Keypads have integrated deep into everyday schedule of an average person that it is almost impossible to carry out working on electronic devices without using them. Computer keyboards, calculators, remote controls, game joysticks, electronic locks and ATM machines are just a few cases where one cannot do even a single task without using keypad. It becomes quite uncanny that how deeply this electronic accessory has embedded into lives of a major part of the population. Various types of keypads exist as per user’s requirements. For instance, computer and some smartphones have QWERTY keypad while calculators and simple telephones come with an alphanumeric keypad.
Though numerous types of keypads are available from various manufacturers, they all follow a common principle of a typical circuit switch. This principle is so simple that one does not even need a power source to build a keypad (we need power only to use it), though it looks like a gizmo. This insight will put you in the tracks of a membrane type keypad.
[header= Graphic Overlay]
Shown in the image above is a 4X4 matrix membrane keypad. Such keypads are used in telephones and simple calculators. Graphic overlay is that part of a keypad over which the symbols corresponding to the keypad are printed. This is the only part of the keypad that is visible to the user and can be highly customized depending upon the configuration of the device and need of the user. A good graphic overlay is desired to provide perceptible interface between the user and the device. It is usually made from polycarbonate or polyester materials which last long and upon which printing can be easily done.
[header= Polymer Layers]
The overlay is attached to the internal keypad structure through an adhesive. The core structure of a membrane keypad is enclosed in flexible polymer layers. Shown in the image above, these translucent polymer layers house the internal circuitry of the keyboard. This layer also acts as an Electrostatic Discharge protection layer. Such charges can develop when keys are used in quick interval of time or are pressed for a longer period. The layers are flexible as well as water and heat resistant. This protects the keypad from getting wet and/or damaged when the device is accidently plunged into water.
[header= Inside the Polymer Layers]
Inside the Polymer Layers
The polymer layer structure comprises of two layers that are closely packed against each other through an adhesive. The layers host two different patterned layers which when superimposed form the basic switching fabric of the keypad. The layers are as thin as a membrane, and hence the name Membrane Type Keypad. Each layer is connected to half of the interconnections on the external connector ribbon.
[header= The Button Mesh Layer]
The Button Mesh Layer
The buttons spread on a polymer layer correspond to the number of keys in the keypad as they are ultimately pressed when the user presses the keypad to generate a signal. On the layer, sections are made to accommodate the buttons. The buttons are made of stainless steel and are quite thin so that once depressed they can easily come back to their original position. The quality of buttons used in a keypad is an important factor deciding the longevity of the keypad.
The image on the right shows the front of the button while that on the left details with the rear part. The front part has a little dimple at the center and little depressions at the periphery. The reverse side of the button has a bulges corresponding to the dimple and the depressions which along with the bulges at the periphery touches the embossed circuitry layer (Notice the similarity of pattern on the circuit layer and the position of bulges on the button). When the button is pressed, the dome shaped button flattens out, such that the central bulge touches the painted circuit and completes an electric path. The buttons are stuck to the polymer layer with the help of a small rectangular piece of adhesive plate.
These buttons are available in various shapes such as circular, quadrilateral and triangular which is shown in the image above. The buttons are chosen corresponding to the circuit pattern that is printed on the other membrane. The structure has beveled edges piercing the thin polymer membrane from sharp edges.
[header= Embossed Circuitry Layer]
Embossed Circuitry Layer
The other half of the polymer layer is the circuit layer. Not that it has a resistors and capacitors soldered over it; this layer has conductive paths embossed on its surface. This saves space and provides economical solutions for mass production. These paths are painted either by using silver or by copper oxide. A pattern is made so that the buttons can easily make contact with the conductive lines. It is designed in such a manner that each key is easily able to send its signal without too many conductive lines being drawn. The parts where risk of short-circuit exits are painted with dielectric. Every button has a blot centered on a semi-circle. The blot and the semi-circle make electrical contact with the conductive lines that convey the signal to a processing unit for detection and decoding purposes. It is these very blot and semicircle where the buttons rest on.
The pattern of the semi-circles makes a matrix structure with a unique location for each in terms of row and column. If the button at the left most side of the top row is pressed, its bulged part will be pushed down to make contact with the semi-circle of the pattern on the other layer. With the help of the matrix pattern, it can be detected that from which part the signal has been generated. The signal is carried to the processing unit which also powers the keyboard to work. The processing unit detects the signal and generates an output corresponding to the location from which the signal has been generated.
Effectively, to detect 16 different signals, we need a matrix of 4×4 buttons where each button represents one unique signal. To identify each signal uniquely, the pattern is drawn by super imposing 4 rows with 4 columns and the corresponding key is identified by two terminals through which current travels. Each pairing combination is unique to every button.
To demonstrate the same, observe how the resistance across two terminals drops to a very low value as soon as the button is pressed. The connection path so made is highlighted (in black) in the image below for simplicity. Additionally, another path (in red) is shown to demonstrate the uniqueness of each path.
[header= Connector Ribbon]
A connector ribbon usually acts as an interface to convey the signal generated by the keypad to the processing unit. The one shown in the image has 8 pins that correspond to the total number of rows and columns in the matrix of the membrane keypad.
Besides membrane keypad, other keypad types are also prevalent. These various types include silicone and the recently introduced, touch screen keypad. Membrane based keypads are relatively old than the ones aforementioned, but are more economic and easy to repair as compared to others.
[header = Free TI Design & Simulation Tools]
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