A switch assembly

Abstract: In the field of tilt switches and shock sensors, it is known to provide a switch comprising a sealed chamber containing two switch terminals and a pool of mercury. On moving of the switch as a result of tilting or the application of an impulse, the mercury moves to close or open a circuit as desired. Such devices have disadvantages arising from the toxity of mercury. The disclosure relates to a switch assembly in which a magnet (26) is mounted for movement in the vicinity of a reed switch (20). A weight (29) is moveable in a direction generally perpendicular to the magnet (26), and is connected thereto by a mechanism (16;38,39,40;44) which translates the movement of the weight to movement of the magnet. The magnet (26) and reed switch (20) are configured so that the reed switch actuates on movement of the magnet. The advantage of the invention is that tilt switches and shock sensors can be constructed without the need for mercury.

Description: This invention relates to a switch assembly, in particular but not exclusively of the kind which operates in response to tilting of the switch (i.e. a tilt switch) or of the kind which is responsive to the application of an impulse to the switch (i.e. a shock sensor). Previously, such devices have relied for their operation upon the movement of a pool of mercury to open or close the contacts of the switch. Numerous arrangements of such switches have been devised. However, all tilt switches and shock sensors which employ a mercury contact are disadvantageous because mercury is an extremely toxic material. Therefore, it is necessary for people manufacturing tilt switches and shock sensors to take precautions against poisoning by inhalation of mercury vapours, ingestion of mercury or touching of mercury. Additionally, the casings of tilt mercury switches and shock sensors must be strongly made and well sealed so that there is no danger of mercury escaping when the switch or sensor is damaged. A further disadvantage of mercury tilt switches and shock sensors is that the mercury is difficult to dispose of safely when the switches and sensors are scrapped. DE-A-1287186 discloses a switch assembly in accordance with the preamble of claim 1. The weight is mounted below the cup. Also, a universal joint is provided between the weight and the cup in order to convert the lateral movement of the weight into longitudinal, sliding movement of the cup so as to move the source of magnetism to actuate the reed switch. According to the invention, there is provided a switch assembly comprising: a support; a reed switch mounted in the support; a source of magnetism moveable between a first position which causes the blades of the reed switch to adopt one configuration and a second position which causes the blades to adopt a further configuration; a weight moveable in a direction generally perpendicular to the direction of movement of the source; and means for interconnecting the weight and the source whereby movement of the weight relative to the support causes movement of the source between its two positions to actuate the reed switch, the means for interconnecting comprising a hollow cup disposed in the support generally surrounding the reed switch, and the source of magnetism being supported in the cup; characterized in that the weight is located at the upper end of the cup whereby lateral movement of the weight causes tilting of the cup which in turn moves the source of magnetism in the cup to actuate the reed switch. (The phrase "actuate the reed switch" is intended to embrace operation of the reed switch by closing the reed blades together, which is known as "Form A" operation, and also by opening the reed blades, which latter mode of operation is known as "Form B" operation.) Preferably, the source of magnetism is a permanent magnet. An advantage of this arrangement is that the switch assembly can be manufactured as either a tilt switch or a shock sensor without the need for mercury. Furthermore, the sensitivity of the switch assembly is easily adjusted, for example by adjusting the mass of the weight, or the material of the source of magnetism. Preferably, the support includes a hollow housing having a base, the reed switch extending upwardly of the base secured to a sub-frame within the housing, and the source of magnetism being moveable along the sub-frame. This has been found to be a particularly efficient arrangement for a tilt switch. Preferably, the sub-frame includes means for constraining movement of the source of magnetism to a region of the sub-frame. A switch assembly including this feature may be inverted without the source of magnetism falling off the sub-frame. Conveniently, the source of magnetism surrounds the reed switch. This feature makes operation of the switch assembly more reliable. It is preferable that the hollow cup has an aperture formed in the base thereof, and is disposed with its base adjacent the base of the housing, the source of magnetism being supported on the base of the cup, the aperture surrounding the sub-frame, and the weight being disposed about the upper end of the cup. In a particularly preferred embodiment, the base of the housing has formed therein a recess for receiving one end of the cup, the upwardly extending walls of the recess being outwardly inclined to permit tilting of the cup. This feature ensures that the cup is correctly located relative to the sub-frame, yet permits tilting of the cup to take place. Preferably, the weight is formed as a cap for the upper end of the cup. The construction of a switch assembly including this feature is advantageously straightforward. Conveniently, the cup tapers towards the base thereof. This feature allows the cup to tilt more readily, and hence permits the switch assembly to be more sensitive. Preferably, the weight and/or the cup taper towards the upper end thereof. This allows the cup to tilt further when enclosed within a housing without contacting the walls of the housing than if the upper end of the cup/weight was squared off. In one embodiment of the invention, the source of magnetism is spaced from the base of the cup by a spacer which transmits motion of the base of the cup to the source of magnetism, whereby in the untilted condition of the cup the source of magnetism lies adjacent the blades of the reed switch and when the cup tilts the source of magnetism moves away from the blades to actuate the reed switch. This arrangement permits Form B operation of the switch assembly. In another embodiment of the invention, the upper end of the cup and/or weight conveniently has formed therein an aperture through which extends at least part of the sub-frame and/or at least part of the reed switch. This feature allows the overall height of the switch assembly to be minimised. The switch assemblies defined herein above may optionally include a plurality of reed switches arranged to be activated by movement of the source of magnetism. It is an optional feature that there is provided a plurality of sources of magnetism arranged to actuate the or each reed switch. The or each source of magnetism may optionally have in excess of two poles. There now follows a description of preferred embodiments of the invention, by way of example, with reference being made to the accompanying drawings in which:- Figure 1 is a cross-sectional view of a switch assembly according to the invention; Figure 2 is a cross-sectional view of a modified form of the switch of Figure 1; Figure 3 is a cross-sectional view of a switch assembly according to the invention designed to have a low overall height; and Figure 4 is a modified form of the switch assembly of Figure 3 shown in a tilted condition. Referring firstly to Figure 1, there is shown a switch assembly in the form of tilt switch 10. Tilt switch 10 comprises a support in the form of hollow, tubular housing 11 and base 12. The upper surface of base 12 has formed therein a central recess 13 the upwardly extending sidewalls 14 of which are inclined to the vertical. A cylindrical cup 16 having its generally closed end 17 lowermost is disposed in recess 13. Cup 16 is shown tilted to one side in Figure 1, although it will be understood that cup 16 normally resides in an upright orientation until some movement of the tilt switch 10 causes it to tilt. The closed end 17 has formed therein a central, circular aperture 18. A vertically extending sub-frame member 19 extends from base 12 upwardly through aperture 18. Sub-frame member 19 has secured therein a conventional reed switch 20 comprising an evacuated glass tube 22 and a pair of reed blades 23, 24 which terminate in the centre of the reed switch at reed contact 25. The other ends of the reed blades 23, 24 pass through the walls of glass tube 22 to form terminals which may be electrically connected (in a manner not shown in Figure 1) to electrical apparatus in which the switch 10 is installed. A source of magnetism in the form of annular magnet 26 encircles sub-frame member 19 within cup 16. Magnet 26 is free to travel up and down sub-frame member 19. A collar 28 is rigidly secured to sub-frame member 19 and limits the extent of travel of magnet 26. The upper, open end of cup 16 is closed by means of a cap-like weight 29. It will be apparent that cup 16, when upright, is in a condition of stable equilibrium but, when the switch 10 is jolted or tilted, cup 16 will tend to overbalance because of the height of its centre of mass occasioned by weight 29. When cup 16 tilts in this manner, the closed end 17 thereof will rise in an inclined manner relative to the base 23 of the switch assembly 10. Since the magnet 26 is ordinarily in contact with closed end 17, magnet 26 will as a result be driven upwardly along sub-frame member 19, the extent of movement of magnet 26 being limited by collar 28. As magnet 26 moves along sub-frame 29, its magnetic field influences the blades of reed switch 20 either to open or close, depending on the initial position of magnet 26 and the nature of field lines associated therewith. In one embodiment, shown in Figure 1, the reed switch is in an open position when the cup is upright, and the reed switch closes when the cup tilts to raise magnet 26. It will thus be seen that the operation of the tilt switch assembly 10 relies upon the conversion of a primarily lateral movement (the tilting of cup 16) into a movement primarily in the perpendicular direction (i.e. the raising of magnet 26) so that magnet 26 influences a reed switch (reed switch 20) to actuate. Figure 2 shows a modified form of the embodiment of Figure 1. In Figure 2, like parts to those shown in Figure 1 are denoted by the same reference numerals. The cup 16 in Figure 2 tapers towards the lower end thereof, although the base 17 of the cup is flat and normally horizontally disposed as in the embodiment of Figure 1. The tapering of cup 16 allows the magnet 26 to rise further up sub-frame member 19 before it fouls the cup 16 or weight 29. This feature allows configuration of the switch assembly 10 for "Form B" operation. This is achieved by the insertion of a spacer 30 between magnet 26 and base 17 so that, in the untilted condition of the cup 16, the magnet 26 is initially disposed adjacent the reed contact 25. When cup 16 tilts, magnet 26 is raised up sub-frame member 19 towards collar 28, and this causes the reed switch to actuate by opening the reed contact which was initially in a closed configuration. Thus, tilting of the device of Figure 2 causes an open circuit to arise. This is useful in some applications. The weight 29 is shaped to allow the cup 16 to tilt further over within housing 11 than is in the case in the embodiment of Figure 1. In particular, the vertically extending side wall of weight 29 tapers inwardly towards the upper surface of the weight, and the upper surface itself is conical, the cone angle being very shallow. It will be understood that, when the cup 16 tilts, it will tilt considerably further than does the cup 16 of Figure 1 before the weight 29 fouls either the side wall or the upper end wall of housing 11. Despite the fact that the magnet 26 rises to a greater maximum height up sub-frame member 19 in the embodiment of Figure 2, the overall height of the embodiment of Figure 2 is less than that of Figure 1 as a result of the design of the cup 16 and the weight 29. In the embodiment of Figure 2 there are shown two electrical terminals 32 intended for connection to the terminals of the reed blades 23 and 24. Wiring 33 is shown to illustrate the manner in which one of the terminals 32 is connected to reed blade terminal 24. A similar arrangement can be devised to connect reed blade 23 to the other terminal 32. Figures 3 and 4 show versions of the switch assembly 10 of Figure 2, modified to reduce the overall height and diameter of the assembly even further. The embodiment of Figure 3 is configured for Form A operation, whilst that of Figure 4 is suitable for Form B operation. The embodiment of Figure 4 is shown in the tilted condition. The primary modification made to the embodiments of Figures 3 and 4 is that the cup 16 and weight 29 are squat in comparison with the corresponding components shown in Figure 2. To accommodate the length of sub-frame member 19 necessary to support the reed switch 20, an aperture 34 is formed centrally in weight 29. The upper end of sub-frame member 19 protrudes through aperture 34. The wall of aperture 34 is chamfered, and the tip of sub-frame member 19 is correspondingly chamfered so that when cup 16 tilts as shown in Figure 4 the weight 29 does not foul on sub-frame member 19. In the embodiment of Figure 4, the collar 28 which limits upward movement of magnet 26 is absent, and weight 29 is shaped in the region 29a to accommodate magnet 26 when cup 16 is in its fully tilted over position as shown in Figure 4. These measures between them allow reduction in height of the embodiments of Figures 3 and 4 as compared with that of Figure 2 to about half the height of the embodiment of Figure 1. The applicants have manufactured a version of the embodiment of Figure 4 the height of which excluding the terminals 32 (i.e. the effective height above a circuit board by which the assembly would protrude) is approximately 11.5mm. The diameter of that version is 15.5mm, as compared with 18.5mm in the manufactured sample of the embodiment of Figure 2. The embodiments of Figures 2, 3 and 4 are the most successful that the applicants have devised, primarily because of their compact configurations and simplicity of assembly. Since there are effectively only three moving parts (cup 16, magnet 26 and, optionally, spacer 30) in addition to the blades of the reed switch 20, operation of the embodiments of Figures 1 to 4 is extremely reliable. In all of the foregoing embodiments, whilst only a limited number of magnets and reed switches has been described, it will be appreciated that various configurations of magnets and reed switches can be devised. For example, versions of the embodiments can be produced in which a plurality of magnets surround a single reed switch; alternatively, a single magnet can be configured to operate a plurality of reed switches 20 within the housing 1. The housing 11 and base 12 can either be formed as an integral, one piece item or can be formed from separate components. The material of the weight is typically brass, because brass is adequately dense to work in the embodiments of the invention without significantly affecting the magnetic field produced by the magnet. It will be appreciated that the sensitivity of the switches can readily be adjusted, in a number of ways. The sensitivity is related to the stability of the weight. Also, the height of the cup (which dictates the overall height of the centre of mass of the tilting member) and the diameter of the cup can readily be adjusted to alter the sensitivity of the device. Embodiments of the invention can be employed as float switches. However, in general, when so configured it is necessary to provide a return mechanism for the tilting member because the effect of gravity in returning the tilting member to its normal, upright position is significantly reduced. Since the embodiments employ a cup to translate lateral motion of the cup to vertical motion of the magnet, a high mechanical advantage results. The current which the assemblies shown in the drawings can switch is limited by the rated current of the reed switches. It is possible to devise versions of the switch assemblies including integral solid state switching devices to handle high currents, with only small currents, within the rated capacities of the reed switches, being passed to the switch assemblies themselves. All the embodiments shown can be configured to operate for tilts in any direction. They can also be configured without modification to operate as shock sensors, when coupled to appropriate shock sensing circuitry. Although in the embodiments shown the source of magnetism has only been described as being a permanent magnet, other sources of magnetism are possible. For example, electromagnets may be used instead.

Claims: A switch assembly comprising: a support (11,12); a reed switch (20) mounted in the support; a source of magnetism (26) moveable between a first position which causes the blades (23, 24) of the reed switch to adopt one configuration and a second position which causes the blades to adopt a further configuration; a weight (29) moveable in a direction generally perpendicular to the direction of movement of the source; and means (16) for interconnecting the weight and the source whereby movement of the weight relative to the support causes movement of the source between its two positions to actuate the reed switch, the means for interconnecting comprising a hollow cup (16) disposed in the support generally surrounding the reed switch, and the source of magnetism being supported in the cup; characterized in that the weight (29) is located at the upper end of the cup (16) whereby lateral movement of the weight causes tilting of the cup which in turn moves the source of magnetism (26) in the cup to actuate the reed switch (20). A switch assembly according to Claim 1 wherein the source of magnetism (26) is a permanent magnet. A switch assembly according to Claim 1 or Claim 2 wherein the support includes a hollow housing (11) having a base (12), the reed switch (20) extending upwardly of the base secured to a sub-frame (19) within the housing, and the source of magnetism (26) being moveable along the sub-frame. A switch assembly according to Claim 3 wherein the sub-frame (19) includes means (28) for constraining movement of the source of magnetism (26) to a region of the sub-frame. A switch assembly according to any of Claims 1 to 4 wherein the source of magnetism (26) surrounds the reed switch. A switch assembly according to Claim 3 or 4 wherein the hollow cup (16) has an aperture (18) formed in the base (17) thereof, and is disposed with its base adjacent the base (12) of the housing (11), the source of magnetism (26) being supported on the base of the cup, the aperture surrounding the sub-frame (19), and the weight (29) being disposed about the upper end of the cup. A switch assembly according to Claim 3, 4 or 6 wherein the base (12) of the housing (11) has formed therein a recess (13) for receiving one end (17) of the cup (16), the upwardly extending walls (14) of the recess being outwardly inclined to permit tilting of the cup. A switch assembly according to Claim 6 or Claim 7 wherein the weight (29) is formed as a cap for the upper end of the cup (16). A switch assembly according to any of Claims 6 to 8 wherein the cup (16) tapers towards the base (17) thereof. A switch assembly according to any of Claims 6 to 9 wherein the weight (29) and/or the cup (16) taper towards the upper end thereof. A switch assembly according to any of Claims 6 to 10 wherein the source of magnetism (26) is spaced from the base (17) of the cup (16) by a spacer (30) which transmits motion of the base of the cup to the source of magnetism, whereby in the untilted condition of the cup the source of magnetism lies adjacent the blades of the reed switch (20) and when the cup tilts the source of magnetism moves away from the blades to actuate the reed switch. A switch assembly according to any of Claims 6 to 10 wherein the upper end of the cup (16) and/or weight (29) has formed therein an aperture (34) through which extends at least part of the sub-frame and/or at least part of the reed switch. A switch assembly according to any preceding claim including a plurality of reed switches arranged to be actuated by movement of the source of magnetism (26).

Inventors: GREEN TERENCE WILLIAM, GREEN, TERENCE WILLIAM
Applicants: HAMLIN ELECTRONICS EUROP LTD, HAMLIN ELECTRONICS (EUROPE)LTD.
Application year: 1992
Main classification: H01H35
IPC codes: H01H35