fig1 is an exploded view of an electromechanical pulse generator or pulse generator according to a preferred embodiment of the invention . the pulse generator 10 comprises a first part 12 and a base part 14 placed inside a cylindrical housing 13 . the first part 12 comprises a user actuatable knob 121 , preferably having a corrugated outer surface for improved grip and facilitating user manipulation . this user actuatable knob 121 is attached to another element 122 which , on a lower surface , comprises a number of projections 123 and valleys 124 positioned between neighbouring projections 123 . the number of projections may vary according to requirements of any specific application , but may be 10 - 20 projections , such as 12 - 15 projections the projections 123 extend radially toward a centre of the lower surface of element 122 to form a substantially circular pattern in order to facilitate rotation . this will be explained in further detail below . depending on the dimensions of the pulse generator , more or less projections may be used . a miniature pulse generator suited for mobile phones or hearing aids may have an outer housing diameter between 2 . 5 and 5 . 0 mm . the base 14 comprises a base element 142 and two electrically conducting terminals 144 and 146 attached thereto . the base element 142 comprises a pie - shaped cut - out or track bounded at its sides by the electrical contacts or terminals 144 and 146 . in addition , a displaceable resilient contact portion 16 or displaceable portion 16 , in the form of a leg of u - shaped electrical conductor 15 is provided — in the pie - shaped track . the u - shaped electrical conductor 15 is fixed at a bent end portion to the base 142 while the other end thereof comprises the displaceable portion 16 and is unsupported and freely displaceable . this displaceable portion 16 is oriented so as to point slightly away from the base 142 . the reason why will become clear further below . thus , the displaceable electrical conductor portion 16 is resiliently displaceable in a first plane substantially parallel to the plane of rotational movement of the first part 12 . the displaceable portion 16 is additionally displaceable in second , substantially orthogonal , axial direction of the electro - mechanical pulse generator 10 . in an assembled pulse generator 10 , rotation of the first part 12 in relation to the base 14 will rotate the projections 123 . the displaceable portion 16 extends outward toward and past a circle formed by the projections 123 and will be biased against the projections 123 or valleys 124 due to the spring effect of the u - shaped electrical conductor 15 and the slight bending away from the base element 142 . the base element 142 additionally comprises a protrusion 145 arranged proximally to an outer peripheral surface of the ( cylindrical or disc shaped ) base 142 and between the electrical contacts 144 and 146 . the protrusion 145 is configured to contact the displaceable portion 16 to prevent rebound thereof against an inactive electrical contact , i . e . contact 144 or 146 as the case may be , during actuation of the pulse generator 10 . the protrusion 145 is preferably provided as an integral part of the base element 142 for example by manufacturing the base element as an injection molded thermoplastic item . naturally , spring constants of the spring formed by the displaceable leg of the u - shaped electrical conductor 15 may be designed with different values in the two above - mentioned substantially orthogonal planes of displacement . this may be accomplished by selecting a non - circular cross section of the displaceable portion 16 . thus , an elongate cross section will facilitate a relatively smaller spring constant in the direction of the shorter dimension compared to the wider dimension . also , the spring constants may be adjusted by an appropriate selection of the material or length of the displaceable leg of the u - shaped electrical conductor 15 . fig2 ( a ) to 2 ( e ) illustrate the dynamic operation of the pulse generator during clock - wise actuation , as indicated by the arrow above fig2 ( b ) during rotation of the control knob 121 . fig2 ( a ) illustrates the displaceable portion 16 in a neutral or centre position and engaging a valley 124 of the element 122 . the displaceable portion 16 is upwardly biased either by its built - in resiliency or spring force or by a flexible member . in the centre position , the displaceable portion 16 does not engage any of the electrical contacts 144 / 146 . by manipulating the user actuatable knob 121 , the projections 123 will rotate to the right and bring the pulse generator 10 to the position in fig2 ( b ), where the displaceable portion 16 engages a side of the projection 123 and is simultaneously forced or displaced in a sideways direction . the sideways displacement continues until the displaceable portion 16 engages the electrical contact 144 , which essentially prevents further sideways movement of the displaceable portion 16 . fig2 ( c ) illustrates a state where further rotation of the first part 12 forces the displaceable portion 16 downwards in order to accommodate the further rotation . fig2 ( d ) and fig2 ( e ) show further progressed states where the displaceable portion 16 is forced along the side of the projection 123 , in frictional engagement with , and along the electrical contact 144 until the displaceable portion 16 reaches the top of the projection 123 . fig2 ( e ) and fig2 ( f ) both illustrate states wherein the displaceable portion 16 contacts and slides past the flat top of the projection 123 moving in a direction opposite to the direction of rotation of the knob 121 . the displaceable portion 16 is seeking to return towards its centre position due to the spring force ( in first plane ) acting on the displaceable portion 16 . this reverse motion of the displaceable portion 16 firstly leads to disconnect of electrical and physical contact to the electrical contact 144 . subsequently , further reverse movement of the displaceable portion 16 toward the centre position is effectively stopped by the base projection 145 as illustrated in fig2 ( f ). the action of the base projection 145 accordingly prevents the displaceable portion 16 from moving towards and contacting the other ( inactive ) electrical contact 146 which would have generated “ false ” or rebound electrical pulses on the latter contact . finally , further rotation of knob 121 will make the displaceable portion 16 slide along the other side of the projection 123 and into the adjacent valley to return the state of the pulse generator to fig2 ( a ). the sliding of the displaceable portion 16 along the surface of the electrical contact 144 provides an advantageous cleansing of the respective surfaces during contact releasing contamination agents such as oxide layers , sweat and cerumen ( hearing aids ) or dust and smoke particles . thus , continued rotation of the first part 12 in relation to the base 14 in the above direction will be detectable as successive electrical connections and disconnections between the displaceable portion 16 and the electrical contact 144 . the angle rotated will be determinable from the number of connections determined . rotation in the opposite direction will have the displaceable portion 16 to successively engage and disengage the opposing electrical contact 146 instead . thus , rotation angle and rotation direction are both detectable by a suitable processor electrically connected to the displaceable portion 16 and the electrical contacts 144 / 146 . the processor may comprise a programmed microcontroller , dsp or digital state - machine . a preferred way sensing or detecting the contact between the displaceable portion 16 and one of the electrical contacts 144 / 146 comprises applying dc voltage difference between the displaceable portion 16 and each of the electrical contacts 144 and 146 . the electrical contacts 144 and 146 may be connected to a dc supply rail through respective pull - up resistors and the displaceable portion 16 to a ground node , or another readily available dc voltage , or vice versa . thus , detecting which one of the electrical contact that is active and counting the number of the voltage pulses provided on one of the active electrical contact will allow a direction of rotation and the rotational angle to be detected / read for example through an input port of a suitably programmed microcontroller or configured digital state machine . both the electrical contacts 144 / 146 and the displaceable portion 16 may be made of virtually any electrically conducting material , such as metals or alloys , e . g . stainless steel , cupper alloys , such as cube , cuni or cuzn , paladium alloys , such as paliney 6 or hera 649 . respective surfaces of the displaceable portion 16 and / or the electrical contacts may be coated by noble metals , such as gold , silver or palladium . the displaceable portion 16 preferably comprises a material with good spring or resilient properties and of suitable hardness . this material may be identical or different from that of the electrical contacts 144 / 146 . in a further embodiment , a further conductive element 18 , illustrated in fig2 ( e ), may be provided . this conductive element 18 may engage the displaceable portion 16 , if the first part 12 is movable in a direction toward the base 14 . thus , displacement in the axial direction of the first part 12 may , in addition to the rotation of the latter part , be sensed . the forcing of the displaceable portion 16 toward the element 18 may be performed by projections 123 or by valleys 124 , depending on the rotational position of the first and second parts when pressing the first part 12 downwards . in order to interconnect the pulse generator 10 to the external world such as a pcb or similar carrier of a portable terminal or hearing instrument , a set of externally accessible terminals or pads 20 are provided on the lower side of the base 14 . respective ones of these externally accessible terminals 20 are electrically connected to the displaceable portion 16 , the electrical contacts 144 and 146 , and optionally to conductor 18 . these externally accessible pads 20 may be provided as respective unitary portions of the displaceable portion 16 and the electrical contacts 144 , 146 , thereby reducing the number of separate parts of the pulse generator 10 . fig3 is an exploded view of a second embodiment of an electromechanical pulse generator 10 according to the invention . the design style of this pulse generator is often referred to as “ roller key ” or simply “ roller ”. components of this embodiment that have similar functions to components of the pulse generator discussed in connection with fig1 have been provided with identical reference numbers to ease comparison and understanding . the first part 12 is provided as a unitary element , preferably an injection molded thermoplastic item , which provides the functionality of the actuatable knob 121 , element 122 and intermediate members 125 , 126 of the pulse generator of fig1 . a corrugated surface 130 of the first part 12 extends to the outside of mating housing portions 13 allowing the user to touch and actuate the pulse generator 10 . the orientation of the corrugated surface 130 and shape and size of the first element 12 relative to the mating housing portions 13 allows the user to manipulate the first part 12 in transversal orientation relative to the axial direction of the pulse generator 10 through the centre of base 142 and first part 12 . this actuation orientation is different from the one applied in pulse generator according to the first embodiment of the invention ( fig1 ) where the actuation orientation of the knob 121 is axially . the base element 142 also comprises a protrusion 145 arranged proximally to an outer peripheral surface of the ( cylindrical or disc shaped ) base 142 and between the electrical contacts 144 and 146 . the protrusion 145 in this embodiment of the invention has the same function as the corresponding protrusion on the base of the pulse generator described in detail in connection with fig1 and 2 . while the present invention has been described with reference to one or more particular embodiments , those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention . each of these embodiments and obvious variations thereof is contemplated as falling within the scope of the claimed invention , which is set forth in the following claims .