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Capacitive sensing

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Implementations

Capacitive keyboards have an interesting characteristic that the capacitive sensing takes place directly under the plunger. This is where the return spring would be in many other switch types. As a consequence, capacitive keyboards tend to have the return spring or buckling rubber sleeve on the outside, underneath the keycap. Topre’s conical spring method is unusual in allowing the rubber dome to be placed safely inside each key.

Capacitive sensing is a technique that requires the keyboard to be designed as a complete assembly. Individual switches don’t operate on their own: a capacitive keyboard takes the form of a special PCB layout, open-bottom switches that allow the capacitive element to approach the PCB, and special sensing circuitry or firmware. While many designs have discrete plunger assembly units for each key station, these units do nothing on their own when removed from the keyboard, as half of the variable capacitor that makes up the capacitive sensor is formed of PCB pads.

There are many ways to introduce a variable capacitor to a switch, as detailed below.

Conductive plastic

Some capacitive designs use a conductive plastic block as the variable element. IBM’s beam spring and Model F designs both take this form.

Foam pad

Foam pad—or “foam and foil”—switches have a foam pad fitted to the bottom of the plunger, affixed onto which is a thin layer of metallic foil. The distance between foil layer and the PCB affects the capacitance registered at that key position. Keystrokes are detected when the foil layer is resting on top of the PCB; the solder mask on the PCB and (where present) the plastic coating on the foil presumably prevent a short circuit. The compressible foam pad allows the plunger to continue being pressed past the point that the foil layer reaches the PCB, to provide overtravel. As such, the complete assembly will not operate without the use of an extra moving part (the foam pad) beyond the plunger itself. Worse, the foam pads are a weakness in the design: as foam can stiffen with age and lose its flexibility, some older keyboards fail due to the foam pad failing to re-expand after a key is struck.

US patent 3965399 “Pushbutton capacitive transducer” indicates that the reason for the flexible foil coating on a foam pad is so that the foil coating can be pressed uniformly against the PCB:

A substantial change in capacitance between the members is effected when the thin dielectric material is disposed across a gap between the capacitance forming member. In order to provide a consistent change in capacitance, a gap of uniform width is preferably maintained. This is provided by the use of a resilient backing which causes the flexible metallic and dielectric films to conform to surface irregularities of the capacitance forming member and its two elements.

The foam pad can also be referred to as an “overtravel” pad: it permits the key to be pressed further after the foil layer makes contact with the PCB. The foam pad design, just like the conductive plastic approaches, appear to require the movable object to rest against the PCB for detection to occur. Topre’s conical spring method is a full analogue sensing method that allows for firmware-configurable pretravel and electronically-defined hysteresis.

Foam pad keyboards include:

Digitran amusingly referred to foam pad keyboards as “sponge-on-a-stick” in an advertisment in Computer Design in January 1979; this was a slight against foam pad designs, as compared to their Golden Touch leaf spring design.

Leaf spring

Leaf spring capacitive sensing uses a metal leaf spring that is pushed down onto the PCB. One side of the leaf spring is attached to the PCB, and the plate hinges down towards the PCB under pressure from the plunger. Overtravel is achieved by a special prong on the leaf spring. The best-known leaf spring capacitive design is that of Digitran’s Golden Touch. Cortron filed a patent for their own design, but in both the one known CP-4550 keyboard and the CP-4550 advertisements, the design is the same as Digitran’s.

Membrane

Although the vast majority of membrane keyboards use conductive sensing, some manufacturers produced conductive membrane keyboards. Manufacturers of capacitive membrane keyboards include Micro Switch with SC and ST Series.

Conical spring

Conical spring switching uses a small, low-force conical spring placed under a rubber dome. As the key is pressed, the rubber dome compresses the conical spring. Topre have made such keyboards since 1983, and continue to produce them to this day. They introduced APC—Actuation Point Changer—to permit the pretravel of the keyboard to be set to any of 1.5 mm, 2.2 mm or 3 mm; this is made possible by the inherently analogue nature of capacitive sensing. See the Realforce website for details and illustrations of Topre keyboards.