Display Circuit for a Camera

1. Field of the Invention

This invention relates to a photographing information display circuit for a camera and more particularly to an information display circuit which performs computation of information relative to exposure.

2. Description of the Prior Art

Heretofore, there have been proposed various types of display circuit arrangements that perform computation of exposure information by means of digital computation circuits and display the results of computation by display circuits through digital circuits. However, such a display circuit is generally composed of seven segments or the like. For display, the computed digital value must be converted into segment informations by means of a decoder circuit, a driver circuit, etc. Therefore, the conventional arrangement is of a complex construction requiring a relatively great amount of electric power for display. For a small-sized camera using a very small battery as power source, the use of such a device results in a great ratio of display power requirement for display to the overall power requirement for the camera. The conventional display circuits are thus not only complex in construction but also require a relatively great amount of power supply.

It is therefore an object of this invention to provide a display circuit which displays results of computation, without using any complex display circuit arrangement, by controlling the duty cycle of pulses based on a digital value obtained through computation in such a way as to drive a meter by the pulses.

It is another object of this invention to provide a display circuit which displays results of computation, without complex display circuit arrangement, by driving a meter with pulses of frequency based on a digital value obtained through computation.

It is still another object of this invention to provide a display circuit which displays results of computation, without complex display circuit arrangement, by driving a meter with an integrated amount of electric charge which is integrated according to a digital value obtained through computation.

The further object, features and advantages of this invention will become manifest from the following detailed disclosure, taken in conjunction with the accompanying sheets of drawings in which:

FIG. 1 is a block diagram illustrating a camera using a display circuit of the present invention as an embodiment example of the invention.

FIG. 2 is a circuit diagram illustrating the details of the embodiment illustrated in the form of a block diagram in FIG. 1 with details shown as applied to shutter control.

FIG. 3 is a circuit diagram illustrating further details of a display circuit of this invention illustrated in FIG. 2.

FIG. 4a is a table showing the relation of film sensitivity to the switches SF1 through SF3 shown in FIG. 2, the diaphragm aperture value and switches SA1 through SA3 also shown in FIG. 2.

FIG. 4b is a timing chart illustrating the operation of the circuit shown in FIG. 3.

FIG. 4c is a circuit diagram illustrating an example of the code conversion circuit shown in FIG. 2.

FIG. 5 is a circuit diagram illustrating a display circuit as another embodiment example of the invention.

FIG. 6 is a wave form drawing illustrating the operation of the circuit illustrated in FIG. 5.

FIG. 7 is a circuit diagram illustrating a modification of the circuit illustrated in FIG. 3.

FIG. 8 is a circuit diagram illustrating a modification of the circuit illustrated in FIG. 5.

Referring to the accompanying drawings, FIG. 1 is a block diagram illustrating an embodiment example using a display circuit arranged in accordance with this invention. In FIG. 1, a photometric circuit 1 measures the brightness of a photographing object by a method , such as a TTL method, and produces an analog information output according to the brightness measured. An A-D converter 2 converts the analog information derived from the photometric circuit into digital information. Digital switching means 3, 4 and 5 are provided for obtaining digital information on shutter time, a diaphragm aperture value and film sensitivity respectively. A reference numeral 6 indicates a digital computation circuit for which various types of computing elements can be used. The digital computation circuit 6 receives digital information on the brightness of the photographing object from the A-D converter 2 and digital information on the shutter time, diaphragm aperture value and film sensitivity from the digital switching means 3, 4 and 5 respectively. The circuit 6 then produces a digital information output required for controlling the shutter time or the diaphragm aperture. Reference numerals 7 and 8 indicate a shutter speed controlling means and a diaphragm aperture controlling means; and 9 indicates a display device arranged in accordance with the invention. The display circuit is composed of a D-A converter 9' which converts the digital information from the digital computation circuit 6 into analog information and a meter 9" or the like which displays the value of the analog information. With the exception of the display device 9, the rest of the circuit arrangement described in the foregoing is similar to various known digital exposure control circuits. Although the A-D converter 2 which is used for the photometric circuit may be arranged to serve double purposes utilizing it also for the D-A conversion in place of the D-A converter 9', a D-A converter must be separately provided for the display circuit in cases where a double integrating method is employed for the photometric circuit. Since a simple circuit arrangement suffices for the displaying D-A converter, it is generally preferable to provide the converter circuit separately from the other converter.

As described in the foregoing, with the display circuit of the invention employed in a digitally controlled camera which performs control by digitally computing exposure information, the result of the digital computation is converted into an analog value to make analog display. In accordance with the present invention, the conversion of the result of digital computation is converted into the analog value by an extremely simplified circuit arrangement without using such a complex ladder network that is employed in the conventional D-A converter. Now referring to FIGS. 2 through 6, further details of the display circuit of the present invention are as described below:

In FIG. 2, which is a circuit diagram illustrating further details of the embodiment illustrated in the form of a block diagram in FIG. 1 with details as applied to shutter control, a diode D.sub.1 is connected in series with a light receiving element cds. A voltage which corresponds to a logarithm of the brightness of a photographing object (voltage corresponding to a value Bv) is produced at a connection node "a" between the light receiving element cds and the diode D.sub.1. A pulse oscillator PG is actuated by the operation of an unillustrated power source switch and is connected to one of the input terminals of an AND gate AND.sub.1 through a switch SW.sub.1, which is interlocked with the power source switch to turn on in response to the operation of the power source switch. A binary counter ct.sub.1 is connected to the output terminal of the AND gate AND.sub.1 to perform binary counting of the pulses obtained through the AND gate AND.sub.1. There is provides a ladder circuit rD which is a known resistance circuit network and is connected to the output terminal of the counter ct.sub.1 to convert the content of the counter ct.sub.1 into an analog voltage. A comparator COM.sub.1 has one input terminal connected to the above stated connection node "a" and another input terminal connected to the output terminal of the ladder circuit rD. The output of the comparator COM.sub.1 is inverted from a high level to a low level when the voltage inputs to the comparator becomes equal with each other. A known analaog-to-digital converter is formed by these components PG, AND.sub.1, ct.sub.1, rD and COM.sub.1. Out of the pulses derived from the pulse oscillator PG, only a number of pulses corresponding to the logarithm of the brightness are transmitted to the binary counter ct.sub.1. Switches SA.sub.1 through SA.sub.3 are provided for setting diaphragm aperture information. A diaphragm aperture value is set in the form of a digital value according to the on-off state of these switches. There is provided a full subtractor SUB.sub.1 which subtracts a digital value of an input terminal B thereof from a digital value of another input terminal A. Switches SF.sub.1 - SF.sub.3 are provided for setting the sensitivity of the film employed. The film sensitivity value is set in the form of a digital value according to the on-off state of these switches. Another full subtractor SUB.sub.2 is identical with the above stated full subtractor SUB.sub.1. There are provided AND gates A.sub.1 - A.sub.3, each of them having one input terminal connected to the output terminal of the full subtractor SUB.sub.2 and having another input terminal connected to the above stated comparator COM.sub.1 through an inverter IN. A register rc.sub.1 stores the digital value which is received through the AND gates A.sub.1 - A.sub.3. Another binary counter ct.sub.2 is connected to another pulse oscillator PG.sub.2 through a switch SW.sub.2 which is interlocked with an unillustrated release button. A code conversion circuit which converts the content of the counter ct.sub.2 into a digital value representing a preset relation. Exclusive OR gates ex.sub.1 - ex.sub.3 and an OR gate NOR.sub.1 constitute a known coincidence detection circuit, which produces a coincidence signal output when the content of the register rc.sub.1 and the output value of the code conversion circuit MC coincide with each other. A transistor Tr.sub.1 has the base thereof connected to the OR gate NOR.sub.1 while a magnet Mg.sub.1 is connected to the transistor. When the transistor Tr.sub.1 is turned on, the magnet is operated to cause the rear diaphragm of an unillustrated shutter to travel. The reference symbol PG.sub.3 indicates a pulse oscillator; and A.sub.4 an AND gate. One of the input terminals of the AND gate A.sub.4 is connected to the above stated inverter IN.sub.1 while the other input terminal is connected to the pulse oscillator PG.sub.3. A counter ct.sub.3 performs binary counting of pulses coming through the AND gate A.sub.4. The reference symbol MCOM indicates a magnitude comparator; LPF a low-pass filter which is connected to the output terminal of the magnitude comparator MCOM and which makes an integrating action; and M a meter.

FIG. 3 is a circuit diagram illustrating in further detail the display circuit of the embodiment of the invention comprising the magnitude comparator MCOM, low-pass filter LPF, counter ct.sub.3 and register rc.sub.1 shown in FIG. 2. Referring to FIG. 3, a magnitude comparator 8269 manufactured by Signetic Co., for example, may be employed as the magnitude comparator MCOM. The magnitude comparator MCOM gives a truth table wherein the output terminal Y produces "1" when A .ltoreq. B and produces "0" when A > B. The reference symbol IN.sub.2 indicates an inverter. The low-pass filter LPF is composed of resistance R.sub.1 and R.sub.2, capacitors C.sub.1 and C.sub.2 and an operational amplifier.

FIG. 4a is a table showing the relation of film sensitivity to the switches SF.sub.1 - SF.sub.3, the diaphragm aperture value and switches SA.sub.1 - SA.sub.3. FIG. 4b is a timing chart illustrating the operation of the circuit illustrated in FIG. 3. FIG. 4c is a circuit diagram illustrating as an example the details of the code conversion circuit shown in FIG. 2. In FIG. 4c, the reference symbols T.sub.1 - T.sub.22 indicate transistors and D.sub.1 - D.sub.12 diodes. Although detailed description of the operation of the code conversion circuit is omitted here as such is not directly related to the present invention, the circuit operates as follows: When, for example, bits 1456 shown at the counter become 1, i.e. when 57 pulses have been counted, the transistors Tr.sub.17 - TR.sub.20 are turned on; a high level output is produced by a line l.sub.2 through the diode 11 and a digital value of 010 is obtained as an output. The input-to-output relation is as follows: The output is 001 when 40 pulses have been counted; 010 when 64 pulses have beeen counted; 011 with 80 pulses counted; 100 with 114 pulses counted; and so on. In this manner, the output arithmetically varies while the input geometrically varies. The embodiment of the invention illustrated in FIGS. 2 and 3 operates in the following manner:

First, the on and off positions of the switches SA.sub.1 - SA.sub.3 are adjusted to a relative condition shown in FIG. 4a according to a desired diaphragm aperture value. The switches SF.sub.1 - SF.sub.3 are also adjusted in the same manner according to a desired value of film sensitivity. With the diaphragm aperture and film sensitivity values having been set in this manner, an unillustrated power source switch is turned on to bring each circuit into an operating state. The switch SW.sub.1 is then turned on by this. The pulses from the pulse oscillator PG.sub.1 are impressed upon the AND gate AND.sub.1 through the switch SW.sub.1, and the counter ct.sub.1 begins to count the pulses. The content of the counter ct.sub.1 is counted up by binary counting. The ladder circuit rD produces an analog voltage corresponding to the content of the counter. The analog voltage is impressed on one of the input terminals of the comparator COM.sub.1. Since a high level output is produced by the comparator COM.sub.1 when the voltage at the connection node a corresponding to a logarithm of the photographing object's brightness is higher than the output voltage of the ladder circuit, the output of the comparator comes to a low level when the content of the counter comes to coincide with the voltage at the node "a", and then a closed state of the AND gate AND.sub.1 takes place. Because of this, a number of pulses corresponding to the logarithm of the brightness is counted up by binary counting at the counter ct.sub.1. Thus, the content of the counter ct.sub.1 is determined by the brightness. Furthermore, since the output of the comparator COM.sub.1 at this time comes to a high level through the inverter IN.sub.1, the AND gates A.sub.1 - A.sub.4 which have remained closed until then are brought into open states respectively. Therefore, the full subtractor SUB.sub.1 then subtracts the content of the counter c.sub.1, i.e. the logarithmic information on the brightness, from the diaphragm aperture value information set through the switches SA.sub.1 - SA.sub.3 ; the operation of AV - Bv takes place; then another full subtractor SUB.sub.2 subtracts the film sensitivity information set through the switches SF.sub.1 through SF.sub.3 from the output of the subtractor SUB.sub.1 ; and the output of the subtractor SUB.sub.2 which has become Av - Bv - Sv = -Tv is transmitted to the register rc.sub.1. As a result of this, the content of the register rc.sub.1 becomes a digital value that corresponds to the shutter time. On the other hand, when the above stated comparator is inverted from a high level to a low level, another counter ct.sub.3 begins the binary counting of the pulses produced by the pulse oscillator PG.sub.3. In the initial stage, therefore, the content of the register rc.sub.1 is greater than the content of the counter ct.sub.3. Then, the input condition to the magnitude comparator becomes A > B and there is produced an output of a low level at the output terminal Y. The output of the output terminal Y is inverted from the low level to a high level when the input condition becomes A .ltoreq. B, i.e. when the content of the counter ct.sub.3 becomes greater than the content of the register rc.sub.1. This condition persists until every bit of the counter ct.sub.3 is set as "1". Then, every bit of the counter ct.sub.3 again is caused to becomes "0" by pulses subsequently arriving at the counter; and again the output of the magnitude comparator MCOM becomes a low level. By this, the output of the magnitude comparator MCOM obtained through the inverter IN.sub.2 is inverted to a high level in a preset cycle T.sub.1 as illustrated in FIG. 4b. The length of time required for the inversion from the high level to the low level, namely, the duty of pulse within the preset cycle T.sub.1 is determined by the content of the register rc.sub.1. The duty becomes to correspond to the shutter time information obtained through computation. Therefore, since the low-pass filter LPF integrates the output of the magnitude comparator MCOM, the output of the operational amplifier AP which constitutes the low-pass filter LPF becomes a value corresponding to the duty and the pointer of the meter M deflects according to the output of analogically display the shutter time information obtained through computation. In this manner, in accordance with this invention, the result of digital computation can be displayed through a very simple circuit arrangement. For controlling shutter time, an unillustrated shutter release lever is turned on to cause a front diaphragm of a shutter to travel and, concurrently with this, the switch SW.sub.2 which is interlocked with the shutter release lever is turned on. Then, the counter ct.sub.2 performs binary counting of the pulses from the pulse oscillator PG.sub.2. The content of the counter ct.sub.2 is converted into a logarithmically suppressed digital information. The coincidence detection circuit which comprises ex.sub.1 - ex.sub.3 and NOR.sub.1 then detects the coincidence of the output of the register rc.sub.1 with that of the code conversion circuit MC. When the two outputs coincide with each other, or in other words, when all of the inputs of the NOR.sub.1 have become "0", to transistor Tr.sub.1 is turned on to actuate the magnet Mg. This causes the rear diaphragm of the shutter to travel to complete the shutter control. In the above described example of embodiment, the output of the magnitude comparator is impressed upon the meter through the low-pass filter LPF. However, the same display operation can be accomplished by impressing the output of the comparator directly upon the meter as shown in FIG. 7, if the meter itself possesses an integrating characteristic.

FIG. 5 is a circuit diagram illustrating another embodiment example of the display circuit of this invention. In FIG. 5 the computation circuits SUB.sub.1, SUB.sub.2 etc. and the shutter control circuit of the coincidence detection circuit, etc. are omitted from the illustration as they are identical with those used in the foregoing embodiment example. However, the display circuit shown in FIG. 5 differs from the one shown in FIG. 2. Unlike the arrangement shown in FIG. 2, the display circuit is formed in the following manner: An input signal coming from a switch SWM is impressed upon the AND gates A.sub.1 - A.sub.3 ; a pulse oscillator PG.sub.10 is connected through an AND gate A.sub.11 to a clock terminal cp of the register rc.sub.1 ; and the output terminal of the register is connected to a conversion circuit which is arranged in accordance with the known method of Sharnnon-Rock and which comprises field effect transistors FET.sub.1 and FET.sub.2, a resistance R.sub.3, capacitors C.sub.3 and C.sub.4 and an operational amplifier OP. The reference symbol IN.sub.3 indicates an inverter; bc.sub.10 a binary counter; and FF a flip-flop circuit. The embodiment illustrated in FIG. 5 operates in the following manner:

Prior to photographing, the switch SWM is turned on by an unillustrated operating means. By this, a power source switch is turned on. Then, in the same manner as in the case of FIG. 2, a digital value corresponding to the shutter time is stored in the register rc.sub.1. Following this, the switch SWM is turned off by operating the above stated operating means. A high level signal is then impressed upon one of the input terminals of an AND gate A.sub.11. By this, the pulses from a pulse oscillator PG.sub.10 is supplied to the clock terminal cp of the register rc.sub.1. The content of the register rc.sub.1 is serially put out, being synchronized with the pulses, and is impressed upon the field effect transistor FET.sub.1 in time series. This causes the transistor FET.sub.1 to make on-off operation based on the digital value of the register rc.sub.1. With the transistor FET.sub.1 being in an "on" condition, the capacitor C.sub.3 is charged with the electric current of a constant current source I and then the capacitor makes discharge through the resistance R.sub.3 before the next pulse from the pulse oscillator enters the register rc.sub.1. This process is repeated by the on-off operation of the transistor FET. Assuming that the content of the register is 1 1 0 as shown in FIG. 6, the second pulse from the pulse oscillator PG.sub.10 turns on the field effect transistor FET.sub.1. The capacitor C.sub.3 is charged up to 8 V and then attenuates by half thus decreasing down to 4 V before the next pulse enters the register. Then, the register rc.sub.1 puts out "1" which turns on the transistor FET.sub.1 ; and the capacitor is charged 4 + 8 V and again attenuates by half and down to 6 V before another pulse enters the register. Following this, with the next pulse, the content of the register is put out. However, since the content is "0", the transistor FET.sub.1 is not turned on. Therefore, the capacitor further attenuates by half and thus attenuating down to 3 V. In this manner, the value of 1 1 0 is converted into 3 V. Furthermore, at this time, that is, when the fourth pulse is put out from the oscillator PG.sub.10, the output of the 3rd bit of the binary counter bc.sub.10 is produced to turn on the field effect transistor FET.sub.4 ; and the voltage of the capacitor C.sub.3 is transmitted to the capacitor C.sub.4 to cause the meter M to display the voltage. Then, the flip-flop circuit FF is set; the binary counter bc.sub.10 is reset; and the transistor FET.sub.4 is again turned off. In this manner, the content of the register rc.sub.1 is converted into an analog value, which is displayed at the meter M. Where the shutter is to be controlled, the switch SWM is again turned on; the output of the full subtractor SUB.sub.2 is again supplied to the register rc.sub.1 to effect shutter release; and, by this, the shutter is controlled in exactly the same manner as in the case of FIG. 2. Furthermore, if a meter that possesses an integrating characteristic by itself is employed as the meter M as shown in FIG. 8, the display can be made in the same manner as in the embodiment example described in the foregoing, even if the meter is directly connected to the output terminal of the register rc.sub.1.

As disclosed in detail in the foregoing, in accordance with the present invention, the digitally computed value of information relative to exposure is displayed by a meter through a very simple display circuit arrangement. With the display circuit simplified, the power comsumption required for display can be reduced to a great extent. The invented display circuit, therefore, has a great advantage when applied to a camera wherein digital computation is performed for display.