JPS59133531A - Photometric circuit - Google Patents

Abstract

PURPOSE:To execute exactly an exposure control by a simple constitution of a circuit by converting a resistance value of a photoconductive element to a current value, and bringing a capacitor to a constant-current charging by its current. CONSTITUTION:An inversion input terminal of an operational amplifier 13 to whose non-inversion input terminal the reference volage 1 is impressed is grounded through a resistance 28. The emitter of an NPN transistor 24 whose base is connected to an output terminal of the amplifier 13 is grounded through a photoconductive element 37. In this way, when a negative feedback is applied to the operational amplifier, and the photoconductive element 37 is connected as a load of its operational amplifier 13, it is possible to bring a time constant capacitor 40 of an exposure controlling circuit to a constant-current charging by an optical current corresponding to a resistance value of the photoconductive element 37, and an exact exposure control can be executed by a simple circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photometry circuit for an exposure control circuit in an electronic shutter of a camera.
The present invention relates to a photometric circuit using a photoconductive element that changes its resistance value depending on the illuminance.

In recent years, cameras, especially so-called lens/shutter cameras, have been pursuing improvements in operability through various automation, compactness,
As efforts are being made to reduce weight, there is a need for raw materials that can be produced industrially at low cost. Therefore, in the field of automatic exposure control technology used in the above-mentioned cameras, it is necessary to make it possible with a simpler and cheaper structure than the current one. Silicon photoelectric diodes (hereinafter referred to as SPD) and photoconductive elements (generally referred to as CdS, hereinafter referred to as CdS) are typical photometric elements currently used in photometric circuits for automatic exposure control in cameras. . SPD has advantages such as quick response and good illuminance-output current proportionality over a wide range, and is almost indispensable especially in high-end cameras.However, on the other hand, it has the following disadvantages. Therefore, it is not necessarily suitable especially for low-priced lens shutters.

Namely, 1) The light intensity-output characteristic is linear, and a half-open shutter that also serves as a shutter aperture, which is common in lens-shutter cameras, allows so-called program exposure control to control only the shutter open time depending on the subject brightness and simultaneously change the shutter speed and aperture. This requires a large-scale circuit or a complicated configuration such as a sub-diaphragm.

11) Since the output current value is extremely small, an amplifier with high input impedance is required, and there is little freedom in integrating the processing circuit.

111) Since the output current value is extremely small, various considerations must be taken during mounting, such as leak prevention.

On the other hand, CdS has a characteristic (hereinafter referred to as γ characteristic) in which the light illuminance-resistance value characteristic has a unique value (γ) of the logarithmic ratio of each amount of change, and is capable of so-called program exposure control. When carrying out the above γ
A shutter mechanism with shutter opening characteristics over time suitable for the characteristics is provided, and CdS is used to set the shutter speed of the mechanism.
Exposure control can be performed without providing a complicated configuration such as a sub-diaphragm by simply associating the γ characteristics of . Therefore, the degree of freedom in arranging the photometric elements increases and the shutter configuration becomes easier.

(2) Since it is a photoconductive element, the current value relative to the light illuminance can be set relatively freely depending on its bias voltage conditions, making the implementation and processing circuit for leakage current easier.

This has the advantage that, in particular, a program exposure control circuit for a half-open shutter can be realized at a low cost. On the other hand,
Although it has a light history phenomenon such as a relatively slow light response speed and poor reproducibility at low illuminance, it is common to incorporate a strobe in the automatic exposure control of popular cameras, and it does not require a wide range of light metering. First, it is fully possible to use CdS as a photometric element while avoiding areas where reproducibility is poor, and the response speed is also sufficiently acceptable.

As mentioned above, programmed exposure control can be executed very easily by using a photometry circuit that uses CdS as a photoconductive element. Connect a time constant capacitor to , charge the capacitor with the output of CdS,
If the shutter is configured to close when the charging potential reaches a predetermined level, the capacitor can be charged with a constant current.In order to perform accurate exposure control, it is necessary to provide a correction circuit or mechanism.

The present invention has been made in view of the above-mentioned matters, and includes connecting a transistor to the output terminal of an operational amplifier, providing a negative feedback path to the amplifier, and connecting a photoconductive element as a load of the amplifier. By doing this, the output current of the transistor is made to correspond to the resistance value of the photoconductive element, and the exposure control capacitor is charged at a constant current with the output current of the transistor, thereby creating a photometry circuit that solves the above problem. This is what we provide.

Next, a photometric circuit according to the present invention will be explained.

Figure 1: A circuit diagram showing one embodiment of the present invention, 37 is a photoconductive element, 13 is an operational amplifier to which a reference voltage 1 is applied to the non-inverting input terminal, and the amplifier is inverted. The input terminal is connected via the resistor 28 [
7 and is grounded. 24 is an NPN transistor whose base is connected to the output terminal of the amplifier 13, and the emitter of the transistor 24 is grounded via the photoconductive element 37. 29 is a variable resistor connected in the feedback path of the amplifier 13. 20 and 21 are PNP run sisters constituting a current mirror, 40 is a time constant capacitor, 9 is a count switch, 16 is a comparator, and the output of the conhalator 16 activates a shutter magnet (not shown) to end the exposure. .

Note that 6 indicates a reference voltage.

Now, the voltage across the photoconductive element 370 is Vsy, and the resistance value of the resistor 28°29 is R? If δH&e is the voltage of reference voltage 1 as Vl, then Vvr= an ” ”” x
It becomes V+.

u NPN) If the resistance value R2 @ tR29 is set so that the current amplification factor) 1f@ of the Run Sister 24 is sufficiently large and the current flowing through the resistor 28.29 can be ignored, then the current amplification factor of the Run Sister 24 Collector current ■c!
4 is ■, assuming that the resistance value of the photoconductive element 37 is R3γ. , 4
=='h11+fall×Vl,

R28R17 Since the transistors 20 and 21Fi constitute a current mirror as described above, the collector currents of transistors 20 and 21 have the same value. Therefore, the collector current of transistor 21 is
4 collector current 1e24, and when the 1st switch 9 is turned off by the release operation, it becomes the capacitor 40.

A current I(44 flows and is charged. Since the current Ic24 is <yt,,+)its×v, as described above, it shows a predetermined constant current determined by R211R37 by the resistor R3T, so the capacitor -4
0 is charged with constant current.

Now, if the capacitance of the capacitor 40 is Cu, the charging voltage of the capacitor 40 is the above reference voltage ■1=C46X
R3,X Rts-. C40, Ru, &o
is a constant R211+R2G, so the resistance value R of the photoconductive element 37 is equal to the exposure time T.
Determined by sy. The resistance value of the photoconductive element can be expressed as R8Lr within a range of good linearity (Re is a constant and L is illuminance). Therefore, the exposure time T until the charging voltage of the capacitor 40 reaches 1 is expressed by βL-' (β is a constant) and is determined depending on the illuminance.

On the other hand, the characteristic of the aperture area with respect to the exposure time of a half-open aperture shutter is generally S (t) = αt (S (t):
(opening area, α constant), so in the above case αtdt=
, -I, T2. Furthermore, as mentioned above, during exposure, when the γ characteristic of the photoconductive element is 1, the deviation E becomes constant. This means that in the circuit shown in Fig. 1, the amount of exposure when the charging voltage of the capacitor 40 reaches Vl is always constant regardless of the illuminance, and the circuit shown in Fig. 1 releases the light. By activating the half-open shutter with the above-mentioned aperture characteristics and activating the shutter magnet to close the shutter when the charging potential of the capacitor 40 reaches Vl, programmed exposure control using the half-open shutter can be executed. .

In this way, in the embodiment shown in FIG. 1, negative feedback is applied to the operational amplifier and the photoconductive element is connected as a load to the operational amplifier, so that the resistance value of the photoconductive element is always adjusted. The time-fixing capacitor of the exposure control circuit can be charged at a constant current with the corresponding photocurrent, and accurate exposure amount control can be executed.

2 is a circuit diagram showing another embodiment of the photometric circuit of the present invention; FIG.

In the figure, 30 and 31 are bleeder resistors that divide the circuit power supply voltage 7, and the output of this resistor is applied to the non-inverting input terminal of the operational amplifier 14.

Connected to the output of the amplifier 14 are NPN transistors 25 and 26 having the same characteristics. The emitters of the transistors 25, 26 are connected to a photoconductive element 38. A feedback path is also provided between the inverting input of the amplifier 14 and the emitters of the transistors 25, 26. A time constant capacitor 41 for controlling the seconds is connected to the collector of the transistor 26, and the output of this capacitor is applied to one input terminal of the comparator 17. Further, a reference voltage from resistors 32 and 33 is applied to the input terminal of the comparator 17, and a shutter magnet is connected to the output of the comparator 1 noter 17. The 34 variable resistors are connected to the collectors of the transistors 25, and the resistor output is connected to one input terminal of the comparator 18.

A reference voltage from a resistor 35.degree. 36 is applied to the input terminal of the comparator 18, and a light emitting diode for low brightness warning is connected to the output of the comparator 18.

In the above configuration, the voltage across the photoconductive element 38 is set to v3.
γ, the resistance value of resistor 30.31 is R8゜HR31, respectively.
+The potential of circuit power supply voltage 7 is V? If so, V3γ=
"Employment-XV," NPN) run R3I, →RMI If the hfe of the sister 25.26 is sufficiently large, the emitter current and collector current can be considered equal, and the base and emitter voltages of the transistors 25 and 26 are connected to the same potential, so the NPN transistor 25. 2
Each collector current Icn * Icta of 6 is I e 2
s = T c 26 = ”X - Riichisemi - 2R3
oR3,R,a (However, the resistance value of the RSS photoconductive element 38) When the switching member 10 changes from closed to open in conjunction with the release operation, charging of the capacitor 41 starts, and its If the time elapsed from the instant is T, then the NP of the capacitor 41 is
N) The potential V41 on the side connected to the collector of the transistor 26 is V4+ = V?, assuming that the capacitance value of the capacitor 41 is C41, since the capacitor 41 is charged with a constant current at Ic, 6. Once one two = y, -x-x-!几－×】
L and T C412C41R are different +R31R38. The potential of the 10 input terminal (non-inverting input terminal) of the comparator 17 is the resistance value of the resistor 32 and 33, respectively.
+ &s = -1--x v, r = 132 +R
33, the output signal 44 of the comparator 17 becomes 'LOW' from the moment the switching member 10 transitions from closed to open.
C411R does not depend on the value of power supply voltage 7 until it changes from "HiGI("), that is, regardless of power supply fluctuations.
If 3o t R3, R3, and R33 are constant, it will be proportional only to the resistance value R38 of the photoconductive element 38.

Therefore, if a photoconductive element having a negative γ value is used as in the embodiment shown in FIG. Programmed exposure control may be implemented.

As described above, in this embodiment, proper exposure can always be obtained regardless of fluctuations in the power supply voltage, so it is suitable for controlling exposure of cameras such as cameras with built-in strobes where the power supply voltage may fluctuate.

Further, the time width of T can be easily adjusted by changing the resistance value of the variable resistor 31, but it may vary depending on the specific resistance value of the photoconductive element 38,
By changing the values of the capacitor 41 and resistors 30, 32, and 33, or by increasing the collector current ratio of the NPN transistor 25, 26, especially by changing the junction area ratio when integrating the circuit, It is possible.

On the other hand, the collector potential V2!1 of the NPN) transistor 25
If the resistance value of the variable resistor 34 is R114, then I
Rs+ v7, V21=VW -I(!211X R34=
V? -Tx R1R81x7. −×Ru. The potential at the input terminal (non-inverting input terminal) of the comparator 18 is R, +R, where the resistance values R31+ of the resistors 35 and 36 are Rss and Ru8, respectively.

The output signal 45 of the comparator 18 is
The resistance value Rsa TH of the photoconductive element 38 when changes from 'LOW' to 'HIGI(' is the above V,, =-su-X
V? By substituting RA87H for Rsa and R3+R31S, &87
H=-xRuxll" If each resistance value is set in advance so that R*sTu is the photoresistance value of the photoconductive element 38 when issuing a low brightness 2 RNA-R111R115 degree warning, the light emitting diode can be turned on using the signal 45. It is possible to provide a lower luminance warning. R55T? (can be adjusted by changing the value of the variable resistor 34, but R (capital) * Rst + R51l
* As with the resistance value change of Rss and the above, Iets and Ie
This can also be done by changing the current ratio of ta.

According to the method shown in FIG. 2, it is possible to easily set the exposure time and low brightness warning simply by selecting resistance values, etc.
It has the advantage of not requiring a reference voltage. Furthermore, there is no need for a switching member that switches the connection of the photoconductive element between the low brightness determination mode and the exposure time control mode, as is often seen in the past. Since the photoresistance value of CdS is low at high illuminance and high at low illuminance, in order to compensate for photometric information, connect a low resistance in series and a high resistance in parallel to the CdS.
It is possible to compensate for the photoresistance value in the same way as in conventional circuits.

Also, it is possible to easily issue a high-intensity warning in the same way as a low-intensity warning, or to display an appropriate brightness display by changing the voltage determining section of the comparator using the method described above.

FIG. 3 is a circuit diagram showing another embodiment of the photometric circuit according to the present invention.

In the figure, reference numeral 15 denotes an operational amplifier, and a film sensitivity switching member 12 selectively applies different reference voltages 3 to 5 to the non-inverting input terminal of the amplifier depending on the film sensitivity. A transistor 270 is connected to the output of the amplifier 15.
A photoconductive element 42 is connected to the emitter of the transistor 27. Also, since the amplifier is provided with a feedback path, the collector current of the transistor 27 is 1c2 Tc27 = R, , (R3g is the resistance value of the photoconductive element, and Vx is connected to the non-inverting input terminal of the amplifier. applied n
voltage). 22.23 is a transistor with a current mirror configuration similar to the diagram @1, and the transistor 2
A time-fixing capacitor 42 is connected to the collector of the transistor 3, and when the switch 11 is opened in conjunction with the release operation, the transistor 42 is charged at a constant current with the collector current of the transistor 23. Since the collector current of the transistor 23 is equal to the collector current of the transistor 27, if the capacitance of the capacitor 42 is Co, the exposure time T
It becomes a con in. 19 is reference voltage 2 and capacitor 4
A comparator that compares the outputs of 2 is used to convert the above voltage 2 to V! Then, the time T from the release operation until the output of the comparator 19 is reversed, the shutter magnet connected to the output of the comparator 19 is activated, and the exposure is completed is T.
= C42Rso X-□.

Vx As mentioned above, since vx indicates a value according to the film sensitivity selected by the switch 12, the exposure time is adjusted according to the film sensitivity. It goes without saying that similar effects can be obtained even if the inverting input terminal is used. It is also an advantageous means to continuously switch the reference voltage at i~ corresponding to the film sensitivity.

As described above, according to the present invention, it is possible to convert the resistance value of a photoconductive element into a current value with an extremely simple configuration, and it is possible to charge a capacitor with a constant current using the current, thereby simplifying exposure control. This can be implemented accurately using the circuit configuration.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of an exposure control circuit using a photometric circuit according to the present invention, and FIG. 2 is a circuit diagram showing another embodiment of an exposure control circuit using a photometric circuit according to the present invention. , 3rd
The figure is a circuit diagram showing still another embodiment of an exposure control circuit using a photometric circuit according to the present invention. 13.14,15... operational amplifier, 24.25,26,27... transistor, 37.3
8,39...Photoconductive element. Patent applicant: Canon Co., Ltd. Procedural amendment (voluntary) Mr. Kazuo Wakasugi, Commissioner of the Patent Office■. Indication of the case 1980 Patent Application No. 8022 2, Name of the invention Photometric circuit 3, Person making the amendment Relationship to the case Patent applicant Office 3-30-24 Shimomaruko, Ota-ku, Tokyo (XL-”-)□ Voice residence 146 Shimomaruko 3-30-25, Ota-ku, Tokyo, Specification subject to amendment 6, Contents of amendment (1) The phrase “It is possible 0” on page 12, line 16 of the specification
” and the word “on the other hand” on page 12, line 17 of the same page.
(PN) If the junction area of the transistor 25 is multiplied by n, the above-mentioned time T becomes as follows.Even if the capacitance value of 041 is reduced without changing the values of Gas and each resistance, the same T can be obtained. If the collector of the NPN transistor 25 is directly connected to the power supply 7, the effect of reducing the capacitor capacity can be used only as an exposure circuit. ” is added. (2) The phrase “Unnecessary” in the second line of page 14 of the specification.
and the word ``also'' in the second line of page 14, K ``By simply changing the transistor junction area ratio, the capacitance value of the button and time constant capacitor can be reduced without changing the potential of each part of the circuit or the CaS characteristics. , and space and cost efficiency can be improved without losing circuit flexibility.'' 179-

Claims (1)

[Claims] A transistor is connected to the output of an operational amplifier to which a voltage is applied to one input terminal, a negative feedback path is provided for the operational amplifier, a photoconductive element is connected as a load of the amplifier, and the 1. A photometry circuit for a camera, characterized in that a signal current corresponding to the resistance value of a photoconductive element is formed as an output current of a transistor, and the signal current is supplied as a charging current of an exposure control capacitor.