Abstract:
The invention provides an improved pressure switch, i.e. a switch that is opened or closed in response to the presence or absence of a pressure in excess of or less than a predetermined value. The pressure switch comprises a switch body mounted on a printed circuit board, a chamber within the switch body, a switch contact plate on the printed circuit board and pressure-responsive means within the chamber that is movable into electrical contact with the switch contact plate.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to pressure switches, i.e. switches that are opened or closed in response to the presence or absence of a pressure in excess of or less than a predetermined value.  
         [0002]     It is an object of the present invention to provide an improved form of pressure switch.  
         [0003]     It is a more specific object of the present invention to provide a pressure switch for switching low voltage and/or low current signals that is economical to produce and operates reliably.  
       SUMMARY OF THE INVENTION  
       [0004]     According to a first aspect of the present invention there is provided a pressure switch comprising a switch body mounted on a printed circuit board, a chamber within the switch body, a switch contact plate on the printed circuit board and pressure-responsive means within the chamber that is movable into electrical contact with the switch contact plate.  
         [0005]     The pressure-responsive means preferably comprises a flexible conductive surface, for example, a flexible metal disc.  
         [0006]     The pressure-responsive means may be arranged to respond to an increase in pressure or to a reduction in pressure. The pressure switch may thus be a vacuum switch.  
         [0007]     A displaceable diaphragm may be contained within the chamber and, if provided, is preferably interposed between the source of pressure and the flexible metal disc or other conductive surface.  
         [0008]     An O-ring seal is preferably interposed between the displaceable diaphragm and the flexible metal disc or other conductive surface.  
         [0009]     According to a second aspect of the present invention there is provided a method of generating an electrical signal from a pressure signal that comprises the use of a pressure switch as defined above. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0010]      FIG. 1  is an exploded perspective view of a pressure switch,  
         [0011]      FIG. 2  is a sectional view of the pressure switch shown in  FIG. 1 ,  
         [0012]      FIG. 3  shows a vacuum switch with the switch shown in its open position, and  
         [0013]      FIG. 4  is a sectional view of the vacuum switch of  FIG. 3 , with the switch in its closed condition. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     The pressure switch shown in  FIGS. 1 and 2  of the drawings comprises a moulded plastic switch body  10  mounted on a printed circuit board  11  and clamped to the printed circuit board  11  by means of a tie rod  12 . There is a chamber within the interior of the switch body  10  and this chamber contains a diaphragm  13 , an O-ring seal  14 , a membrane switch disc  15  and an insulator ring  16 . There is a switch contact plate  17  that is soldered in position on the printed circuit board  11  and there is an air signal pipe  18  connected to the switch body  10  so that the pressure in the air signal pipe  18  is supplied to the chamber.  
         [0015]     The insulator ring  16  normally maintains the membrane switch disc  15  clear of the switch contact plate  17  and the circuit containing the terminals on the printed circuit board  11  is, therefore, normally open.  
         [0016]     When there is an increase in the pressure in the air signal pipe  18 , for example, a pressure within the range of from 2 to 6 bar, the pressure acting on the upper surface of the diaphragm  13  causes the diaphragm  13  to be displaced downwardly to compress the O-ring seal  14  and to apply a pressure to the upper surface of the membrane switch disc  15 , which comprises a flexible conductive metal disc. The application of pressure to the membrane switch disc  15  produces downward distortion of the central part of the disc  15  such that the disc  15  makes contact with the switch contact plate  17  to complete the circuit containing the terminals on the printed circuit board  11 . Thus, when the switch senses the presence of a pneumatic pressure, for example, a normal shop air pressure within the range of from 2 to 6 bars, an electrical signal is generated.  
         [0017]     When the air pressure is exhausted from the switch body  10  via the connecting pipe or tube  18 , the resilience of the flexible membrane  15  causes it to revert to its original relaxed state and thus move away from the conductive plate  17  and thereby disconnect the electrical signal.  
         [0018]     The particular form of switch shown in the drawings is designed to switch low voltage TTL or CMOS signals. Although the specific form of switch shown in  FIGS. 1 and 2  of the drawings is designed to operate within the range of from 2 to 6 bars, it can also be used for pressures of up to 10 bar.  
         [0019]     In addition, although the specific form of switch shown in the  FIGS. 1 and 2  of the drawings is designed to sense the presence of normal shop air pressures, the invention is also applicable to the sensing of pressures in other fluids, for example, hydraulic pressures.  
         [0020]      FIGS. 3 and 4  show a switch that is designed to respond to a reduction in pressure and can thus be used as a vacuum switch. It includes a housing having a main body member  20  and a top member  21  secured to a printed circuit board  22  by means of tie rods  23 . There is a chamber  24  within the housing main body member  20  and this chamber  24  is connected by a line  25  to a potential vacuum source (not shown). The surface of the main body member  20  in contact with the printed circuit board  22  is formed with a circular rebate within which there is an O-ring seal  26 . An airtight seal is thus provided between the printed circuit board  22  and the housing main body member  20 .  
         [0021]     The face of the housing top member  21  in contact with the printed circuit board  22  is formed with a central circular rebate within which a conductive membrane  27  is contained. There is a terminal-carrying switch contact plate (not shown) that is soldered in position on the printed circuit board  22  and the conductive membrane  27  is normally in the position shown in  FIG. 3 , in which it is spaced from the switch contact plate. The periphery of the conductive membrane  27  is bonded to an insulating ring  28  that, in turn, is bonded to the adjacent surface of the printed circuit board  22 . There is a central hole  29  bored through the printed circuit board  22  so that the chamber  24  is in communication with the underside of the conductive membrane  27  and there is a central hole  30  drilled through the top member  21  of the housing so that the upper side of the conductive membrane  27  is in communication with the atmosphere.  
         [0022]     The arrangement is such that, when the pressure in the line  25  falls below a predetermined value, and there is thus a predetermined difference between the pressures on the two sides of the conductive membrane  27 , the central portion of the conductive membrane  27  will be deflected, as shown in  FIG. 4 , so that it comes into contact with the terminals of the switch contact plate on the printed circuit board  22  and thus generate an electric signal.  
         [0023]     The design of the conductive membrane  27  and the material from which it is made will be so chosen as to ensure that the electric signal is generated whenever the pressure within the line  25  falls below a pre-selected value.  
         [0024]     Advantages of the pressure switch of the present invention include: 
    a) its low cost,     b) its small size in comparison with conventional pressure switches,     c) its ability to be placed on, and soldered to, a printed circuit board,     d) its ability to be screwed into a cabinet,     e) its ability to be piped with 2 mm. outside diameter highly flexible 10 bar pneumatic pipe, that can actually be welded chemically into the cylinder and supplied to a specific cut length, and     f) it can be manufactured in multi-switch strips in the form of a an in-line bank of switches that can be designed to be used in multiples on a single strip or cut into smaller single switches or two, three or more switches.    
 
         [0031]     In an alternative embodiment of the invention, the flexible diaphragm is coated on its underside with a conductive layer that thus provides the electrical contact. This removes the need for a separate membrane switch layer and insulating annulus beneath the flexible diaphragm. This alternative embodiment will thus comprise:  
         [0032]     a connecting micro-bore pipe attached to a switch body,  
         [0033]     a flexible one-side conductive diaphragm,  
         [0034]     an O-ring seal,  
         [0035]     a PCB base complete with an electrical two-pole conductive contact layer, and  
         [0036]     a tie rod or other simple fixing means for attaching the switch body to the PCB.  
         [0037]     In a further alternative embodiment, the pressure switch does not employ a membrane switch but, instead, includes a conductive plastic layer—approximately 1 mm. thick—that is sandwiched between the flexible diaphragm and the two-pole conductive PCB contact layer. With no applied pressure, the conductive plastic layer, or plastic “pill”, has a very high resistance. With the application of pressure to the diaphragm, the plastic pill will be squeezed against the contact layer on the PCB base. With increasing pressure, the electrical resistance of the plastic pill will decrease and the plastic pill will become an electrical conductor. Under pressurised conditions, therefore, the contacts will conduct and the circuit will change from open circuit to closed circuit conditions. The plastic pill can also be used as a proportional conductive layer such that its resistance changes from a very large value—in the megohm range—to a very small value—a fraction of an ohm depending on the size of the air pressure signal acting on the switch diaphragm.