The fundamental element required in the capacitive touch sensing application described is the variable capacitor itself. This capacitor should be easy to construct as well as sensitive to human touch in order to enable this as an alternative to mechanical buttons and switches. Such a touch sensitive sensor element can be constructed by “opening up” a capacitor structure so that the electric field can be interfered with by a conductive foreign object, in this case, a finger. Figure 1.1 shows the top and cross-sectional views of such a capacitive sensor as implemented in the printed circuit board itself.
As shown, a PCB-based capacitor is formed between the center copper pad and the ground pour surrounding it. The electric field is allowed to leak into the area above the capacitor. The interaction of this sensor pad and the surrounding ground pour (also the ground plane underneath) create a baseline capacitance that can be measured. The base capacitance of such a sensor is in the range of ~10 pF for a finger-sized sensor. When a conductor, e.g., a finger, comes into the area above the open capacitor, the electric field is interfered with causing the resulting capacitance to change. The coupling of the conductive finger into the capacitive sensor increases the capacitance of the structure beyond the baseline capacitance, the capacitance of the sensor with no touch. By continuously measuring the capacitance of the sensor(s) in the system and comparing each result to a predetermined baseline capacitance, the system microcontroller can determine not only on/off button functions for each sensor element but also “amount” of press used for more complex interfaces such as positional sliders.
The sensitivity of this sensor is dependent on the gap between the surrounding ground and the sensor plate. A gap of around 0.5 mm is recommended. In addition, PCB thickness plays into the overall sensitivity as well: when it is very thin as in the case of a flexible PCB, this increases the tight coupling between the sensor and the ground plate beneath it and decreases its sensitivity. A standard FR4 PCB with 1-mm to 1.5-mm thickness is ideal.
The sensor pad size of around 10-mm diameter is typically used. This size is similar to the surface area of a human finger when pressed down. Such a sensor using the above care abouts typically has ~5 pF to 10 pF of capacitance untouched.
The highlighted ground plane underneath the sensor aids in shielding it from potential interference generated by other electronics in the system. It also helps to maintain a more constant baseline capacitance needed as a reference for each measurement.
The base capacitance of such a design is affected by stray capacitances on the PCB as well as potentially other environmental effects such as temperature and humidity. Therefore, the detection system needs to constantly monitor and track this variation for correct comparison to touch events.
This application report discusses the design of a single-touch capacitive sensor interface using the MSP430 microcontroller. With ultra-low power features and integrated peripherals, integrating a single-touch user interface into MSP430 applications can be readily accomplished. This application report provides an overview of the technology, details about system careabouts and details for different methodologies ofcapacitive touch sensing implementations using the MSP430 family.