referring initially to fig1 to 5 , there is shown an auto - injector device comprising a front housing part 10 and a rear housing part 12 , having respective screw threaded portions 14 , 16 to allow the parts to be screwed together as seen in fig1 to 3 . the front and rear housing parts may be seen clearly in fig4 . the front housing part is provided at its forward end with a captive depth adjustment cap 18 having an aperture 20 at its forward end through which the syringe needle may project . the forward housing part 10 is provided with an internal collar 22 which slideably receives a syringe carrier 24 which is biased rearwardly by a spring 26 . the syringe carrier 24 has an internal bore designed to receive a syringe 28 including a syringe plunger 30 and a needle 32 . the bore in the syringe carrier 24 may be designed to be an interference fit at its forward end with a part of the syringe so as lightly to grip the syringe . the syringe carrier 24 is provided at its forward end with a rib ( not shown ) that limits rearward movement of the syringe carrier under the influence of the spring 26 . the rearward housing part 12 contains a constant force drive spring 34 wrapped around a drum 36 . the outer end of the constant force spring is secured to an anchorage ( not shown ) on the inner part of the rearward housing part 12 . the drum 36 is mounted for rotation in a drive piston 38 and the constant force spring is designed to be energised as the drive piston 38 is moved to its rearward position as shown in fig1 . the drive piston 38 may be latched in its cocked position and released by any suitable mechanism , but this is done in the present embodiment by means of a trigger 40 provided on the rearward housing part 12 . the drum 36 defines a recess 42 in which is confined a spherical magnet 44 having north and south poles . in the base of the recess 42 is provide a soft iron keeper element 46 which , in the absence of other magnetic influences , retains the spherical magnet against the base of the recess 42 . the rearward housing part 12 has provided in a wall thereof a captive , fixed , magnet 48 positioned at the forward end of the operational stroke of the drive piston 38 . the captive magnet 48 is positioned such that magnetic attraction between the captive magnet and the moveable magnet 44 overcomes the attraction between the moveable magnet 44 and the keeper 46 when the drive piston has moved to a position in which the injection is complete . the resultant ‘ click ’ as the moveable magnet impacts the fixed magnet therefore gives an audible indication that the injection is complete . the impact will also generate a tactile signal that can be felt by the user . in other embodiments , not shown , different types of audible , tactile or visual indicators may be provided . for example , co - acting magnets may be provided which cause an element to project or retract into the housing so that the user can feel for an indication of injection complete . an important feature of the magnetically - operated indicator is that it does not divert energy from the main constant force spring that expels the dose . in fact energy is required during priming of the device to separate the magnets but this does not significantly diminish the energy available for expelling the dose . an injection cycle will now be described . the forward and rearward housing parts are unscrewed and the drive piston 38 pushed back to the cocked position by reversing the forward housing part and pushing the piston back using the nose of the cap 18 until it is latched by the trigger . as the drum 36 moves rearwardly , the spherical magnet 44 is pulled away from the captive magnet 48 and is pulled back towards the keeper 46 by magnetic attraction . a syringe is then loaded into the syringe carrier 24 in the forward housing part 10 and the housing parts screwed together . the cap is removed during or after this operation by suitable means ( such as is described below ). the auto - injector is then in the condition shown in fig1 ready to be fired . on firing , the drum 36 and drive portion 38 are released for forward movement under the influence of the drive spring 26 so that the drive piston 38 moves the syringe plunger 30 forward to expel the dose . towards the end of the stroke , when the required amount of the drug has been expelled , the drum 36 is close enough to the captive magnet for the captive magnet to attract the spherical magnet with a force greater than that exerted by the keeper so that there is a loud click indicating injection complete . referring now to fig6 to 16 , there is now described an arrangement for facilitating removal of the needle sheath . as seen in fig6 and 7 , when assembled with the syringe in place prior to injection , a typical needle sheath 50 extends only a short way forward of the cap 18 . this is not enough to enable someone to grip the needle sheath sufficiently tightly to remove it . there is a risk therefore that the user circumvents this problem by removing the sheath 50 before inserting the syringe into the syringe carrier , with the attendant risk of needle - stick injury . accordingly , as seen in fig8 to 16 , a spring finger 52 is provided that is anchored to the forward housing part or a component affixed thereto and extends forwardly and inwardly . the spring finger 52 is designed so that its forward end can latch behind the rearward annular rib of the needle sheath 50 when the syringe has been inserted into the syringe carrier 24 and the syringe carrier pushed forwardly against the bias of the spring 26 as seen in fig1 . the spring finger retains the syringe and sheath in this forward position leaving a much greater portion of the sheath available to be gripped and removed . as soon as the sheath is removed by easing it off , and thus overcoming the friction fit between the needle sheath and the needle hub of the syringe , the syringe shoots back rearwardly under the influence of the spring 26 so that the needle is safely within the cap 18 . the device is then ready for use as previously described .
an autoinjector is provided with a magnetically operated “ injection complete ” indicator . a moveable magnet associated with the drive piston is kept at one end of a recess by a soft iron keeper until the drive piston reaches its formal position where the moveable magnet is attracted to impact a magnet on the housing to generate an audible click . also described is a spring finger that slips behind the rear edge of the syringe boot temporarily to hold the end of the boot well clear of the housing to that it can be gripped for removal .