DE2053001C3 - Circuit arrangement for controlling a camera shutter - Google Patents

Description

One advantage of the outlined solution is that d <e prior display of the exposure time to be expected becomes possible.

Embodiments of the invention are explained with reference to the drawing. It shows

F i g. 1 a first circuit and

F i g. 2 shows a second circuit according to the invention.

After the circuit in FIG. 1, a photoresistor 1 as a photoelectric converter and one or more diodes 2 are connected in series with a voltage source 3 and form a measuring circuit. It is known that the current flowing in the diode 2 with the voltage V _{d applied to it} by the relationship

is connected, where i _{s is} the saturation current in the reverse direction and h is a value that is dependent on the absolute temperature.

Since the amount of i _{s is} extremely small and 1 can be neglected if i is more than a few nanoamps (10 " ⁹ A), the following applies approximately

i + K ₀ .

(H)

If the voltage of the voltage source is V and the resistance of the photoresistor 1 is denoted by R , then the following applies

V = h log i + V ₀ + iR.

(IU)

The relationship between the resistance R of the photoresistor and the brightness B of the object to be photographed is given by

R = kB ~ \ (IV)

The invention relates to a circuit arrangement for controlling a camera shutter according to the preamble of claim 1.

where γ = constant can be assumed as a property of a photo resistor. From relationships (III) and (IV) it follows

I / = ft logί + V ₀ + ikB- ^y . (V)

Since the third link opposite the second and

third is very small, it can be neglected so that it follows

Since 1 in the brackets can be neglected, the result is

(XIIl)

log / = log— £ -2 +, .log B.

(VI)

IO

Inserting equation (VII) into equation (II) results

K ^ / iylogß + Jilog -; --- + F ₀ . (VIII)

κ "'

This means that the logarithm of the brightness β of the object to be recorded is proportional to the voltage across the diode. In general, the voltage V _im across a series circuit of m diodes is given by

This means that the voltage across the capacitor is largely proportional to the logarithm of the charge time. The relationship between V _c and t when charging the capacitor 4 via η diodes connected in series can be represented generally by

K _c = V-nV _Q + nh \ og

nCh

(XIV)

If the difference between V _in and V _{c reaches} a predetermined value, e.g. B. V- V _in = V _c , the shutter is closed. Then t, i.e. the exposure time, can be linked to the brightness B of the object as follows:

ηV ₀ -nh log - ^ r = ^mh Y

^{mh lo} 8 + ^mV o-

A timing circuit includes a timing capacitor 4, a switch 5 that is parallel to the capacitor 4, and one or more diodes 6.
If switch 5 is used in conjunction with the ", / k \

opening movement of the shutter is opened, 30 B ^ ⁷ t = CnH (~. rr)

the timing capacitor 4 charges itself from the \ F - mV ₀ J

Voltage source 3 on. The charging current is given

by If γ = 1 and in = n, then

^constant

dt '

(X)

C = capacitance of the capacitor, V ₁ . = Voltage on capacitor, t seconds after

the store means.
From equations (II) and (X) it follows

Bt =

mChk

= constant.

Oil)

F-UiF ₀
If ,. = 1, then m and η can be chosen so that

that - =.,. ·. thats why

V =

F ₀ ,

(XI)

Bt = Cnh

V-mV ₀

• e

= constant.

(XVl)

45

di = C-e

VV, -V _n

VV ₀ CV,

t = ce "" Ä ~ Jp dF "

v_v _o K = Che h + A.

For f = 0, V _c = 0 and therefore

A = ■■ -C / je ~

ν - K, Γ V '

5- ■]

(XII)

V, = hlog I ^ e /,

In this way, the condition for the automatic control of the shutter exposure time can be set depending on the brightness B.

_{Based on the principle given above, a differential amplifier formed by the field effect transistors Tj and T 2} is provided in the first embodiment of the circuit, so that [V - V _dn ) and V _c are fed to two inputs and switching transistors T ₃ and T _4, d »e can be switched depending on the output variables of the differential amplifier. The voltage [V - V _11n ) is fed to the control electrode of the field effect transistor T ₁ , ie one of the inputs of the differential amplifier circuit, via a switch 8 when a voltage source switch 7 is closed. In the case of the internal measurement, this is stored in a capacitor 9, which is located between the control electrode of the field effect transistor T ₁ and the negative pole of the voltage source 3. Namely, if the photoresistor 1 is arranged behind a pentaprism of a single-lens reflex camera, the must immediately

light intensity measured before folding up the mirror can be saved. The switch 8 is opened immediately before the mirror is folded up. If a switch 10 is now closed in connection with the shutter, the capacitor 4 is still short-circuited by the switch 5, so that the potential at the control electrode of the field effect transistor T _{2 is} much lower than that of the field effect transistor Ti. That means that the field effect transistor T ₂ blocks almost completely. _{The transistor T 3} connected downstream is therefore non-conductive, while the transistor T ₄ connected downstream is conductive. The current flows through a winding 11 of a magnet in the collector circuit of the transistor T ₄ , so that the shutter cannot close. As a result of the opening of the shutter, the switch 5 is opened so that the capacitor 4 is charged via the diodes 6. When the voltage V _c on the capacitor 4 is almost the same (V - V _in ) , current suddenly begins to flow through the field effect transistor T ₂ , so that a voltage drop occurs across a resistor 12, which lowers the base potential of the transistor T ₃ . Therefore, the transistor T ₃ becomes conductive, while the base potential of the transistor T ₄ increases, so that the emitter potential falls. As a result, the transistor T ₄ blocks, so that the magnet 11 is de-energized and the shutter closes. The exposure time t controlled in this way depends, as demonstrated by equations (XV) and (XVI), on the brightness B of the object to be photographed.

From equation (XVI) it can be seen that if n / m = γ is made, the behavior of the photoresistor 1 can be appropriately corrected or calibrated. This means that photoresistors with different properties can be used; however, if possible, the number of diodes 2 and 6 should match in terms of the stability of the circuit in the event of voltage and temperature fluctuations. The γ of the CdS and other photoresistors is less than 1, so that in the first, in FIG. 1, a source electrode resistor 13 is provided on the field effect transistor T _{2 in} order to bring about the correction of γ by setting the source electrode resistor 13 appropriately. In detail, the input voltage of the field effect transistor T ₂ and thus the suction electrode current increase when the capacitor 4 is charged. However, the voltage drop across the resistor 13 between the control and source electrodes is introduced as negative feedback, so that the increase in the suction electrode current can be suppressed. In other words, the apparent input voltage of the field effect transistor T ₂ is lower than that of the field effect transistor T ₁ . Since the resistor 13 can be set very precisely, photoresistors with any γ can advantageously be used.

Factors that influence the exposure, such as the aperture setting of the taking lens, the film sensitivity, etc., can be set by a potentiometer 14, the center tap of which is coupled, for example, with the setting of the aperture value, the film sensitivity approximately so that {5 a release level is changed . Since the quantities log B or log f enter the amplifier as input voltages, the amount of change in the output quantity of the amplifier is constant when the exposure factor B or t changes by one step. Therefore, control in a wide range becomes possible by adjusting the potentiometer 14.

With a long exposure time and a correspondingly long charging time, the charging current of the capacitor 4 decreases, so that the problem of leakage c shows up. In order to eliminate this difficulty, there is a relatively high resistance in parallel with the diodes 6 15 switched.

In the second embodiment according to FIG. 2, the logarithmic device of the measuring circuit is formed from a diode sequence 16 and a photoresistor 17 as a photoelectric converter and a switch 18, and the measured value is stored in a capacitor 19. The voltage on the capacitor 19 is applied to the control electrode of a field effect transistor T _{5 which} , together with a further field effect transistor T _{6, constitutes} a first differential amplifier. At a further input, namely at the control electrode of the field effect transistor T ₆ , there is a voltage dropping across an adjustable resistor 21; The resistor 21 is in series with a diode sequence 20, which comprises as many diodes as the diode sequence 16. Since the adjustable resistor 21 is adjusted according to the exposure factors (for example aperture setting of the taking lens, film sensitivity, etc.), the one falling across the resistor 21 is and the voltage applied to the control electrode of the field effect transistor T _{6, of} course, in relation to the exposure factor. A voltage drop across an output resistor 22 of the field effect transistor T ₅ is therefore a function of the brightness of the subject and the aperture setting as well as the film sensitivity. In the suction electrode circuit of the field effect transistor T ₆ , a display device 23 is arranged, which displays the expected exposure time. The display device 23 can also be located in the suction electrode circuit of the field effect transistor T ₅ or between the suction electrodes of the field effect transistors T ₅ and T ₆ .

The logarithmizing device of the time circuit contains a diode sequence 24, a capacitor 25 and a switch lying parallel to the capacitor.: 6, as in the first embodiment. It would be possible to compare the voltage on the capacitor 25 with that on the output resistor 22, but in the second embodiment, with regard to voltage fluctuations of the voltage source 3 and changes in temperature, the voltage on the capacitor 25 is transmitted to a second differential amplifier made of field effect transistors T. ₇ and T ₈ , to which a voltage divider consisting of a resistor 30 and a diode sequence 31 is assigned for inputting a comparison voltage, the voltage drop across the output resistor 27 being used for the comparison with the measured variable.

The drop across the output resistors 22 and 27 voltages are the inputs of a third differential amplifier consisting of transistors T ₉ and T ₁₀ is supplied, since the switch 26 short-circuits the capacitor 2f before the shutter operation, dei transistor T is locked ₇ and the drop across the resistor 22 voltage is greater than the voltage drop across the resistor 2 ", so that the input variable ar to the transistor T _{9 is} greater than that at the transistor T _{1 (} and no current flows through the transistor T ₁₀

There is therefore no voltage drop across resistor 28. As a result, the base potential of a transistor T _{11 of} a circuit formed from this and a further transistor T _{12 is} approximately zero. Therefore, the transistor T ₁₁ blocks while the transistor T ₁₂ conducts, so that the magnet 29 is excited, whereby the closing movement of the shutter is prevented. If the switch 26 is opened because the shutter is opened, the capacitor 25 is charged so that the current suddenly flows through the transistor T ₁₀ . Therefore, the voltage drop across resistor 28 increases, and transistor T _n becomes conductive, while transistor T ₁₂ becomes non-conductive. Then will

the magnet 29 is de-energized, so that the closing movement of the shutter begins.

In the second embodiment, the voltage gain can be precisely adjusted by the variable resistor 22, so that the second embodiment has the advantage that the voltage stored in the capacitor 19 can be linearly expanded and compressed to a corrective door because of the one other than one γ of the photo resistance to allow. It is also advantageous that the preceding display of the automatically running exposure time can take place in a simple manner, as described above.

1 sheet of drawings

Claims (6)

Patent claims: 1. Circuit arrangement for controlling a camera shutter, including a measuring circuit a photoelectric converter arranged behind the taking lens and one Logarithmizing device for forming a first output voltage which corresponds to the logarithm of the Object brightness is at least approximated, and optionally a storage capacitor for Storage of this first output voltage, with a timing circuit including a timing capacitor, a charging resistor and one controlled by the beginning of the exposure time Switch, which time circuit a second output voltage approximated to the logarithm of the exposure time emits, and with a trip circuit that connects the first and second output voltage to each other compares and emits the signal to terminate the exposure time, characterized in that that the logarithmic device of the measuring circuit by one or more series-connected diodes (2, 16) and the logarithmic device of the time circuit by one or more series-connected diodes (6, 24) as load resistance are formed. 2. Circuit arrangement according to claim 1, characterized in that the measuring circuit has a first differential amplifier (T ₅ , T ₆ ) to which the first output voltage and an input voltage for film sensitivity and / or set aperture value are applied. 3. Circuit arrangement according to claim 2, characterized in that a voltage divider-like Input device consisting of an adjustable resistor (21) and one or more diodes (20) in series connection for inputting the input voltage for film speed and / or aperture value is provided. 4. Circuit arrangement according to claim 3, characterized in that the first differential amplifier (T ₅ , T ₆ ) has an adjustable resistor (22) for adjusting the level of its output voltage. 5. Circuit arrangement according to one of claims 2 to 4, characterized in that the timing circuit has a second differential amplifier (T ₇ , T ₈ ) to which the second output voltage and a comparison voltage are applied, and that the trip circuit for forming the comparison between the first and the second output voltage has a third differential amplifier (T ₉ , T ₁₀ ) to which the output signals of the first and second differential amplifiers are fed. 6. Circuit arrangement according to claim 5, characterized in that a voltage divider-like Device made up of a resistor (30) and one or more diodes (31) connected in series for entering the equivalent stress. In a proposed circuit arrangement of this type (DT-PS 1 772 106) the logarithmic device of the measuring circuit is formed by a voltage divider-like circuit and inclusion of the photoelectric converter, an approximation of the logarithm is achieved according to the function y = 1/1 + χ , and the logarithmic device of the time circuit is formed by a KC element, which follows the function ζ = 1 - e '' The approximation of these functions to the logarithmic function is only given in a limited range, so that the proposed circuit is only for a very limited brightness - or time range is applicable. The invention is based on the object of providing a circuit arrangement of the type specified at the beginning to be trained so that the control of the camera shutter over a larger measurement and time range done correctly. The object is achieved in that the logarithmizing device of the measuring circuit by a or several diodes connected in series and the logarithmizing device of the time circuit by a or several diodes connected in series are formed as a load resistor.