Exposure time control circuit

An exposure time control circuit for a camera includes a level selecting circuit between a light measuring circuit and a memory circuit to have a selected output memorized in the memory circuit. The level selecting circuit is provided with two inputs, one of which is the output of the light measuring circuit indicative of the scene brightness and the other of which is the output of a maximum exposure time output source indicative of the maximum exposure time acceptable to the camera. The memory circuit memorizes the output selected by the level selecting circuit which output indicates a shorter exposure time between the two.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an exposure control circuit for a camera, and 
more particularly to a circuit for controlling an exposure time in a 
camera in accordance with the information memorized by a memory means 
which is connected with a maximum exposure time output means and a light 
measuring circuit. The memory means memorizes the output of either the 
maximum exposure time output means of the light measuring circuit. 
2. Description of the Prior Art 
In the automatic exposure control cameras, there have been various measures 
for controlling the exposure time under low scene brightness. Therefore it 
is now possible to take pictures of low brightness with a long exposure 
time. However, it is often undesirable to take pictures with a long 
exposure time, since the long exposure time control markedly consumes the 
electric power of the camera. Further, it is impossible to take pictures 
of high quality with such a long exposure time unless the camera is 
supported on a tripod. 
Therefore, it has been known in the art to limit the exposure time in view 
of the practical photographing conditions. Thus, it is known to connect a 
high resistance in parallel with a photodetector to measure the scene 
brightness so that the exposure time control circuit may be changed over 
from a light measuring circuit to the high resistance to effect a 
predetermined maximum exposure time when the scene brightness is below a 
predetermined level. 
This type of the exposure time limiting circuit is disadvantageous in that 
the photodetector having high accuracy is not made use of effectively. In 
other words, in this circuit, the photodetector is not used at all when 
the camera is operated to take pictures with the maximum exposure time. 
SUMMARY OF THE INVENTION 
In view of the above mentioned disadvantages of the prior art, the primary 
object of the present invention is to provide an exposure time control 
circuit for a camera which makes use of the photodetector even when the 
scene brightness is low. 
Another object of the present invention is to provide an exposure time 
control circuit for a camera which controls the exposure time with high 
accuracy even when the exposure time controlled is almost equal to the 
maximum exposure time. 
The exposure time control circuit in accordance with the present invention 
is characterized in that a memory circuit in accordance with the output of 
which the exposure time is controlled is connected with a maximum exposure 
time output means and a light measuring circuit, and that the memory 
circuit selects one of the outputs from the former and the latter that is 
shorter than the other. When the output exposure time give by the maximum 
exposure time output means is shorter than the output exposure time given 
by the light measuring circuit, the maximum exposure time is selected to 
be memorized by the memory means and the exposure time is controlled in 
accordance therewith.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing which shows an exposure time control circuit for 
use in a single lens reflex camera, a photodiode PD is connected between 
the positive input terminal and the negative input terminal of a first 
operational amplifier OP1. The positive input terminal thereof is 
connected with a reference voltage source Vr1 indicative of the manually 
controlled diaphragm information and the film sensitivity. A log 
conversion diode D1 is connected between the negative input terminal and 
the output of the first operational amplifier OP1. Thus, a light measuring 
circuit is constituted. 
A third operational amplifier OP3 (A second operational amplifier OP2 will 
be described hereinafter.) has its negative input terminal connected with 
the output terminal thereof and a memorizing capacitor C1 is connected 
with the positive input terminal thereof. The memorizing capacitor C1 is 
grounded. Thus, a memory circuit is constituted. 
A fourth operational amplifier OP4 is connected at its negative input 
terminal with the output terminal of said third operational amplifier OP3 
by way of a time expansion diode D2, the anode of which is connected with 
the negative input and the cathode of which is connected with the output 
of the third operational amplifier OP3. An integrating capacitor C2 is 
connected between the negative input terminal and the output terminal 
thereof. A start switch SW2 which is opened in response to start of the 
leading shutter blind of the camera is connected in parallel with the 
integrating capacitor C2. Thus, a time expansion circuit is constituted. 
A fifth operational amplifier OP5 is connected at its positive input 
terminal with the output terminal of said fourth operational amplifier 
OP4. The negative input terminal thereof is connected with a reference 
voltage source Vr4 and the output terminal thereof is connected with a 
trailing shutter blind holding magnet Mg. The fifth operational amplifier 
OP5 serves as a comparator which compares the output from the fourth 
amplifier OP4 with the reference voltage Vr4 and operates to deenergize 
the magnet Mg when the level of the output from the fourth operational 
amplifier OP4 has reached the level of the reference voltage Vr4. 
The above described circuit elements OP1, OP3, OP4 and OP5 are well known 
in the art wherein the positive input terminal of the third operational 
amplifier OP3 is connected with the output terminal of the first 
operational amplifier OP1. Accordingly, the detailed description thereof 
is omitted here. 
The exposure time control circuit in accordance with the present invention 
is characterized in that a second operational amplifier OP2 and switching 
elements MOS1 and MOS2 are connected between the light measuring circuit 
including the first operational amplifier OP1 and the memory circuit 
including the third operational amplifier OP3. The second operational 
amplifier OP2 is connected at its negative input terminal with a reference 
voltage Vr2 determined in accordance with the maximum exposure time by way 
of a resistor R2 and at its positive input terminal with the output of 
said first operational amplifier OP1 by way of a resistor R1. Further, the 
negative input terminal of the second operational amplifier OP2 is 
connected with the positive input terminal of the third operational 
amplifier OP3 by way of a MOS-FET (Metal Oxide Semiconductor type Field 
Effect Transistor) MOS2 the gate of which is connected with the output 
terminal of the second operational amplifier OP2 and a mirror-up switch 
SW1 which is opened in response to the swing-up of a mirror in the single 
lens reflex camera. The output terminal of the second operational 
amplifier OP2 is further connected with the positive input terminal of the 
third operational amplifier OP3 by way of an inverter IN, another MOS-FET 
MOS1 the gate of which is connected with the inverter IN, and said 
mirror-up switch SW1. Further, the output of said first operational 
amplifier OP1 is connected with the positive input terminal of the third 
operational amplifier OP3 by way of the MOS-FET MOS1 and the switch SW1. 
Thus, a selecting circuit A and a switching circuit B are constituted. 
In operation of the above described exposure time control circuit, the 
scene brightness is measured by the photodetector PD. The first 
operational amplifier OP1 operates to give an output indicative of the 
desirable exposure time in accordance with the input from the 
photodetector PD and the reference voltage Vr1 determined by the diaphragm 
information and the film sensitivity. When the output exposure time from 
the first operational amplifier OP1 is shorter than (i.e. the level of the 
output is lower than) the maximum exposure time (i.e. the reference 
voltage Vr2), the output voltage of the second operational amplifier OP2 
becomes low and accordingly said MOS1 is turned on and said MOS2 is turned 
off so that the output of the first operational amplifier OP1 is memorized 
in said memorizing capacitor C1. To the contrary, when the output of the 
first operational amplifier OP1 is higher than the reference voltage Vr2, 
the output voltage of the second operational amplifier OP2 becomes high, 
and accordingly, the MOS1 is turned off and the MOS2 is turned on so that 
the reference voltage Vr2 is memorized in said memorizing capacitor C1. 
Then, upon depression of a shutter release button (not shown) which results 
in swing up of a mirror and start of the leading shutter blind, said 
mirror-up switch SW1 and said start switch SW2 are opened and said 
integrating capacitor C2 starts to be charged in accordance with the 
information memorized in said memorizing capacitor C1 which memorizes 
either the output of the first operational amplifier OP1 or the reference 
voltage Vr2. The output of the operational amplifier OP1 indicates the 
exposure time corresponding to the scene brightness and the reference 
voltage indicates the maximum exposure time. The output of the integrating 
capacitor C2 is compared with the reference voltage Vr4 at the fifth 
operational amplifier OP5 and the trailing shutter blind holding magnet Mg 
is deenergized when the time determined by the information charged in the 
integrating capacitor C2 has lapsed. Thus, the exposure time is controlled 
in accordance with the output of the light measuring circuit when the time 
controlled is not longer than the predetermined maximum exposure time.