Although small in size, keypads play a major role in the function of a human machine interface (HMI). Unless the HMI features a touchscreen interface, it will most likely rely on a keypad through which the operator can input commands. But not all keypads are made the same, as some feature different technologies than others.
Among the most popular types of keypads is membrane. Often found in household appliances as well commercial applications, they typically consist of three layers: a top layer with the labels and keys printed on the front; a space layer; and a back layer that features conductive stripes. So when the operator presses down on a key, it forces the upper and bottom layers together. The computer or HMI will then register the keypress.
Another type of keypad that’s used widely in commercial applications is dome-switched. Technically speaking, it’s a combination of a flat-panel membrane and mechanical-switch keypad. They feature two circuit board traces embedded under a silicone keypad with a dome-shaped switch. These domes create tactile feedback when pressed, making them particularly useful in commercial and work-related applications. Furthermore, dome-switch keypads have a high level of reliability, often lasting for as many as 5 million cycles.
A third type of keypad is the scissor-switch. It lives up to its namesake by featuring keys that are designed like scissors. The scissor-switch keys contain two separate plastic pieces that lock together like a scissors. It also features a rubber dome like the aforementioned dome-switched keypads, but the scissor design improves functionality by keeping the keys connected together. While there are several different ways to design a scissor-switch keypad, most contain three layers.
There are some potential downsides to scissor-switch keypads, however, including the cost. You can expect to pay more for a scissor-switch keypad than membrane or dome-switch. Furthermore, they have a tendency to attract dirt and debris under the keys, reducing their life expectancy unless cleaned on a regular basis.
Capacitive keypads use capacitance technology to register keypresses. When you press down on a key, it chances the capacitance of the capacitor pads. They are typically designed with foam element keys that are finished with aluminum. Capacitive keypads aren’t as popular as membrane keypads, although they are still a viable choice for certain applications.