Circuitry for controlling shutter time in accordance with a manually adjustable resistor
BACKGROUND OF THE INVENTION 
     In general, this invention relates to camera shutter arrangements; more particularly, it relates to a control circuit therefor which enables manual adjustment of the exposure time of the shutter. 
     Prior art manual shutter control circuits determine the exposure time depending on the time constant of a series circuit consisting of a capacitor and a variable resistor. It is known, however, that the control circuits of this type involve several disadvantages. The variable resistor can provide only a reduced adjustment accuracy in a low resistance area and hence the control circuit of this type presents an unsatisfactory voltage variation characteristic at low voltages. Furthermore, when it is required to control the shutter time over a wide range, the manual shutter control resistor has to be of such a type that can be varied over a wide range, from a very high resistance value to a very low resistance value. As is well known, it is difficult to manufacture such a resistor. Still another disadvantage involved in the prior art control circuit is that, when semiconductor devices are used, the construction of the circuit becomes complicated and that difficulties are encountered in providing a control circuit having good temperature characteristics because of the cumulative influence of the ambient temperature over the semiconductor devices used. 
     SUMMARY OF THE INVENTION 
     This invention is embodied in a camera shutter arrangement of the type including an electromagnet, a timing capacitor, a timing switch for enabling charging current to develop a voltage across the capacitor, and switching circuit means connected to respond to the voltage across the capacitor to switchably energize the electromagnet. 
     In accordance with the improvement of this invention, there is provided a constant current source that preferably is implemented by a self-bias connected field effect transistor. A first transistor has a base-emitter junction and a collector electrode connected to the constant current source. A second transistor has a base-emitter junction and a collector electrode connected to the capacitor. First and second resistors are each connected in parallel with the base-emitter junction of a respective one of the first and second transistors. First and second current amplification circuit means are provided. The first current amplification circuit means is connected between the constant current source and the first resistor so as to serve as a feedback circuit for the first transistor. The second is connected between the constant current source and the second resistor. Owing to their symmetrical arrangement and similar characteristics, these current amplification means provide good perfomance even under the influence of temperature variations. The currents they respectively supply to the first and second resistors develop voltages across the base-emitter junctions of the first and second transistors. As will be shown in a mathematical treatment below, by manually adjusting the resistance value of one of these resistors there is caused a substantial change in the charging current to the capacitor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a block and schematic drawing of the preferred embodiment of this invention. 
    
    
     DETAILED DESCRIPTION 
     In the drawing, a field effect transistor 1 (hereinafter referred to as FET) is connected in a self-bias circuit in which a resistor 2 is connected across the source and gate electrodes of the FET 1. The resistor 2 is also connected to the collector electrode of an NPN transistor 3. The emitter electrode of the transistor 3 is connected to the negative or ground terminal of a DC power source 17. The positive terminal of the DC power source 17 is connected to the drain of the FET 1. A transistor 4 has its base electrode connected to the gate of the FET 1, its collector electrode connected to the positive terminal of the DC power source 17, and its emitter electrode coupled to the base of the transistor 3 through a diode 5. A manually variable resistor 6 is connected across the base-emitter junction of the transistor 3. In a similar manner to the series circuit of the transistor 4, diode 5 and variable resistor 6, there is provided a transistor 7, diode 8 and semi-fixed resistor 9 which are serially connected to each other and to the DC power source 17. The base of the transistor 7 is connected to the base of the transistor 4. 
     A timing switch 10 and a timing capacitor 11 are connected to each other and connected at one end thereof to the positive terminal of the DC power source 17 and at the other end thereof to the collector of a transistor 12 which serves a logarithmic expansion function. The transistor 12 has its emitter electrode connected to the negative terminal of the DC power source 17 and its base electrode connected to the junction of the diode 8 and the semi-fixed resistor 9. Thus, the resistor 9 is in parallel with the base-emitter junction of the transistor 12. A Zener diode 13 is connected between the positive terminal of the DC power source 17 and a resistor 14. A conventional switching circuit 15 has two input terminals which are respectively connected to the junction of the Zener diode 13 and resistor 14 and to the junction of the timing capacitor 11 and the collector of the transistor 12. The output of the switching circuit 15 is connected to one end of a shutter actuation electromagnet coil 16. The other end of the coil 16 is connected to the positive terminal of the DC power source 17. 
     In the shutter control circuit having said construction, the self-bias-connected FET 1 functions as a constant current source, and the transistor 3 receives at its collector electrode a constant current from the FET 1 and a feedback signal is applied to the base of the transistor 3 by way of the current amplifying transistor 4 and the diode 5. 
     Now the principles and operation of the above-described arrangement will be explained. If the gate-source voltage of the FET 1 is represented by V GS  and the resistance value of the resistor 2 is represented by R 1 , then the source current i s  of the FET 1 will be expressed by the following formula: 
     
         i.sub.S = V.sub.GS /R.sub.1 . . .                          (1) 
    
     this formula shows that the FET 1 and resistor 2 form a constant current source. The collector current i C  of the transistor 3 is determined by the base-emitter voltage V BE1  of the transistor 3 and feedback signal is applied to the base of the transistor 3 by way of the transistor 4, diode 5 and variable resistor 6. Under these conditions the base-emitter voltage V BE1  of the transistor 3 will be expressed as ##EQU1## where, K: Boltzmann&#39;s constant 
     T: absolute temperature 
     q elementary electric charge 
     i o  : constant 
     V o  : base-emitter voltage of the transistor at the time when the collector current i C  is i o . 
     In Formula (2) if KT/q is represented by K 1 , then the following formula will be obtained: 
     
         V.sub.BE1 = K.sub.1 log i.sub.S /i.sub.o + V.sub.o . . .   (3) 
    
     On the other hand, if the resistance value R 2  of the variable resistor 6 is represented by R 2 , then the emitter current i 1  of the transistor 4 will be expressed as 
     
         i.sub.1 = V.sub.BE1 /R.sub.2 . . .                         (4) 
    
     the base-emitter voltage V BE2  of the transistor 4 and voltage V D3  across the terminals of the diode 5 will be expressed as follows: 
     
         V.sub.BE2 = K.sub.1 log i.sub.1 /i.sub.o + V.sub.o . . .   (5) 
    
     
         V.sub.D3 = K.sub.1 log i.sub.1 /i.sub.o + V.sub.o . . .    (6) 
    
     Similarly, if the collector current of the transistor 12 is represented by i 2 , then the base-emitter voltage V BE4  of the transistor 12 will be expressed as 
     
         V.sub.BE4 = K.sub.1 log i.sub.2 /i.sub.o + V.sub.o . . .   (7) 
    
     If the resistance value of the semi-fixed resistor 9 is represented by R 3 , then the emitter current i 3  of the transistor 7 will be expressed as 
     
         i.sub.3 voltage  V.sub.BE4 /R.sub.3 . . .                  (8) 
    
     therefore, the base-emitter voltaage V BE5  of the transistor 7 and the voltage V D6  across the terminals of the diode 8 will be expressed respectively as follows: 
     
         V.sub.BE5 = K.sub.1 log i.sub.3 /i.sub.o + V.sub.o . . .   (9) 
    
     
         V.sub.D6 = K.sub.1 log i.sub.3 /i.sub.o + V.sub.o . . .    (10) 
    
     It is found that the shutter control circuit having above-mentioned construction satisfies the following relation. 
     
         V.sub.BE1 + V.sub.BE2 + V.sub.D3 = V.sub.BE4 + V.sub.BE5 + V.sub.D6 . . (11) 
    
     substitution of Formula (3), (5), (6), (7), (9) and (10) into Formula (11) will give the following relation: 
     
         i.sub.2 = (i.sub.S × i.sub.1.sup.2)i.sub.3.sup.2 . . . (12) 
    
     It will be appreciated from this Formula that the collector current i 2  of the transistor 12 which serves as a timing current can be doubled by increasing i 1  by √2 times while maintaining i S  and i 3  at constant levels. In other words, the timing current can be varied in accordance with a geometrical progression of 2 by varying the resistance value R 2  of the variable resistor 6 in accordance with the following relation: 
     
         R.sub.2 = R.sub.o 2.sup.1/2K . . .                         (13) 
    
     where, R o  : constant. 
     When the timing switch 10 is closed the electromagnet coil 16 is energized by way of the switching circuit 15 to thereby open the shutter. Upon opening the timing switch 10 the capacitor 11 starts to be charged. When the voltage across the capacitor 11 reaches a level corresponding to the voltage across the Zener diode 13, the switching circuit 15 is reversed to deenergize the electromagnet coil 16, so that the shutter is closed and a predetermined exposure time can be obtained. 
     When a feedback circuit incorporated in the circuit arrangement, the manual shutter control circuit of this invention provides an improved voltage variation characteristic at low voltages. The symmetrical arrangement of the group consisting of the transistors 3 and 4 and diode 5 and the group consisting of the transistors 7 and 12 and diode 8 cancels the influence of the ambient temperature over these devices and thus contributes to improve the temperature characteristic of the circuit. Besides, the shutter control circuit of this invention has an ability of adjusting the shutter time over a wide range by varying the resistance value only within a substantially narrow range.