DE2209670C3 - Exposure control circuit for a camera shutter - Google Patents

Description

The invention relates to an exposure control circuit for a camera shutter according to the preamble of claim 1.

In a known device of this type (DT-OS 06 392) the object brightness is called electrical Voltage measured and must because of the internal measurement used (measurement through the taking lens through) before the exposure time with the Camera shutter is formed. To save the operating voltage source (battery) of the camera, this is only switched on when the camera is actually being used (see also US Pat. No. 33 24 779).

To store the exposure measurement voltage, capacitors are regularly used, but they have one more or have less pronounced leakage current, which falsifies the measurement result Film capacitors achieve small leakage currents that

ίο Film capacitors are suitable because of their Size not for installation in a camera. Electrolytic capacitors have a relatively low Size, but their leakage current is very high under the operating conditions described.

The invention is based on the object of a Design exposure control circuit according to the preamble of claim 1 so that its leakage current behavior when using an electrolytic capacitor is improved and that of the capacitor stored object brightness-dependent voltage value even with a longer delay between the When to measure and when to actually perform the exposure remains unadulterated

This object is achieved with the presupposed exposure control circuit with the characterizing features of claim 1 Refinements and developments are given in the subclaims.

In the exposure control circuit according to the invention, the storage capacitor is not, as in known circuits, when the exposure measuring circuit is switched on, it is charged from the value 0 to an object brightness-dependent value, but it is from a constantly applied high voltage value, namely the voltage value of the voltage source Switching on the exposure metering circuit to a lower one, namely the one that is dependent on the brightness of the object Discharge voltage value. The storage capacitor is therefore always in the formed state and therefore has only a very low leakage current, so that the object brightness-dependent voltage value stored by it is practically not falsified. Therefore, not only electrolytic condensate at all for a reliable exposure control tion, but it is even possible to use tantalum electrolytic capacitors, which due to a high capacity per unit volume have particularly small size, but a very show high leakage current when they are in the non-formed or only slightly formed state. There however, also tantalum electrolytic capacitors after a longer formation time, which in the case of the invention Exposure control circuit is always given just yet have very small leakage currents, it is through the The present invention has become possible due to its small size for camera purposes per se very attractive tantalum electrolytic capacitors to use.

In the following, the invention will be explained in more detail using an exemplary embodiment. In the Drawing shows

F i g. 1 shows a circuit of a known device for controlling the exposure time,

hs Fig. 2 a circuit of the egg foundling one direction,

J shows a leakage current-time diagram of an electrolytic capacitor from the application of a voltage to the

Capacitor.

The known exposure metering and storage circuit of Figure 1 includes a power source such as a Battery 1, a photoelectric converter 2 and diodes 3 and 4 connected to each other in series and via a Switches 5 are connected in parallel to the battery 1. the Diodes 3 and 4 can be replaced by resistors. In the case of a single lens reflex camera with internal measurement, the photoelectric converter 2 is temporarily darkened, e.g. B. because he is either on the opposite sides of an eyepiece on the pentaprism or is attached in front of the film surface so that the Photoelectric converter 2 can be illuminated by light that passes through the taking lens is

When the switch 5 is closed, a current flows from the battery 1 to the photoelectric converter 2 and the diodes 3 and 4 , is proportional. A storage capacitor 6 is connected in parallel to the diodes 3, 4 via a normally closed switch 7 which is designed to open precisely at the moment before the light falling on the photoelectric converter 2 causes the shutter to operate is interrupted. The storage capacitor 6 is charged to the voltage applied to the diodes 3, 4. The connection point between the storage capacitor 6 and the switch 7 is connected to a circuit A which actuates the shutter magnet and which comprises a magnet for holding the shutter open and controls the exposure time according to the size of the voltage stored in the storage capacitor 6.

A further switch 8 is provided, via which the circuit A can be supplied with energy from the current source 1.

It is assumed that the input current of the circuit A is zero, that the change in the voltage stored in the capacitor 6 when the brightness of the light source doubles is 36 mV and that the capacitor 6 has a capacitance of 33 μΡ. In order to control an exposure time of 10 seconds within an error range of Vs step length (ie: 10 · 2 ± '"= 8.71; 11.49 see), the leakage current / from capacitor 6 is:

3.3 · 10
"10

= 2.4 nA.

Capacitors with such a small leakage current can be found under film capacitors, they but are not found under electrolytic capacitors. As mentioned, however, it is a film capacitor unsuitable for installation in a camera due to its large dimensions. That's why it was studied the leakage current of a tantalum electrolytic capacitor which is compact and has good working properties having. Such an electrolytic capacitor has been found to be extremely downsized Leakage current leads if it is a capacitor with high dielectric strength and this is operated at a low charging voltage, and that the leakage current reduction corresponds to the lapse of time since the voltage was applied to the capacitor is proportional. F i g. 3 graphically illustrates the time-leakage current relationship of a tantalum electrolytic capacitor with a dielectric strength of 35 V, which is 2 V has been charged at two ambient temperatures, namely 60 ° C and 20 ° C.

From the graph it can be seen that at the two ambient temperatures the described condition for the leakage current through a charging or Formation time of about 82 or 36 seconds fulfilled can be. In the case of FIG. 1, this means that from the time at which the switch 5 is closed, up to

ίο time at which the lock is activated, a period of time of 82 or 36 seconds must elapse, which is an impossible condition therefore it is proposed to connect the capacitor 6 according to FIG. 2 directly to the negative terminal of the Connect power source 1.

In the circuit according to FIG. 2, the capacitor 6 is normally over the photoelectric Converter 2 charged to the voltage value of current source 1, with the (constantly flowing) leakage current on is reduced to an extremely small value. When the switch 5 is closed, a measurement of the To carry out the intensity of the light incident on the photoelectric converter 2, the parts 2, 3 act and 4 as a voltage divider, and the capacitor accepts the voltage drop across the diodes 2, 3. Excess charge on the capacitor 6 is discharged through the diodes 3 and 4, so that the voltage at the capacitor 6 approaches that voltage which is due to the voltage divider from the photoelectrical

jo tric converter 2 and the diodes 3, 4 has set. Then, when the shutter release button (not shown) is pressed, the switch 7 opens through the Effect of a shutter release member (in the case of a single-lens reflex camera by the effect of the Mirror adjustment member), so that the capacitor 6 is disconnected and stores a voltage that the intensity of the light corresponds to exactly before opening the switch 7 on the photoelectric Converter 2 has occurred. This maintains a condition that is independent of the Intensity of the light, which after opening the switch 7 on the photoelectric converter 2 hits. As is known, the photoelectric converter 2 after opening the switch 7 by the folding up mirror darkened.

In the course of the further action of the shutter release member, the switch 8 is closed. The shutter is then opened to expose the film. After a period of time corresponding to the voltage stored in the capacitor 6, the shutter is closed by the action of the circuit A to stop the exposure of the film.

When the camera is in the dark, the resistance value of the photoelectric converter becomes 2 probably so great that no charging current for the capacitor 6 flows. To avoid this, a Resistor 9 connected in parallel to photoelectric converter 2 with a large resistance value will. If the camera is in the dark,

f> o is the capacitor 6 via the resistor 9 with Energy supplied.

Accordingly, the present invention enables the use of a compact, inexpensive Tanialelektrolytkondens, ators, its leakage current on a very

ds low value is reduced. what on the one hand to one economic advantage leads and on the other hand to the possibility of such a capacitor in a camera to be built in.

lilatl /.ci

Claims (5)

Patent claims: 1. Exposure control circuit for a camera shutter, with an exposure control circuit which one comprising a series circuit of a photoelectric converter and an impedance Has voltage divider, one end fixed to a pole of a voltage source and the other end is connected to the other pole of the voltage source via a first switch and which, when the first switch is closed, has an object brightness-dependent voltage at a voltage divider point emits, with a storage capacitor that connects to a pole of the voltage source and one end the other end is connected to the voltage divider point via a second switch, and with one Exposure time control circuit scanning the voltage value across the storage capacitor, characterized in that the storage capacitor (6) is closed when the second switch (7) and the light metering circuit switched off at both ends is connected to the voltage source (1) and charged to its voltage value and after turning on the exposure metering circuit by closing the first switch (5) to the object brightness-dependent voltage is discharged at the voltage divider point and that it is at the storage capacitor is an electrolytic capacitor 2. Exposure control circuit according to claim 1, characterized in that it is the Storage capacitor (6) is a tantalum electrolytic capacitor 3. Exposure control circuit according to claim 1 or 2, characterized in that the storage capacitor (6) when the second (7) and closed open first (5) switch with the photoelectric converter (2) one end with the Voltage source (1) connected series circuit and when the first (S) and closed second (7) switch with the impedance (3, 4) one between one pole of the voltage source (1) and forms parallel connection lying at the voltage divider point 4. Exposure control circuit according to one of claims 1 to 3, characterized in that the Series connection of the second switch (7) and storage capacitor (6) parallel to the series connection of impedance (3, 4) and the first switch (5) lies. 5. Exposure control circuit according to one of claims 1 to 4, characterized in that a Resistor (9) is connected in parallel to the photoelectric converter (2).