MEMBRANE KEYBOARDS
CONDUCTIVE CONNECTIONS AND SWITCHES
Membrane keyboard is usually composed of two polyester films to which silver conductive connections and connective surfaces are placed through the method of screenprinting. A self-adhesive foil of suitable thickness (spacer foil) with cuttings where connective surfaces are separates both key poles and forms a firm foil composite. The cuttings enable an electrical contact between both poles of a key.

GRAPHIC LAYER
The upper layer of the composite is equipped with foil which is the carrier of distinct information for each key. We name it »graphic foil« because it has to do with graphic procedures of printing colours. It is important that the colours (symbols and signs) are printed on the bottom side of the graphic foil – on the side which is fixed to the basic composite. This procedure prevents the user from coming in touch with the printed colour which enables durability of the printed information.
An appropriate quality of a graphic foil also enables the use of membrane keyboards in extremely demanding environments (the presence of dust, chemicals and high humidity) especially where maintenance and cleaning is needed. This is of extreme importance in the case of medical devices where the membrane keyboard has become almost irreplaceable.

ASSEMBLING ADHESIVE
The bottom side of the basic composite is equipped with assembling adhesive the purpose of which is to fix the keyboard to the complete keyboard area. This is why the use of membrane keyboards assures high IP standards.

TACTILE ELEMENT
The insertion of tactile elements in the basic composite creates the physical feeling - so called 'click' effect - when pressing the key which is synchronized with the electrical function of a key. It is an important feature of the membrane keyboard because it abolishes the need for additional signalization (sound, light). Some kind of signalization is necessary in the case of a keyboard without the feeling of switch because it informs the user that the key when pressed did its task. The feeling can be created with metal tactile elements (metal domes) or an appropriate embossing of the graphic foil (poly-dome). The presence of the tactile feeling provides for the basic division of membrane keybords: keybords with and without a 'click'.


EMBOSSING AS AN OPTION
The ergonomy and practicability of the keyboard can be additionaly perfected by selective embossing of keyboard's graphic foil's active surfaces – keys. By so doing the keys become more attainable and the switching feeling better. It also enhances the attractiveness and appearance of the entire application. The shape of the embossed areas is arbitrary.


TRANSPARENT SURFACES
Indispensable parts of almost any interface between the user and the device are the display and the neccessary light signalization. A broad specter of various treatments of a graphic foil enables the integration of transparent surfaces of different quality within every membrane keyboard. Selective adjustment of graphic foil's certain surfaces makes possible for the use of LCD and LED displayes. A perfect clearness and transparency of the keyboard can be achieved if needed as well as only semi - transparency in various colour shades.

CHANGEABLE INFORMATION
By inserting information stripe pockets adjusting of inscriptions and funtions of any key and changing of arbitrary inscriptions on the keyboard in possible. This way the feasibility of making a basic keyboard type which can be modified separately on any device is formed. The pocket is equipped with a transparent window on a graphic foil and with an entry for the carrier of changable signs. One can find the entry on the side or at the back of the keyboard which preserves IP standards. The pocket position is conditionally arbitrary and depends on the keyboard configuration. In some types of keyboards the pocket can be found at the position of keys.

ELECTRONIC COMPONENTS INTEGRATION
Miniaturisation of electronic components has additionaly improved the usefullness of mebrane keyboards because it enabled their insertion in the keyboard. The insertion of SMD LED diodes simplificates the mechanic construction of the device and is very widespread. The additional increase in level of integration brings the option of combining the membrane keyboard with PCB which is the carrier of conductive connections between keys and which enables the integration of other electronic components.

Keys and SMD LED’s are on same electric layer, graphic foil must be emboss on SMD LED areas. Keys and SMD LED’s are on separate electric layer, graphic foil can be flat on SMD LED areas.

 


STANDARD AND OPTIONAL PROTECTION
Standard protection which is applied to any membrane keyboard:

  • graphite layer as a protection of silver conductive connections on contact surfaces of a connection tail and on contact surfaces of tactile elements,
  • holding foil which creates a security chamber and prevents movements of tactile elements within the keyboard,
  • isolation lacquer which protects tactile elements on a connective tail prevents unwanted electric contact within the device,
  • two-sided protection of clear windows (LCD windows) with protective foil;

Optional protection which can be inserted on the basis of client's special requests:

  • optional foil protection of conductive connections on the connective tail which is necessary in cases of demanding environments and in cases of pasting a keyboard on metal carriers,
  • EMV protection realized through an optional conductive layer (silver or graphite layer) which can be contacted on a connective tail of a keyboard or with a separate connective tail;
CONNECTION TAIL
The membrane keyboard is usually equipped with a connection tail, whose length can be adjusted to device demands. The connection with electronics inside the application can be created through a broad selection of suitable connectors. In the case of 2,54 mm pitch the connectors can be crimped on the connection tail. In the case of smaller pitches the connection tail of the keyboard is prepared for the use of FFC-FPC connectors which are parts of electronics and are not delivered together with the keyboard.

ELECTRICAL CHARACTERISTICS
VOLTAGE 25 VDC max *
CURRENT 50 mA max *
POWER 0,6 W max *
CIRCUIT RESISTANCE < 100 Ohms **
INSULATION RESISTANCE 5 x 106 Ohms
CONTACT BOUNCE < 20 mS max
MECHANCIAL CHARACTERISTICS
DIMENSIONAL TOLERANCES +/- 0,2 mm
ACTUATION FORCE 2 - 4 N
SWITCH TRAVEL 0,15 - 0,2 mm
KEYBOARD THICKNESS 1,5 mm max
BENDING RADIUS CONNECTION TAIL > 3 mm
OPERATING LIFE
WITHOUT TACTILE ELEMENT > 4.000.000 operations
METAL DOME TACTILE > 1.000.000 operations
POLYESTER DOME TACTILE > 2.000.000 operations
ENVIRONMENTAL CHARACTERISTICS
OPERATING TEMPERATURE RANGE - 20 do + 70 oC
STORAGE TEMPERATURE RANGE - 40 do + 80 oC
  * switch rating for optimum life: 20 VDC, 10 mA, 0,25 W
** depend on length of conductive lines,
     conductive line resistance: < 8 Ohm at 100mm length, 1mm width


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