Abstract:
The present invention relates to a braking device for use with a vehicle which moves along a surface, the braking device being operated by an area of magnetic material, and is particularly, but not exclusively, for use with a wheeled trolley or cart to deter its removal from a designated area. The braking device comprises a first portion for mounting on the vehicle, a braking portion movable relative to the first portion from an inoperative position to an operative braking position in braking contact with the surface, and retaining portion for retaining the braking portion in its inoperative position. The retaining portion comprises a blocking portion and trigger portion, the trigger portion being movable between a primed position and a triggered position by passage in close proximity to the magnetic material, and in the primed position, holding the blocking portion in a blocking position preventing movement of the braking portion. In moving to its triggered position, the trigger portion releases the blocking portion to allow it to move to its unblocking position to allow movement of the braking portion to its braking position on passage of the device past the magnetic material. To prevent inadvertent release of the braking portions, the braking portion cooperates with the blocking portion to prevent movement of the blocking portion from its blocking position to its unblocking position in the absence of the magnetic material, cooperation being released by application of a force to the braking portion or the blocking portion.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a braking device for use with a vehicle which moves along a surface, the braking device being operated by means external to the vehicle, and particularly but not exclusively to a braking device for use with a wheeled trolley or cart to deter its removal from a designated area. 
     A particular problem with supermarket-type shopping trolleys or carts is the removal of these trolleys or carts from designated areas, such as the supermarket and/or its car park. Although various systems have been proposed for placing physical obstructions such as grids and/or narrow stalls at the exit points from these designated areas, which obstacles are intended to interact with the trolley&#39;s castor wheels or with the trolley itself to deter its removal, these known systems are easily overcome and are also unsuitable for certain applications, such as vehicle entrances for supermarket car parks. 
     A braking device designed to overcome these problems is described in International Patent Application No. WO92/15476. The device is designed to be activated as it passes over a member or area of magnetic material to cause a braking member to be applied to the ground to impede the trolley&#39;s continued movement. The braking device is conveniently associated with a castor wheel of the trolley. However, the environment of dust and heavy knocks which a trolley castor wheel has to withstand produces a weakness and unreliability in the locking and unlocking of this device. The present invention relates to an improvement of the type of device described in the above referred-to application. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a braking device for mounting on a vehicle movable alone a surface comprising a first portion for mounting on the vehicle, a braking portion movable relative to the first portion from an inoperative position to an operative braking position in braking contact with the surface, and retaining means for retaining the braking portion in its inoperative position the retaining means being operable by external means, for example comprising magnetic material, to release the braking portion, the braking portion and retaining means being adapted to cooperate to prevent operation of the retaining means to release the braking portion in the absence of the external means. 
     A portion of the retaining means may abut a surface of the braking portion in such a way that relative movement of the retaining means and braking portion are prevented, the cooperation between the braking portion and retaining means being removed by application of a force to one of the components. 
     The retaining means may include blocking means and trigger means, the trigger means being moveable between a primed position and a triggered position by passage in close proximity to the external means. In the primed position, the trigger means may hold the blocking means in a blocking position preventing movement of the braking portion and, in moving to the triggered position, the trigger means may release the blocking means to allow the blocking means to move to its unblocking position to allow movement of the braking portion to its braking position on passage of the device past the external means, the braking portion cooperating with the blocking means to prevent movement of the blocking means from its blocking position to its unblocking position in the absence of the external means. 
     The blocking means may include a portion having a surface on an end portion which, in the blocking position of the blocking means, abuts a surface of the braking portion such that the blocking means prevents movement of the braking portion to its braking position and the braking portion prevents movement of the blocking means to its unblocking position. The braking portion may be adapted to be moved by the presence of the external means in a direction opposite the direction of its movement to its braking position, so as to release cooperation between the blocking means and the braking portion to permit movement of the blocking means to its unblocking position. 
     The device may include a stabilizing element which, in the primed position of the trigger means bears against the blocking means in the region of its contact with the trigger means so that, when the trigger means moves to its triggered position, the stabilizing element engages the trigger means to hold it in its triggered position. 
     For releasing cooperation between the blocking means and the braking position, as an alternative to movement of the braking portion, the stabilizing element may be arranged to apply a force to the blocking means when the trigger means moves to its triggered position. 
     In relation to the foregoing and what follows, the term “magnetic material” is used to denote a material which is affected by a magnetic force and can either be a magnet or a permanently magnetised material or a material, e.g. mild steel, which is attracted to a magnet or permanently magnetised material. It will be understood that where there are two separate elements of “magnetic material” which are to interact, one element may be permanently magnetised or a magnet and the other element may be made of a material which is attracted to it or may be a magnet or permanently magnetised to either be attracted to it or repelled by it. In the following description the term “attracted” is used but it will be understood that in a correspondingly modified arrangement, the two elements may be repelled. 
     According to another aspect of the present invention, there is provided a braking device for braking a vehicle movable along a surface including a first portion mounted on the vehicle and a further or braking portion movable relative to the first portion between an inoperative position and an operative braking position for contacting the surface to brake the vehicle, blocking means mounted on the first portion for blocking movement of the braking portion to its operative position and trigger means movable between a primed position and a triggered position in which the blocking means is released to allow movement of the braking portion to its braking position, the trigger means being moved from its primed position to its triggered position by passage of the device in close proximity to magnetic material, wherein to prevent inadvertent operation of the device, the braking portion co-operates with the blocking means to prevent movement of the blocking means to its unblocking position in the absence of the said magnetic material. 
     Cooperation between the braking portion and the blocking means may be released either by applying a force to the braking portion or to the blocking means. 
     The trigger means preferably includes a member mounted on the first portion which is arranged to move from its primed position to its triggered position as it passes in close proximity to magnetic material, e.g. a magnetic material strip mounted in the surface over which the vehicle moves. The member is advantageously pivotally mounted on the first portion and biassed to its primed position so that it will return to that position after operation. The trigger member may be pivotally mounted about its centre of gravity and biassed to its primed position by a spring. The member may be made of magnetic material or incorporate an element of magnetic material and may be generally elongate with one end closer to the surface over which the vehicle moves than the other. 
     The blocking means may also be pivotally mounted on the first portion. 
     The braking portion may include a member pivotally mounted on the first portion and arranged to be biassed by gravity towards its braking position. The member may have an eccentric circumferential surface portion which engages the surface over which the vehicle moves when the braking member is moved to its braking position. The member is arranged to pivot in a plane parallel to the direction of movement of the vehicle and in a direction such that continued movement of the vehicle will continue angular movement of the member to a position in which it moves the first portion away from the surface. 
     The blocking member in its blocking position may hold the braking member in its inoperative position against the bias of gravity. The contact surfaces of the blocking means and braking member are preferably arranged such that the blocking means is deterred from moving to its unblocking position while contact between it and the braking member is maintained. To enable the device to be operated the braking member may be caused to move slightly in the direction opposite to that in which it moves to its braking position, to free the blocking means for movement. This may be achieved by providing the braking member with a magnetic element which is attracted to the magnetic material in the ground which operates the device to cause the required unblocking movement of the braking member simultaneously or substantially simultaneously with movement of the trigger means to its triggered position. Alternatively a force may be applied to the blocking means to move it out of abutment with the braking portion. 
     To protect the device as far as possible from dust the trigger means may be mounted in a first closed chamber defined in part by the first portion and the braking member may be mounted in a second chamber which is open at its lower end to allow the braking member to project therethrough in its braking position. The blocking means may comprise two members, one subject to the trigger member in the first chamber and the other subject to the braking member in the second chamber, the two members being pivotally mounted for joint pivotal movement. 
     The device may include resetting means. The resetting means may operate magnetically or mechanically. 
     The trigger means may be biassed to its primed position and return to that position once the device has been operated. With such an arrangement, the resetting means may operate to return the blocking means and braking portion to their blocking and inoperative positions respectively. Where resetting is effect magnetically, both may be provided with magnetic material elements which interact with an externally applied magnetic material element to return both to the required positions. Where resetting is effected mechanically, both may be provided with lateral projections which are engaged by a resetting member. The resetting member may be permanently mounted in the first portion and movable between an inoperative and resetting position by a removable key. 
     The resetting means may be arranged to move the stabilizing element out of engagement with the trigger means to permit the trigger means to be returned by the resetting means to its primed position, the resetting means simultaneously moving the blocking means back to its blocking position engaging the trigger means and the stabilizing device. Preferably the resetting means is arranged to simultaneously return the braking portion to its inoperative position and the blocking means to its position cooperating with the braking means to prevent the braking means from moving to its braking position and the blocking means to its unblocking position in the absence of the external means. 
     According to a further aspect of the invention, there is provided a braking device for a wheeled trolley or cart or the like including any feature disclosed herein and not disclosed in my prior International patent application No. WO92/15476. 
     Further features and advantages of the present invention will become apparent from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a side elevation of a castor carrying an embodiment of a braking device according to the present invention, in its inoperative condition; 
     FIG. 2 is a side elevation of the castor of FIG. 1 showing the device in its operative condition; 
     FIG. 3 is a view of the castor of FIG. 1 in the direction of arrow III in FIG. 1; 
     FIG. 4 is a section through the braking device of FIGS. 1 and 2 on the line IV—IV of FIGS. 5 and 6; 
     FIG.  4   a  is a section similar to that of FIG. 4 showing a modification of the braking member its operative braking position; 
     FIG. 5 is a section on the line V—V of FIG. 4; 
     FIG. 6 a section on the line VI—VI of FIG. 4; 
     FIG.  6   a  is a section on the line VI a —VI a  of FIG. 6; 
     FIG.  6   b  is a section similar to that of FIG. 6 but including a modification; 
     FIG. 7 is a section on the line VII—VII of FIG. 4 but with the resetting member omitted; 
     FIG. 8 is a section on the line VIII—VIII of FIG. 4 showing the resetting means in its resetting position; 
     FIG. 9 is a section similar to that of FIG. 8 showing the device in its operative position and the resetting means in its inoperative position; and 
     FIGS.  10   a  and  10   b  are a plan view and end elevation of a key for the resetting means of FIGS.  9  and  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The braking device shown in the drawings includes a first portion or body  1  and a further portion  2  arranged to move relative to the first portion to effect the braking function by contacting the surface over which the device moves to lift the first portion of the device away from the surface. In the embodiment, the first portion  1  is fixed to a part of a trolley or cart adjacent the surface or ground over which the trolley moves. Preferably, it is fixed to part of a fork  4  of a caster wheel  5  which is rotatable about the axis of fixing means  3 , which may be a nut and bolt, by which the wheel  5  is fixed to the fork. The portion  1  may be attached to the fork by the same fixing means. 
     As shown, the first portion  1  forms a casing in which the movable components of the braking device are housed. Specifically, the first portion  1  is formed with a laterally opening recess  6 . The inner part of the recess (see FIG. 4) is closed by a first plate  7  to define a first chamber  8  and the opening of the recess is closed by a second plate  9  to define a second chamber  10  between the two plates  7 ,  9  and the first portion. The first chamber  8  is closed so as to protect the components housed therein from dust and dirt, but the second chamber  10  is open along its lower periphery, for reasons which will appear. 
     The further portion or braking member  2  is mounted in the second chamber  10  for angular movement about a pin  11  mounted by its ends in the two plates  7 ,  9 . The centre of gravity of the further portion is offset from the axis of the pin  11  so that gravity biases the further portion to rotate from its inoperative position shown in full lines in FIG. 5, in which the centre of gravity is to one side of the pin, in a clockwise direction to its operative braking position shown in broken lines in FIG. 5, in which part of the portion  2  projects through the open lower periphery of the chamber  10  into contact with the surface  12  on which the castor wheel  5  moves. 
     As shown in FIGS. 4 and 5, the further portion or braking member  2  is in the form of a flat disc and is provided with a high friction finish to grip surface  12  on that part  2   a  of the periphery which contacts surface  12 . To increase the braking effect or adhesion of member  2 , the width of the part of the member which contacts surface  12  may be increased as shown in FIG.  4   a  and that part  2   e  of the member may be made of rubber and bonded to a rigid plastics component  2   f  by which it is supported and mounted on shaft  11 . This reshaping of member  2  is also designed to counteract the lateral forces applied to member  2  during braking because it is off the axis  3   a  of the castor wheel on which it is mounted. 
     Because of the self-aligning action of castor wheels, the castor wheel  5  will track by rotating about its vertical axis  3   a  to face in the direction of travel, which is shown by arrow A in the drawings. Additionally, portion  2  is arranged so that continued movement of the castor in the direction of arrow A will cause the portion  2  to continue to rotate in the clockwise direction to its fully operative position. Part  2   a  of the periphery is curved or arcuate but eccentric relative to the axis of pin  11  so that, as the portion  2  rotates in the clockwise direction, the distance between the axis of pin  11  and that region of the peripheral part  2   a  of portion  2  in contact with surface  12  increases to the point that the castor wheel  5  is lifted off surface  12  so that the trolley is supported on surface  12  by the further portion  2  and not by the associated castor wheel, to thereby impede further progress of the trolley. 
     Means are provided to limit angular movement of the portion  2  in the direction from its inoperative position to its operative position to ensure that it remains in a fully operative position if attempts are made to continue to push the trolley in the direction A. As shown, the portion  2  is shaped so that in its fully operative position, part of the periphery of portion  2  comes into abutment with part of the wall of portion  1  defining the second chamber to prevent further rotation. 
     For controlling movement of portion  2  from its inoperative position to its operative position, the device includes retaining means for retaining the braking portion  2  in its inoperative portion. The retaining means includes blocking means  13  which is normally arranged to prevent movement of the further portion  2  from its inoperative position, and trigger means  14  for releasing the blocking means to allow movement of the further portion  2  to its operative position. The trigger means  14  is arranged to be operated by passage of the device in close proximity to, e.g. over, a member of magnetic material. As shown, a strip  12   a  of magnetic material is arranged in the surface  12 , e.g. across the exit to a supermarket or its car park. 
     The trigger means  14  is mounted in the first chamber  8  and includes an elongate member or bar which is normally in a primed position shown in full lines in FIG.  6  and is, preferably, biased to that position. It is deflected from that position as it passes the magnetic material strip  12   a.  As shown, the trigger member  14  is mounted for pivotal movement about the axis of a pin  15  which passes through the bar substantially centrally between its ends, preferably at the centre of gravity of the bar. The bar extends in the vertical plane but is inclined to the vertical with the lower end  14   a  extending forwardly in the direction of movement of the device and the upper end  14   b  extending rearwardly. The bar is biassed to the primed position shown in FIG. 1 by a leaf spring  16  fixed to portion  1 . 
     The member  14 , or its lower end  14   a,  is made of magnetic material such that the lower end will be attracted to the magnetic material strip  12   a.  As the device passes over the magnetic material strip  12   a,  the lower end  14   a  of the member will tend to move relative to portion  1  in a clockwise direction about the axis of pin  15  as shown in FIG. 1, against the bias of the leaf spring  16 . 
     The blocking means  13  includes a first component or member  17  which is arranged in the first chamber  8  and mounted for pivotal movement about the axis of a pin  18  which is fixed in one end  17   a  of the member. The member  17  extends generally horizontally from the pin  18  and is arranged with its other free end  17   b  resting on the upper end  14   b  of the member  14  of the trigger means, the overlap between the two components being such that, when the trigger member  14  is moved from its primed position to its triggered position, the upper end  14   b  of the member  14  moves clear of the end  17   b  of the blocking member  17  which is then freed to fall under gravity, rotating in a clockwise direction about the axis of the pin  18 . 
     The blocking means  13  includes a second member  19 , fixed to and for rotation with the pin  18  and member  17 , which is housed in the second chamber  10  and interacts with the further portion  2  to hold the further portion  2  in its inoperative position when the trigger means is in its primed position. As shown, the member  19  is fixed at one end  19   a  to the pin  18  and extends generally downwardly from the pin  18 , its lower end  19   b  abutting a surface  2   b  on the further portion  2  to prevent clockwise rotation of the further portion when the member  19  of the blocking means is in its blocking position. When the first blocking member  17  is released by the trigger member  14 , it is freed to rotate in a clockwise direction under gravity. Rotation of member  17  causes the member  19  to rotate in a clockwise direction to move the end  19   b  of the member out of the path of the further portion  2  which is then free to rotate in a clockwise direction to its operative braking position. 
     It will be appreciated that, in use of the braking device on a trolley or cart, the braking device will inevitably be subject to jarring and vibration and it has been found essential to ensure that the conditions of normal usage of the trolley or cart will not cause inadvertent operation of the braking device. To this end, the further portion  2  is arranged to cooperate with the blocking means  13  somewhat in the way the trigger means  17  does to prevent freeing of the further portion if the trigger means inadvertently frees the blocking means. As shown, the blocking member  19  is arranged to contact the further portion  2  in such a way that the two are in effect jammed together so that, even if vibration or jarring causes the trigger member  14  to momentarily rotate to its triggered position, engagement between the member  19  of the blocking means and the further portion  2  will maintain member  17  of the blocking means in its upper position permitting the trigger member  14  to return to its primed position with its upper end  14   b  underneath the end of the blocking member  17 . As shown, this jamming is obtained by appropriate shaping of surface  2   b.  Additionally, because of the offset of the centre of gravity of the further portion  2  from its axis of rotation, the further portion is biased by gravity to bring the contact surface  2   b  into forced abutment with the end surface of the member  19 . 
     However, the jamming between the further portion  2  and member  19  of the blocking means has to be removed to enable the device to operate when required. For this, the further portion  2  may be slightly rotated in a counter-clockwise direction as or when the trigger member  14  has been moved to its triggered position, so that the blocking means will then be free to move to its unblocking position. To achieve this, the further portion  2  may be provided with an element  20  of magnetic material on the side of the axis of rotation opposite that with the centre of gravity G. This element  20  is intended to interact with the magnetic material strip  12   a  in the surface  12  so that. as the device is moved over the strip  12   a,  element  20  is attracted to the strip  12   a  to rotate the further portion  2  counter-clockwise. At the same time, the trigger member  14  is moved to its triggered position by attraction of its lower end  14   b  to strip  12   a.  This frees both of the blocking members to rotate clockwise to remove member  19  from the path of portion  2  which then rotates to its operative position. 
     Even with this provision to prevent inadvertent operation by jarring or vibration, it is found that, if the device is subject to a shock, e.g., it falls for example when the trolley is taken over a curb, the device may be inadvertently operated. To prevent this, means may be provided for holding or retaining the trigger member  14  in its inoperative position. The means may positively stop or block rotation of the trigger member towards its triggered position and be released as the device passes over the magnetic strip  12   a.  Alternatively, the means may simply non-positively engage the trigger member to “damp” any tendency on its part to move and so tend to hold it in its inoperative position. Both alternatives are shown in FIG.  6 . As shown, the positive retaining means may comprise a non-ferrous spring  14   c  mounted by one end on the casing portion  1  and arranged with its other end abutting the lower end  14   a  of trigger member  14  so as to hold the lower end of trigger member  14  against spring  16 . The spring  14   c  carries an element  14   d  of magnetic material so that, as the device moves over strip  12   a,  spring  14   c  is deflected downwardly clear of the lower end of trigger member  14  so that trigger member  14  is free to pivot under the effect of the strip  12   a.  When the device is returned to its inoperative position strip  14   c  simply returns to its operative position blocking movement of trigger member  14 . 
     The non-positive retaining means includes a ramp or wedge-shaped portion  14   e  mounted on or moulded onto the wall of casing portion  1  as shown in FIGS.  6  and  6   a.  In its inoperative position, the upper end portion  14   a  of trigger member  14  is engaged by the portion  14   e  which tends to “damp” any movement of member  14  towards its operative position and therefore tends to retain member  14  in its inoperative position. 
     To further increase the reliability of operation of the device, a stabilizing device in the form of a pivotal catch  37  may be provided which, as shown in FIG.  6   b,  in the inoperative condition of the device has its free end  39  resting on blocking member  17 . When trigger member  14  moves into its triggered position by passing over a magnetic strip  12   a,  the catch  37  rotates anti-clockwise about the axis  38  to hold trigger member  14  in its triggered position. Catch  37  may be made of a spring material to be biased in an anti-clockwise direction or may be biased in an anti-clockwise direction by a spring, so that it exerts a force on blocking member  17  such that, once the member  17  is released by the movement of member  14 , catch  37  forces member  17  to rotate. The force applied by the catch  37  to the member  17  of the blocking means may be used to force the member  19  out of cooperation with braking portion  2  to release braking portion  2 . Element  20  of magnetic material can then be omitted. Catch  37  holds member  14  in its triggered position against anti-clockwise rotation after the force of magnetic strip  12   a  has been removed. The catch  37  may have very little mass of its own and this reduces the possibility of the member  14  being inadvertently released from engagement by catch  37  by vibration of the device enabling the device to be reset by unauthorised manipulation of the further portion  2 . 
     To enable a trolley or cart to be put back into normal use once the device has been operated, it is necessary to arrange for the device to be able to be reset to its inoperative position. This cannot simply be achieved by manually pushing the further portion  2  back into the chamber  10  in an anti-clockwise direction to its inoperative position because that will not reset the blocking means. Accordingly, the device cannot readily be reset in this way by an unauthorized person who is endeavouring to remove the trolley from the designated area. 
     The device in the form shown in FIG. 6, ie without catch  37 , may be arranged to be reset magnetically by placing a member of magnetic material above the operative parts of the device in a pre-selected position. To achieve magnetic resetting, the further portion  2  is provided with a second element of magnetic material in a position where it will be more strongly attracted to the resetting magnetic member than will the first element  20 , and by providing the blocking member  17  with an element of magnetic material or by making the blocking member  17  of magnetic material. When the resetting member is placed above the device, the further portion  2  is rotated anti-clockwise to its inoperative position and at the same time the first blocking member is rotated anti-clockwise to its blocking position. During this movement, the free end  17   b  of the blocking member  17  contacts the trigger member  14  and pivots it against the bias of spring  16  in a clockwise direction out of its path to its blocking position above the upper end  14   b  of the trigger member  14 . The free end  17   b  of the blocking member  17  is rounded so that there is a camming action between it and trigger member  14  to facilitate movement of the blocking member to its blocking position. This movement simultaneously caused movement of the second blocking member  19  to its blocking position ready to abut surface  2   b  of portion  2  as soon as the magnetic resetting member is removed. For convenience, the authorised person may leave the resetting member on top of the device while the trolley is being moved back to the designated area so that when it passes over the magnetic strip  12   a  at the exit to the designated area, it will not be operated again, but will be retained in its inoperative condition. 
     However, it is preferred for resetting to be effected mechanically using a key  30  which is inserted into an opening  24  in the first portion  1  to engage with and operate a resetting member  25  which is mounted in the device and movable between an inoperative, withdrawn, position and a resetting position in which the further portion  2  and blocking means  13  are returned to their inoperative position and blocking position respectively. 
     As shown, the resetting member  25  is mounted in the first portion  1  to be moved linearly into the second chamber  10  to engage with lateral projections  26  and  27  on the further portion  2  and the blocking member  19  respectively to rotate the further portion  2  anti-clockwise from its operative to its inoperative position and to then rotate the member  19  in an anti-clockwise direction from its unblocking position to its blocking position. Rotation of member  19  to its unblocking position rotates member  17  to its position above the trigger member, the trigger member being moved out of the way by the member  17  as it moves to its unblocking position against the bias of the spring  16 . 
     The resetting member  25  carries a rack  28  which is engageable by teeth  29  on the end of a key  30  for moving the resetting member between its withdrawn position (FIG. 8) to a position in which the device is reset (FIG.  9 ). The device having been reset, the key is then operated to move the resetting member  25  back to its withdrawn position. However, the person operating the key may, to prevent inadvertent operation of the device when the trolley is being returned to the designated area, leave the resetting member in its resetting position until the trolley has been returned to its designated area. 
     The resetting member  25  is guided in its linear movement between its withdrawn and resetting positions by a spine  31  on that part provided with the rack which is received in and slides along a groove  32  in portion  1 . The member  25  is retained in its withdrawn position until engaged and moved by the key, by engagement of a detent  33  in a corresponding notch  34 . The detent is provided on the end of a flexible finger  35  which is flexed to permit the detent  33  to move out of the notch  34  when the member  25  is moved to reset the device. The projections  26 ,  27  are provided on the portion  2  and member  19  in positions to be appropriately engaged by camming surfaces  25   a  and  25   b  on member  25  which accommodate relative movement with the respective projection. 
     Another advantage of the modification shown in FIG.  6   b  is that, with this modification, the device can only be reset by a positive resetting mechanism, for example as described above. With this resetting mechanism, when the key  30  is rotated it acts (as previously described) through resetting member  25  to bear down on projection  27  and cause the member  17  to rotate anti-clockwise to its reset position. As the member  17  rotates it comes up against the end  39  of catch  37  and forces the catch to rotate clockwise; this releases the member  14  which is returned to its primed position. When the resetting member is returned to its withdrawn position, the device is again ready to be activated. 
     The first and second portions  1 ,  2  together with the blocking means  17 ,  18 ,  19  and resetting means  25  may be made of any suitable materials. For example, the first portion may be made of zinc, aluminium or plastics. The blocking and resetting members  17 ,  18 ,  19 ,  25 , together with the plates  7 ,  9 , may be made of plastics material. The further portion may be made of polyurethane or rubber. Those parts or elements made of magnetic material, such as trigger member  14 , may, as previously indicated be made of a material which is attractive to a magnet, such for example as mild steel. The strip  12   a  is preferably made of a permanently magnetised material or a permanent magnet.