Keypad design 101 (or: a 22 step guide to getting it right!)
At a user level a keypad is a very simple and intuitive thing. You press a key, known in the keyboard trade as a ‘knob’, and a letter, number or symbol is registered (normally on a screen). Keypads have revolutionised the facility and speed of communication, both in writing, speaking to others via a device, or even changing your TV channel. Simply brilliant, and their flexibility has enabled them to remain contemporary and intuitive whatever the technology.
In its most basic form, a keypad is a flat panel membrane less than 1mm thick, consisting of one or more keys. Each key consists of an open contact between two tracks (or wires) that exit at the back of the panel. The tail that can then be connected directly to a PCB (Printed Circuit Board).
Each key is graphically represented on the top panel. To operate, all that is required is a simple press of a finger on the designated key to momentarily short the contact. Membrane Keypad technology has been around for more than 20 years. It has proven to be simple, reliable, economical and above all attractive both in design and function.
Even despite advances in new manufacturing processes, the basic functional design and working principles of the membrane keypad have not changed.
At one level the simplicity of the keyboard is what makes it so brilliant. So for many keyboard designers it initially comes as a surprise when we ask 22 questions before we can even think about quoting.
Here they are – and we’ll explain a bit more about the complexity after you’ve seen then:
- Inside dimensions (Xmm)
- Outside dimensions (Ymm)
- Number of keys/buttons
- Metal domes required? Y/N
- Polydomes required? Y/N
- Number of solid colours including background
- Number of clear LCD windows
- Number of semi-clear LED windows
- Number of semi-clear LED ports
- Tail Exit Position and length
- Type of connector required, if any
- Number of prototypes
- Quantity required
- Embossing? Y/N
- Integrated LEDs? Y/N
- Total number of integrated LEDs
- EMI Shielding? Y/N
- Mounting Holes? Y/N
- Number of holes and sizes
- Cut outs? Y/N
- Number of cut outs and sizes
- Number of transparent colours
Whilst it may look exhausting, it is the exhaustive nature of the design capability that has let the membrane keypad stay modern in the present day and age. The possibilities are endless and also very exciting. Many of the above questions will have clear answers, but some need more careful design consideration.
Good keypad design adds value to any device
Back in the day when devices were huge and everything on a monitor was black and white keypads were ‘industrial’ in design at best. But we now live in a fully technicolour digital world where ‘knobs’ can not only be any colour you want but in any combination you fancy.
Indeed, the graphic user interface has endless possibilities. Because the raw material is clear and colours are printed individually, it is possible to leave clear areas in the graphic that can create any shape of window for displays or LED ports. As such colour combinations are endless – but always bear in mind that colours are individually printed, resulting in cost implication.
Most basic units typically start with dual colours, one colour for the detail and the second colour for the background. Typically keyboards have between two and five colours and, particularly around TV, video and gaming, there has been some standardisation with certain colours common across platforms.
Actually there is no limit. Solid colours are more cost effective as half-tone printing is limited and can lead to extra expense. But it’s up to the imagination of the designer. The printing process may currently seem a little restrictive, but digitally printed graphics with full colour resolution may soon be possible. For colour reference we use the universal pantone colour chart as a guide to the finished product – and with 2161 different pantone colours to choose from there is no limit to creativity.
It is also possible to raise certain areas of the graphic to create either ridges around the keys or the whole key itself. The process is mostly cosmetic but is very popular and the results are aesthetically pleasing. And in this business ‘aesthetically pleasing’ equals ‘value added’.
The electrostatic finger conundrum
In this digital world, electrostatic discharge can interfere with or damage sensitive electronic components, and human ‘digits’ can build up an electrostatic charge. Therefore an extra layer, either solid or in a grid pattern, can be incorporated between the graphic and the upper circuit. This layer facilitates the cancellation or dissipation of electrostatic build up on a person’s finger. At the same time this can act as an RFI shield, regularly required on non-conducting composite casing materials to shield sensitive electronic devices from radio frequency electromagnetic interference.
Circuit design options
As discussed above the circuit assembly quite simply consists of two layers, an upper circuit and a lower circuit, separated by a spacer material. As we know the keys only make contact when the upper circuit is pressed down to short the lower circuit. Both circuits are joined at a single connector on the end of the tail.
Unlike PCB boards, the tracks are printed with conductive ink onto a clear polyester film. This allows many options but also limitations on spacing of keys and circuit layout. So far, so simple.
However a membrane switch with up to 7 keys can have a single common track on one layer and then 7 more tracks, one to each key on the other layer resulting in a tail with 8 tracks leading off the membrane keypad.
Where more keys are required for example a 12 key membrane it is preferable to use an X Y matrix configuration. In other words, one layer will have 3 tracks for the columns and the other layer will have 4 tracks for the rows resulting in a 3×4 matrix and 7-track tail. In this situation your microprocessor will interpret which key has been activated and the resulting response processed.
Actually the great thing if you work with us is that you don’t need to ‘worry about the math’ – this is bread and butter stuff to us, and we can recommend the optimal option.
You won’t need to chase your tail
The design and configuration of the keyboard tail is important. After all it’s what connects the keypad to your interface, or processor. The tail can be manufactured to any length (normally 50mm-200mm) and have various connection options. The most popular option is the female type with housing at 2.54mm pitch and is suited to plug into a standard header on a PCB.
Male solder tabs are also available to directly solder into a predrilled board. There are several PCB mount type connectors where the tail end can push fit into a connector already mounted on the PCB. Some of these PCB connectors offer a 1mm pitch for tight spaces and this would result in a narrower width of the keypad tail.
Again, we are happy to advise and have the experience to do so.
Feel the feedback
Standard construction of any membrane keypad offers no tactile feedback. Where deemed important an audio or visual display is used to satisfy most users. Where an actual physical feedback is required, tactile keys are an extra feature. These consist of metal clickers incorporated inside the membrane assembly that offer a positive and reassuring ‘click’ when pressed.
Lighting it up with integrated LEDs
Surface mounted LEDs can be located inside the membrane keypad. They can either be with their own circuit and tail or incorporated with the key circuit layer. Normally, the graphic is embossed over the LED with the option of printed transparent colours to enhance the look of the LED on the graphic.
Keyboards are our day job
We manufacture keyboards all the time. The clients we work with, across many sectors of technology, value our experience, expertise and advice. To be honest we’ve seen all of the mistakes people can make and understand the minutiae. The earlier you involve us, the less heavy lifting you need to do.