Abstract:
A single use camera having an electronic flash assembly including a flash charging circuit and a flash illumination circuit is provided with one or more access openings in the camera cover. The access openings are aligned with contact terminals in the flash charging circuit to allow insertion of a shorting device that acts to disable operation of the flash charging circuit. This is useful during a film loading operation to prevent charging of the flash capacitor which might otherwise be caused by electrostatic start up of the flash charging circuit. Preventing operation of the flash charging circuit in preference to shorting of the flash capacitor directly preserves battery power and assures the flash charging transistors will not be damaged by attempting to charge into a short circuited flash capacitor.

Description:
CROSS-REFERENCE OF RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 08/798,402, filed Feb. 7, 1997, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of photographic cameras, and in particular to single use cameras having flash assemblies. More particularly, the invention relates to a method and apparatus for controlling the functioning of the flash assembly during film loading operation to prevent inadvertent exposure of the film by an element of the flash assembly caused by build-up of electrostatic charge during film loading. 
     BACKGROUND OF THE INVENTION 
     Typically during the manufacture of a single use camera incorporating a flash assembly in the camera, the flash assembly with its printed circuit board and a battery are mounted on the camera body. A front cover is then assembled to the body covering the flash assembly and its printed circuit board. After testing for proper operation of the flash assembly, the camera is placed in a dark environment during which the photographic film is loaded by withdrawing the film from its cassette and pre-winding it on a take up spool. The back cover is then assembled to the camera to form a light tight camera assembly. The camera is then removed from the film loading station and is ready for sale. It is also common practice to place a label on the camera that covers some of its physical features that are non-functional during normal picture taking. 
     U.S. Pat. No. 34,168 discloses a single use camera of the above type that incorporates a pair of holes in the front cover to provide access by a shorting bar to terminals of a flash capacitor in the flash assembly during the film loading operation and before the label is affixed to the camera. The described purpose of this feature is that by keeping the shorting bar inserted in the access openings, the capacitor is maintained fully discharged until the film cassette and rolled unexposed film are fully loaded and the back cover is put in place to provide light tight film cassette receiving and film takeup chambers. In this way, inadvertent exposure of the film during the film loading operation is avoided. 
     Commonly assigned U.S. Pat. No. 5,506,646, discloses a single use camera having a flash assembly and which also is provided access holes in the front cover to allow access to the flash capacitor by a battery supply for fast charging of the flash capacitor as part of a flash firing test of the flash assembly. The flash charging circuit in this patent employs a high gain cascaded transistor oscillation feedback to maintain oscillation during charging of the main flash capacitor. With this arrangement, relatively low levels of energy feedback from the flash circuit during discharge of the flash tube cause automatic restarting of the flash charging circuit for recharging of the flash capacitor in preparation for the next picture taking event. It has been found that with this flash circuit, electrostatic charge during handling of the camera under some circumstance can cause the flash charging circuit to start operation even if the camera user does not particularly want this to occur. In commonly assigned allowed U.S. application Ser. No. 08/575,658--Constable, filed Dec. 21, 1995, this unwanted starting of the flash charging circuit during normal camera use is eliminated by provision of a resistor between the base of the first oscillation transistor and ground, the effect of which is to bleed off electrostatic charge build up and thus prevent the electrostatic charge from unwanted starting of flash charging operation. However, during a film loading operation in a camera assembly operation, electrostatic charge conditions are more serious than would be encountered by a camera user walking on a carpet. If electrostatic charge causes the flash charging circuit to start operation during film loading, the flash capacitor can become charged to sufficiently high potential to cause the typical &#34;flash ready&#34; light to be turned on causing exposure of the film being loaded. If the shorting bar feature as described in the &#39;168 is employed, the flash capacitor will be prevented from charging up to charge voltage needed to turn on the flash ready light and the problem can thereby be avoided. However, a problem with the &#39;168 patent feature is that while shorting out the flash capacitor prevents charging of the flash capacitor, the flash charging circuit remains operating at full current conditions. Continued operation under these conditions can damage the oscillation transistors and also needlessly serves as a drain on the single use camera battery. There is therefore a need for an arrangement operative during film loading in a single use camera which will prevent build up of charge on the flash capacitor and at the same time prevent the flash charging circuit from operating. 
     SUMMARY OF THE INVENTION 
     Thus, in accordance with one aspect of the invention, there is provided a method of loading a roll of unexposed film into a single use camera having an electronic flash assembly including a flash illumination circuit and a flash charging mounted behind a front cover, wherein the method comprises inserting shorting means through access openings provided in the front cover in such a manner as to disable operation of the flash charging circuit during the film loading operation. 
     In another aspect of the invention, a single use camera is provided wherein the camera has a roll of unexposed film, a taking lens, an electronic flash assembly including a flash charging circuit and a flash illumination circuit and a cover enclosing said film roll and said flash assembly in light tight manner, the camera characterized by the cover having at least one access opening allowing insertion of short circuit means to contact electrical contacts in the flash charging circuit to disable operation of the flash charging circuit and preferably to prevent the start of self oscillations in the flash charging circuit. 
     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a front perspective view of a single use camera used in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a an exploded front perspective view of the single use camera illustrated in FIG. 1; 
     FIG. 3 is a an electrical schematic diagram of a flash charging and control circuit of the electronic flash assembly depicted in FIGS. 1 and 2; 
     FIG. 4 is a plan view of a printed circuit board implementing the schematic circuit shown in FIG. 3; and 
     FIG. 5 is a partial plan view of the inside of the camera cover shown in FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1-2, there is shown a single use camera 10 having a plastic molded body 3, including a frame section 5 defining a middle portion of the camera which is used to retain the majority of photographic components of the camera 1, as described briefly below. A front cover section 7 and a rear cover section 9 are snap fitted together to the front and rear of the frame portion 5, respectively, by snaps 11, 13 in the manufacturing assembly of the camera 1. A pair of molded chambers 15, 17 are provided in the rear of the frame portion 5 and the rear cover section 9 for housing a film cassette 19 and a take-up spool 21, respectively. First and second access openings 24a, 24b are provided in the front cover 7 to provide access to electrical contacts on a flash assembly circuit board 39 described below. When finally assembled, a decorative label 23 is attached to the exterior of the camera body 3 and covers the access openings 24a, 24b. 
     The camera 1 also includes a taking lens 25 and a viewfinder 27, as are commonly found in other photographic cameras, as well as a depressable shutter release button 29 located on the top of the camera body 3. The filmstrip 31 contained within the film cassette 19 is prewound onto the take-up spool 21 for advancement back into the cassette by means of a film advance wheel 33 which pulls film from the take-up spool over a sprocket (not shown) across an exposure gate (not shown) and into the cassette. 
     Still referring to FIGS. 1-2, an electronic flash assembly 35 is attached to the frame portion 5 of the camera 1. Briefly, this assembly includes a flash head 37; a circuit board 39; a momentary switch 41 attached to the circuit board; a flash capacitor 43 mounted lengthwise beneath the taking lens 25; and a direct current power source 45, such as a battery 47. First and second electrical contacts 49a, 49b are provided on the facing surface of the circuit board 39, the purpose of which are described below. When assembled, the camera 1 includes a cantilevered portion 42 of the front cover 7 which serves to access the momentary switch 41. 
     Referring to the electrical schematic of FIG. 3, the flash assembly 35 includes a charging and control circuit 44, including the battery 47; a self-oscillating flash charging circuit 48; an oscillation arresting circuit 56, and a flash illumination circuit 58, including main flash capacitor 43. 
     Referring specifically to the electrical schematic of FIG. 3, the self-oscillating charging circuit 48 includes a voltage converting transformer 60 having primary and secondary windings 52,64, respectively; the previously referred to momentary switch 41 for initiating oscillations in the circuit; a resistor 68 placed in series with the momentary switch 41; ganged transistors 70, 74, acting as switching elements for supporting and maintaining the oscillations; and a diode 76 for rectifying current induced in the secondary windings 64 of the transformer. 
     Normal charging of the flash assembly 35 is initiated by a single depression of the momentary switch 41, thereby establishing current flow through the resistor 68, transistors 70, 74, and the primary transformer winding 52. The switch 41 connects the base of the transistor 70 to the battery 47 through the resistor 68. Current flowing from the battery 47 into the base of the transistor 70 is multiplied by a transistor gain of fifty (50) and flows to the base of the transistor 74. The current is multiplied again at the transistor 74, with a gain of two hundred (200), and flows through the collector of transistor 74 and the transistor primary winding 52. As the current flow builds in the primary winding 52, it inductively induces current flow in the secondary winding 64. Current flows out of the secondary winding, charging the flash capacitor 43, and into the base of the transistor 70, providing positive feedback which sustains oscillations in the charging circuit. Once initiated, the oscillations are self sustaining even after momentary switch 41 is opened. 
     The oscillation arresting circuit 56 includes a zener diode 78 connected from the junction of neon ready light 82 and resistor 86 to the base of a digital pnp transistor 80. The charge voltage on flash capacitor 43 is sensed by neon ready light 82 which begins conducting at two hundred seventy volts (270 v.), whereupon the voltage drop across the ready light falls to two hundred and twenty volts (220 v.) when it is conducting. The voltage sensor 78, which includes the ready light 82 and the zener diode in series, begins conducting at about three hundred and thirty volts (330 v.), which also represents a predetermined or full charge desired on flash capacitor 43. When the flash capacitor 43 is fully charged, the zener diode 78 begins to conduct, applying current to the base of the transistor 80, switching the transistor 80 on, and grounding the self-oscillating charging circuit 48. Oscillations in the circuit 48 are thereby arrested, and charging stops. 
     The flash trigger circuit 58 includes a triggering capacitor 62, a voltage converting transformer 90, a flash triggering electrode 92, and a synchronizing switch 94. The triggering capacitor 62 is charged by current flow through the secondary winding 64 at the same time and in the same manner as the flash capacitor 43. In normal operation, the synchronizing switch 94 is closed by the camera shutter button 29 at the proper time in the exposure sequence. The capacitor 62 discharges through the primary windings of the voltage converting transformer 90, inducing about four thousand volts (4 kv.) in the triggering electrode 92, and ionizing the gas in the flash tube 50. The flash capacitor 43 then discharges through the flash tube 50, exciting the ionized gas and producing sufficient flash illumination. An additional capacitor 72 according to this embodiment provides filtering on the base of the transistor 70 to keep the circuit from inadvertently turning on due to undesirable noise, such as the neon ready light 82 turning off or from battery bounce. 
     Typically, the flash charging circuit 48 is off (not oscillating) when a picture is taken and the flash tube is fired. Due to the relatively high gain of the oscillation transistors 70,74, electrical energy from firing of the flash tube and fed back to the base of transistor 70 via secondary winding 64 causes transistor 70 to begin conducting which, in turn, restarts conduction of transistor 74 thereby desirably restarting the charging operation of the flash charging circuit. Because of the high gain sensitivity of the oscillation transistors 70, 74, relatively small electrical effects, such as caused by battery bounce resulting from jostling of the camera or electrostatic charge induced when a camera user walks on a dry carpet, can cause inadvertent start up of the camera. Capacitor 72 from the base of transistor 70 to ground provides a bypass of electrical impulses resulting from battery bounce. Similarly, a resistor 75 connected from the base of transistor 70 to ground serves to bleed down any small amounts of electrostatic charge build caused by normal handling of the camera thereby preventing inadvertent restart of the flash charging circuit 48. 
     However, during film loading in the process of camera assembly, it is possible for excessive electrostatic charge to be generated that may be too great for resistor 75 to bleed down quickly enough to prevent restarting of the flash charging circuit. In this event, it may be possible that the flash charging circuit would turn on and cause charging of flash capacitor 43 and consequent turn on of ready light 82 causing fogging of the film being loaded. If, as taught by the aforementioned &#39;168 patent, a shorting bar were placed across the terminals of the flash capacitor as represented in FIG. 3 by terminals 96, the flash capacitor would not charge and exposure damage to the film would be avoided. However, the flash charging circuit 48 would continue operating into effectively a short circuit across the secondary winding 64. This would cause the trahigstors 70,74 to conduct at very high current levels and could cause damage, such as burn out of the transistors. Even if the transistors were not seriously damaged, the high current conduction would constitute an unnecessary drain on the battery 17 which could cause unreliable operation of the camera during normal picture taking operations. 
     In accordance with the invention, electrical contacts are provided between the base of one of the oscillation transistors 70,74 to ground, preferably between the base of transistor 70 and ground. For this purpose, referring jointly to FIGS. 4 and 5, access openings 24a, 24b are provided in front cover 7 in such locations as to be in alignment with contact terminals 49a, 49b on the printed circuit board 39. In FIG. 4, contact 49b is the collector terminal of transistor 80 which is connected to ground trace 102 to which the positive terminal of flash capacitor 43 is also connected. Contact 49a is connected from the emitter terminal of transistor 80 via trace 101 and a jumper wire 103 to the base terminal of transistor 70. FIG. 5 shows a cutaway portion of the inner surface of front: cover 7 illustrating the positions of access openings 24a, 24b. Also shown is a raised wall 100 which isolates the opening 24a to ensure that a shorting probe wire 99 (FIG. 3) inserted through the opening is limited to the contact point 49a. Although not expressly shown, it will be appreciated that a similar raised wall surrounding access opening 24b may also be employed for the same purpose. While the shorting of the base of either transistor 70 or transistor 74 to ground will be effective to prevent the starting of charging circuit 48 (with the alignment of access opening 29a being positioned accordingly over the printed circuit board 39), it is preferred to use the base of transistor 70. This will ensure that both transistors 70 and 74 are disabled from conducting so long as the shorting bar is inserted in the access openings. If only transistor 74 had its base shorted to ground, it might be possible that momentary switch 41 could be closed during the film loading operation thus placing the transistor collector-emitter junctions directly across battery 47 with potential damage to the transistor. 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     PARTS LIST 
     1 single-use camera 
     3 plastic body 
     5 frame portion 
     7 front cover 
     9 rear cover 
     11 snap engagement feature 
     13 snap engagement feature 
     15 chamber 
     17 chamber 
     19 film cassette 
     21 film spool 
     23 label 
     24a,b access openings 
     25 taking lens 
     27 viewfinder 
     29 shutter release button 
     31 filmstrip 
     33 film advance wheel 
     35 electronic flash assembly 
     37 flash head 
     39 circuit board 
     41 momentary switch 
     42 cantilevered portion of front cover 
     43 flash charge capacitor 
     44 flash charging and control circuit 
     45 direct current power source 
     47 battery 
     48 self-oscillating flash charging circuit 
     49a,b oscillation transistor shorting contacts 
     50 flash tube 
     52 primary transformer winding 
     56 oscillation arresting circuit 
     58 flash trigger circuit 
     60 charging circuit transformer 
     62 triggering capacitor 
     64 secondary transformer winding 
     68 resistor 
     70 transistor 
     72 capacitor 
     74 transistor 
     75 resistor 
     76 rectifying diode 
     78 zener diode 
     80 digital transistor 
     82 neon ready light 
     86 resistor 
     90 transformer 
     92 flash triggering electrode 
     94 synch switch 
     96 flash capacitor contact terminals 
     100 raised wall inside front cover 
     101 transistor 70 base contact trace 
     102 ground trace 
     103 jumper wire