Double pole membrane switch having preferred sequence closing feature

Membrane switch device comprises first and second parallel insulating supports having opposed first and second surfaces. A first surface central contact and a peripheral contact are provided on the first surface, the peripheral contact extending around and being isolated from the first surface central contact. The first surface central contact and the peripheral contact have commoning extensions which project towards each other and have adjacent free ends. The free ends of the extension define a commoning locus that surrounds the center of the switch site. The second surface has a second surface central contact thereon which is opposed the first surface central contact and a commoning contact which is opposed to and, conforms to, the commoning zone. The shorting contact is electrically isolated from the second surface central contact. Circuit conductors extend to both contacts on the first surface and to the second surface central contact only on the second surface. When the switch is closed, one closing sequence is excluded; the second surface central contact can not be connected to the first surface peripheral contact before it is connected to the first surface central contact.

FIELD OF THE INVENTION 
This invention relates to membrane switches of the type in which two 
contacts on one surface are engaged with one contact on a second surface 
when the switch is closed. The invention is particularly concerned with 
the sequence in which the switch contacts are engaged with each other when 
the switch is closed. 
BACKGROUND OF THE INVENTION 
A widely used type of membrane switch serves to connect two circuit 
conductors on one surface to one surface conductor on a second surface. A 
switch of this type is of the double pole single throw type in that when 
one of the membranes is pressed towards the other membrane, one conductor 
is connected to two conductors on the other membrane. 
In known types of membrane switches of the double pole single throw type, 
the two switch contacts on the one surface will engage or contact the one 
switch contact on the second surface substantially simultaneously in most 
instances when the switch is closed. However, it is always possible that 
one of the contacts on the first surface will engage or contact the single 
contact on the second surface before the second contact on the first 
surface is brought into engagement with the single contact on the second 
surface. Sequential closing of this type can occur, for example, when the 
person operating the device on which the switch is provided presses the 
switch site with a pointed instrument such as a pencil point so that the 
closing force is applied only to a localized area. Most membrane switches 
are designed to be closed by finger pressure, and if finger pressure is 
applied, the closing force is distributed over a relative extensive area 
rather than concentrated in a very limited area. 
In many electronic devices, it is of no consequence if the contacts of a 
double pole single throw switch do not engage or contact each other at the 
same instant. However, it is important in some devices that certain 
sequences of closing be avoided if the device is to function in its 
intended manner. If the two contacts only of a double pole single throw 
switch are engaged with each other in some electronic devices, a totally 
unacceptacle result will follow and the operator may conclude that there 
is a malfunction in the device. The probability of improper closing 
sequence in a given switch may be very slight, but it is nonetheless 
important that the switch be designed to exclude improper closing sequence 
entirely. 
The present invention is directed to the achievement of a double pole 
single throw membrane switch which, when closed, will positively exclude 
the possibility of one of the contacts being connected to a second one of 
the contacts before it is connected to the third contact. Stated another 
way, the invention is directed to the achievement of a membrane switch in 
which a preferred contact closing sequence will be followed or all of the 
contacts will be pressed into engagement with each other simultaneously. 
A membrane switch device in accordance with the invention comprises first 
and second parallel spaced-apart insulating supports which have opposed 
first and second surfaces and have opposed contact means on the opposed 
surfaces forming an electrical switch. At least one of the supports is 
flexible so that the supports can be moved relatively towards and against 
each other until the opposed contacts are against each other. A switch in 
accordance with the invention is particularly characterized in that the 
contact means on the first surface comprises two electrically separate and 
adjacent contacts, one of the contacts having a first surface main contact 
portion and a commoning portion. The other contact on the first surface 
has a commoning portion which is adjacent to the commoning portion of the 
main contact portion. The two commoning portions define a commoning zone 
on the first surface. The contact means on the second surface comprises a 
second surface main contact portion and a second surface commoning portion 
which is electrically isolated from the second surface main contact 
portion. The second surface commoning portion is located such that it is 
against the commoning zone on the first surface when the second surface is 
moved relatively against the first surface. The second surface main 
contact portion is against the first surface main contact portion when the 
surfaces are against each other. A second surface circuit conductor on the 
second surface extends to the second surface main contact portion and 
first surface circuit conductors on the first surface extend to the two 
contacts on the first surface. Upon relative movement of the second 
surface towards the first surface the second surface circuit conductor 
will be electrically connected to both circuit conductors on the first 
surface and the sequence of connection will exlude the possibility of the 
second surface circuit conductor being connected to the other contact on 
the first surface prior to its being connected to the first surface main 
contact portion. 
In accordance with further embodiments, the contact means on the first 
surface comprises a first surface central contact and a peripheral contact 
which surrounds the central contact. The commoning portions comprise 
commoning extensions which extend towards each other. 
In accordance with further embodiments, the first surface peripheral 
contact, the commoning zone, and the second surface commoning portion are 
substantially circular.

As shown in FIGS. 1 and 2, a switch assembly 2 in accordance with the 
invention comprises a base member 4, a first insulating support 6 having 
contact means 22 thereon, a separator 8, a second insulating support 10 
having contact means 24 thereon and a cover 12. The cover is somewhat 
flexible and has the switch site indicated at 14 by a numeral or other 
marking. The support 6 has an upper surface 16 which is opposed to the 
lower surface 18 of the upper support 10. The separator 8 has an opening 
20 therein at the switch site so that the flexible upper support 10 can be 
moved downwardly to close the switch. The supports 6, 10 may be of 
polyester or other suitable film and the conductors, including the switch 
contacts, may be metallized conductors produced by electro-deposition, or 
they may be screened on conductive inks. In the following description, the 
surface 16 is referred to as the first surface and the surface 18 as the 
second surface. 
Referring now to FIG. 3, the switch contact means 22 on the surface 16 
comprises a first surface central contact 26 and a peripheral contact 28. 
The peripheral contact is generally circular and surrounds the central 
contact. First surface circuit conductors 30, 32 extend to the central 
contact 26 and the peripheral contact 28, respectively. These circuit 
conductors normally extend to the electrical devices controlled by the 
switch. 
The first surface central contact 26 comprises a main contact bar 34 which 
extends through the center of the switch site and which merges with the 
circuit conductor 30. Commoning extensions 36 extend from the main contact 
bar 34 as branches extending normally of the axis of the main contact bar. 
Additional commoning extensions 38 extend from the main contact bar 
adjacent to the ends of the bar 34 and the extensions 38 are somewhat 
shorter than the extensions 36 for reasons which will become apparent. 
Additional branches 40 extend laterally from the main contact bar but 
these branches do not participate in the commoning function carried out by 
the extensions 36, 38; the extensions 40 are present to insure that the 
second surface central contact 50 will engage the first surface central 
contact 26. 
The peripheral contact 28 is not a complete circle but has ends 42. A 
plurality of peripheral contact commoning extensions 44 extend from the 
peripheral contact inwardly towards the bar 34 of the first surface 
central contact. 
The free ends of the extensions 36, 38 and 44 are adjacent to each other 
and define a circular shorting zone indicated at 48 by phantom lines in 
FIG. 3. In the disclosed embodiment, these free ends of the commoning 
extensions 36, 44 overlap and in any event, the free ends should be 
sufficiently close to each other to permit them to be electrically 
connected to each other by a commoning conductor 58 on the surface 18 as 
will be described below. 
The contact means 24 on the second surface 18, that is the lower surface of 
the second insulating support 10, comprise a second surface central 
contact 50 and a commoning contact 58 which surrounds the central contact 
50. The central contact 50 is circular in form but has an open center 
through which extend conductors 56. This design is used in order to reduce 
the amount of ink required for the circle. As explained previously, the 
extensions 40 on the bar 34 are contacted by the circular portion 50 and 
the bars 56 of the second surface central contact when the switch is 
closed. 
The commoning conductor 58 is generally circular but is an incomplete 
circle and the circuit conductor 52 extends from the central conductor 
through the resulting gap in the commoning conductor 58. The conductor 58 
is opposed to and conforms in shape and size to the commoning zone 48 on 
the surface 16. 
In use, when the active area 14 of the cover 12 is pressed, the membrane 
support 10 is flexed downwardly and the contact means 24 on the surface 18 
are brought into engagement with the contact means 22 on the surface 16 so 
that the second surface circuit conductor 52 is connected to the first 
surface circuit conductors 30, 32. 
If the closing force is applied uniformly to an extensive portion of the 
areas of active zone 14, the probability is that the contacts 50, 58 on 
the surface 18 will simultaneously engage the contacts 26, 28 on the 
surface 16. If simultaneous engagement is achieved, the second surface 
central contact 50 will contact the first surface central contact 26 and 
the commoning conductor 58 will move into the commoning zone 48 so that it 
will extend over the free ends of the extensions 36, 44. The commoning 
conductor will thus provide a conductive path from the central contact 26 
on the first surface to the peripheral contact 28 on the first surface 16. 
If, however, a localized closing force is applied to the zone 14 and the 
closing force is not on the center of the zone, it is possible that the 
commoning conductor 58 will be moved against, and into contact with, the 
free ends of the extensions 36, 44 and the central contact 50 on the 
second surface will not contact the central contact 26 on the first 
surface 16. If this happens, however, the circuit conductor 52 will not be 
connected to either of the circuit conductors 30, 32 for the reason that 
the commoning conductor 58 is electrically isolated from the second 
surface central contact 50. If the operator does not achieve closure of 
the switch, it will soon be realized and a more uniform force will be 
applied to the zone 14. When the additional force is applied, the contact 
50 will engage contact 26 and circuit conductor 52 will be connected to 
circuit conductors 30, 32 simultaneously. 
It will be apparent from the foregoing description that it is impossible to 
connect the circuit conductor 52 to the circuit conductor 32 prior to its 
being connected to the circuit conductor 30. The only possible 
non-simultaneous sequence is the connection of the circuit conductor 52 to 
the circuit conductor 30 followed by connection of the circuit conductor 
52 to the circuit conductor 32 while connection to circuit conductor 30 is 
maintained. 
A switch in accordance with the invention can have contacts having shapes 
significantly different from the shapes of the contacts shown in the 
drawing and described above. The location of the central and peripheral 
contacts in the disclosed embodiment is probably the most logical 
arrangement of contacts in accordance with the invention. However, the 
main contact on the first surface can simply have shorting or commoning 
extensions projecting laterally from the center of the switch zone and the 
second contact on the first surface can simply have commoning extensions 
inter-digitated with commoning extensions extending from the first 
contact. The commoning contact on the second surface would, as described 
above, conform in shape and size to the commoning zone on the first 
surface and be electrically isolated from the main contact on the second 
surface to which the second surface circuit conductor extends.