JPH0112251Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic exposure control circuit, and more particularly, to an exposure control circuit in which a limiter mechanism is added to the exposure time.

In an electronic shutter camera, the ideal relationship between subject brightness and exposure time is shown by straight line A shown in FIG. In the figure, the horizontal axis is the logarithmic value of the subject brightness, and the vertical axis is the logarithmic value of the exposure time. For example, in a dark state with almost no subject brightness, the ideal exposure time would be infinite seconds;
Cameras configured with exposure time relationships are inconvenient in practical use. For this reason, in cameras that conventionally perform automatic exposure control, a limiter mechanism is added that closes the shutter even if the exposure is insufficient when a preset time is reached, or a limiter mechanism is added selectively in consideration of the use of a tripod. An exposure control circuit has been developed that allows the use of

Figure 2 is described in Japanese Patent Publication No. 53-40893.
This is a conventionally developed exposure control circuit with a limiter function, in which a reference current source and a photocurrent source are combined with a diode, and a current equal to the difference between the photocurrent and the reference current is applied to one terminal of the diode. The other terminal of the diode is coupled to an integrator circuit, to which the photocurrent is also coupled. When the photocurrent is greater than the reference current, such as during normal brightness, the difference current represents a value that reverse biases the diode. Therefore, only the photocurrent is integrated, resulting in an exposure interval between the minimum exposure interval and the maximum exposure interval. When the photocurrent is lower than the reference current, the difference current represents a value that forward biases the diode and is added to the photocurrent to produce a composite current whose magnitude is equal to the reference current. Therefore, only the reference current is integrated and a constant maximum exposure interval is obtained for all levels of copy brightness below a predetermined value.

That is, when the above circuit operates, as shown by the straight line B in FIG. 1, accurate exposure conditions corresponding to the straight line A are given at seconds shorter than the level of the preset exposure time B'. If the exposure time is longer than the B′ level, the preset fixed
It works like this, giving B′ level seconds.

The conventional circuit described above can provide a certain limiter function, but since the current is controlled by superimposing the current generated in proportion to the incident light and an independent reference current, near the maximum exposure interval of the B′ level, As shown by curve C in FIG. 1, ideal characteristics could not be obtained and there was a drawback that errors occurred.

The present invention eliminates the above-mentioned drawbacks and provides an exposure control circuit capable of performing a shutter operation with ideal limiter characteristics, and will be described in detail below with reference to examples.

In FIG. 3, PD is a photodiode that detects the brightness of an object and outputs a photocurrent.The anode of the photodiode PD is connected to the cathode side of the battery, and the cathode is connected to the first transistor consisting of transistors Q ₁ and Q ₂ . Current mirror and transistors Q ₃ , Q ₄
A second current mirror is connected to the second current mirror.
Transistors Q ₅ and Q ₆ are circuits that create a reference current I _R.
This reference current I _R is connected to two reference currents I _R1 and I _R2 (I _R ≒ I _R1 ≒
I _R2 ) is derived. One of the derived currents
I _R1 is connected to the connection point between transistor Q ₁ of the first current mirror and photodiode PD, and the other current I _R2
are applied to the connection points between the transistor Q ₂ and the capacitor C of the first current mirror. The other end of the capacitor C is connected to the cathode side of the battery, and a switch SW is provided in parallel to the capacitor C.
The switch SW is closed in conjunction with the shutter operation, sets initial conditions, and is closed at the same time as the shutter is opened. One input terminal of a comparator Comp is further connected to the connection point between the capacitor C and the transistor Q ₂ to which the above-mentioned current I _R2 is applied, and the voltage of the integrating circuit applied to the input terminal is applied to the other input terminal V It is compared with the voltage provided by _ref to form an output signal. The output signal of the comparator Comp is given to the coil Mg for driving the relay, and the shutter operation is controlled.

The photocurrent I _L detected by the photodiode PD is applied to the first current mirror after subtracting the reference current I _R1 derived by the transistor Q ₃ .

Here, when the photocurrent I _L of the photodiode PD is larger than the reference current I _R1 (I _L ≧ I _R1 ), the collector current of the transistor Q ₁ of the first current mirror is
I _L ′ becomes I _L ′ = I _L − I _R1 , and since it is a current mirror, the above current I _L ′ draws a current of I _L ″≒I _L ′ to the transistor Q ₂ side, and to this, the transistor Q ₃ Another reference current of
I _R2 is added, and the integrated current becomes I _L ″+I _R2 ≒ I _L − I _R1 + I _R2 = I _L.

Eventually, a current with the same value as the photocurrent of the photodiode PD flows into the capacitor C and reaches the level shown in Figure 1.
The relationship between the exposure time and subject brightness in a state where DC A is larger than B' can be obtained.

On the other hand, when the photocurrent I _L of the photodiode PD is smaller than the reference current I _R1 (I _L < I _R1 ), the transistor Q ₂ is cut off and the collector current I _L ″ of the transistor Q ₂ is zero, I _L ″=I _L '=0.

At this time, the current flowing into the capacitor C becomes another collector current of the transistor _Q3 , that is, the reference current I _R2 , and below a predetermined photocurrent level, the reference current I _R2 flows into the capacitor C.
A steep limiter action can be obtained.

When the integrated voltage reaches the voltage value provided by V _ref at the other end of the comparator Comp, an output signal is formed and the electromagnet Mg is activated to terminate the shutter.

In the above exposure control circuit, we have given an example in which a photodiode is used to detect the brightness of the subject, so leakage at the photodetector is not a big problem, but the above circuit is especially useful in cases such as CdS. be. That is, if there is a leak in the photodiode PD, and if the leakage current is Id, then when I _L ≧ I _R1 , I _L ′ = I _L + I _d − I _R1 , and in this case, I _R1 = I _R2 + I _d . By choosing the collector currents I _R1 and I _R2 of transistor Q ₃ as follows,
The integrated current becomes I _L ″+I _R2 ≒ I _L , and a circuit operation can be performed that cancels out the leakage current Id.The above transistor _Q3-2
It is easy to design a circuit with a difference between the two collector currents I _R1 and I _R2 .

As described above, according to the present invention, exposure control with excellent response characteristics without error in limiter operation at a limit level can be performed with a relatively simple circuit.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between subject brightness and exposure time, FIG. 2 is a conventional exposure control circuit diagram, and FIG. 3 is an exposure control circuit diagram according to the present invention. PD: Photodiode, Q ₁ , Q ₂ : 1st current mirror, Q ₃ , Q ₄ : 2nd current mirror, C: Capacitor, Q ₅ , Q ₆ : Reference current circuit, Comp: Comparator, SW: Switch, Mg: Electromagnet.

Claims (1)

[Claims for Utility Model Registration] Connected between the first node and the second node,
a photodetection circuit that detects the brightness of the subject and outputs a photocurrent; a capacitor connected between the third node and the second node; and a reference current that corresponds to a predetermined brightness to the first a reference current circuit input to the node and the third node; a current mirror comprising a reference transistor connected to the first node and an output transistor connected to the third node; A photographic exposure control circuit comprising: a circuit for detecting that a predetermined value has been reached and controlling a shutter operation.