The basic description of EC switches is described in the original patent from Topre Corporation:
“... provide a keyboard switch wherein constant hysteresis can be obtained without changing the operation characteristics even if an operation value is arbitrarily set, thereby properly preventing chattering, and wherein an operator can know that the operation member has been depressed to a depth corresponding to the operation value.”
The operational principle of EC switches is very different from your common mechanical switch, in the way that there’s no mechanical contact happening, keypresses are registered through an electric circuit in the form of a capacitive sensing mechanism.
The main components of an EC switch (Topre based) are the following:
The 3 main parts that we can identify are:
The rubber dome is the part responsible for the switch tactility and resistance and doesn’t have a contribution to the keypress registering mechanism. On the other hand, the spring and PCB are the essential part of EC switches.
Following the same patent we have a very clear and detailed description of the functioning principle:
“ [...] there is provided a keyboard switch comprising an insulating substrate, a first electrode laid on the insulating substrate, a second electrode facing the first electrode, a dielectric disposed between the first and second electrodes, said second electrode being formed of a conical coil spring, and being positioned on the dielectric so that one end portion of the spring may be in contact with the surface of the dielectric, a capacitance between the first and second electrodes being mainly defined by the contact area between the dielectric and second electrode, a button on the other end portion of the second electrode for compressing the second electrode toward the first electrode to change the contact area when the button is depressed toward the first electrode, said capacitance varying with the change in the facing area which changes substantially in proportion to the depth of depression of the button, and a means for giving snap feeling to an operator when the button is depressed and the capacitance exceeds a given value, whereby the switch is capacitively coupled for a switching operation and the snap feeling is given to the operator when the capacitance exceeds the given value.”
Here’s an example showing the compression of the aforementioned spring:
I’ll pass over the technical aspect of capacitor theory and move on with the actual PCB reverse effort and design.
Go to the next page: Sensing Circuit