or not touched by the finger. Each touch button is associated to its dedicated capacitive sensor.

Shape

• Round is best while oval or square with round corners are also acceptable
• Any other shape with acute angles is not recommended
• Possibility to put a hole for reverse mount SMD LED in the middle (will reduce a little bit the sensor surface, can be compensated with higher sensitivity setting or bigger sensor)

Size

• 1cm diameter is recommended
• Above 1.5cm diameter is useless due to finger tip surface
• Below 1cm is possible at the expense of higher sensitivity setting hence higher consumption

Pitch

• 1.5cm is recommended as minimum
• Below 1.5cm is possible but reduces user friendliness and improves the risk of side touch effects.

Examples



Slider

Introduction

Similarly to the mechanical sliders they intend to replace, touch sliders monitored by Semtech products provide of course the position information but also an ON/OFF state (i.e. respectively slider touched or not touched by the finger) as well as movement information (move-up or move-down).

Each touch slider is made by several capacitive sensors placed back to back on the PCB.
For good position resolution a slider usually requires interpolation (i.e. number of positions not limited to the number of sensors) which requires a layout ensuring that the finger always touches at least 2 sensors.

Shape

Slider

• Straight shape is usually recommended for better user friendliness but other shapes are also possible
• Mechanical guide on overlay improves user friendliness and robustness especially for exotic shapes

Sensors

• Rectangular shape implies a lot of sensors to ensure interpolation (max half surface of finger per sensor)
• Chevron shape is recommended as it provides good interpolation with relatively low number of sensors

Size

Slider

• 1cm recommended width
• Above 1.5cm width is useless due to finger tip surface
• Below 1cm width is possible at the expense of higher sensitivity setting hence higher consumption
• The number of sensors depends on the length required and resolution targeted

Sensors

• The smaller the better, typically below 0.5cm2 recommended (half surface of finger)
• Bigger is possible but requires more complex layout (more interpolation required to ensure good resolution)

Pitch

• The smaller the better to maximize interpolation
• Ground clearance recommendations apply (see §2.3)

Example

Wheel

Introduction

A wheel can be seen as a slider with round shape, as such it has similar layout constraints and also provides position, ON/OFF, and movement information.
Each touch slider is made by several capacitive sensors placed back to back on the PCB.
Similarly to a slider, a wheel usually also requires interpolation but because of its “infinite length” nature, the position precision requirement is usually not as critical as for a slider. (movement detection may be more important)

Shape

Wheel

• Round is usually recommended for better user friendliness but other shapes are also possible
• Mechanical guide on overlay improves user friendliness and robustness especially for exotic shapes

Sensors

• Rectangular shape implies a lot of sensors to ensure interpolation (max half surface of finger per sensor)
• Chevron shape is the best but may be complex to design inside a wheel
• Whirl shape is recommended as it gives a good compromise between the number of sensors required and layout complexity

Size

Wheel

• 1cm recommended width
• Above 1.5cm width is useless due to finger tip surface
• Below 1cm is possible at the expense of higher sensitivity setting hence higher consumption
• The number of sensors depends on the wheel diameter required and the resolution targeted

Sensors

• The smaller the better, typically 1cm2 recommended for whirl shape (see below)
• Bigger is possible but requires more complex layout (more interpolation required)

Pitch

• The smaller the better to maximize interpolation
• Ground clearance recommendations apply (see §2.3)

Examples