JPS6283733A - Low luminance informing circuit - Google Patents

Abstract

PURPOSE:To obtain the correct signal corresponding to the limiter function by deriving the output signal of the saturated detecting circuit and making it into the deciding signal concerning whether or not the light electric current is at the limiter operating area. CONSTITUTION:A light electric current Iin of a phototransistor Q1 after the direction of the electric current is changed through current mirror circuits (Q8-Q10) for the electric current to deduct an electric current Ic (Q2) of a transistor Q2, is added to an output electric current Ic (Q13) of the second electric current source by a current mirror and comes to be the integrating electric current to a capacitor C. Since the transistor Q2 is saturated, the base electric current is increased. For example, when the collector electric current of a transistor Q3 is selected to the value smaller than a transistor Q5, the transistor Q2 is saturated, and then, a transistor Q6 held ever to off is turned on, a transistor Q17 is turned on and a signal X is formed which informs that the light electric current of the phototransistor Q1 enters the limiter operating area.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a low voltage sensor that forms a signal that determines whether a photocurrent generated in a photoelectric conversion element in an electronic shutter circuit is at a level lower than a preset value. This relates to a brightness notification circuit.

Background technology: Recent cameras equipped with lens shutters driven by electronic control do not provide detailed displays for exposure settings, and focus only on whether the current exposure status is within the camera shake limit. A mechanism is adopted in which the flash is displayed, and if the camera shake limit has been reached, the camera can manually switch to flash or automatically switch to flash. Therefore, even if detailed exposure information (information related to shutter speed and aperture) for shutter control is not displayed, a signal for not automatically switching to flash or a notification signal for manual switching is required.

On the other hand, as an electronic shutter circuit equipped with a limiter mechanism for photographing such a low-luminance object, a circuit as described in Japanese Utility Model Application Publication No. 57-90427 has been developed.

<Problems to be Solved by the Invention> In the conventional electronic shutter control circuit described above, when the photocurrent does not reach a preset current level due to low brightness, the integrated current of the capacitor is set to a constant current. At low brightness below the level, even if the exposure amount during actual imaging is insufficient, the shutter is closed and the imaging is completed, and imaging is performed using the b limiter function.

However, the conventional electronic shutter control circuit described above charges a capacitor with a constant current during low brightness to generate a magnet drive signal for shutter control. Therefore, there is no display especially when the flash is switched manually or automatically, which is very inconvenient. Furthermore, even if a display were to be displayed, a circuit separate from the signal for the limiter function was added to display the signal.

<Means for solving the problem> The present invention applies a first constant current to the outflow point of a photocurrent source generated in a photoelectric conversion element, and calculates the current difference between the two currents generated at the added point. In addition to the current mirror, a second constant current is added to the inflow current that is the output of the current mirror to form an integral current, and this integrated current is supplied to a capacitor to provide limiter characteristics. This is a low illuminance notification circuit that detects the saturation of the generated constant current source and forms a signal that determines whether the photocurrent generated in the photoelectric conversion element corresponding to the illuminance of the subject is within the IJ mitter operation region.

<Embodiment> In the figure, a phototransistor Q1 is provided as an element that generates a photocurrent corresponding to the illuminance of an object. At the current outflow point of the phototransistor Qt is an NPN transistor Q2. Q4. A first constant current source consisting of a current mirror Q7 is connected, and an NPN transistor Q7 is further connected to the connection point.
A first current mirror circuit consisting of s + Qs + QCs is connected. As will be described later, a current equal to the difference between the photocurrent and the first constant current generated by the fourth constant current source is obtained as the collector current of the transistor Q1o of the first current mirror circuit.

The transistor Q! A second constant current source formed by a current mirror consisting of NPN transistors Qtt + Q12 + Qt3 is connected to the collector of the transistor o. The collector of the transistor Q7 included in the first constant current source is PNP), and the collector of the transistor Q+3 included in the second constant current source is PNP.
The collector of transistor Q14 is connected to the transistor Q5. The transistor Q+4 constitutes a current mirror circuit together with the PNP transistors Qts and QCs, and a constant power source 〇 is connected to the collector of the transistor Qrs and the base of the transistor Qrs.

The integrating capacitor C is inserted between the power supply vcc and a node A to which the transistor Qzo, which obtains the output current of the current mirror circuit, and the transistor Qu, which supplies the second current, are connected. The node A is applied to a comparator C8m as a terminal for applying the potential of the capacitor C, and the level is compared with the comparison voltage Vref inputted to the other side, and the output signal is applied to the transistor Q+s to drive the shutter driving magnet M. control the behavior of

In addition to the circuit for forming the signal for controlling the magnet M6, a circuit for deriving the limiter operation notification signal is provided.

That is, a PNP transistor Q3 whose bases are connected to the transistor Q5 included in the first constant current source and whose collector is connected to the collector of the transistor Q4, and a PNP whose base is connected to the connection point between the transistor Q3 and the transistor Q4. ) transistor Q6, and the transistor Q
A PNP) transistor Ql? whose base is connected to the collector of 6 through a dividing resistor R1. The limiter operation notification signal X is transmitted from the collector of the transistor Q+7.
is output.

In the circuit having the above configuration, the phototransistor Q1
The photocurrent Iin is the current ■ of the transistor Q2. (Q2
) is the current mirror circuit (Q8. Q9
, Qro), the output current of the second current source by the current mirror ■. (Q13) and becomes the integrated current to capacitor C.

That is, when I in > I c (Q2), (I in
-I c (Qz)) is connected to the current mirror circuit (Qs
, Q9, QIO). Therefore, although the direction is reversed, the output current 'c (Q
lo) is I c(Qro) ”I 1n-Ic((h
), the integrated current of the capacitor is I i
n −I c (Qz) + I c (Qll).

The first and second constant current sources are a common constant current source (Q+4
．． Since it is a current mirror circuit connected to QCs, Qta), the transistor IC (Qz)"■.

(Qll), and the above integrated current can be rewritten as follows, and the amount of incidence on the phototransistor is large, I
If in > Ic((h), the capacitor is charged with a current equal to the photocurrent of the phototransistor Q,.

l1n-Ic(Q2) +Ic(Qll)=Iin On the other hand, when fin≦'c(Qz), transistor RO2 is saturated and the collector current of transistor Qto becomes zero.

Therefore, the integrated current is the collector current I of transistor Q+t
C (Qll), and in the case of low illuminance, the current is constant regardless of the photocurrent. Capacitor C is charged at (Qll),
A limiter is applied.

At this time, since the transistor Q2 is saturated, the base current increases. For example, set the collector current of transistor Q3 to a value smaller than that of transistor Q5 (for example, 1/□0
), when transistor Q2 is saturated, transistor Q6, which had been kept off until then, is turned on. This turns on the transistor Q17 and forms a signal X that indicates that the photocurrent of the phototransistor Q1 is within the limiter operating range.

A switch SW inserted in parallel with the capacitor C opens its contacts in conjunction with the shutter opening operation to enable charging of the capacitor.

In the above embodiment, the collector of the transistor Q4 detects saturation to notify the operation of the 1,1 mitter.
The transistor Q2 is not limited to the above embodiment.
Detect the collector voltage level of, for example, transistor Q2
It is possible to compare and detect the potentials using a comparator to which a reference potential is given in advance.

In addition, when the circuit is constructed using transistors with opposite polarities, the transistor Q2 is replaced with a collector 1 and a collector 2 as described in the specification of Japanese Patent Application No. 60-8193.
By connecting a PNP transistor with a heavy collector structure, a similar limiter operation notification signal can be immediately derived from the second collector.

<Effects> According to the present invention, it is possible to form a signal for notifying limiter operation with a simple configuration by using a circuit for obtaining a limiter function in an electronic shutter circuit, and there is no need to add a separate circuit. Moreover, an accurate notification signal corresponding to the limiter function can be obtained, and the high-performance electronic shutter circuit can be made easier to operate.

[Brief explanation of drawings]

Claims (1)

[Claims] 1) Adding a first constant current to the photocurrent generated in the photoelectric conversion element and inputting the difference between the two currents to a current mirror circuit;
In a shutter control circuit that adds a second constant current to the output current of the current mirror circuit to form an integral current for shutter control, a circuit that detects saturation of a constant current source that forms the first constant current. A low brightness notification circuit, comprising: deriving the output signal of the saturation detection circuit and using it as a determination signal for determining whether or not the photocurrent is in a limiter activation region. 2) The circuit for detecting saturation includes an NPN transistor into which the output current of the first constant current source is input as a base, and detects an increase in the base current of the saturated transistor to form a determination signal. Characteristic claim 1
The low brightness alarm circuit described in Section 1.