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David mentions capacitive touch buttons in EEVblog #130 - The uCalc Credit Card Scientific Calculator / Computer and indeed I have read about capacitive touch buttons before like in the documentation of de kompasklok which unfortunately is only available in Dutch.
But how do they work and can I easily make my own with a little microcontroller programming (and a PCB)?
You can very easily make your own capacitive touch buttons. Texas instruments MSP430 has a good code base and PCB layout considerations for you to start right away.
The way they work, as better explained by the PDF linked above is as follows,
"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."
I recommend starting with the MSP430 LaunchPad and then getting the capacitive sense booster pack. You could be playing with capacitive touch for $15.
Capacitive sensors work by governing an RC oscillator when C is the capacitance between the PCB pads. PCB pads create a touch sensor that creates an oscillator and that oscillator is monitored by the micro-controller. When a human touch is encountered on the pads the capacitance on the pads alters the frequency generated by the oscillator. Hence a touch is detected. Many microcontrollers including Microchip PIC and TI MSP430 have capacitive inputs.
The capacitive sensors are highly dependent on the PCB layout patterns. A cross-hatch pattern serves better than the solid pattern.
