JPS61287332A - Exposure control circuit of camera - Google Patents

Abstract

PURPOSE:To improve the transient response at application of a power which by applying an initial signal shortening a capacitor charge time of a filter means provided to a DA conversion means of the PWM system to the DA converting means tentatively in place of a pulse train at application of the power switch of a camera. CONSTITUTION:An initial signal shortening the charging time of a capacitor C1 being the filter means provided to the DA conversion means 5 is fed to the DA conversion means 5 in place of the pulse train at application of the power switch S 100 so as to detect that the DA conversion output is a predetermined analog quantity in response to the set exposure information, e.g., film sensitivity information directly or indirectly, and when the prescribed analog quantity is detected directly or indirectly, a pulse train in response to the exposure information from the pulse train generating means in place of the initial signal if fed to the DA conversion means 5 by an initial control circuit. Suppose that the capacitor C1 is charged at the low logic of the pulse train, then the charging time of the capacitor C1 is shortened, where T is the pulse period having a duty ratio in response to the exposure information, t0 is the time of low logic and the initial signal is a pulse train having a low logic time of t0<t1.

Description

[Detailed description of the invention] (Invention ql↑ field) The present invention utilizes a so-called pulse width modulation method (hereinafter referred to as "pulse width modulation method").

The present invention relates to a dew ratio control circuit for a camera equipped with a PWM digital-to-analog converter (hereinafter referred to as a DA converter).

(Background of the Invention) A PWM type DA converter is used, for example, to convert film sensitivity information in the form of a digital value into an analog value corresponding to the film sensitivity. Figure 14 shows this kind of DA
An example of an exposure control circuit for a camera having a converter is shown, and the film sensitivity setting circuit 15 includes three switches 8W1 to SW.
3, and eight types of film sensitivity can be set depending on the state of each switch. The film sensitivity set by each switch is converted by the microprocessor 17 into film sensitivity information of a predetermined digital value. The microprocessor 17 generates a pulse signal with a duty ratio corresponding to the digital value and supplies it to the DA converter 5. DA converter 5
The transistor Q1 turns on and off according to the pulse signal, and a DC voltage of a corresponding level is applied to the filter circuit (
C1, R7) and taken out as a DA conversion output.

Note that 3100 represents a power switch and E represents a battery.

To explain the DA conversion operation in detail, the power switch STO.

When input, the microprocessor 17 starts operating.

A pulse train having a duty ratio corresponding to the film sensitivity set by the film sensitivity setting circuit 15 is output to the boat o3. If the output of port 03 is "1", transistor Q1 is turned on, so the charge on capacitor 01 is transferred to resistor R.
7 is discharged throughout.

Also, if the output of boat 03 is "0", transistor Q
Since the capacitor C1 is turned off, the capacitor C1 is charged through the resistors R6 and R7. While repeating such charging and discharging, the charging voltage of the capacitor 01 converges to an analog voltage level corresponding to the film sensitivity.

Therefore, if the capacitor C1 is completely discharged when the power switch S too is turned on, it will take a considerable amount of time for the analog output to reach the voltage level corresponding to the film sensitivity. Therefore, the power switch S
Even if you want to operate the camera immediately after turning on the 160, you will have to wait until the voltage of capacitor C1 settles to a predetermined value, so you will miss the photo opportunity, and if you operate the camera before it has settled down, you will not be able to get the correct exposure. is not obtained.

In short, as shown in Figure 14, the D
The A converter requires a filter circuit (CI, R7) to remove output ripple.

Due to the time constant of this filter circuit, there is a drawback that transient response when the power switch is turned on is poor.

(Purpose of the invention) The purpose of the present invention is to solve these problems.

An object of the present invention is to provide an exposure control circuit for a camera that improves the transient response of a DA converter when a power switch is turned on.

(Summary of the Invention) When the power switch is turned on, the present invention supplies an initial signal to the DA converting means instead of a pulse train to shorten the capacitor charging time of the filter means provided in the DA converting means.
Directly indicates that the A conversion output has reached a predetermined analog amount predetermined according to set exposure information, for example film sensitivity information.

or an initial control circuit that supplies a pulse train corresponding to the exposure information from the pulse train generating means to the DA converting means instead of the initial signal when the predetermined analog amount is detected directly or indirectly. ing.

Assuming that the capacitor is charged on the low logic side of the pulse train, and if the period of the pulse with the duty ratio according to the exposure information is T and the low logic time is to, then the initial signal has a low logic time of tl)tO. If it is a pulse train, the charging time of the capacitor can be shortened. Preferably, the initial signal is a low logic signal.

In this case, charging time will be the fastest.

(Embodiments) First Embodiment - FIG. 2 shows an embodiment of the present invention. The output of an absolute temperature proportional voltage generation circuit 1 serving as a power supply circuit is connected to an analog-to-digital converter (hereinafter referred to as an AD converter) 3. and each reference signal input terminal of a digital-to-analog converter (DA converter) 5. The output of the photometry circuit 7 is input to the AD converter 3 via a cancellation circuit 9. The output of the exposure factor setting circuit 11 is also input to the AD converter 3. The output of the AD converter 3, the output of the initialization circuit 13, and the output of the film sensitivity setting circuit 15 are input to the microprocessor 17. Further, control signals corresponding to the aperture value and exposure time calculated by the microprocessor 17 are input to the magnet drive circuit 19. The input signal from the film sensitivity setting circuit 15 is converted into a binary number by the -f microprocessor 17, and a corresponding pulse train is sent to the DA converter 5.
is output to. The output of the DA converter 5 is the adder circuit 21
The signal is input to the photometric circuit 7 via the photometric circuit 7, and is also input to the AD converter 3. Furthermore, the TTL dimming control circuit 23
The output of is also input to the photometric circuit 7.

Each of these elements will be described in further detail with reference to FIG.

The reference voltage generation circuit 1 includes an operational amplifier A1 and a voltage dividing resistor R1.
~R4, diodes Di, and D2.

They are connected to output a voltage signal proportional to the absolute temperature T. That is, the output voltage VAI of operational amplifier A1 is.

However, it can be expressed as: Boltzmann constant q: electronic charge T: absolute temperature. Here, the electrical characteristics of diodes D1 and D2 are the same.

The exposure factor setting circuit 11 includes two variable resistors VRI and V
It is composed of B2. Here, the variable resistor VRI is linked to the movement of the aperture ring of the lens, and therefore a voltage signal corresponding to the amount of change in the aperture can be extracted. Further, a voltage signal corresponding to the opening of the lens F1 is taken out from the variable resistor VR2.

The initialization circuit 13 includes a resistor RIO and a capacitor C2, and sets the microprocessor 17 to an initial state when the power is turned on. In this embodiment, the film sensitivity setting circuit 15 is composed of three switches SWI to SW3, and can set eight types of film sensitivity as shown in FIG. 3 according to the on/off states of the switches 8W1 to SW3. Note that rOJ becomes "1" when the switches SW1 to SW3 are on, and "1" when they are off. The microprocessor 17 is of a well known type and includes a read-on memory ROM, random access memory I.
It has JRAM and a central processing unit CPU.

The magnet drive circuit 19 includes resistors R8, R9, transistor Q2. Q3. Magnet MG1. MGZ, the base of the transistor Q2 is connected to the output terminal 01 of the microprocessor 17 via the resistor R8, and the transistor Q
The base of 3 is connected to the microprocessor 1 through resistor R9.
It is connected to the output terminal o2 of 7. magnet MG1
is for the aperture control magnet, and this magnet) MC
)1, the aperture can be stopped at any position. The magnet M()2 is a magnet for controlling the rear curtain of the shutter, and by demagnetizing it at any time after the front curtain of the shutter has run, it is possible to start running the rear curtain of the shutter.

The DA converter 5 uses a pulse width modulation method (hereinafter referred to as PVVM).
transistor Q of the DA converter 5
The base of 1 is connected to the microprocessor 17 through resistor R5.
The emitter of the transistor Q1 is grounded, and the collector is connected to the power supply circuit 1 through a resistor R6.
Connected to the output terminal of That is, the temperature signal from the temperature proportional voltage generation circuit 1 is supplied to the DA converter 5 as a reference voltage.

Further, the collector of the transistor Q1 is connected to one input terminal of the operational amplifier A4 of the adder circuit 21 via a filter circuit consisting of a resistor R7 and a capacitor C1.

Furthermore, the analog output of the DA converter 5 is connected to the operational amplifier A4.
The signal is input to AD converter 30 channel CH4 via.

The photometric circuit 7 is well known and includes a photodiode FD interposed between two input terminals of an operational amplifier A2, and two diodes D3 . D
4, and a photodiode, where vA4: adder circuit 2
1 output voltage S: can be expressed as the reverse saturation current of the diode D31D4.

A cancellation circuit 9 that cancels the temperature-dependent characteristics of the diodes D3 and D4 includes two diodes D5 inserted in opposite directions in the operational amplifier A3 and the feedback circuit.
．． D6, which cancels out the influence of the temperature characteristics of the diodes D3 and D4 in the output from the photometric circuit 7.

The output voltage VA3 of the canceling circuit 9 is.

However, work 2 can be expressed as the current flowing into the cancellation circuit.

The AD conversion circuit 3 is of a sequential comparison type having four input terminals CHI to CH4, and its reference voltage terminal VREF
A temperature signal from the temperature proportional voltage generation circuit 1 is inputted to. As mentioned above, the temperature signal is used in common with the DA converter 50 reference voltage. And channel CHI
A variable resistor VRI is connected to the channel CH2, a variable resistor VR2 is connected to the channel CH2, an output terminal of the cancel circuit 9 is connected to the channel CH3, and an output terminal of the adder circuit 21 is connected to the channel CH4.

In FIG. 1, switch S100 is the main switch f, and E is the battery.

Next, while explaining the operation of each element in Figure 1, the film sensitivity (SV), photometric value (BV), apex value AV of the aperture value, and apex value AVO of the aperture value necessary for exposure factor control are explained.
I will explain in detail.

1) Film Sensitivity (SV) Film sensitivity information can be input to the microprocessor 17 from a film sensitivity setting circuit 15 having three switches 8W1 to SW3. The desired film sensitivity is set according to the on/off state of each switch as shown in FIG.

The microprocessor 17 converts the input data into 8-bit data as shown in FIG. This 8-bit digital film sensitivity data is processed by the microprocessor 17.
The data is then converted into analog data by the DA converter 5 as follows. In this embodiment, digital-to-analog conversion is performed using the pvvM method.

In the PWM method, a constant pulse output waveform is generated with a repetition period T as shown in FIG. Assuming the minimum unit time when the pulse waveform is rOJ as 10, 6 g, 8 h) OD A conversion! TOX2”=256-tO=
The relationship T holds true. Furthermore, when the digital data of film sensitivity shown in FIG. 4 is expressed as rDJ, the relationship D+D=2 5 6 holds true between it and its complement rDJ. Therefore, microprocessor 1
Using the timer in 7, first turn on output port 03, load the value "D" into the timer, interrupt at the rDJ cycle of the counter, turn off output port o3 when the first interrupt occurs, and then turn on the timer. Load the value ``stone'' into , and similarly interrupt at the ``stone'' cycle. Thereafter, by repeating the process, a periodic pulse train of "D" cycle on and "D" cycle off can be obtained.

When the pulse train thus obtained is supplied to the DA converter 5, the analog code obtained from the DA converter 5, that is, the input voltage waveform of the operational amplifier A40 becomes as shown in FIG. In FIG.

VC indicates the average voltage, and ML indicates the ripple voltage.

If the time constant of the filter circuit (R7, C1) of the DA converter 5 is set sufficiently large, the ripple voltage VL can be ignored. Here, the average voltage VC is.

It can be expressed as

The 8-bit data rDJ is set so that when the film sensitivity changes by IE'V as shown in Figure 4, the data value increases or decreases by 6, so as is clear from equation (4), the DA conversion LgB of the circuit is a voltage equivalent to %Ev. FIG. 7 shows the input voltage VC corresponding to each film sensitivity at an ambient temperature of 25°C.

2) Apex value AV of the aperture value, Apex value AVO of the aperture value at the time of shooting of the photographing lens and Apex value AVO of the aperture value at the time of shooting of the photographing lens are used to input to the input channel CH1 of the AD converter. be done. The voltage VC'HI set by the variable resistor VRI is expressed as follows. This formula indicates how many aperture steps deviate from the maximum aperture of the photographic lens at which photography is to be performed. Further, the voltage VCH2 representing the open F value of the photographing lens is set by the variable resistor VR2.

It can be expressed as This voltage VCH2 is input to the input 9@cH2 of the AD converter 3.

2) Photometric value (BY) If the photocurrent corresponding to a certain amount of incident light on the photodiode PD is ILL, then when the amount of incident light is doubled, the photocurrent becomes 2IL1. Therefore, the output voltage VA2*VA2' of the operational amplifier A2 corresponding to each amount of light is respectively.

becomes. Therefore, if the change in the output voltage of the operational amplifier A2 due to the change in the amount of light is ΔVA2.

It can be expressed as In other words, when the amount of light doubles, the output voltage of operational amplifier A2 becomes 2KT, n2 (V)
will only increase. Since a doubling of the change in light amount is nothing but an increase in the IEV equivalent light amount, in this example, the changes in factors related to exposure equivalent to %IRV at an ambient temperature of 25°C.

It will change at the rate of KT Jn 2 +36 Cmv)/Ev. IEVI7) A resolution of 3A is sufficient for camera exposure control, etc.
The voltage corresponding to I and SB of the Di converter.

It would be a good idea to choose. Assuming that the resolution of the AD converter is 8 bits, the absolute reference input voltage VAI of the AD converter is:

Then, a resolution of -E''/ can be obtained.

to Here, the photocurrent IL of the photodiode FD is.

Using the apex value BV of the subject brightness and the apex value AVO of the maximum aperture value of the photographic lens.

, = , , 2(BY-AVO) ・・・・・・・・・・・・・・・・−(11) However, α can be expressed as a constant, and when +11) equation is substituted into equation (3), .

The output of the photometric circuit 7 via the cancellation circuit 9, that is, the output voltage VA3 of the operational amplifier A3 is:

It can be expressed as Therefore, if we rewrite the n3 equation using equations (1) and (4).

・・・・・・・・・・・・n3 becomes.

Let us consider whether data such as B can be obtained by AD converting the output voltage VA3 expressed as in Equation 91 using the reference input voltage VREF proportional to temperature.

If you divide the face expression by (I・expression).

becomes. Here, the first term D indicates film sensitivity data. In addition, the second term can be an integer whose minimum unit is the unit of H in (BV-AVO) by appropriately determining the constant α, and the denominator 256 represents the data length of 8 bits. .

The value of the numerator itself can be considered as AD-converted data.

Similarly, regarding the apex value of the aperture value during shooting (
By dividing Equation 5) and Equation (6) regarding the apex value of the open F value by Equation 60, the digital values of (Av-AvO) and AVO can be obtained.

Next, an example of a procedure for camera exposure control will be described with reference to flowcharts shown in FIGS. 8 to 10.

-Main routine- In step S1, the initialization circuit 13KJ shown in FIG.
The t+-P microprocessor 17 is initialized and its operation is started. Next, in step S2.

Film sensitivity information is read from the on/off states of the switch groups SW1 to SW3. In step S3, the read film sensitivity information is changed into binary data and stored in the memory. In step S4, the program jumps to a subroutine and performs initial control processing when the power is turned on. The subroutine will be described later. In step S5, the AD converter 3
The three pieces of information are AV-AVO, information about the step from the converted aperture F value of the photographic lens to the current aperture value, AVO, which represents the aperture value, and information BV-AVO+8V, which is the sum of the photometric value and film sensitivity. The data is loaded into the microprocessor 17. In step S6, it is determined whether or not the camera has been released, and if a negative determination is made, step S2
Return to , repeat the above-mentioned operations, and wait.

If an affirmative determination is made in step S6, the process proceeds to step S7,
To find the EV value immediately after release, EV = BY-AVO+ 8V+AvO=BV+S
V -------Performs functional calculations. That is, information obtained from channels CHI and CH3 of the AD converter 3 is added. This embodiment describes a program control type camera, and the combination of aperture height AV and shutter speed TV for a predetermined EV value is determined in advance.

Therefore, the scheduled aperture direct AV is performed at step SS.

Car I A'V=-EV+1 ・・・・・・・・・・・・・・・
- Calculated as αe. Next, in step S9, the rate ! TV = EV-AM =-EV-1...
・・・・・・ Calculate αη and calculate the planned shutter speed TV
seek.

In step 810, the AD converter 3 is used to continuously AD convert the output VA3 of the operational amplifier A3. Immediately after the release, the aperture of the photographing lens is open, so the AD conversion output of analog data VA3 is.

BV-AVO+ eV. Here, if the aperture when the photographic lens is being narrowed down is expressed in AV, the AD conversion output at any given point in time is.

It can be expressed as B'V-AV + SV. Since BY and Sv are constant even immediately after release, the number of stops from opening (AV-AV
O') is.

AM-Avo=(BV-Avo+5v)-(BV-Av
+5v)-----・al. A.V.
Since the value of O is input to the microprocessor 17 from the AD converter 30 channel CH2.

The aperture value AV when the aperture is being narrowed down can be determined from the calculation of (AV-AVO)+AVO. Next, in step 811, it is determined whether the aperture value AV calculated in this way matches the planned aperture value AV, and if the determination is negative, the process proceeds to step 810.
Returning to step 1, the output of operational amplifier A3 is again continuously AD converted. If the determination is affirmative, the process proceeds to step 812, where the boat 01 is turned on to turn on the aperture control magnet MC)1.
is set to high level, thereby turning on transistor Q2. As a result, the aperture control magnet MCI is turned on, the aperture is locked, and the aperture is controlled to the predetermined aperture value AV. Then, in step 813, the seven-point speed AV is loaded into the microprocessor 170 counter in response to the start of travel of the shutter leading curtain.

In step S14, it is determined whether the count value of KARA/RI has reached the expected value AV. If a negative determination is made, the process waits until the count value of the counter reaches TV, and if a positive determination is made, the boat 02 is set to a low level in order to turn off the shutter magnet MG2 and run the shutter rear curtain. As a result, transistor Q3 turns off and magnet M
G2 is turned off, and the shutter trailing curtain starts running.

-Pulse Train Generation Routine- Next, a pulse train generation routine for generating a pulse train with a duty ratio corresponding to the film sensitivity will be described with reference to FIG.

When this program is started at a predetermined timing, step 830 turns on flag 1 of the internal timer and proceeds to step 831. In step 831, the port 03 is set to high logic, and in step 832, the film sensitivity data M(D) stored in the memory is set in a timer. When the timer measures the data M(D), in other words, when a timer interrupt occurs, flag 1 is reset in step 833 and the process proceeds to step 834. In step 334, HOT 03 is brought to a low logic, and in step 335.

A memory value M(D) indicating the complement of D is set in the timer. Then, when a timer interrupt occurs, step 83
Return to 0. Thereafter, by repeatedly performing these steps, a pulse train having a duty ratio corresponding to the film sensitivity data is obtained.

- Subroutine - Next, a subroutine for initial control processing will be described with reference to FIG. 10.

When jumping to this subroutine from step S4 of the main routine, in step 821, a first pass flag 2 (hereinafter referred to as flag 2) is set to identify whether or not this routine is passed for the first time. Step S2
In step 2, the state of flag 2 is detected, and if it is the first pass, the process advances to step 823, and if it is the second or subsequent pass, the process jumps to step 829. In step 823, the port 03 of the microprocessor 17 is brought to an "L", ie low logic state, turning off the transistor Q1 of FIG. That is, a low level signal is supplied to the DA converter 5 as an initial signal. In step S24, channel CH4 of the AD converter 30 is selected, and the charging voltage of capacitor C1 in FIG. 1 can be monitored via the output of operational amplifier A4. In step 825, the switch SW
An analog voltage VF set in advance according to the film sensitivity input from SW1 to SW3 is read and stored in a predetermined area. Next, in step 826, the AD conversion operation is started and the charging tt reading of the capacitor C1 is started. In step 827, the analog voltage vi corresponding to the film sensitivity read in step 825 is compared with the AD conversion value of the DA conversion output, and if they match, the process proceeds to step 828 and the AD conversion operation is stopped. And step 829
Then, turn off flag 2 and return to the main routine.

Next, the transient response characteristics when the power switch is turned on will be explained with reference to FIG. 11.

FIG. 11 represents the charging voltage of capacitor C1 in FIG. 1 versus time, and curve A shows the curve A when baud)03 of microprocessor 177 is in the rLJ, i.e., low logic state (transistor Q1 is off). 2 shows a charging characteristic in which the capacitor C1 is charged to the AD converter 50 reference voltage VREF by the resistors R6 and R7. Curve B shows the charging characteristic of the capacitor when the capacitor C1 is charged with a pulse train having a duty ratio of 1 determined by a certain film sensitivity.

In curve B, for example, it takes TO time to reach a voltage of 1.342 VK corresponding to film sensitivity SO100. In other words, when the power switch SIOQ is turned on, if a pulse with a duty ratio corresponding to the set film sensitivity 15O100 is output from the boat o3, the charge voltage of the capacitor C1 reaches 1.342V. It will take time TO.

On the other hand, looking at curve A, we see that the same film sensitivity is 5O100.
The time it takes for the charging voltage to reach the voltage corresponding to is TX, and the relationship is TX(To).

This means that 1 hour TX is resistor R6 + R7 and capacitor C
This is because the current is determined by the balance between the discharging time constant determined by the resistor R7 and the capacitor C1 when the charging time constant is determined only by the resistor R7 and the capacitor C1.

1. Second embodiment - subroutine shown in FIG. 10, microprocessor 1
Since everything is the same as the 10th embodiment except for the absence of the 70 input channel CH4 and its connection, the second embodiment will be described below.
Only the subroutines of this embodiment will be explained.

In the first embodiment, the charging voltage of the capacitor C1 is monitored by the AD converter 3, but in the second embodiment, the curve A in FIG.

Paying attention to the fact that it is expressed as, the capacitor CI and the resistor R6
, R7 is constant, the charging voltage vC1 of the capacitor C1 depends only on time t.

The charging voltage of the capacitor C1 is indirectly monitored based on the time. As shown in FIG. 7, the input voltage of the operational amplifier A40 corresponding to the film sensitivity is determined, so instead of monitoring the charging voltage of the capacitor C1, which is indicated by the curved line in FIG. The charging voltage of the capacitor C1 can be indirectly checked by monitoring the passage of time after the processor 17 starts operating with a timer.

Here, C1=0,22/jF, 'R6=4.
7Kn and R7=75Km, the relationship between film sensitivity and charging time is determined from Equation 19 as shown in FIG. 12. In Figure 12, the theoretical value for the film sensitivity of ISO3200 is L520'/, but in this case, according to equation 19, it would take an infinite amount of time to converge to 1.5207, so here Then, 1.517V
is used to find the charging time.

That is, 0.003V (1,520-1,517) is a voltage equivalent to πmV with respect to ISO 3200, but in practice, an error of about πgv has a small effect on exposure and can be ignored.

The 70-chart of the second embodiment will be explained with reference to FIG. FIG. 13 corresponds to the subroutine of FIG. 10.

At step 341, a first pass flag 2 is set to identify whether or not this routine is the first time. In step 842, the state of flag 2 is detected;
If it is the first pass, proceed to step 843; if it is the second pass or later, jump to step 847 and set flag 2.
Reset. In step 843, the microprocessor 17 node 03 is set to rLl to turn off the transistor Q1 of FIG. In step 844, a timer in the microprocessor 17 is set to a time TS corresponding to the set film sensitivity. In step 845, a timer is activated, and in step 846, it is determined whether or not the timer time T has reached the set time T8. If the determination is affirmative, the process proceeds to step 847, where flag 2 is turned off and the main routine of FIG. Return to step S5.

In the second embodiment, the channel CH4 of the microprocessor 17 is not required, and the connection between the DA converter 5 and its channel CH4 is also not necessary, and the circuit is simplified compared to the first embodiment.

In the above explanation, a low level initial signal is supplied to the base of the transistor Q1 when the power is turned on, but a pulse train with an off time t1 longer than the off time 10 in a pulse train with a duty ratio corresponding to the set film sensitivity. may be supplied to the transistor Q1 as an initial signal. Furthermore, the information to be subjected to DA conversion is not limited to film sensitivity information, but also includes other exposure information such as an open F value, an aperture value, and an aperture value.

(Effects of the Invention) According to the present invention, when the power switch of the camera is turned on, P
An initial signal that shortens the capacitor charging time of the filter means provided in the WM type DA converting means is temporarily supplied to the DA converting means instead of a pulse train, so that the capacitor charging time is shortened and *source switch Improves transient response when turning on.

[Brief explanation of drawings]

FIG. 1 is an exposure control circuit diagram showing one embodiment of the present invention, and FIG.
The figure is a block diagram, Figure 3 is a diagram showing the relationship between the switch status of the film sensitivity setting circuit and film sensitivity, Figure 4 is a diagram showing 8-bit data corresponding to film sensitivity, and Figure 5 is a diagram showing the relationship between the switch status of the film sensitivity setting circuit and film sensitivity.
The figure is a waveform diagram for explaining the operation of a PWM type DA converter, Figure 6 is a waveform diagram showing an example of the DA conversion output, Figure 7 is a diagram showing analog 'turret pressure corresponding to film sensitivity, and Figure 8 is a waveform diagram showing an example of the DA conversion output.
The figure shows an example of the main routine of the exposure control program.
9 is a flowchart showing an example of a pulse train generation routine, FIG. 10 is a flowchart showing an example of an initial control routine, FIG. 11 is a chart showing charging characteristics of capacitor C1, and FIG. Diagram showing time. FIG. 13 is a flowchart showing another example of the initial control routine, and FIG. 14 is a circuit diagram showing an example of a PWM type DA converter. 1...Temperature proportional voltage generation circuit. 3...AD converter, 5...DA converter. 7...Photometering circuit, 11...Exposure factor setting circuit. 15... Film sensitivity setting circuit. 17...Microprocessor. 19... Magnet drive circuit, A1-A4... Operational amplifier. R1~RIO...Resistor, C1~C2...Capacitor. PD: Photodiode. VRI, vR2...variable resistor. DIND6...Diode. Q1 to Q3...Transistor, MGl, M()2...Magnet. 8W1~SW3...Switch. S100...Power switch. Figure 7 Figure 3 Figure 1O Figure 12 Figure 1. ．． Figure 3

Claims (1)

[Scope of Claims] 1) In an exposure control circuit for a camera that converts exposure information in the form of a digital value into an analog value and performs exposure control using at least the analog value, (a) a duty according to the exposure information; (b) filter means consisting of a capacitor and a resistor, and modulates the supplied reference voltage into an analog value according to exposure information according to the duty ratio of the pulse train. (c) a digital-to-analog conversion means for extracting a modulated analog signal via the filter means; The initial signal to be shortened is
In addition to supplying the digital signal to the analog converting means,
When it is directly or indirectly detected that the analog output of the analog conversion means has reached a predetermined analog amount predetermined according to the set exposure information, and when the predetermined analog amount is directly or indirectly detected. and initial control means for supplying the pulse train to digital-to-analog conversion means in place of the initial signal. 2) The camera according to claim 1, wherein the initial signal is a pulse train signal that shortens the charging time of the capacitor compared to a pulse train with a duty ratio according to set exposure information. exposure control circuit. 3) The camera exposure control circuit according to claim 1, wherein the initial signal is a high logic signal or a low logic signal.