Abstract:
In the drive system disclosed, a first power source supplies a driving current to drive a moving member of an electromagnetic shutter driving device in one direction or in the reverse direction a second power source device consisting of a DC booster circuit, a capacitor, etc.; a photo-to-electric converter circuit discerns the brightness level of an object to be photographed and controls the outputs of the first and second power source devices to shift between these two outputs. An automatic change-over supplies the output of the second power source to the electromagnetic shutter driving device when the level of the output of the photo-to-electric converter circuit is equal to or above a predetermined level and supplies the output of the first power source to the electromagnetic shutter driving device when the output level of the photo-to-electric converter circuit is lower than the predetermined level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a drive system for an electromagnetically driven shutter which is electromagnetically opened and closed. 
     2. Description of the Prior Art 
     The driving force required for opening and closing the shutter of a camera generally has heretofore been derived from stored mechanical energy such as a spring force or the like irrespective as to whether the shutter is a lens shutter or a focal plane shutter. This has necessitated the use of a complex mechanism for driving a shutter with the stored mechanical energy. Accordingly, a complex mechanism has been also required for opening and closing the shutter. Such mechanisms have required the use of numerous component parts. In view of this, various kinds of electromagnetically driven shutters for producing a shutter driving force have recently been proposed. In these electromagnetically driven shutters, a driving force for opening and closing the shutter is electromagnetically generated by a drive mechanism. Therefore, the use of such a drive mechanism obviates the necessity of the type of mechanism required for a mechanical cocking action. Also, such an electromagnetically driven shutter permits replacement of all the mechanisms required for exposure control with electronic circuits. Thus, the electromagnetically driven shutter has many advantages over conventional mechanically driven shutters. 
     However, the structural arrangement of electromagnetically driven shutters has a number of drawbacks. For example, electromagnetically driven shutters in general use a moving coil type electromagnetic drive device having a permanent magnet and a moving coil. Between the pole piece and the yoke of the permanent magnet, is a bobbin on which a coil is wound. The bobbin is interlocked with the shutter. Therefore, when the coil is energized, an interaction between the current thus supplied and a magnetic flux generated by the permanent magnet between the yoke and the pole piece causes a force to be exerted on the bobbin in one direction to open the shutter. When the direction of the current supplied to the coil to energize it is reversed, the force is exerted on the bobbin in the reverse direction to close the shutter. For placing an electromagnetic drive device of this type within a camera, the size of the electromagnetic drive device is limited by a limited space available for accommodating it within the camera. Therefore, even where an alnico magnet is employed as permanent magnet, the magnetic flux generated in the gap between the yoke and the side face of the pole piece is 3000 to 5000 gauss. The electric current to be supplied to the coil is 0.5 to 1.0 A at the most, even with a single-3 alkaline battery used as battery. Further, in order to obtain stable current supply after the battery has been drained to some degree, the current becomes less than the aforementioned value. 
     Therefore, the driving force obtainable from an electromagnetic drive device is only several ten grams at the most. With such a driving force used for a shutter, it takes as much as 20 to 50 m sec. before the shutter is fully opened after the coil is energized. Then, for fully closing the shutter, a length of time 20 to 50 m sec. is also required. An example of the characteristic curves of such a shutter operation is illustrated by a curve a in FIG. 1 of the accompanying drawings. Furthermore, where an available space necessitates the use of a button type silver oxide or mercury cell as battery, the current available from such a battery is not more than several tens of m A. Therefore, in such a case, the length of time required for fully opening the shutter becomes still longer. To operate the shutter at a high speed of 1/500 or 1/1000 sec. with such an electromagnetic drive device, therefore, the shutter must be closed before it is fully opened. Then, the maximum aperture value becomes extremely small (less than f 32). A shutter characteristic curve of such a shutter operation is as represented by a curve b in FIG. 1. This tends to result in increasingly inconsistent of exposure values due to uneven cut of the aperture, etc. Also, diffraction might take place because of the extremely small aperture. Therefore, a shutter drive device of this type is not usable for a high speed shutter operation. 
     To solve this problem, U.S. Pat. No. 4,072,965 has disclosed a system for obtaining a driving force in which a capacitor is employed as current source for energizing the coil of an electromagnetic drive device; the capacitor is charged with a boosted voltage obtained by boosting the voltage of a battery; and the electric charge thus obtained is applied to the coil to obtain a driving force. 
     In this system, a driving current is obtained through a capacitor, so that a driving current which is several times as large as the driving current obtainable from a battery can be produced to permit a high speed shutter operation. However, in accordance with the system of this prior art, a large capacitance is required for accommodating the capacitor and this presents a problem with regard to an available space, etc. Assuming that a shutter is to be driven by the electric charge of the capacitor up to a low speed time of about 1/15 sec., for example, and assuming that the electric current I to be supplied to the coil is about 1 A, the quantity of electric charge Q of the capacity to required for flowing this current for a period of time t 1 , 60 m sec. is: 
     
         Q=IT=1×60×10.sup.-3 =0.06 (coulomb) 
    
     Then, assuming that the capacitor is charged with a voltage of 20 V, the capacitance C of the capacitor required for obtaining this much electric charge is: ##EQU1## 
     To have that high a capacitance, the capacitor must be of a size measuring about 20 to 30 mm in diameter and 30 to 60 mm in length. Then, such a large size makes it very difficult to place the capacitor within a limited space available in a camera. 
     The present invention is directed to the elimination of the above stated drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a drive system for an electromagnetically driven shutter suitable for a camera in which the coil of an electromagnetic drive device is arranged to be energized by a capacitor for a high speed shutter operation and by a battery in the case of a low speed shutter operation. 
     It is another object of the present invention to provide a drive system for an electromagnetically driven shutter arranged to detect the voltage of the electric charge of a capacitor and to inhibit a shutter release operation when the electric charge voltage is below a predetermined value. 
     It is a further object of the invention to provide a drive system for an electromagnetically driven shutter arranged to increase the shutter closing speed by discharging the electric charge of a capacitor to close the shutter by a large driving force obtained thereby. 
     These and other objects, features and advantages of the invention will become more apparent from the following description of embodiments thereof when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS: 
     FIG. 1 is a graph showing the characteristic curves of conventional electromagnetically driven shutters. 
     FIG. 2 is a schematic illustration of the structural arrangement of the electromagnetically driven shutter to which the present invention is applied. 
     FIG. 3 is an electrical circuit diagram showing as an embodiment of the invention a drive control circuit of the electromagnetically driven shutter shown in FIG. 2. 
     FIG. 4 is a graph showing the characteristic curves of the electromagnetically driven shutter shown in FIGS. 2 and 3. 
     FIG. 5 is an electric circuit diagram showing, as another embodiment of the invention, a drive control circuit of the electromagnetically driven shutter shown in FIG. 2. 
     FIG. 6(a) is a graph showing a characteristic curve of the electromagnetically driven shutter shown in FIGS. 2 and 5. 
     FIG. 6(b) is a graph showing the characteristic of the shutter driving current of the electromagnetically driven shutter shown in FIGS. 2 and 5. 
     FIG. 7 is an electric circuit diagram showing, as a further embodiment of the invention, a drive control circuit of the electromagnetically driven shutter shown in FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 2 which shows the structural arrangement of an electromagnetically driven shutter embodying the present invention, a moving coil type electromagnetic drive device 10 includes a coil bobbin 13 and a coil 9 movable up and down in a gap provided between a yoke 12 and a pole piece 11 of a permanent magnet Mg such as an alnico or rare earth magnet which is magnetized in the vertical direction. The coil 9 is wound on the coil bobbin 13 concentrically therewith. The bobbin 13 has a protrudent or protruding part 13&#39;. A pin 14 is secured to the protruding part 13&#39;. The protruding part 13&#39;  is connected by the pin 14 and a slot 15&#39; to an L-shaped rotative (i.e., rotatable) lever 15 which is arranged to rotate on a center shaft 16. A pin 17 is secured to one end of the lever 15 to connect the L-shaped rotative lever 15 to one end of another rotative (i.e., rotatable) lever 18 which is arranged to be rotatable on its center shaft 19. A pin 20 is secured to the other end of the lever 18. A shutter is composed of two thin plates or blades 21 and 22. The shutter 21 and 22 is provided with shutter openings 23 and 24 and also with auxiliary diaphragm openings 28 and 29. The above stated pins 17 and 20 are disposed at the ends of the shutter blades 21 and 22. 
     The electromagnetically driven shutter arranged in this manner operates as follows: When the coil 9 is energized, the energizing current interacts with a magnetic flux generated by the permanent magnet Mg in the gap between the yoke 12 and the poles of the permanent magnet to exert a force on the bobbin to move it either upward or downward according to the direction in which the energizing current is applied to the coil 9. With the force thus exerted on the bobbin 13, the bobbin begins to move. The movement of the bobbin causes the L-shaped pin 14 to articulate the rotative lever 15 about the rotation center shaft 16. Assuming that the bobbin moves upward, the L-shaped lever 15 rotates clockwise. The clockwise rotation of the lever 15 causes the pin 17 to articulate the rotative lever 18 clockwise on the rotation center shaft 19. This rotation of the lever 18 causes the shutter blade 21 to slide leftward and the other shutter blade 22 to slide rightward. The openings 23 and 24 of the shutter blades 21 and 22 then form an overlapped opening 25 between them and the auxiliary diaphragm openings 28 and 29 also form an overlapped opening 30 as shown in FIG. 2. These overlapped openings are gradually enlarged as the shutter blades 21 and 22 move further. A surface of film which is not shown is thus exposed to a light coming through the overlapped opening part 25. 
     When the quantity of the light exposing the film reaches a predetermined value, the direction of the current applied to the coil 9 is reversed to exert a downward force on the bobbin 13. Then, the bobbin 13 begins to move downward. The downward movement of the bobbin 13 causes the L-shaped rotative lever 15 and the rotative lever 18 to rotate counterclockwise and the shutter blades 21 and 22 begin to slide respectively in the directions opposite to the directions mentioned in the foregoing. The sliding movements of the shutter blades 21 and 22 in the reverse directions close the overlapped opening 25 between their openings 23 and 24 and the overlapped opening 30 between the auxiliary openings 28 and 29 and hence terminate the exposure of film to the light. 
     In FIG. 3 which shows, as an example of embodiment of the invention, a drive control circuit of the electromagnetic driven shutter shown in FIG. 2, a reference symbol E 1  indicates a power source battery; and SW 1  indicates a main switch of the camera. A constant voltage circuit 31 is arranged to produce a constant voltage Vc. An operational amplifier OP 1  amplifies the output of a light sensitive element 34 such as a silicon photo cell (SPC). The latter is connected to the two input terminals of the operational amplifier OP 1  and is provided for discerning the brightness of an object to be photographed. The light sensitive element 34 is attached to the front face of the camera for the purpose of measuring outside light. 
     A diode 36 which is provided for logarithmic suppression is connected to the negative feedback route of the operational amplifier OP 1 . A voltage dividing resistor 32 and a variable resistor 33 bias to give the non-inverting input terminal of the operational amplifier. Information on the ASA sensitivity of a film being used is supplied to the variable resistor 33. The output terminal of the operational amplifier OP 1  is connected to an inverting input terminal of an operational amplifier OP 2  which is provided for forming a comparison circuit. Voltage dividing resistors 37 and 38 are arranged to give a voltage level to the non-inversion input terminal of the operational amplifier OP 2 . A switching transistor TR 1  has its base connected to the output terminal of the operational amplifier OR 2  through a base resistor 35. When the output level of the operational amplifier OP 1  becomes higher than the potential at the voltage dividing point of the voltage dividing resistors 37 and 38, the output level of the operational amplifier OP 2  changes from a high level to a low level to turn on the transistor TR.sub. 1 thereby. 
     A reference numeral 39 indicates a DC booster provided for boosting its input voltage; 40 and 41 indicate diodes provided for preventing a charging current from flowing in the reverse direction; 42 and 43 indicate resistors arranged to limit the charging current; and symbols C 1  and C 2  indicate capacitors arranged to supply energizing currents to the coil 9 of the aforementioned electromagnetic drive device 10. The capacitor C 1  is arranged to supply a current required for opening the shutter and the capacitor C 2  a current required for closing the shutter. 
     Numerals 44 and 45 indicate voltage dividing resistors provided for detecting the charging voltage of the capacitor C 1  and numerals 46 and 47 voltage dividing resistors for detecting the charging voltage of the capacitor C 2 . Each of these resistors has a high resistance value. Each of two comparators CP 1  and CP 2  detects the potential of the voltage dividing point of the resistors 44 and 45 or that of the resistors 46 and 47 and, when the detected potential becomes higher than a predetermined level established by voltage dividing resistors 72 and 73 or 74 and 75, the output level of the affected comparator changes from low to high. An AND gate AND 1  is arranged to obtain a logical product of the outputs of the comparators CP 1  and CP 2 . A numeral 70 indicates a differentiation circuit; OR 1  indicates an OR gate which is arranged to obtain the logical sum of the output of a flip-flop circuit 71 and the output of the above stated operational amplifier OP 2  ; and IN 1  indicates an inversion circuit connected to the output terminal of the operational amplifier OP 2 . 
     An operational amplifier OP 3  amplifies the output of a light sensitive element 50 such as a silicon photo-cell (SPC) connected between the two input terminals of the operational amplifier OP 3  for the purpose of measuring light. The light sensitive element 50 is located to measure incident light through the overlapped opening 30 between the auxiliary diaphragm openings 28 and 29 in the electromagnetic shutter blades. Resistors 48 and 49 bias the non-inverting input terminal of the operational amplifier OP 3 . A variable density filter 50&#39; is arranged in front of the light measuring light sensitive element 50 to enter the ASA sensitivity of the film being used. The variable density filter 50&#39; is places an ND filter of a higher transmission factor in front of the light sensitive element 50 as the ASA sensitivity of the film increases. 
     A transistor TR 2  has a base terminal connected to the output terminal of the operational amplifier OP 3  while the collector terminal thereof is connected to the collector terminal of another transistor TR 3  through a resistor 66. The collector and the base of the transistor TR 3  are shortcircuited. The base terminal of the transistor TR 3  is connected to the base terminal of a transistor TR 4 . The collector of the transistor TR 4  is connected to a time constant determining capacitor 51. 
     A timer operation circuit TM 1  is arranged to be set by a set signal applied to its terminal 2 and to be maintained at a high level for a given length of time determined by the charging time of the above stated time constant determining capacitor 51. The timer operation circuit TM 1  is, for example, is formed on a single chip integrated circuit such as the Analog IC 555 manufactured by RCA or the like. A differentiation circuit is formed by resistors 52 and 54 and a capacitor 53. The output pulse of the differentiation circuit is supplied to the set terminal 2 of the timer operation circuit TM 1  and the set terminal S of an RS flip-flop circuit 55. The logical product of the output of the timer operation circuit TM 1 , the output of the RS flip-flop circuit 55 and that of the OR gate OR 1  is produced by an AND gate AND 2  which is arranged to receive three inputs. A shutter opening signal So is produced from the output terminal of the AND gate AND 2 . Numerals 56, 57 and 58 indicate resistors and a capacitor which constitute a differentiation circuit. A symbol TM 2  indicates another timer operation circuit which works in the same manner as the timer operation circuit TM 1  and is arranged to be set by the output pulse of the differentiation circuit formed by the resistors 56 and 57 and the capacitor 58. 
     The time constant of the timer operation circuit TM 2  is determined by resistors 59 and 59&#39; and a capacitor 60. A switching transistor TR 14  is connected to the resistor 59&#39;. The output BL of the operational amplifier OP 2  is applied to the base terminal of the switching transistor TR 14  through a resistor 67. Resistors 61 and 62 and a capacitor 63 form a differentiation circuit. The RS flip-flop circuit 71 is arranged to be reset by the output pulse of the differentiation circuit. An AND gate AND 3  produces a logical product of the outputs of the timer operation circuit TM 2  and the OR gate OR 1 . A shutter closing signal Sc is produced from the output terminal of the AND gate AND 3 . 
     A group of switching transistors TR 5  -TR 13  control the power supply to the coil 9 of the aforementioned electromagnetic drive device 10. These transistors TR 5  -TR 13  are interconnected as illustrated in FIG. 3 and are arranged to perform the switching described below: 
     (1) When power is to be supplied to the coil 9 from the battery (for a low shutter speed): 
     a. To open the shutter: 
     Transistors TR 9 , TR 7 , TR 11  and TR 5  . . . on 
     Other transistors . . . off 
     b. To close the shutter: 
     Transistors TR 9 , TR 12 , TR 6  and TR 10  . . . on 
     Other transistors . . . off 
     (2) When power is to be supplied to the coil 9 from the capacitor (for a high shutter speed): 
     a. To open the shutter: 
     Transistors TR 5 , TR 8  and TR 11  . . . on 
     Other transistors . . . off 
     b. To close the shutter: 
     Transistors TR 10 , TR 13  and TR 6  . . . on 
     Other transistors . . . off 
     In order to perform the above listed switching actions, the shutter opening signal So is impressed on the bases of the switching transistors TR 5  and TR 11  through resistors 76 and 77 respectively; and the shutter closing signal Sc is impressed on the bases of the transistors TR 10  and TR 6  through resistors 79 and 78 respectively. The output of the inversion circuit IN 1  is impressed on the base of the transistor TR 9  through a resistor 80. Numerals 81-84 indicate base resistors of the transistors TR 8 , TR 9 , TR 12  and TR 13 . With the circuit arrangement described in the foregoing, a current flows to the coil 9 of the electromagnet drive device 10 in the direction of OD for opening the shutter and in the direction of CD for closing the shutter. 
     The circuit described in the foregoing operates as follows: 
     (1) When the brightness of an object to be photographed is high and the shutter must be operated at a high speed: 
     With the main switch SW 1  of the camera closed, a constant voltage Vc is produced in the output of the constant voltage circuit 31 to render the subsequent circuits operative. Since the brightness of the object is high, a large quantity of light falls on the light sensitive element 34. Accordingly, a large amount of photoelectric current proportional to the quantity of the incident light flows from the light sensitive element 34. The photoelectric current is then logarithmically suppressed by the logarithmic suppression diode 36 and a voltage corresponding to the logarithm of the brightness of the object is produced at the output terminal of the operational amplifier OP 1 . The high brightness of the object to be photographed causes the output level of the operational amplifier to be higher than the voltage value of the voltage dividing point of the voltage dividing resistors 37 and 38. Therefore, the level of the output signal BL of the operational amplifier OP 2  which forms a comparison circuit becomes low to turn on the switching transistor TR 1 . With the transistor TR 1  turned on, a current is supplied from the battery E 1   to the DC booster circuit 39 to produce a high DC voltage output at the booster circuit 39. Then, the capacitors C 1  and C 2  are charged with this high DC voltage through the diodes 41 and 40 and the resistors 43 and 42 respectively. 
     a. In cases where the shutter release operation is performed prior to completion of the charging process on the capacitors C 1  and C 2  : 
     Before completion of the charging process on the capacitors C 1  and C 2 , the voltage level of the voltage dividing point of the resistors 44 and 45 arranged to detect the charge voltage of the capacitor C 1  and that of the point of the voltage dividing resistors 46 and 47 arranged to detect the charge voltage of the capacitor C 2  are low. The output voltages of the comparator circuits CP 1  and CP 2  are therefore low so that the output of the AND gate AND 1  remains low. Hence, no differentiation pulse is produced by the differentiation circuit 70 and the output of the RS flip-flop circuit 71 also remains low. Further, since the output level of the operational amplifier OP 2  is low as mentioned in the foregoing, the output level of the OR gate OR 1  is also low. 
     In this condition, when a shutter release button is depressed, the normally open switch SW 2  is closed and a negative differentiation pulse is produced as output of the differentiation circuit formed by the resistors 52 and 54 and the capacitor 53. The RS flip-flop circuit 55 is set by the pulse and the level of the output voltage of the flip-flop circuit becomes high. Further, the negative differentiation pulse output of the differentiation circuit 52, 53 and 54 sets the timer operation circuit TM 1  to make the level of its output high. Thus, of the three inputs of the AND gate AND 2 , the levels of two inputs become high. However, since the output of the OR gate OR 1  is at a low level, the output of the AND gate AND 2  remains low. Moreover, with the capacitors C 1  and C 2  not completely charged and with the output level of the OR gate OR 1  thus being low, the level of one of the two inputs of the AND gate AND 3  is low. The output level of the AND gate AND 3 , therefore, remains low irrespective of the level of the other input. 
     Accordingly, the shutter opening signal So and the shutter closing signal Sc are kept at low levels. Then, since the switching transistors TR 5  -TR 13  provided for controlling the current to be applied to the coil 9 remain off, the coil 9 is not energized. 
     In accordance with the arrangement of this embodiment, therefore, shutter opening and closing actions are not performed while the capacitors C 1  and C 2  have not been completely charged, even if a shutter releasing operation is performed, because: if the coil 9 is allowed to be energized before completion of the charging process on the capacitors C 1  and C 2 , a sufficient driving force for a high shutter speed can not be obtained and then correct exposure might not be effected. 
     b. Operation where a shutter release occurs after completion of the charging process on the capacitors C 1  and C 2  : 
     Upon completion of the charging process on the capacitors C 1  and C 2 , the potential level of the voltage dividing point of the voltage dividing resistors 44 and 45, which are provided for detecting the charge voltage of the capacitor C 1 , and that of the dividing point of the voltage dividing resistors 46 and 47, which are provided for detecting the charge voltage of the capacitor C 2 , become high. Therefore, the output voltage levels of the comparator circuit CP 1  and CP 2  become high and the level of the output of the AND gate AND 1  also becomes high, and positive differential pulses are generated from the differentiation circuit 70. These pulses set the RS flip-flop circuit 71 and its output is inverted to a high level. This in turn causes the output level of the OR gate OR 1  to go high. When a shutter release operation is performed under this condition, the RS flip-flop circuit 55 is set through the process described in the foregoing and the output level of the flip-flop circuit 55 becomes high. At the same time the timer operation circuit TM 1  is set to make the level of its output high. With the timer operation circuit TM 1  set, the time constant determining capacitor 51 begins to be charged. 
     Since the levels of the three inputs to the AND gate AND 2  are high under this condition, the output level of the AND gate AND 2  also becomes high to make the level of the shutter opening signal So high. Therefore, the switching transistors TR 5  and TR 11  are turned on. Also, because the level of the brightness discerning signal BL is low at this time, the level of the output of the inversion circuit IN 1  L is high and the switching transistor TR 9  is off. Therefore, when the switching transistor TR 5  turns on, the switching transistor TR 8  is also turned on. Then, a current flows from the capacitor C 1  through the switching transistors TR 8  and TR 11  to the coil 9 in the direction of OD to cause the shutter to open. 
     When the aforementioned shutter release operation causes the differentiation circuit 52, 53 and 54 to produce a negative differentiation pulse, the timer operation circuit TM 1  is set by the pulse and the time constant determining capacitor 51 begins to be charged. When the shutter begins to open as mentioned in the foregoing, the auxiliary diaphragm disposed in front of the light measuring light sensitive element 50 also begins to open and a light from the object to be photographed strikes the light sensitive element 50. A photoelectric current proportional to this incident light then begins to flow from the light sensitive element 50. The photoelectric current then becomes an emitter current of the transistor TR 2  connected to the output terminal of the operational amplifier OP 3 . A collector current which is about equal to the emitter current flows to the transistor TR 2 . Then, the collector and the base of the transistor TR 3  is shortcircuited and the transistor TR 3  serves as diode that produces a voltage corresponding to a voltage between the base and the emitter of the transistor TR 4 . Therefore, this causes a collector current equal to the collector current of the transistor TR 2  to flow through the collector of the transistor TR 4 . The time constant determining capacitor 51 is thus charged with a current equal to the photoelectric current of the light measuring light sensitive element 50. When the voltage of this electric charge exceeds the threshold value of the timer operation circuit TM 1 , the level of the output of the timer operation circuit TM 1  becomes low. This causes the level of the output (the shutter opening signal So) of the AND gate AND 2  to become low. 
     In this manner, the shutter opening signal So remains at a high level for a length of time determined by the timer operation circuit TM 1 , i.e. during the length of time T 1  shown in FIG. 4. During this period, the coil 9 of the electromagnetic drive device 10 is supplied with a current in the direction of OD and the shutter proceeds to open in a manner as represented by a curve OC 1  in FIG. 4. 
     When the output of the timer operation circuit TM 1  changes from a high level to a low level to cut off the current supply to the coil 9 of the electromagnetic drive device 10 in the direction of OD, the differentiation circuit 56, 57 and 58 produces a negative differentiation pulse to set the timer operation circuit TM 2 . Then, the time constant determining capacitor 60 begins to charge. Concurrently with this, the output of the timer operation circuit TM 2  changes to a high level. Then, since the output level of the OR gate OR 1  is high, the output of the AND gate AND 3  also changes to a high level. This causes the level of the shutter closing signal Sc to become high and the switching transistors TR 6  and TR 10  are turned on. Since the switching transistor TR 9  remains in an off state when an object to be photographed is in a highly bright state as mentioned in the foregoing, the switching transistor TR 13  turns on with the switching transistor TR 10  turned on. Therefore, the capacitor C 2  supplies through the switching transistors TR 13  and TR 6 , the coil 9 of the electromagnetic drive device 10 with a current in the direction of CD which is reverse to the direction in which the current is supplied for opening the shutter. With the coil 9 supplied with the current, the shutter beings to close. Meanwhile, since the object to be photographed is very bright, the level of the brightness discerning signal BL is low and, accordingly, the switching transistor TR 14  is in an on state, the capacitor 60 is charged in accordance with a time constant determined by the parallel resistance values of the resistors 59 and 59&#39; and the capacity value of the capacitor 60. When the voltage of the electric charge of the capacitor 60 exceeds the threshold value of the timer operation circuit TM 2 , the output of the timer operation circuit TM 2  changes to a low level. This causes the output of the AND gate AND 3  to change to a low level. The switching transistors TR 6  and TR 10  are turned off and then the switching transistor TR 13  is also turned off to cut off the current supply to the coil 9 of the electromagnetic drive device 10. In other words, the coil 9 of the electromagnetic drive device 10 is supplied with the current in the direction of CD for a length of time T 2  determined by the timer operation circuit TM 2  as indicated in FIG. 4. Then, the shutter closes in a manner as represented by a curve CC 1  in FIG. 4. 
     With the output of the timer operation circuit TM 2  changed to a low level, the output terminal of the differentiating circuit 61, 62 and 63 produces a negative differentiation pulse, which resets the RS flip-flop circuits 55 and 71 to make the outputs of these flip-flop circuits low. The sequence of actions required for an exposure operation of the camera are completed through the steps described in the foregoing. 
     (2) Operation when the brightness of an object to be photographed is low and the shutter must run at a low speed: 
     Since the brightness of the object is low, the quantity of light striking the light sensitive element 34, provided for measuring an outside light is small. Therefore, the output level of the operational amplifier OP 1  does not increase beyond the potential of the voltage dividing point of the voltage dividing resistors 37 and 38. The output signal BL of the operational amplifier OP 2  which forms a comparison circuit is now high and the switching transistor TR 1  remains off. Accordingly, the booster circuit 39 does not work and the capacitors C 1  and C 2  are not charged. Further, the level of the output of the inversion circuit, i.e., inverter, IN 1  is low. 
     When a shutter release operation is performed under these circumstances, the normally open switch SW 2  closes and, as described in the foregoing, the RS flip-flop circuit 55 and the timer operation circuit TM 1  are set so their respective outputs change to high levels. 
     Further, since the output of the operational amplifier OP 2  is high and the output of the OR gate OR 1  is also high, all of the three inputs to the AND gate AND 2  are high. Therefore, the output level of the AND gate AND 2  becomes high and raises the shutter opening signal So to a high level. This causes the switching transistors TR 5  and TR 11  to turn on. Meanwhile, since the inversion circuit IN 1  has its output at a low level, the switching transistor TR 9  is turned on. The on switching transistor TR 5  turns on the switching transistor TR 7 . Since the capacitors C 1  and C 2  have not been charged, the switching transistor TR 8  does not turn on. Through these processes, a current flows from the constant voltage circuit 31 to the coil 9 of the electromagnetic drive device 10 through the switching transistors TR 9 , TR 7  and TR 11  in the direction OD to open the shutter. Then, since the current flowing to the coil 9 is supplied at a low voltage of the constant voltage circuit 31, the current is much smaller than when a current is supplied from the capacitor C 1 . Therefore, compared with the shutter driving operation performed by the electric charge of the capacitor C 1 , the shutter in this case is opened at a much slower speed to have a shutter opening characteristic as represented by a curve OC 2  in FIG. 4. 
     Concurrently with setting of the timer operation circuit TM 1 , the time constant determining capacitor 51 begins charging. However, since the current with which the time constant determining capacitor 51 is to be charged is determined by the quantity of the light incident upon the photometric light sensitive element 50 as mentioned in the foregoing, the capacitor 51 is charged with a small current when the brightness of the object to be photographed is low. Therefore, a long period of time is required for charging the time constant determining capacitor 51. In this case, therefore, the output level of the timer operation circuit TM 1  remains high for a longer period of time and the shutter opening current flows for a length of time T 1  &#39; as shown in FIG. 4. Next, when the output of the timer operation circuit TM 1  changes to a low level, the timer operation circuit TM 2  is set as mentioned in the foregoing and the output level of the circuit TM 2  remains high for a period of time determined by the length of time required for charging the time constant determining capacitor 60. 
     The high level of the brightness discerning signal BL turns off the switching transistor TR 14 . Therefore, the time constant determining capacitor 60 is charged only through the resistor 59. This makes the time constant determined by the capacitor 60 longer than a bright object. Therefore, a shutter closing current flows for a length of time T 2  &#39; as shown in FIG. 4 along a shutter closing characteristic CC 2  in FIG. 4. As described in the foregoing, in cases where the brightness of an object to be photographed is low, the constant voltage circuit 31 supplies a current to the coil 9 of the electromagnetic drive device 10 over a long period of time T 1  &#39;+T 2  &#39; as shown in FIG. 4 to drive the shutter at a slow speed. 
     As described in detail in the foregoing, in this embodiment, the brightness of the object is discerned and a large current is supplied from a capacitor to the coil 9 of the electromagnetic drive device 10 only when the shutter must be operated at a high speed. When a high speed shutter operation is not required, a current is supplied from a battery to the coil 9. The coil 9 of the electromagnetic drive device 10 is thus arranged to obtain a current from a capacitor only for driving the shutter at a high speed. This arrangement, therefore, does not require a large quantity of electric charge and serves to permit the use of a capacitance of a small capacity. Therefore, a capacitor of a small size can be used and can be easily placed within a limited space available in a camera. Besides, in accordance with the arrangement, the length of time required for charging the capacitor also can be shortened. 
     In accordance with the invention, therefore, it is possible to obtain an electromagnetically driven shutter that can be operated at a high speed and also can be easily placed within a camera. Compared with the conventional mechanical shutter, the electromagnetically driven shutter obtained in accordance with the present invention is simple in structure and can be formed with a less number of parts. These are salient advantages in terms of practical applications and manufacture. 
     FIG. 5 illustrates another embodiment example of the drive control circuit of the electromagnetically driven shutter of the invention shown in FIG. 2. In FIG. 5, a reference symbol E 2  indicates a power source battery; SW 11  indicates a power source switch; a reference numeral 141 indicates a constant voltage circuit; OP 11  indicates an operational amplifier which forms a SPC head amplifier; and 144 indicates a light sensitive element such as a silicon photo cell (SPC). The light sensitive element 144 is connected between the two input terminals of the operational amplifier OP 11 . Voltage dividing resistors 142 and 143 are arranged to give a voltage level to a non-inversion input terminal of the operational amplifier OP 11 . A transistor TR 21  has its base connected to the output terminal of the operational amplifier OP 11 . A symbol TR 22  indicates a transistor which is arranged to serve as diode with the base and the collector thereof shortcircuited. The collector of the transistor TR 22  is connected to the collector of the transistor TR 21  through a resistor 139. The base of the transistor TR 22   is connected to the base of a transistor TR 23 . A time constant determining capacitor 145 is connected to the collector of the transistor TR 23 . 
     A differentiation circuit D 1  is formed by resistors and a capacitor 146, 147 and 148. A symbol SW 12  indicates a normally open switch which closes in response to a shutter release operation; and TM 11  indicates a timer operation circuit. The timer operation circuit TM 11  is formed into an analog IC chip and is arranged in the following manner: When the terminal T of the circuit receives a trigger pulse, the output level of the circuit becomes high and, concurrently with this, the circuit begins to charge the capacitor 145. When the voltage of the electric charge of the capacitor 145 reaches a predetermined level, the output level of the circuit TM 11  becomes low. Therefore, the output of the timer operation circuit TM 11  is kept at a high level just for a length of time determined by the time constant circuit. 
     Another differentiation circuit D 2  is formed by resistors and a capacitor 149, 150 and 151. There is provided another timer operation circuit TM 12  which operates in the same manner as the other timer operation circuit TM 11 . The time constant of the timer operation circuit TM 12  is determined by a resistor and a capacitor 152 and 153. The timer operation circuit TM 12  is arranged to be triggered by the output of the above stated differentiation circuit D 2 . 
     A differentiation circuit D 3  is formed by resistors and a capacitor 182, 183 and 154. A numeral 155 indicates a DC booster; 156 indicates a diode which is arranged to prevent the electric charge of the capacitor from flowing in the reverse direction; 157 indicates a resistor arranged to restrain a current with which the capacitor is charged; 162 indicates a capacitor which is arranged to supply a shutter closing current; and 158 and 159 indicate voltage dividing resistors which are provided for detecting the voltage of the electric charge of the capacitor 162. 
     When the potential of the voltage dividing point of the above stated voltage dividing resistors 158 and 159 reaches a given level determined by voltage division by resistors 138 and 137, the output of a comparison circuit COM 1  changes to a low level. A numeral 160 indicates a differentiation circuit; 161 indicates a RS flip-flop circuit which is arranged to be set by the output of the differentiation circuit 160. An AND gate AND 11  is arranged to obtain a logical product of the output Q of the flip-flop circuit 161 and the output of the timer operation circuit TM 11 . There is provided another AND gate AND 12  which obtains a logical product of the output Q of the flip-flop circuit 161 and the output of the timer operation circuit TM 12 . A shutter opening signal OS is obtained from the output terminal of the AND gate AND 11  and a shutter closing signal CS from that of the AND gate AND 12 . These signals are arranged to be supplied to a drive circuit of an electromagnetic driving coil which is described hereinafter. There are provided a groups of switching transistors TR 24  -TR 29  which are arranged to control current supply to the coil 9 of the electromagnetic drive device 10. The base of the transistor TR 25  is connected to the output terminal of the AND gate AND 11  through a resistor 136 while the bases of the transistors TR 24 , TR 27  and TR 29  are connected to the output terminal of the AND gate AND 12  through resistors 133, 134 and 135. An operational amplifier OP 12  forms a constant current circuit. A numeral 165 indicates a resistor provided for detecting the value of a shutter opening current; 163 and 164 indicate a voltage dividing resistor and a voltage dividing variable resistor which are arranged to give a level to the non-inversion input terminal of the operational amplifier OP 12 . The value of the shutter opening current of the coil 9 of the electromagnetic drive device 10 are adjustable by adjusting the variable resistor 164. The inversion input terminal of the operational amplifier OP 12  is connected to the collector of the transistor TR 26  while the output of the operational amplifier OP 12  is arranged to be applied to the base of the transistor TR 28  through a resistor 132. Numerals 130 and 131 indicate resistors. In this embodiment example, information on the ASA sensitivity of a film to be used is arranged to be obtained by changing the transmission factor of a variable density filter 120 which is disposed in front of the photometric light sensitive element 144. The operation of the circuit arrangement which is described in the foregoing will be understood from the following description with reference to the operation characteristic curves shown in FIG. 6(a) and (b): 
     When the main switch SW 11  of the camera is closed, the DC booster 155 is actuated to have the capacitor 162 charged by the output voltage of the booster 155 through the diode 156 and the resistor 157. When the voltage of the electric charge of the capacitor 162 comes to exceed a predetermined level, the output of the comparison circuit COM 1  changes to a low level and a differentiation pulse is produced at the output terminal of the differentiation circuit 160 to set the RS flip-flop circuit 161. The output Q of the flip-flop circuit 161 then changes to a high level. When a shutter release operation is performed under this condition, the switch SW 12  is closed and a negative differentiation pulse if produced at the output terminal of the differentiation circuit D 1  to trigger therewith the timer operation circuit TM 11 . The output level of the timer operation circuit TM 11  becomes high and, concurrently with this, the time constant determining capacitor 145 begins to be charged. Since the output level of the RS flip-flop circuit 161 has become high with the output of the timer operation circuit TM 11  changed to a high level, the output level of the AND gate AND 11  also become high to produce the shutter opening signal OS. The switching transistor TR 25  is turned on by this signal. Since the level of the shutter closing signal CS has become low at this time, the switching transistors TR 24 , TR 26 , TR 27  and TR 29  have turned off and the transistor TR 28  has turned on. Therefore, the coil 9 of the electromagnetic drive device 10 is suppllied with a current from the battery E 2  in the direction of OD through the switching transistors TR 28  and TR 25  and the shutter begins to open as mentioned in the foregoing. Then, since the voltage of the current detecting resistor 165 has been fed back to the invention input terminal of the operational amplifier OP 12  at this time, the current supply to the coil 9 is kept constant. When the auxiliary diaphragm opening 30 is formed as the shutter opening movement further proceeds, a light comes to fall on the photometric light sensitive element 144 from the opening 30. Then, a current proportional to the quantity of this incident light flows to the collector of the transistor TR 21 . The transistor TR 22  then acts to cause a current equal to this collector current to flow to the collector of the transistor TR 23  to charge the time constant determining capacitor 145 with this current. When the voltage of the electric charge of the capacitor 145 reaches a given level, the output level of the timer operation circuit TM 11  turns low. Then, the output OS of the AND gate AND 11  changes to a low level to turn off the switching transistor TR 25  and thereby the current supply to the coil 9 in the direction of OD is cut off. Thus, as shown in FIG. 6(b), a current in the shutter opening direction is allowed to flow to the coil 9 for a length of time T 1  which is proportional to the brightness of the object to be photographed. When the output of the timer operation circuit TM 11  turns to a low level, a negative differentiation pulse is produced by the differentiation circuit D 2  concurrently with this. The pulse causes the output of the timer operation circuit TM 12  to change to a high level and, at the same time, the time constant determining capacitor 153 begins to be charged. Further, with the output of the timer operation circuit TM 12  changed to a high level, the output level of the AND gate AND 12  becomes high to produce the shutter closing signal CS. This signal causes the switching transistors TR 24 , TR 27  and TR 29  to turn on. With the switching transistor TR.sub. 27 thus turned on, the transistor TR 28  turns off. Further, with the switching transistor TR 24  thus turned off, the switching transistor TR 26  comes to turn on. Therefore, through these switching transistors TR 26  and TR 29 , the capacitor 162 supplies the coil 9 with a current in the direction of CD and the shutter begins to close. Since the current in the shutter closing direction is supplied from the capacitor which has been charged by the booster circuit, the value of the current is greater than the current which is used for opening the shutter. Therefore, the shutter is quickly closed in a manner as represented by a curve CC shown in FIG. 6(a). 
     When the voltage of the electric charge of the time constant determining capacitor 153 reaches a given level, the output of the timer operation circuit TM 12  changes to a low level. This in turn causes the output of the AND gate AND 12  to change also to a low level. All of the switching transistors TR 24 , TR 26 , TR 27  and TR 29  are then turned off to cut off the current supply to the coil 9. Further, with the output of the timer operation circuit TM 12  having turned to the low level, a negative differentiation pulse is produced at the output terminal of the differentiation circuit D 3 . The RS flip-flop circuit 161 is then reset by the pulse and the output Q of the flip-flop circuit changes to a low level. 
     The shutter closing current thus flows to the coil 9 for a length of time T 2  as shown in FIG. 6(a). Since this closing current is supplied from the capacitor 162 which has been charged with a boosted voltage, the value of the current is much greater than the value of the current supplied for opening the shutter. Accordingly, the shutter closing characteristic becomes steep as represented by a curve CC in FIG. 6(a). This steepness makes the characteristic less inconstant to give a stable shutter operating characteristic. 
     Since the RS flip-flop circuit 161 is not set and the output Q of the circuit remains at a low level before completion of a charging process on the capacitor 162, the outputs of the AND gates AND 11  and AND 12  are also kept at their low levels before the charging process is completed. Before completion of the charging process on the capacitor 162, therefore, no current supply is effected to the coil 9 and the shutter can not be operated even if a shutter release operation is performed under such a condition. 
     FIG. 7 is a circuit diagram showing a further embodiment example of the invention. The embodiment shown in FIG. 7 differs from the embodiment shown in FIG. 5 in the following point: A charging process on a capacitor for closing the shutter is initiated by a shutter release operation and is automatically stopped when the voltage of the electric charge of the capacitor reaches a predetermined level. Then, concurrently with the completion of the charging process, a shutter releasing action begins. In FIG. 7, the same parts are indicated by the same reference symbols or numerals as in FIG. 5 while the embodiment shown in FIG. 7 also operates in a manner similar to the embodiment shown in FIG. 5. Therefore, the following description covers only the component parts and their actions that differ from those of the embodiment shown in FIG. 5 while the rest are omitted. 
     Referring to FIG. 7, a normally open switch SW 12  is arranged to close in response to a shutter release operation. A differentiation circuit D 3  is formed by resistors 170 and 171 and a capacitor 172. A numeral 173 indicates a RS flip-flop circuit. A switching transistor TR 31  controls a current supplied from a battery E 2  to a DC booster 155. An operational amplifier OP 11  forms a SPC head amplifier. A light sensitive element 144 is connected to the two input terminals of the operational amplifier OP 11  while a diode 144° for logarithmic suppression is connected to a negative feedback route thereof. A resistor 142 and a variable resistor 143 are arranged to give a potential to the non-inversion input terminal of the operational amplifier OP 11 . The variable resistor 143 is also arranged to be supplied with information on the ASA sensitivity of a film to be used. A voltage follower is formed by an operational amplifier OP 13  the output terminal of which is connected to the base of a transistor TR 30  provided for logarithmic expansion. The voltage VS of the voltage dividing point of the resistor 142 and the variable resistor 143 is arranged to be supplied to the emitter of the transistor TR 30 . A time constant determining capacitor 145 is connected to the collector of the transistor TR 30 . The operation of this circuit arrangement is as described below: 
     When a shutter release operation is performed by closing the main switch SW 11  of the camera, the switch SW 12  is closed and a negative differentiation pulse is produced by the differentiation circuit D 1 . The RS flip-flop circuit 173 is set by the pulse and the output Q of the flip-flop circuit 173 changes to a low level. This causes the switching transistor TR 31  to turn on to effect power supply to the DC booster 155 which in turn produces a boosted DC voltage. Then, the capacitor 162 is charged with this boosted voltage through the diode 156 and the resistor 157. When the voltage of this electric charge reaches a predetermined level, the output of the comparison circuit COM 1  changes to a low level and the differentiation circuit 160 produces a negative differentiation pulse to reset thereby the RS flip-flop circuit. The output Q of the flip-flop circuit then changes to a high level. This causes the switching transistor TR 31  to turn off. The DC booster 155 ceases to work and a charging process on the capacitor 162 comes to a stop. 
     Further, the negative differentiation pulse produced by the differentiation circuit 160 also triggers the timer operation circuit TM 11  to make the output level of the circuit TM 11  high. A shutter opening signal OS is produced by this. Then, concurrently with this, the time constant determining capacitor 145 begins to be charged. The shutter opening signal OS causes a driving current to flow to the coil 9 in the direction of OD and shutter opening movement begins. When this movement causes a light to fall on the light sensitive element 144 from the auxiliary diaphragm opening 30, the light sensitive element 144 produces a current proportional to the incident light. This current is then logarithmically suppressed by the logarithmic suppression diode 144&#39; and the output level of the operational amplifier OP 11  changes in accordance with the logarithmic value of the quantity of the incident light. The output of the operational amplifier OP 11  then also appears in the output of the operational amplifier which forms a voltage follower. The output voltage of the amplifier OP 13  is then logarithmically expanded by the transistor TR 30 . The time constant determining capacitor 145 is charged with the logarithmically expanded current thus obtained. When the voltage of the electric charge of the capacitor 145 reaches a predetermined level, the output level of the timer operation circuit TM 11  becomes low to cause a negative differentiation pulse to be produced from the output terminal of the differentiation circuit D 2 . The pulse then triggers the timer operation circuit TM 12  to make the output level of the circuit TM 12  high and a shutter closing signal CS is produced. When the shutter closing signal CS is thus produced, concurrently with this, the time constant determining capacitor 153 begins to be charged. When the voltage of the electric charge of the capacitor 153 reaches a predetermined level, the output level of the timer operation circuit TM 12  becomes low. 
     The switching transistors TR 24  -TR 29  act in the same manner as in the case of FIG. 5 while the shutter opening signal OS and the shutter closing signal CS are produced. Their actions during these periods are therefore omitted from description here. 
     As will be understood from the foregoing detailed description, in this embodiment, the driving current required for closing the shutter is supplied from the capacitor which has been charged with a boosted voltage. Therefore, compared with arrangement to supply a current from a battery, a much greater driving current is obtainable by the arrangement of the embodiment. The shutter closing characteristic thus becomes steep to prevent inconstancy of the characteristic of the shutter due to a frictional force, etc. so that exposure can be effected at a high degree of accuracy. 
     It is conceivable to obtain a driving current from a capacitor also for opening a shutter. However, for opening the shutter, it is possible to correct inconstancy by the use of an auxiliary diaphragm light measuring system. Besides, such arrangement makes the length of time required for charging the capacitor longer and would not bring about such a salient improvement attainable in the shutter closing characteristic in accordance with the invention. 
     In each of the embodiments described in the foregoing, a moving coil type electromagnetic drive device is employed as electromagnetic drive device. However, the invention is not limited to the use of the electromagnetic drive device of the type employed in these embodiment examples. It is therefore to be understood that the present invention is applicable also to electromagnetic drive device of other types.