GB2058458A - Key switch - Google Patents

Abstract

A key switch, for instance for data entry keyboards, has a housing (11-16) supporting a snap-action spring (32) with a movable contact portion (35) positioned adjacent a fixed contact (58) on the housing. An actuator leaf spring (48) is anchored at one end of the housing, and has a free end (50) which is movable to drive the snap-action spring (32) between first and second bi-stable positions to operate the switch. A keystem (65) is slidably supported in the housing, and has a tab portion (72) in contact with the actuator spring (48). Hence depression of the keystem (65), via a key cap (95), displaces the actuator spring (48) via the tab (72) to drive the snap-action spring (32). The tab (72) gives a mechanical advantage in switch operation, which gives improved operational characteristics. Such switches can be readily assembled to make keyboards. <IMAGE>

Description

SPECIFICATION Key switch The present invention relates to key switches, of the snap-action type.

Key switches of a variety of styles are in widespread use to enable manual entry of alphanumeric information or other instructions in data-processing equipment, typewriters and teletypes, calculators, and similar equipment. A number of individual key switches, typically of a single-pole single-throw type, are usually clustered together in a common frame to form a keyboard for data entry. Each switch has a key cap which displays a symbol representing the switch function, and which is depressed by an operator to actuate the switch.

Key switches are often the only moving parts in a data-entry system, and it is important that these components be sufficiently rugged to withstand heavy service over a long operating life. It is equally important for the switch to have an actuating stroke which is comfortable for the operator, and which provides a tactile sensing of switch actuation as the key cap is depressed. The switch should also have a hysteresis characteristic in the actuating stroke to avoid switch jitter or Multiple actuations of the operator "teases" the switch at the operating point qf the actuating stroke. These functions are described in greater detail in U.S. Patent 3,777,090. An object of the invention is to provide an improved key switch of the above type.

According to the present invention there is provided a key switch including a housing; first and second terminals secured to the housing, the first terrminal having a portion defining a fixed switch contact; a snap-action conductive spring secured to the housing and in electrical connection with the second terminal, and having a portion defining a movable switch contact positioned adjacent the fixed contact; an actuator spring secured to the housing and having a free nd movable to bring a portion of the actuator spring into bearing contact against the snapaction spring; and a keystem slidably mounted on :he housing and having a portion bearing on the ree end of the actuator spring to apply an 3ctuating force through the actuator spring to the ;nap-action spring, the direction of the actuating force being generally parallel to the direction of system motion.

Thus, we have a new style of key switch which uses relatively few component parts for reduced assembly complexity and manufacturing cost, and which has an overall geometry enabling a number of switches to be grouped together in a compact keyboard. The switch provides a mechanical advantage in that the actuating force sensed by the operator is lower than the spring force urging the contacts together when the switch is closed.

The switch also incorporates the desired characteristics of tactile sensing of the actuation point, and hysteresis to prevent "teasing" of the switch closure.

Briefly stated, a key switch embodying the invention includes a hollow housing which preferably includes a base, and a cover fitted over and engaged with the base. An elongated keystem is slidably mounted on the housing, and preferably has an upper portion in bearing relationship with the cover and a lower portion in bearing relationship with'the base to provide good isolation from keystem side loads. The keystem has a central axis which defines the direction of its motion during switch operation.

A pair of leaf springs have first ends secured together and to the housing by a terminal pin, the springs being cantilevered across the housing interior to free second ends. One of the springs is a snap-action element carrying a movable contact positioned adjacent a fixed contact on the housing. An actuating force applied to an intermediate portion of the snap-action spring between its first and second ends drives the spring between first and second bi-stable positions in which the switch contacts are open and closed respectively.

The second leaf spring is an actuator spring having an intermediate portion bearing on the snap-action spring to apply the actuating force.

The second end of the actuator spring is laterally displaced from the central axis of the keystem, and is in contact with a tab extending laterally from the keystem. Depression of the keystem deflects the second end of the actuator spring, and the resulting force is applied to the snap-action spring to actuate the switch.

The switch is characterized by a small number of simple parts (housing base and cover, keystem, two springs, and two terminal pins), and by a compact, low-profile housing geometry. Keystem side loads are minimised for low-friction operation, and the lower portion of the keystem on the keystem axis preferably passes through clearance openings in the two springs into a blind bore in a supporting and guiding boss on the housing base.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is an elevation, partly in section, of a key switch embodying the invention; Fig. 2 is a bottom view of a housing cover on line 2-2 of Fig. 1; Fig. 3 is a side elevation of the key switch on line 3-3 of Fig. 1; Fig. 4 is a top view of the key switch on line 4-4 of Fig. 1; Fig. 5 is an exploded view of a base assembly for the key switch; Fig. 6 is an exploded view of a base assembly, keystem, and housing cover; Fig. 7 is a top view of a snap-action spring formed as a fiat blank; Fig. 8 is a side view of the spring shown in Fig.

7 after final forming; Fig. 9 is a top view of an actuator spring formed as a fiat blank; Fig. 10 is a side view of the actuator spring after final forming; Fig. 11 is a pictorial bottom view of a portion of a keyboard housing cover for another key switch embodying the invention, used as one element of an array of switches forming a keyboard; Fig. 12 is a top plan view of the keyboard housing cover; Fig. 13 is a bottom view, partly broken away, of a keyboard assembly; Fig. 14 is a top plan view of a portion of a strip defining multiple snap-action springs; Fig. is a view on line 15-15 of Fig. 14; Fig. 16 is a view on line 16-l6ofFig. 14; Fig. 17 is a view on line 17-17 of Fig. 14;; Fig. is a view on line 18-l8ofFig. 14; Fig. 19 is a top plan view of a portion of a strip defining multiple actuator springs; Fig. 20 is a view on line 20-20 of Fig. 19; Fig. 21 is a sectional elevation of an individual key switch used in the keyboard; Fig. 22 is a sectional view on line 22-22 of Fig. and Fig. 23 is a pictorial view of a keystem.

A key switch 10 embodying the invention is shown in Figs. 1, 3 and 4, and the asembly is also shown in exploded view in Fig. 6. The operating components of the key switch are largely enclosed within a housing 11 which includes a base 12, Figs. 1 and 5. Although the total switch assembly is basically square in planform as suggested by phantom lines 13 in Fig. 4, two opposing corners of the housing base are truncated to give the base a roughly hexagonal shape, see Fig. 5.

The base is moulded from an insulating plastics material such as an ABS. The base has a floor 15, and a continous sidewall 1 6 extending upwardly from the floor to define the perimeter of the base and is internally thickened at opposed corners 17 (Fig. 5) to enable the formation of a pair of terminal-receiving slots 1 8 extending vertically through the sidewall.

One of the thickened sidewall corners is further extended inwardly toward the centre of the base to form a spring-supporting pillar 20. The floor of the base also defines a centrally positioned and generally cylindrical boss 21 projecting both above and below the floor, see Fig. 1. A central keystem-receiving blind bore 22 is formed in the boss. A pair of spring-supporting ledges 23 (Fig. 5) extend from opposite sides of the boss, the ledges protruding slightly above the top of the boss to form a central notch or recess 24.

Locking tabs 25 are integrally formed on opposite sides of the base 12, and the tab portion which joins the base is slightly resilient so the upper ends of the tabs can be flexed toward and away from each other. The central portion of each tab is laterally extended to define pairs of lower shoulders 26 and upper shoulders 27. A short post 28 extends from the upper surface of each tab.

Base 12 is one component of a base assembly 31, shown in exploded view in Fig. 5, and in assembled form at the bottom of Fig. 6. Another component of the base assembly is a snap-action contact spring 32 (Figs. I and 5-8). The spring is shown in Fig. 7 in a flat form as cut from sheet metal stock, and Fig. 8 shows the spring after a bending operation to provide the desired stresses for bi-stable snap-action.

Referring to Fig. 7, the snap-action spring is generally rectangular in plan, and has tapered first and second ends 34 and 35. A rectangular slot 36 is punched through the spring adjacent one end 34, and a central circular opening 37 is also formed through the spring. A pair of narrow, generally parallel slots 38 extend along the length of the main body of the spring, and these slots have end portions 39 which converge toward each other, but do not intersect. Slots 38 partition the spring into a central arm 41 and a pair of side members 42 positioned on opposite sides of the central arm.

After the several openings described above are formed through the spring, a relatively sharp bend - is made in central arm 41 along line 43 (Fig. 7) to deform the arm, see the side view of Fig. 8. The bend in effect shortens the central arm, and flexes the main body of the spring upwardly, see Fig. 8.

That is, the bend places central arm 41 in tension, and side members 42 in compression, with the bent portion of the central arm extending below the main plane of the spring, and the rest of the central arm being bowed above the surface of the upwardly-flexed side members.

When thus formed, spring 32 is a snap-action element which, when anchored at first end 34, snaps between two bi-stable positions in response to a force applied or released on the central arm as suggested by arrow 44 in Fig. 8. The spring is preferably formed from a highly resilient sheet material, such as full-hard Type 302 stainless steel of about 0.004-inch thickness, and in a typical form, is approximately 1/4-inch in width and about 11/1 6-inch in length.

The base assembly 31 also includes an actuator spring 48, as shown in Figs. 1, 5-6, and 9-10.

This spring is also preferably formed from a resilient sheet-metal material such as full-hard Type 302 stainless steel of about 0.005-inch thickness. Spring 48 is an elongated leaf having a first tapered end 49 and an elongated second end 50 of reduced width. A rectangular slot 51 is formed through the spring adjacent its end 49, and an elongated racetrack-shaped opering 52 is cut through the central part of the spring, see Fig. 9. After these openings are formed, the spring is then bent about fold line 54 (Fig. 9) so the plane of second end 50 is angled upwardly approximately 50 degrees from the plane of the original flat stamping.

Base assembly 31 is completed by a pair of terminal pins 56 and 57, which provide electrical connections for the switch, as well as securing the several parts of the assembly together. Each pin has a shank 58 with an enlarged head 59 having an upper surface which is centrally recessed to form a pair of pads or contacts 60. The shank is reduced in width at shoulders 61, and includes a pair of laterally-extending locking tabs 62 below these shoulders. As seen in Fig. 1 , the upper surface of the locking tabs is spaced from the under-surface of enlarged head 59 by a distance corresponding approximately to the height of terminal-receiving slots 1 8 in the base 12.The terminal pins are stamped from a conductive material such as 70-30 cartridge brass (onequarter to one-half hard), and pin 57 should be gold plated to provide a good electrical contact surface on contacts 60.

The components thus far described are assembled to form base assembly 31, see Figs. 5 and 6. The first ends 34 and 49 of the two springs are positioned over spring-supporting pillar of the base with rectangular slots 36 and 51 in alignment with terminal-receiving slot 1 8 adjacent the pillar. Terminal pin 56 is then inserted through the rectangular slots in the springs, and into the terminal-receiving slot of the base.

With the parts thus supported, as shown at the bottom of Fig. 6, the lower end of shank 58 is twisted about 45 degrees to move locking tabs 62 out of alignment with slot 18, see Fig. 1. The twisting action locks the pin to the base and secures the two springs against the top surface of the base against the pillar 20. Terminal pin 57 is inserted and locked in the same fashion, but this pin serves solely as a terminal and switch-contact surface, and thus does not pass through and clamp the springs.

The next component of the switch 10 is an elongated keystem 65, Figs. 1, 3-4 and 6. The keystem is preferably integrally moulded from a plastic material such as "Delrin" (Registered Trade Mark). The keystem defines a cross-shaped or cruciform head 66, and an enlarged cruciform bearing portion 67 extending axially below the head. The four arms of the head 66 are uniformly dimensioned and spaced with respect to the axial centreline of the keystem, see Fig. 4. The arms of the bearing portion 67, however, are non-uniform in dimension and include an extended arm 68 which serves a keying function, see below.

Fillets 69 (Fig. 6) are integrally formed between the lower ends of the arms of bearing portion 67, and the upper surface of the fillets serve as extension stops for the keystem when the switch is assembled. A cylindrical pin 71 extends axially from the undersurface of the bearing portion. A paddle-shaped tang or tab 72 extends laterally from pin 71 below the bearing portion.

Keyswitch 10 is completed by a cover 75 which fits over the base 12 to form housing 11.

The cover is a hollow, moulded plastics component which is generally similar in planform to base 12, and is preferably made from an ABS plastic material. The cover has a central, generally hexagonal platform 76, and a sidewall 77 depends from the undersurface of the platform. The upper ends of the sidewall on four of the six sides of the platform is laterally extended to define flanges 78.

Short tabs 80 extend downwardly from the undersurface of sidewall 77, which tabs are useful for positioning the key switch against a circuit board when a number of switches are mounted together to form a keyboard. The inner surface of the sidewall also defines four inwardly-extending ribs 81 (Figs. 1 and 2) which terminate about midway toward the lower end of the sidewall. The bottoms of the ribs define stops, against which the upper surface of base 12 is abutted when the base and cover are assembled.

A pair of upwardly extending notches 83 are formed in the lower end of opposite sides of sidewall 77, see Figs. 2 and 6. A wedge-shaped projection 84 extends outwardly from each end of each notch 83, and the upper surface of each projection defines a shoulder 85.

A generally cylindrical hollow turret 88 extends upwardly from the centre of platform 76 of the cover, and a top wall 89 extends across the top of the turret. A keyed cruciform slot 90 is formed through top wall 89 and is so dimensioned that the bearing portion 67 of the keystem 65 makes a mating slip fit through the slot. A turret extension 92 projects laterally from one side of turret 88, and the undersurface of the extension defines a recess 93 (Figs. 1 and 2) which provides a clearance space for keystem tab 72 when the switch is assembled, see Fig. 1. A terminal clamping post 94 extends downwardly from the undersurface of platform 76 adjacent sidewall 77 and opposite recess 93.

To assemble the key switch, keystem 65 is inserted into the interior of the cover 75 until head 66 and bearing portion 67 extend through the top of turret top wall 89. The keystem cannot be improperly positioned due to the keying action of bearing portion 67 in slot 90 of the turret top wall.

Fillets 69 at the lower end of the keystem bearing portion seat against the undersurface of top wall 89 to provide a limit stop for upward movement of the keystem. When thus assembled, see Fig. 1, keystem tab 72 is within recess 93 of the turret extension.

Base assembly 31 is then fitted into the bottom of the housing cover, with keystem pin 71 fitted into the blind bore 22 of the base. The base and cover make an interference fit, and the parts are pressed together until the upper surface of the base abuts the underside of ribs 81. When the base is fully seated in this position, locking tabs 25 on the base snap inwardly to place lower shoulders 26 over and against shoulders 85 of wedge-shaped projections 84 on the cover, thus locking the base and cover together. A conventional key cap 95 (shown in phantom line in Fig. 1), bearing a designation (not shown) of the switch function, is then pressed over head 66 to be engaged with the keystem.

When thus assembled, keystem tab 72 bears against the cantilevered free second end of actuator spring 48, see Fig. 1, and the combined restoring forces exerted by the actuator and snapaction springs (which are slightly compressed in this position), urge the keystem into a fully extended position in which the switch is open. The two springs are clamped to the base by terminal pin 56, and are also supported by the upper surface of spring-supporting pillar 20 and ledges 23 of the base. The remaining length of the two springs is cantilevered from these clamped portions across the interior of the switch housing, and pin 71 of the keystem passes through spring openings 37 and 52.The lower end of post 94 is pressed against the head of terminal pin 56 to ensure that the clamping action of the pin against the springs is not loosened by, for example, application of heat to the terminal shank when the switch is soldered into a circuit.

To actuate the switch, keystem 65 is depressed by finger pressure against the cap 95, driving downwardly keystem tab 72 and the free second end 50 of the actuator spring. The portion of the actuator spring adjacent its fold-line 54 bears against the central part of arm 41 of the snap-action spring.

Continued downward displacement of the central arm of the snap-action spring causes the central arm to snap "through centre", snapping end 35 of the snap-action spring downwardly against contacts 60 on terminal pin 57, thus closing an electrical circuit between the two terminal pins.

Continued downward movement of the keystem has no effect on the switch closure, and such movement is limited when keystem pin 71 abuts the bottom blind bore 22 in the base.

The keystem stops (fillets 69 in the extended position, and the bottom of bore 22 in the fully depressed position) are positioned to provide a total keystem stroke of about 0.160 inch. The springs are configured to actuate the switch when the keystem has been depressed about 0.100 inch, so the keystem has about 0.060-inch overtravel beyond the actuation point as is desirable in this style of switch.

When the switch is not actuated, a restoring force or pre-load of about 30 grams is exerted on the keystem by the slightly deflected actuator and snap-action springs. This eliminates the need for a separate return spring to drive the keystem to the fully extended position upon release of the key cap by the operator. The springs are configured to require an actuating force of about 75 grams on the key cap to drive the keystem to the actuating point of about 0.100-inch travel. When the snapaction spring snaps into the actuated position, the restoring force exerted by the springs on the keystem abruptly decreases by about 5 to 10 grams, providing the operator with the desired tactile signal of switch actuation.

It is characteristic of snap-action spring 32 that the force applied to the spring to close the switch is considerably higher than the force at which the spring will snap back to the open or non-actuated position. In terms of keystem displacement (which is directly related to the force applied by the actuating spring to the snap-action element), the keystem must be permitted to raise toward the extended position about 0.030 inch above the closure actuation point before the snap-action element returns to its normal position to open the contacts. The switch is thus provided with the desired hysteresis characteristic which prevents contact jitter if the keystem is held at about the downstroke actuation point.

The cantilevered configuration of the springs and keystem tab 72 provides several important advantages in the functioning and reliability of the switch. Relatively long leaf springs can be used to avoid high local stresses which can produce fatigue failure in the spring metal. In this respect, advantage is taken of the oblong shape of the housing interior by orienting the springs along the longest dimension of the housing.

Another significant advantage is that the leveraged arrangement of the keystem tab and actuator spring provides a mechanical advantage, permitting use of a relatively heavy snap-action element to maintain high contact pressure when the switch is closed. The snap-action spring is designed to require about 125 grams of force on central arm 41 to drive the spring through centre and close the switch contacts. If the keystem acted directly on the snap-action element, the full load of 125 grams would have to be applied by the operator.The disclosed arrangement, however, applies the operator's relatively light finger pressure through cantilevered keystem tab 72 to the free second end of the actuator spring, causing a larger force to be applied to the central arm through a relatively small displacement which is adequate to drive the snap-action spring "over centre" to close the switch contacts.

While the switch has been disclosed in a presently preferred form, it is to be understood that other styles of snap-action elements can be used. For example, a suitable alternative spring can be formed by crimping the side members of the spring to place the central arm in compression in either case, the side members being adequately supported by ledges 23 on the housing base, and recess 24 between the ledges providing clearance for movement of the central arm when the switch is actuated.

The keystem supporting arrangement is especially advantageous as accurate guidance is provided by bearing portion 67, and by pin 71 as fitted into the bore 22. These components provide bearing surfaces which are well spaced apart to provide the switch with a smooth, low-friction stroke even though keystem side loads may be applied by the operator. The overall geometry of the switch, and the extension of boss 21 beneath the floors of the base, provides this functional separation of the bearing surfaces without affecting the desirable "low profile" short projection of the upper part of the switch above mounting-panel flanges 78.

Internal friction-producing loads on the keystem are also minimised, because the direction of the force applied to the snap-action spring is generally parallel to and in alignment with the direction of keystem axial motion.

Mounting of the switch in a keyboard array is conventional, and the housing is dimensioned so a plurality of switches can be arranged on centres which are spaced apart by three-fourths inch or less. Tabs 80 can be used for switch positioning on a printed-circuit board or the like, and electrical connections are made directly to the protruding shanks of the two terminal pins. In a typical mounting arrangement, flanges 78 of the housing cover abut a panel which is apertured to receive the lower end of the housing, and the switch is secured by the panel which is captive between the flanges and upper shoulders 27 of locking tabs 25 (clearance for which is provided by truncating the two opposed corners of the otherwise basically square housing planform).

Key switch 10 provides all of the desired operating characteristics for such a component, with a significant reduction in the number of parts required, and without need for adherence to close dimensional tolerances. Use of coil springs (which are awkward to handle and tend to tangle during assembly operations) is avoided, and the need for a seperate return spring is eliminated. Assembly of the switch is essentially a "snap together" operation after the multi-function terminal pins are twisted to be locked in place.

A second embodiment is shown in Figs.

11-23, and this embodiment is useful in providing an integrated array of individual key switches forming a keyboard. The individual key switches are mounted in a housing which includes a cover 100, see Figs. 11-12. The cover is an integrally moulded plastics part which accommodates enough individual key switches to form, for exampre, a keyboard for a typewriter teletype, computer-entry device, and the like.

Cover 100 has a top panel 101 with downwardly extending longitudinal walls 102 and lateral walls 103 which intersect to define individual cells or compartments 104 to accomodate the individual key switches.

Extending from the inner surface of each of the four walls defining a compartment 104 are a pair of tabs 106 which are spaced apart to define a channel 107 (Figs. 11 and 13) in which a keystem.

is received as described below. A hollow and centrally positioned turret 108 extends from the upper surface of panel 101 above each compartment 104, see Figs. 12 and 21, and the top surface of each turret defines a keyed cruciform slot 109 (Fig. 12).

One of the longitudinal walls of each compartment 104 has a downwardly extending locating pin 111, and one of the lateral walls of each compartment has a downwardly extending locking pin 112. Enlarged bosses 113 are periodically positioned along the housing cover in the lateral walls, and each boss has a central threaded recess 114 to receive a screw (not shown) when the keyboard is assembled.

Longitudinal walls 102 are extended beyond the matrix of compartments 104 to provide mounting arms 11 6 with openings 11 7 therethrough enabling the keyboard assembly to be secured to another device.

Each row or compartments has a spring-metal strip 120 which defines a plurality of snap-action springs 121 (Figs. 14-18).Thestriphasa longitudinally extended base 122 having evenly spaced lateral extensions 123. The end of each lateral extension is bi-furcated and curved (Fig.

1 5) to form a pair of contact fingers 124. The body of each lateral extension also has a generally rectangular opening 125 with a short tab 126 extending there-into. Circular openings 127 are formed through base 1 22 intermediate each pair of lateral extensions 123, and these openings are positioned to receive locating pins 111 on the housing cover.

Each snap-action spring 121 is supported at the end of a tab 129 which extends longitudinally from one side of its lateral extension 123. After being stamped from a larger sheet of spring metal, the snap-action spring is a generally rectangular flat panel. A pair of longitudinally extending grooves 1 30 are then formed at opposite ends of the panel to deform it into a stressed and arcuately bowed snap-action spring.

When viewed from the side, Fig. 18, the spring is similar in shape to a steel measuring tape, and pressure applied at the central part of the top surface of the spring deforms the spring into a second bi-stable position, providing the desired snap-action. A pair of elongated contacts 132 are partially severed from the main body of the spring, and are bent to extend slightly below the undersurface of the spring, Fig. 17.

Referring to Figs. 1 9 and 20, a second spring metal strip 135 defines actuator springs 136. The strip has a longitudinally elongated base 137 having spaced circular openings 138 (similar to openings 127 in base 122 of the strip of snapaction springs) to receive locating pins 111. Strip 135 is shown as a flat blank in Fig. 19 as stamped from a larger sheet of spring metal. Longitudinally spaced lateral extensions 140 extend from the base, each having a circular opening 141 to receive a locking pin 112 on the housing cover.

Each spring 136 extends longitudinally from its extension 140, and each spring has a generally rectangular end portion 143 integrally joined at a fold line 144 with a tapered intermediate portion 145 which is integrally joined to the end of the extension 140.

The originally flat actuator spring is then bent into its final form, Fig. 20, by several further steps.

First, a downwardly extending dimple or depression 147 is formed adjacent fold line 144 in the position indicated by phantom line 148 in Fig.

19. A free terminal end 149 of the spring is then bent into a curved configuration (Fig. 20) along phantom line 1 50 in Fig. 1 9. Finally, the actuator spring is bent upwardly along fold line 144, and also where the intermediate portion joins lateral extension 140 as shown in Fig. 20.

Each key switch of the array is fitted with a keystem 1 53 (Figs. 13 and 21-23) which is preferably integrally moulded from a plastics material. The keystem has a base 154, and a pair of stops 1 55 extend from opposite sides of the base to terminate in downwardly extending tips 1 56. A guide tab 1 57 extends from a third side of the base, and a second guide tab 1 58 of enlarged width extends from the base opposite tab 1 57.

A post 1 59 extends upwardly from the top of base 1 54, and a pair of guide blocks 1 60 are formed on opposite sides of the post to give the upper end of the post a generally cruciform cross section which makes a slip fit within cruciform slot 109 in the housing cover. An angled cruciform head 161 extends from the top of post 1 59 to receive a key cap 162 (Fig. 21) of the type already described. Angulation of the head may be eliminated if no slope of the top of the key cap is desired.

Fig. 13 is a multiply broken-away bottom view of housing cover 100 showing the addition of the various components which are assembled to form a complete keyboard assembly 1 64. Portion A of Fig. 13 is a bottom view of a portion of the housing cover alone. Portion B shows the appearance of the assembly after a plurality of keystems 1 53 have been fitted into the housing cover with stops 1 55 and guide tabs 1 57 received in channels 107, and keystem posts 1 59 positioned within turrets 108 so the upper ends of the posts project beyond the top of the turrets with heads 1 61 accessible for the fitting of key caps.

The next step in the assembly process is to position a plurality of strips 135 over locating pins 111 and against the undersurface of the housing cover to position an actuator spring 136 over each compartment 104, see Portion C of Fig. 13. Strips 1 35 are further positioned by locking pins 112 which fit within circular openings 141 in each strip of actuator springs.

Referring to Portion D of Fig. 13, strips 120 of snap-action springs are then superimposed over the strips 135, with circular openings 127 receiving locating pins 111 to ensure proper alignment of the strips. Rectangular openings 1 25 in strip 1 20 make an interference fit over locking pins 112, and tabs 126 in each opening 125 grip the locking pins to hold the strips of snap-action springs in place.

Referring now to Portion E of Fig. 13, a sheet 1 65 of an insulating plastics material is fitted over the superimposed strips of actuator and snapaction springs. The sheet has openings 166 to receive pins 111 and 112 on the housing cover to ensure proper alignment of the sheet with respect to the several compartments 104.

Relatively large circular openings 1 67 are formed through the dielectric sheet, and each opening 167 is centrally positioned over an associated compartment 104 to expose contacts 132 on the snap-action springs. At least one circular opening 1 68 is also formed through the sheet for each row of large openings 1 67, and opening 1 68 is aligned with one set of contact fingers 124 on each strip of snap-action springs.

The final step of the assembly process is to fit a printed-circuit board 1 70 over insulating sheet 165, the board having small openings 171 to receive locating pins 111 and locking pins 112 to ensure proper alignment of the board with the other components of the keyboard. The printedcircuit board is secured in place by screws 1 73 threaded into recesses 114 in the housing cover.

The printed-circuit board thus forms a base for'the keyboard, and completes the housing for each individual key switch.

As is conventional in keyboard assemblies of this type, the upper surface of the printed-circuit board bears plated conductive terminal strips 1 75 (shown in phantom line in Fig. 13), each strip being centered over an opening 1 67 in the insulating sheet to underlie contacts 132.

Common contact terminal strips 176 are also positioned on the board in alignment with openings 1 68 and in electrical contact with contact fingers 124. The common contact strips thus place the strips of snap-action springs in connection with the circuit board, and individual switch closures are completed when a snap-action spring is driven "through centre" by depression of the associated key cap and keystem to snap movable switch contacts 132 against the underlying individual conductive strip 1 75 which serves as a fixed switch contact.

Upward movement of the keystem is limited by the top surface of keystem base 1 54 which abuts the undersurface of housing-cover top panel 101 (Fig. 21) until the keystem is depressed.

Downward movement of the keystem is limited by stop tips 1 56 which straddle the snap-action spring to abut the top surface of the printed-circuit.

board. As in the first key switch embodiment described above, the actuator and snap-action springs are configured to close the switch before the keystem bottoms, providing the desired characteristics of overtravel, tactile sensing of switch closure, and hysteresis to prevent "teasing" of the switch contact at the closure point.

Claims (14)

1. A key switch including a housing; first and second terminals secured to the housing, the first terminal having a portion defining a fixed switch contact; a snap-action conductive spring secured to the housing and in electrical connection with the second terminal, and having a portion defining a movable switch contact positioned adjacent the fixed contact; an actuator spring secured to the housing and having a free end movable to bring a portion of the actuator spring into bearing contact against the snap-action spring; and a keystem slidably mounted on the housing and having a portion bearing on the free end of the actuator spring to apply an actuating force through the actuator spring to the snap-action spring, the direction of the actuating force being generally parallel to the direction of keystem motion.

2. A switch as claimed in claim 1, wherein the keystem portion which contacts the actuator spring is laterally extended to contact the actuator spring at a position laterally displaced from a central axis of the keystem, the central axis being generally parallel to the direction of keystem motion and passing through the approximate point of actuating force application on the snap-action spring, the free end of the actuating spring being displaced about a lever arm with respect to the point of actuating force application.

3. A switch as claimed in claim 2, wherein the actuator-spring portion which contacts the snap action spring is disposed between the free end of the actuator spring and that portion of the actuator spring which is secured to the housing.

4. A key switch, including a housing; a fixed switch contact on the housing; an elongated snapaction spring having a first end secured to the housing, and a cantilevered portion extending away from the first end to a free second end, the spring defining a movable switch contact in the cantilevered portion which can be displaced between first and second bi-stable positions in which the contacts are open and closed respectively in response to an actuating force applied to the snap-action spring; an elongated actuator spring having a first end secured to the housing, a cantilevered portion extending away from the first end to a free second end, and an intermediate portion which acts against the snapaction spring to apply said actuating force; and an elongated keystem slidably mounted on the housing and having a portion in contact with the actuator spring to deflect the actuator spring in response to keystem movement and thereby to deflect and actuate the snap-action spring.

5. A switch as claimed in claim 4, wherein the keystem portion is a tab extending laterally away from a central axis of the keystem, said tab being arranged to drive the free end of the actuator spring through a larger displacement than a displacement of the snap-action spring needed to drive the snap-action spring from the first position to the second position, thereby providing a mechanical advantage enabling keystem actuating force to be lower than the actuating force of the snap-action spring.

6. A switch as claimed in claim 5, wherein the keystem includes a head configured to receive a key cap, a bearing portion adjacent the head and in sliding engagement with the housing, and an elongated pin extending from the bearing portion and having an end in sliding engagement with the housing.

7. A switch as claimed in claim 6, wherein the springs have clearance openings, and the keystem pin extends through the openings, the spring being between the bearing portion and pin end of the keystem.

8. A switch as claimed in claim 7, wherein the housing includes a base having a generally planar undersurface, the base defining a boss with a blind bore and extending beneath said undersurface, the keystem pin end being received in the bore.

9. A switch as claimed in claim 8, wherein the boss further defines an upper ledge for supporting a portion of the snap-action spring at a position generally intermediate the first and second ends of the snap-action spring.

1 0. A switch as claimed in claim 9, wherein the housing has a generally oblong hollow interior, the springs extending the longest dimension of the oblong interior.

11. A switch as claimed in claim 10, and including a terminal pin secured to the housing and having an enlarged head which clamps the first ends of the two springs together and to the housing.

12. A keyboard assembly, including a housing; and a plurality of key switches supported on the housing; wherein each key switch has a conductive snap-action spring carrying a movable contact normally spaced apart from a fixed contact on the housing, an actuator spring having a free end movable to deflect a portion of the actuator spring against the snap-action spring, and a keystem movably mounted on the housing and having a portion bearing on the free end of the actuator spring to apply an actuating force through the actuator spring to the snap-action spring to close the contacts, the direction of actuating force being generally parallel to the direction of keystem motion.

13. An assembly as claimed in claim 12, wherein at least some of the key switches are arranged in rows, the key switches in a given row having snap-action springs which are integrally connected together, and actuator springs which are integrally connected together.

14. An assembly as claimed in claim 13, wherein each of the integrally connected actuator springs has an anchored end secured to the housing, the portion of the actuator spring which contacts the snap-action spring to close the contacts being between the anchored end and the free end.

1 5. A snap-action key switch substantially as described with reference to Figs. 1 to 10, or Figs.

14 to 23 of the accompanying drawings.

1 6. A keyboard substantially as described with reference to Figs. 11 to 13 of the accompanying drawings.