US3800104A

US3800104A - Low profile keyboard switch assembly with snap action cantilever contact

Info

Publication number: US3800104A
Application number: US00306303A
Authority: US
Inventors: W Lien; M Pobog
Original assignee: Becton Dickinson and Co
Current assignee: Becton Dickinson Electronics Co; Becton Dickinson and Co
Priority date: 1972-11-13
Filing date: 1972-11-13
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26
Also published as: DE2356024A1; JPS49133879A; FR2213571B1; FR2213571A1; NL7314379A; GB1451970A; JPS5419066B2; DE2356024C3; DE2356024B2; IT996435B

Abstract

A snap-action switch is formed from a thin resilient sheet that is elastically stressed into a three-dimensional configuration. One or more bi-stable segments are provided wherein relative movement of the end of a bi-stable segment causes a snap action into a reversely-curved configuration. Switch contacts are only engaged or disengaged after the switch has snapped into a stable configuration.

Description

United States Patent [191 Lien et a1.

LOW PROFILE KEYBOARD SWITCH ASSEMBLY WITH SNAP ACTION CANTILEVER CONTACT Inventors: Wallace A. Lien, Palos Verdes Estates; Mathew Pobog, Newport Beach, both of Calif.

Becton, Dickinson Electronics Company, Ritherford, NJ.

Filed: Nov. 13, 1972 Appl. No.: 306,303

Assignee:

US. Cl. 200/5 A, 200/67 DA, 200/159 A, 200/166 BH Int. Cl. HOlh 13/44 Field of Search 200/1 R, 5 R, 5 A, 6 BA, 200/17 11,18,159 B, 166 BH, 67, 6 BB, 159 R, 159 A References Cited UNITED STATES PATENTS 9/1956 Verkuil ZOO/159 A [4 1 Mar. 26, 1974 3,250,882 5/1966 Campbell et a1 200/159 A X 3,636,284 1/1972 Wilkes 4 200/67 DA 3,600,528 8/1971 Leposavic 200/159 B X 3,290,439 12/1966 Willcox et al.. 200/5 A X 3,697,711 10/1972 Tetrick 200/3 A X Primary ExaminerJames R. Scott Attorney, Agent, or Firm-Marvin H. Kleinberg [57] ABSTRACT A snap-action switch is formed from a thin resilient sheet that is elastically stressed into a threedimensional configuration. One or more bi-stable segments are provided wherein relative movement of the end of a bi-stable segment causes a snap action into a reversely-curved configuration. Switch contacts are only engaged or disengaged after the switch has snapped into a stable configuration.

12 Claims, 17 Drawing Figures 'MENHED UAR 2 6 i974 SHEET UF 5 LOW PROFILE KEYBOARD SWITCH ASSEMBLY WITH SNAP ACTION CANTILEVER CONTACT This invention relates to snap-acting devices of a type in which a gradual motion of one portion or a part is translated into a sudden motion of another portion or part, and more particularly, to a snap-action switch which employs this phenomenon.

In recent years, the need has arisen for keyboards for use with substantially pocket-size electronic calculators. In attempting to achieve thinner and thinner profiles for the calculator, the design constraints imposed upon the keyboard have been such that very limited key travel is available to establish contact for data entry purposes. From a human factors standpoint, it has been deemed desirable in key-operated devices to-provide an audible and/or tactile signal when the key has been actuated.

In those applications where substantial key travel is available, the movement of the key provides the tactile signal. Yet other tactile signals include audible clicks or mechanical detents which can be felt while operating the key.

In satisfying the need for a relatively thin device with only limited key travel available, it has been deemed desirable to provide audible and tactile feedback which could provide a signal, within the limited distance of key travel available.

Prior art devices have employed snap action devices. One such device includes a concave disc positioned above contacts on a printed circuit board. When a key is depressed, the concave disc is depressed into an overcenter condition which provides a conductive link between the contacts. Upon release of the key, the disc is restored to its normal concave shape and the contact is broken.

Other devices employing a snap action principle have been disclosed in the several patents to D. F. Wilkes, U. S. Pat. Nos. 3,479,624; 3,543,595; 3,539,742; and 3,636,284, among others. It is noted, however, that the principles employed in the Wilkes devices add a degree of complexity which is unnecessary in a simple keyboard application.

It has been deemed desirable to adapt the basic principle ofa deformed elastic member which has two positions of stability to the needs ofa key switch and, more particularly to a full keyboard wherein a plurality of switches could be fabricated as a single assembly.

According to the present invention, a single sheet of a conductive material is fabricated to include a cantilever arm that has two portions. A first portion is deformed to provide a relatively shorter support arm. A remaining, relatively longer, tongue member is, as a result stressed into a convex or concave configuration to allow for the difference in length. At the free end of the cantilever there is provided a contact which is adapted to engage a terminal. In a preferred embodiment, the contact is bifurcated.

As a force is applied near the free end of the cantilever, the cantilever, as a unit, is bent. A fulcrum or support is provided under the relatively longer, tongue portion which bends until the forces holding it in one of its stable curved configurations are overcome, causing it to snap to its opposite curvature. An additional force and motion is imparted to the free end of the cantilever member in the direction of motion, closing the contact.

The material, in a preferred embodiment, is a spring material, such as phosphor bronze, beryllium copper or stainless steel, that provides an audible click as the tongue portion snaps into its second stable configuration. This imparts to the operator both an audible and a tactile signal that the switch has been operated and that the contact has been made. As the key is released, changes on the forces on the tongue portion cause it to snap to its opposite or initial curvature. The snap upon opening again provides an audible and tactile signal to the operator that the contact has been opened.

'The switch is carefully designed so that contact can not be made and released by teasing" the switch with slight increases or decreases of pressure on the key mechanism. Further, the snap action provides a more or less standardized force of contact closure, substantially independent of the mode of operating the key. Once the switch is closed, after the snap action has taken place, additional pressure on the key will not increase the pressure between the movable and stationary contacts of the switch since it is desirable to have contact force independent of operator force.

In alternate configurations, a plurality of snap action switch members may be fabricated from a single sheet of material to faciliate assembly into a keyboard. The movable contacts can be designed to cooperate with various types of stationary contacts. In different applications, one may employ a flat contact pad on a printed circuit board, a post or terminal on a base plate or board, or an elevated contact area on a printed circuit board.

By appropriate shaping of the movable contacts, each type of stationary contact can be accommodated. Further, the sheet can be designed so that groups of switches can be electrically isolated. from other switches or groups of switches. Contacts can easily be supplied to establish a separate electrical circuit to each key or group of keys.

The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

FIG. 1 is a perspective of a keyboard employing the present invention;

FIG. 2 is a side sectional view of the keyboard of FIG. 1, FIGS. 2A-2D, inclusive, illustrating keys in actuated and unactuated conditions;

FIG. 3 including FIGS. 3A and 3B is a front section view of an actuated and unactuated key 12C of FIG. 2, taken along line 3-3 of FIG. 2C taken in the direction of the appended arrows;

FIGS. 4A and 4B are a perspective view of a switch in the open and closed positions, respectively;

FIG. 5 is a top view of a plurality of switch members according to one employment of the invention at an early state of fabrication;

FIG. 6 is an enlarged view of one of the switches of FIG. 5;

FIG. 7 is a side view of the switch of FIG. 6 at a later stage in fabrication; and

FIG. 8, including FIGS. 8A, 8B, and 8C, is an end view of a key switch combination, illustrating an alternative contact configuration.

Turning first to FIG. 1, there is shown in perspective view, a low profile keyboard 10 according to the present invention. Included are a plurality of individual keys 12 which are arranged in groups such as a numerical group 14 and

different function groups

16, 18. In the illustrated embodiment, a printed circuit extends from the end of the keyboard 10 and is adapted to be coupled to an appropriate connector or wires (not shown) which electrically interconnect the keyboard 10 to a system.

According to the present invention, each key 12 has a limited vertical travel available to it, and, accordingly, must provide adequate tactile and auditory clues to the user when a key 12 is either actuated or released.

Turning next to FIG. 2, there is shown in side section a plurality of differing keys 12A-12D, inclusive. Each of these keys is separately illustrated in the enlarged sectional views of FIGS. '2A-2D, inclusive, respectively. It will be noted that each of the keys 12A through 12D differs slightly in design detail and have been included as illustrative of alternative approaches that can be taken to the design of a particular key.

In FIG. 2, there can be clearly seen the several ele ments that combine to form the keyboard. A frame member 22 overlies an insulating sealer membrane 24 and a switch plate 26, includes all of the movable switch contacts. A spacer plate 28 supports the switch plate 26 and a printed circuit board 30, which contains the stationary contacts, acts as a bottom or base plate.

In FIG. 2A, a first key switch 12A is shown having projecting lugs at the front and rear of the key. A rear lug 32 rides in an appropriate slot 34 and the key 12A is mounted to pivot about that point of attachment. A forward lug 36 is provided to limit upward key travel and cooperates with an overhanging lip 38 in the key plate 22.

A switch cam 40 engages a movable switch member 42, which is an integral part of the switch plate 26. The movable switch member 42 is actuated through the insulating sealer membrane 24. As the key 12A is depressed, the cam 40 exerts a downward force on the movable switch portion 42 until a stationary switch contact 44 is engaged.

In the illustrated embodiment, the stationary contact 44 is an elevated portion of the base plate printed circuit board with suitable contact material 46 integral therewith to complete a circuit with the movable switch portion 42.

FIGS. 2B through 2D, inclusive, illustrate alternative switch configurations in both actuated and unactuated positions. FIG. 2B, shows a key 12B which lacks a front lug but provides, to service the same function, side lugs 368 which limit the upward travel of the key 12b. All other elements of the key assemblies of FIGS. 28 through 2D are substantially the same as in FIG 2A. The same reference numerals with the suffix 8" appended thereto have been applied to the components of FIG 2B. The remaining FIGS. 2C and 2D have not been described in detail.

A front view of the key 12C, for example, is shown in FIG. 3 which includes FIG. 3A corresponding to the key 12C in the unactuated configuration, and FIG. 3B which is a view of the key 12C when actuated. As is shown, a movable contact 42C is disengaged from stationary contact 44C in FIG. 3A, but is shown in contact in FIG. 3B, as the key 12C is shown in the depressed or actuated configuration.

In FIG. 4, including FIGS. 4A and 4B, a single switch 48 according to the present invention is shown in perspective view. FIG. 4A shows the switch 48 in the open configuration while FIG 4B shows the switch 48 in the closed configuration. The key, frame member and sealer membrane of FIG. 2 have been omitted for clarity of explanation of the operation of the switch 48.

A switch plate 50 is made of a conductive material which is also resilient such as phosphor bronze, beryllium copper, or stainless steel. Th movable switch portion 52 is cut or otherwise fabricated from the main plate 50. The switch plate 50 rests on a spacer plate 54 which, in turn, rests on a printed circuit board 56. In this embodiment, the printed circuit board 56 has a conductive pad 58 as the stationary switch contact.

The movable switch member 52 includes a bifurcated contact pair 60 at the free end of the movable switch member 52. In this embodiment, the contact pair 60 is bent toward the base plate to assure good electrical contact with the pad 58.

A pair of slots 62 running from the fixed end toward the movable end of the movable switch member 52 divides it into a central tongue portion 64 and supporting arm portions 66. A crimp or bend 68 is placed in the supporting arm portion 66 near the point of attachment to the switch plate 50.

The crimp 68 has two purposes. First, it imparts a generally upward bias to the switch member 52 and, second, it shortens the supporting arms 66 relative to the tongue portion 64, thereby introducing a curvature into the tongue portion 64 extending out of the plane of the movable switch member 52. The spacer plate 54 provides a support tab 70 which influences or controls the operation of the tongue, as to where it snaps, and how crisply it does so.

As the movable switch member 52 is cammed downward by the action of the key, the arm portions 66 pivot about the point of attachment behind the crimp 68.

The tongue portion 64, however, is constrained to pivot about the stiffening tab 70 acting as a fulcrum. Accordingly, the movable arm 52 moves downward. The free end continues to bend until the tongue portion 64 is substantially straight. As the free end continues to move downward, the tongue portion 64 snaps into a reverse curvature, reducing the force applied to the key top by the finger.

The dimensions and placement of the movable switch members 52 are such that the continued downward movement of the free end does not engage contact 60 with pad 58 until after the tongue portion 64 has reversed its curvature. The contact pair 60 can be bent or shaped to adjust the point of switch travel at which the contact is made."

While FIGS. 4A and 48 have shown a flat pad, stationary contact 58 in cooperation with a bent contact pair 60, other alternative embodiments utilizing a raised stationary contact such as is shown in FIGS. 2 and 3 would operate in substantially the same fashion,

with the movable switch arm contacts being coplanar with the body of the arm.

Turning next to FIG. 5, there is shown a switch plate 126, fabricated from a single sheet of material. As shown, sheet 126 can be punched, stamped or chemically etched or milled into the configuration shown wherein a plurality of movable switch members 152 are provided. A plurality of contact tabs 130 are provided to engage conductors on a printed circuit base plate (not shown) to complete the electrical circuit of individual switches.

As shown, the switch members are adapted to be isolated from each other in groups corresponding to the groups shown in FIG. 1, such as a numerical group 114, and the

function groups

116 and 118. This would be accomplished by severing the connections at the parting lines so that each switch within a group would be electrically interconnected but that switch groups would be isolated from each other. As shown, a separate contacting tab 130 is provided for each of the potential groups. I

Turning to FIG. 6, there is shown in somewhat greater detail, one of the movable switch segments of the plate 126 of FIG. 5. As shown, and corresponding to the reference numerals of FIG. 4, the movable switch member is initially formed from a planar blank and has the contact 160 at the free end of a cantilever arm. A pair of slots 162 divide the cantilever into central tongue portion 164 and supporting arms 166.

In FIG. 7, which is an enlarged side view of the switch of FIG 6, a crimp or bend 168 is made in the arm portions 166 which imparts a bias to the arm 152. As shown, the supporting arms I66 are in the plane of the movable member while the tongue portion 164 has a curvature imposed upon it out of that plane.

Turning finally to FIGS. 8A, 8B and 8C, there are shown alternative stationary contact configurations suitable for use in cooperation with the movable switch of the present invention. FIG. 8A shows a nail head or rivet contact 180 which can be electrically connected to circuits from the bottom side of the base plate member 182. FIG. 88 illustrates a contact 184 which is raised and formed from the base plate material, and which is covered with a conductive material 186, as shown in FIGS. 2 and 3. FIG. 8C illustrates a flat pad contact 188 as shown in FIG. 4.

Thus there has been shown a novel switch configuration which provides a tactile and audible signal when actuated. A movable cantilever member includes a prestressed portion that has a first curvature with the cantilever in a first position. As the cantilever moves to a second position through a center position, the prestressed portion reverses its curvature with an audible snap or click and which accelerates the actuation of the switch. Releasing the switch, the cantilever moves to the first position and the prestressed portion again reverses its curvature to its first curvature.

A plurality of similar switches are easily fabricated from a sheet of spring material using mass production tecnhiques. A keyboard can be quickly fabricated using such a switch plate together with a printed circuit board containing stationary contacts, and a spacer plate with keys and a frame member added. The combination is easily assembled.

We claim:

1. A snap action switch swinger adapted to cooperate with a stationary switch contact comprising:

a cantilever arm member extending from a conductive spring body member and said arm member having a fixed end and a free end;

separating means for dividing said cantilever arm member into a support portion and a tongue portion, intermediate said fixed and free ends;

means for introducing a difference in length as between said tongue portion and said support portion, said means imparting a curvature to the relatively longer portion, biasing said cantilever arm member into a first, stable, non-actuated configuration; and

contact means at said free end, said cantilever arm member being operable, in response to movement in a direction opposite to the curvature of said longer portion, to reverse the curvature of said longer portion, biasing said cantilever arm member into a second, stable actuated configuration with said contact means in operational proximity to the stationary contact.

2. The snap action switch swinger of claim 1, above, wherein said separating means include a pair of slots extending from said fixed end toward said free end, the elongated portion intermediate said slots being considered said tongue portion and said means for introducing a difference in length include a crimp in said support portion thereby shortening said support portion relative to said tongue portion.

3. The snap action switch swinger of claim 1, above, wherein said contact means include a pair of opposed cantilever arm members, extending in a direction substantially orthogonal to said cantilever arm member direction of extension.

4. A snap action switch assembly including in combination:

cantilever arm means extending from a conductive spring body member, said cantilever arm means having a fixed end and a free end;

separating means for dividing said cantilever arm means into an elongated support portion and an elongated tongue portion, intermediate said fixed and free ends;

means for introducing a difference in length as between said tongue portion and said support portion for imparting a curvature to the relatively longer portion and for biasing said cantilever arm means into a first, stable, non-actuated configuration in a first direction;

contact means at said free end of said cantilever arm means; and

spacer support means below said conductive spring body member and including a tab portion extending under said longer portion to provide a bending point for said longer portion different from the bending point of said shorter portion in response to motion applied to said cantilever arm means;

whereby motion of said cantilever arm means in a second direction, opposite to said first direction, causes said longer portion to engage said tab portion, forcing said longer portion to reverse its curvature into a second table actuated configuration for said arm means, reducing the magnitude of the force required to continue motion of said cantilever arm means in said second direction.

5. The snap action switch assembly of claim 4, above,

including base plate means adjacent said spacer support means, said base plate means including fixed contact means operatively positioned to engage said contact means when said cantilever arm means are in the second stable actuated configuration.

6. The snap action switch assembly of claim 4, above, further including push button means adjacent said cantilever arm means for imparting motion in said second direction to said cantilever arm means, for actuating the switch assmebly and for maintaining the switch assembly in the second stable actuated configuration against the bias in said first direction, in response to continued operation of said push button means.

7. The snap action switch assembly of claim 4, above, wherein said relatively longer portion is said tongue portion and said relatively shorter portion is said support portion.

8. A snap action switch assembly including in combination:

a. a conductive switch plate having a plurality of cantilever arms extending therefrom, each having a fixed end and a free end;

b. means for providing each of said cantilever arms with a tongue portion and a support portion, and for introducing a difference in length as between said tongue portion and said support portion to impart a curvature to said tongue portion in a primary stable configuration;

c. spacer support means adjacent said conductive switch plate including a plurality of tab portions, each underlying the tongue portion ofa one of said arms, each said tab portion supplying a fulcrum to its associated tongue portion different from the corresponding support portion; and

d. base plate means, adjacent said spacer support means, having a plurality of stationary contact elements each adapted to be engaged by a corresponding one of said cantilever arm free ends, whereby movement ofa one of said cantilever arms in the direction of said base plate causes said tongue portion to engage said tab portion, reversing the curvature of said tongue portion into a secondary, stable configuration with said cantilever free end engaging said stationary contact element corresponding thereto, and whereby in the absence of a force holding said arm in said secondary, stable configuration, said arm returns to said primary stable configuration.

9. The snap action switch assembly of claim 8, above, further including:

a plurality of keys, each adjacent a one of said cantilever arms for imparting motion to said arms, thereby placing said arms in the secondary stable configuration with said stationary contact engaged with the corresponding arm free end. 10. A keyboard assembly comprising, in combination:

a. actuator means comprising a plurality of individually operable key members adapted to be depressed; b. a plurality of cantilevered, movable contact members having free ends and fixed ends, mounted adjacent said actuator means, each in operable proximity to a corresponding one of said plurality of key members, each of said movable contact members having an upward bias and a first, relatively stable configuration in a nonactuated, up position, and a second, relatively stable configuration in an actuated, down position; c. support means comprising a frame containing said plurality of movable contact members, and a plurality of fulcrum elements, each positioned adjacent and below the fixed end of a corresponding movable contact member; and d. a plurality of fixed contacts spaced apart from said plurality of movable contacts, each adapted to be engaged by a corresponding one of said plurality of movable contact members free ends, each of said movable contact members being operable in response to a limited depression of a corresponding key member to partially pivot against said fulcrum element against said upward bias to change from said first to said second relatively stable configuration, for electrically engaging said fixed and movable contacts,

whereby engagement of said contacts is independent of further depression of said key member.

11. The keyboard assembly of claim 10, above, wherein each of said cantilevered movable contact members comprise separating means for dividing said contact member imto a support portion and a tongue portion intermediate said free ends including a pair of separate slots extending between said fixed and free ends; and means for introducing a difference in length as between said tongue portion and said support portion, imparting a curvature to the relatively longer portion, which in said first relatively stable configuration is a downward curvature and in said second relatively stable configuration is an upward curvature.

12. The keyboard assembly of claim 11, above, wherein said means for introducing a difference in length comprises a crimp in said support portion adjacent said contact member fixed end, whereby said tongue portion is the relatively longer portion, said crimp also providing said upward bias.

Claims

1. A snap action switch swinger adapted to cooperate with a stationary switch contact comprising: a cantilever arm member extending from a conductive spring body member and said arm member having a fixed end and a free end; separating means for dividing said cantilever arm member into a support portion and a tongue portion, intermediate said fixed and free ends; means for introducing a difference in length as between said tongue portion and said support portion, said means imparting a curvature to the relatively longer portion, biasing said cantilever arm member into a first, stable, non-actuated configuration; and contact means at said free end, said cantilever arm member being operable, in response to movement in a direction opposite to the curvature of said longer portion, to reverse the curvature of said longer portion, biasing said cantilever arm member into a second, stable actuated configuration with said contact means in operational proximity to the stationary contact.

4. A snap action switch assembly including in combination: cantilever arm means extending from a conductive spring body member, said cantilever arm means having a fixed end and a free end; separating means for dividing said cantilever arm means into an elongated support portion and an elongated tongue portion, intermediate said fixed and free ends; means for introducing a difference in length as between said tongue portion and said support portion for imparting a curvature to the relatively longer portion and for biasing said cantilever arm means into a first, stable, non-actuated Configuration in a first direction; contact means at said free end of said cantilever arm means; and spacer support means below said conductive spring body member and including a tab portion extending under said longer portion to provide a bending point for said longer portion different from the bending point of said shorter portion in response to motion applied to said cantilever arm means; whereby motion of said cantilever arm means in a second direction, opposite to said first direction, causes said longer portion to engage said tab portion, forcing said longer portion to reverse its curvature into a second table actuated configuration for said arm means, reducing the magnitude of the force required to continue motion of said cantilever arm means in said second direction.

5. The snap action switch assembly of claim 4, above, including base plate means adjacent said spacer support means, said base plate means including fixed contact means operatively positioned to engage said contact means when said cantilever arm means are in the second stable actuated configuration.

8. A snap action switch assembly including in combination: a. a conductive switch plate having a plurality of cantilever arms extending therefrom, each having a fixed end and a free end; b. means for providing each of said cantilever arms with a tongue portion and a support portion, and for introducing a difference in length as between said tongue portion and said support portion to impart a curvature to said tongue portion in a primary stable configuration; c. spacer support means adjacent said conductive switch plate including a plurality of tab portions, each underlying the tongue portion of a one of said arms, each said tab portion supplying a fulcrum to its associated tongue portion different from the corresponding support portion; and d. base plate means, adjacent said spacer support means, having a plurality of stationary contact elements each adapted to be engaged by a corresponding one of said cantilever arm free ends, whereby movement of a one of said cantilever arms in the direction of said base plate causes said tongue portion to engage said tab portion, reversing the curvature of said tongue portion into a secondary, stable configuration with said cantilever free end engaging said stationary contact element corresponding thereto, and whereby in the absence of a force holding said arm in said secondary, stable configuration, said arm returns to said primary stable configuration.

9. The snap action switch assembly of claim 8, above, further including: a plurality of keys, each adjacent a one of said cantilever arms for imparting motion to said arms, thereby placing said arms in the secondary stable configuration with said stationary contact engaged with the corresponding arm free end.

10. A keyboard assembly comprising, in combination: a. actuator means comprising a plurality of individually operable key members adapted to be depressed; b. a plurality of cantilevered, movable contact members having free ends and fixed ends, mounted adjacent said actuator means, each in operable proximity to a corresponding one of said plurality of key members, each of said movable contact members having an upward bias and a first, relatively stable configuration in a nonactuated, ''''up'''' position, and a second, relatively stable configuration in an aCtuated, ''''down'''' position; c. support means comprising a frame containing said plurality of movable contact members, and a plurality of fulcrum elements, each positioned adjacent and below the fixed end of a corresponding movable contact member; and d. a plurality of fixed contacts spaced apart from said plurality of movable contacts, each adapted to be engaged by a corresponding one of said plurality of movable contact members free ends, each of said movable contact members being operable in response to a limited depression of a corresponding key member to partially pivot against said fulcrum element against said upward bias to change from said first to said second relatively stable configuration, for electrically engaging said fixed and movable contacts, whereby engagement of said contacts is independent of further depression of said key member.