in the easy - to - clean keypad device in accordance with the present invention , parts or dimensions which are identical in the figures are labeled with the same reference numerals . for clarity , the components referred to in the description are usually labeled with reference numerals only once , even when the components appear in a figure more than once . fig1 shows a first exemplary embodiment of the present invention that is designed as an emergency call device 1 , in a cross - sectional , three - dimensional illustration from the front . the lower piece of emergency call device 1 is formed by a base shell 2 , which includes receptacles for base feet 3 on the underside , to ensure that emergency call device 1 may be placed securely on a support surface . base shell 2 is formed , e . g ., as a first injection - moulded piece . a chassis 4 is mounted on base shell 2 , e . g ., via a snap - in connection , and base shell 2 encloses chassis 4 in a retaining manner . chassis 4 may also be designed as an injection - moulded piece , and it is oriented parallel or substantially parallel to the support surface in its two - dimensional extension . a circuit board 5 , which is oriented parallel or substantially parallel to chassis 4 in its two - dimensional extension , is mounted on underside of chassis 4 , which faces the support surface . emergency call device 1 , is closed on its top side via an upper shell or closing cover 6 , which may also be designed as an injection - moulded piece , and on which four keys 7 a , b , c and d are installed . upper shell 6 and keys 7 a , b , c form a first assembly , which , in the preinstalled state , may be removed entirely from the second , preinstalled assembly , which is formed by base shell 2 , chassis 4 , and circuit board 5 . when emergency call device 1 is disassembled , it is separated into the two assemblies . in particular , the installation of keys 7 a , b , c and d is designed such that they do not fall out even when upper shell 6 is removed . since the first assembly is designed as a purely mechanical assembly , it may be cleaned easily after disassembly . by separating the function carrier of the second assembly , chassis 4 in particular , and design element chassis 6 with keys 7 a , b , c and d , the appearance may also be easily changed . the number of keys may be changed easily via this separation by using a replacement upper shell with fewer than four receptacles for keys . in addition , the color and geometry of the upper shell may be changed as requested by the customer or for a specific application , without needing to intervene in the base device , i . e ., the second assembly . as shown in fig2 , which shows upper shell 6 and keys 7 a , b , c and d in a schematicized , three - dimensional exploded view from the inside of the housing , keys 7 a , b , c and d are snapped into upper shell 6 from below , i . e ., from the interior of the housing , against the actuating direction . keys 7 a , b , c and d include four guide bolts 8 for this purpose , which are integrally formed on keys 7 a , b , c and d such that they extend perpendicularly to the actuating direction . to receive guide bolts 8 , guide bushings 9 are integrally formed in upper shell 6 , which guide keys 7 a , b , c and d in the actuating direction . oblong slots 10 , which are open toward the interior of the housing and are tapered in the inlet section , are formed in guide bushings 9 . gaps 11 in guide bushings 9 are formed on either side of slots 10 . slots 10 are therefore formed by freestanding blades 12 located between gaps 11 and slots 10 . when keys 7 a , b , c and d are installed , guide bolts 8 are snapped into assigned slots 10 by bending freestanding blades 12 elastically apart while inserting guide bolt 8 . after keys 7 a , b , c and d are snapped in place , they are captively held , and they may move freely along oblong slots 10 in the actuation direction of keys 7 a , b , c and d . as shown in the top view in fig3 , from the inside of the housing toward the same region as shown in fig2 , each of the keys 7 a , b , c and d includes two guide bolts 8 , which may be inserted in related slot 10 . optionally , more guide bolts 8 may be provided for each key 7 a , b , c , and d . guide bolts 8 are not located opposite to each other . they form an intermediate angle that is not 180 °. this design ensures that the correct installation position — not upside - down , in particular — is attained during initial assembly and during reassembly after cleaning . with keys having the same or a similar diameter , the intermediate angle between the guide bolts is different , to ensure that a key is not installed in the wrong position . as shown best in fig1 , keys 7 a , b , c , and d each include a circumferential , annular segment 13 on the free end extending into the housing . when keys 7 a , b , c , and d are installed , segment 13 is located parallel to guide bushing 9 . each segment 13 encloses an annular sealing collar 14 integrally formed with chassis 4 . even in the non - actuated state of keys 7 a , b , c and d , segment 13 and sealing collar 14 are located such that they overlap in the axial direction , i . e ., in the direction of actuation of keys 7 a , b , c and d . segment 13 serves to carry away — in a controlled manner — any fluid that may have entered keypad formed by keys 7 a , b , c and d . this fluid may be directed via segments 13 to chassis 4 . sealing collars 14 ensure that the fluid may not reach circuit board 5 , which is located underneath chassis 4 . instead , the fluid is diverted to the side via the shape of chassis 4 , e . g ., via discharge channels . as further protection against fluid or the like that may enter , circuit board 5 is located under chassis 4 , where it is protected . as is also shown best in fig1 , a key mat 15 is installed between keys 7 a , b , c and d and circuit board 5 . key mat 15 extends over entire circuit board 5 or at least over the region of the circuit elements of circuit board 5 and is fixed in position or clamped between circuit board 5 and chassis 4 via an enclosing section of chassis 4 . if keys 7 a , b , c and d are actuated , key mat 15 is compressed or deformed , which directly actuates the circuit elements , which are formed by key mat 15 and circuit board 5 . to transfer the compression forces , every key 7 a , b , c and d includes four compression punches 16 , which are located inside segment 13 ( see fig2 and 3 ). compression punches 16 are located on transfer bridges 17 of key mat 15 . transfer bridges 17 are deformed elastically when pressure is applied , and they come in contact via a contact tip or a carbon tip 18 with contact fields ( not shown ) located on circuit board 5 , thereby resulting in electrical contact being established . in a modified embodiment , only two of the four compression punches 16 serve to transmit a switching impulse , and the remaining two compression punches 16 are used to stabilize keys 7 a , b , c and d . in another exemplary embodiment , different , replaceable keys are used . the keys differ according to the number or design of compression punches 16 , thereby enabling a different selection of circuit elements to be actuated . depending on the selection of circuit elements , different programmed functions of emergency call device 1 are activated . the possible functions of emergency call device 1 may therefore be changed easily by replacing a current key with a replacement key , which , e . g ., actuates a pressure point on the key mat and , therefore , another circuit pattern on circuit board 5 , thereby triggering a different function . illumination means , leds 19 in this example , are located in the center on the underside of circuit board 5 , underneath particular key 7 a , b , c and d , the illumination means being designed to illuminate keys 7 a - d . the illumination takes place through an opening in circuit board 5 and chassis 4 , so that an end section 20 — which is flat or concavely curved , in particular — of keys 7 a , b , c and d is illuminated . the light passes through flat end section 20 and is guided via light guidance through particular key 7 a , b , c and d to the center or edge section of key 7 a , b , c and d , which is visible to the user , thereby illuminating it and making it easier to see in dark surroundings . fig4 a through d show an exemplary embodiment of a key 7 e of the type that may be used in the device shown in fig1 through 3 . fig4 a shows key 7 e in a top view from below , so that the circular basic cross section is easy to see . flat end section 20 is located in center , and it is surrounded by four compression punches 16 , which are distributed evenly azimuthally . annular segment 13 is also shown . it encloses end section 20 and compression punch 16 . two guide bolts 8 are integrally formed such that they extend radially outwardly or protrude . in their cross section that is perpendicular to their longitudinal extension , guide bolts 8 are circular or elliptical in shape . in addition , two guide segments 21 may be integrally moulded in place parallel to guide bolts 8 , and which engage in bilateral gaps 11 in the installed state . as shown in fig4 b , which shows keys 7 e in a top view from above , the two guide bolts 8 are not located diametrical to each other , but rather form an intermediate angle of 170 °. this intermediate angle ensures that key 7 e may not be installed upside - down . in addition , keys with the same outer diameter may be coded mechanically via the intermediate angle to prevent incorrect installation , at the wrong insertion point in particular . fig4 c shows key 7 e in a schematic cross - sectional drawing along a line of intersection a - a in fig4 a . it is clear that the upper side of key 7 e forms a receptacle for an actuating element 23 , which is shown inserted in fig4 d . key 7 e is composed of a hard material , e . g ., polycarbonate . actuating element 23 is injected , bonded , or mechanically snapped into receptacle 22 , and it is also formed of a hard material , e . g ., polycarbonate or abs , to impart a positive tactile impression to the user . a symbol or the like may be provided on the upper side of actuating element 23 . to transmit the light emitted by the leds , the material of which key 7 e is made is translucent or transparent , and it may be optionally colored , e . g ., red , blue or yellow . the surfaces of key 7 e are polished , to minimize the interfacial losses of the light . actuating element 23 is opaque , so that the edge region of key 7 e is illuminated in this embodiment . in an alternative embodiment , an opaque key includes a central opening , into which a transparent or translucent actuating element is injected , bonded , or snapped in place mechanically . the actuating element includes an end section , so that the light from leds 19 is guided through the actuating element . in an alternative embodiment , the center of the key is therefore illuminated . it will be understood that each of the elements described above , or two or more together , may also find a useful application in other types of constructions differing from the type described above . while the invention has been illustrated and described as embodied in an easy - to - clean keypad device , it is not intended to be limited to the details shown , since various modifications and structural changes may be made without departing in any way from the spirit of the present invention . without further analysis , the foregoing will so fully reveal the gist of the present invention that others can , by applying current knowledge , readily adapt it for various applications without omitting features that , from the standpoint of prior art , fairly constitute essential characteristics of the generic or specific aspects of this invention .