Abstract:
The present invention is a lighting device for an electronic or electromechanical apparatus such as a timepiece of the wristwatch type. The invention including a device displaying time-related or other information, this lighting device including a light source for lighting the display device. The lighting device is characterized in that the light source is also able to measure the intensity of the ambient light. The invention also concerns a method for controlling the lighting of an apparatus of the aforementioned type.

Description:
FIELD OF THE INVENTION 
   The present invention concerns a device for lighting a portable electronic device such as a wristwatch in order to facilitate the user&#39;s reading of the data provided by the device. The present invention also concerns a method for implementing such a device. 
   BACKGROUND OF THE INVENTION 
   Watches whose dial is lit using a light source to allow a user to read the time in the dark have been known for very many years. These watches differ from each other in the intensity of the lighting provided by the light source. For part of them, the light source brightly illuminates the watch dial. This can prove advantageous if a salesperson wishes to demonstrate the qualities of his product to a potential purchaser and show him the appearance the watch will have during nighttime use. The user will thus be able to see the watch display in its illuminated state, despite the lighting prevailing in the sales point. However, if the user of the watch wishes to check the time during the night, the great intensity of the lighting is likely to dazzle him. Moreover, this solution has the considerable inconvenience of consuming a large amount of energy, which constitutes a serious handicap in the case of portable electronic objects of small dimensions such as a watch whose energy storage capacities are necessarily limited. In order to overcome this drawback, it has been proposed to illuminate the watches less brightly but still sufficiently, of course, for the user of such a watch to be able to read the time-related or other information in the dark. This second solution has the main merit of being economical from the point of view of electric power consumption. However, it is practically impossible, unless one withdraws to a poorly lit place, to demonstrate the illuminating qualities of the watch at the sales point, since the illumination is too slight to be able to be seen in the light of day. 
   A new step was made in the state of the art by proposing, as is done, for example in U.S. Pat. No. 4,995,016 in the name of the Seikosha company, to provide the watch with a light sensor capable of detecting the various levels of intensity of the ambient lighting and adapting the illumination of the display device with which the watch is provided, as a function of the detected level of light. 
   A device of this type means that one no longer has to choose, during construction of the watch, between intense or low lighting, for example of the display device for the data provided by said watch. Thus, when the ambient lighting is weak (in the dark or semi-dark), lighting of the display device itself is weak, which, from the point of view of electric power consumption, is very favourable and enables the user of the watch, nonetheless to consult it at any time, particularly in the middle of the night. However, when the ambient lighting is intense, the light sensor deactivates the watch lighting means. However, when the ambient lighting is strong, the light sensor deactivates the watch lighting means. However, the watch is provided with a switch which, when it is activated, allows the watch to be brightly illuminated even in full daylight, for example in a watch boutique, in order to allow the salesperson to demonstrate the qualities of the product to a potential purchaser and show him the appearance the watch will have during night time use. The addition of an extra component in the form of a light sensor is not however without certain problems. This represents an extra cost both from the point of view of the number of components to be used and from the point of view of the assembly and manufacturing time, and introduces a new source of possible failure, which may, at more or less long term, be detrimental to the reliability of the electronic watch thereby equipped. Further, this system of detection is directive and its efficiency is a function of the location of the sensor. A shadow produced, for example, by a shirtsleeve, falsifies measurement of the degree of ambient lighting. In order to overcome these problems, those skilled in the art have no other choice than to increase the detection surface. However, this measure considerably harms the aesthetic appearance of the watch and increases its size. 
   It is an object of the present invention to overcome the drawbacks of the prior art in addition to others by providing a lighting device for a portable electronic object, which allows the illumination of data displayed by the electronic apparatus to be controlled reliably and inexpensively as a function of the intensity of the ambient lighting. 
   SUMMARY OF THE INVENTION 
   The present invention thus concerns a lighting device for a portable electronic or electromechanical apparatus such as a timepiece of the wristwatch type including a display device for time-related or other data, this lighting device including a light source for lighting the display device, said device being characterized in that the light source is also able to measure the intensity of the ambient lighting. 
   Owing to these features, the present invention provides a lighting device whose light source is capable both of illuminating the data display device of the apparatus to which it is fitted, and of detecting the degree of intensity of the ambient lighting. The present invention thus enables one to avoid using an independent light intensity sensor, which, as will easily be understood, is very advantageous insofar as it is thus possible to limit the number of components used, to simplify construction and thus limit costs. Further, the reliability of a lighting device according to the invention is improved with respect to those of similar known prior art devices. 
   The lighting device according to the invention can advantageously be used in combination with the optical elements used to illuminate the display device like those disclosed in European Patent Application No. EP-A-0 860 755. Indeed, the optical elements which are used to distribute the light produced by the light source over the surface, for example of a watch dial, can be used reversibly to collect the ambient light owing to the principle in accordance with which the optical paths travelled by the light are reversible. The use, in combination, of the lighting source and the elements originally used to diffuse the light produced by said lighting source to collect the ambient light provides the sensor with more reliable information as regards the degree of intensity of the ambient light than if the sensor alone was used. Indeed, this sensor includes a limited active surface and the detection signal that it provides can easily be disturbed by a passing shadow. 
   According to another feature of the invention, the light intensity provided by the light source is adapted to the measured ambient light intensity. Thus, if the lighting device is activated while the apparatus to which it is fitted, particularly a timepiece of the wristwatch type, is in full light, said lighting device will provide strong illumination. Consequently, a jewellery salesperson will be able to demonstrate the features of the watch to his client and show him the appearance the watch will have when the client uses his watch, for example at night. Conversely, if the lighting device is activated in a dark place, it will provide less strong illumination than in full daylight. The user will thus not be dazzled if he consults his watch during the night, and the electric power consumption will be limited, which increases the life time of the batteries powering the watch. 
   According to yet another feature of the invention, account is taken of the time necessary for the human eye to become accustomed to the dark. Thus, if a user passes quickly from an illuminated place to a dark place and he wishes to consult his watch immediately afterwards, the lighting device will consider that the user&#39;s vision has not yet adjusted to the new lighting conditions and will illuminate the watch display device brightly. However, if the user wishes to consult his watch after a longer period of time, when it will be deemed that said user&#39;s vision has adjusted to the night vision conditions, the lighting device will shine weakly. 
   The present invention also concerns a method for lighting a display device for time-related or other information for an electronic or electromechanical apparatus such as a timepiece of the wristwatch type, including a light source for lighting the display device, characterized in that the light source is also used to measure the intensity of the ambient light. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will appear more clearly from the following detailed description of an embodiment example of the lighting device according to the invention, this example being given purely by way of non-limiting illustration, in conjunction with the annexed drawings, in which: 
       FIG. 1  is a circuit diagram of the lighting device according to the invention, and 
       FIG. 2  is a histogram of showing the evolution of the voltage across the terminals of different elements of the lighting device according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention proceeds from the general inventive idea that consists in using the same lighting source, not only for lighting the display device of an electronic or electromechanical apparatus such as a wristwatch, but also as a detector of the ambient lighting intensity to adapt the intensity of the display device lighting to the environmental conditions. Owing to this feature, the number of components to be used is limited and the manufacture of such a display device is made simpler and thus more economical. Moreover, a display device of this type has improved reliability. 
   The present invention will be described with reference to an electronic apparatus of the wristwatch type. It goes without saying that the invention is not limited by the type of display device used. It may be a dial above which hands move or a liquid crystal cell. Likewise, the invention is not limited to the horological field and can be applied to any other type of portable apparatus such as a wireless or portable telephone or other. 
   Illumination of a wristwatch can occur using various means, amongst which the following can be cited: 
   electroluminescent sheets on which designs are printed or which are used in combination with a partially transparent dial; 
   a light guide, for example a ring-shaped light guide, as disclosed in European Patent No. EP-A-0 860 755; 
   a planar light guide that is either arranged on the dial (front lighting), or above it when the dial is partially transparent (back-lighting); 
   the hands of the watch as is disclosed, for example, in U.S. Pat. No. 4,995,022; 
   lighting sources as disclosed in U.S. Pat. No. 6,106,127. 
   The methods succinctly described above are more particularly suited for implementing the present invention. Of course, these methods remain valid if the watch dial is partially or entirely formed by a liquid crystal display cell. 
   Reference will be made first of all to FIG.  1 . During the standby phase, light emitting diode D 3 , does not play the role of lighting means, but, conversely, operates in a mode in which it detects the degree of ambient luminosity. Within the scope of the present invention, one could use, for example, the diodes marketed by the Agilent company under the references HSMB-190C and HSMC-S690 or the diode marketed by Stanley under the reference FR1111C. Diode D 3 , connected to the gate of a transistor T 1 , thus forms therewith a measuring stage that works like a current generator whose intensity will depend on the degree of ambient luminosity. One thus has a current source controlled by LED D 3 . The current produced by this current source passes through a resistor R 1  which is connected to the drain of transistor T 1  and which creates a voltage drop proportional to the current produced by said current generator. One thus has a voltage which is a function, on the one hand of the current produced by the current source controlled by diode D 3 , and on the other hand of the actual value of resistor R 1 . As will be better understood in the following part of the description, the choice of the value of resistor R 1  will allow a voltage threshold to be fixed below which diode D 3 , when energised, will produce intense lighting, and beyond which diode D 3  will produce limited lighting. 
   As can be seen in the circuit diagram, switching means comprising a transistor T 2  are connected to the common point between transistor T 1  and resistor R 1 . During the standby phase, transistor T 2  is still conductive and thus allows the voltage present across the terminals of said resistor R 1  to be quickly applied to a capacitor C 3 . This capacitor C 3  is mounted in parallel with resistor R 1  via a non-return diode D 1 , which prevents said capacitor C 3  from discharging through resistor R 1 . Likewise, capacitor C 3  is associated with a resistor R 9  with which it forms an RC circuit whose time constant determines the speed at which capacitor C 3  can discharge through resistor R 9 . This RC circuit thus forms a memory stage, which will store a state corresponding to a weak or strong ambient luminosity level as a function of the electric signal produced by the measuring stage. It will be seen hereinafter that the value of the time constant of the circuit formed by capacitor C 3  and resistor R 9  is adjusted as a function of the time necessary for the human eye to adjust to modifications in the ambient lighting. 
   The elements described up to now thus define two time constants. The first of these constants corresponds to the very short time that is necessary to charge capacitor C 3  via anti-return diode D 1 , the latter having a very low resistance. The second time constant defined by the elements described hereinbefore corresponds to the time necessary for capacitor C 3  to be discharged into resistor R 9 . This time is longer than the time necessary to charge capacitor C 3  and is adjusted as a function of the human vision parameters as already mentioned. Consequently, when the device according to the invention is in intense ambient lighting conditions, the current generator formed by LED D 3  and transistor T 1  associated therewith will very quickly charge capacitor C 3 . If then, the device according to the invention passes into an environment where the ambient lighting is weaker, capacitor C 3  will gradually be discharged through resistor R 9 . It should be understood, in fact, that if the device according to the invention passes from a place that is brightly illuminated to a place that is less so, at the moment of this transition, there is a situation in which the potential drop created by resistor R 1  which is, it should be recalled, proportional to the current produced by diode D 3 , is less than the potential of capacitor C 3 . Consequently, the current generator formed by said diode D 3  and transistor T 1  cannot recharge capacitor C 3 . This will only be possible again at the moment when capacitor C 3  is sufficiently discharged and its potential becomes less than the potential present across the terminals resistor R 1 . Thus, the fluctuations in voltage present across the terminals of capacitor C 3  are the faithful reflection of the variations in intensity of the ambient lighting. 
   As can be seen in  FIG. 1  annexed to the present Patent Application, RC circuit formed by capacitor C 3  and resistor R 9  is connected to the logic input D of a flip-flop  1 . This flip flop  1  constitutes a stage which, when a signal commanding the light source to be switched on is produced, adapts the intensity of light provided by said light source as a function of the electric state stored in the memory stage. More precisely, flip flop  1  will consider that its input D is at a high logic level “1” or low logic level “0”, depending on whether the voltage applied at this input is greater than a first given value, for example 1.7 volts, or less than a second given value, for example 1.2 volts. In a conventional manner, flip flop  1  has the function of applying, without modification, the logic state in which it finds its input D to its output Q via the effect of external solicitation? In the case of the present invention, this external solicitation takes the form of an application of pressure on a push-button PB 1  which, at moment t1 (see  FIG. 2 , “start” curve) sets output Q of a timing circuit  2  at a high level “1”. As can be seen in the circuit diagram, output Q of timing circuit  2  is directly connected to clock input CLK of flip-flop  1 . Thus, when push-button PB 1  is pressed at moment t 1 , timing circuit  2  is switched on, which has the effect of transferring the logic state of input D of flip flop  1  towards output Q thereof, and holding said output Q in this state for a certain period of time after push-button PB 1  has been activated. This period of time is imposed by timing circuit  2  and corresponds to the time interval t1-t3 on the “tempo” curve of FIG.  2 . The state of output Q of flip-flop  1  is thus the image of the ambient lighting conditions at the moment when push-button PB 1  was activated. 
   Output Q of timing circuit  2  is also connected to the gate of transistor T 2 . As was already mentioned previously, transistor T 2  is conductive during the entire duration of the standby phase of the device according to the invention, and capacitor C 3  is connected to the current generator formed by LED D 3  and transistor T 1  via anti-return diode D 1  and said transistor T 2 . However, as soon as diode D 3  is no longer being used as the ambient light intensity sensor, but as a lighting source, the memory stage (capacitor C 3 , resistor R 9 ) must immediately be uncoupled from the measurement or integration stage (diode D 3 , transistor T 1 , resistor R 1 ) to avoid falsifying the charge state of said capacitor C 3 . This is the role of the signal generated at the output Q of timing circuit  2  which has just made transistor T 2  non-conductive. 
   When push-button PB 1  is pressed to command LED D 3  to be switched on, this has the effect of setting logic output Q of timing circuit  2  to “1”. Then, the logic state of input D of flip flop  1  is transferred to its output Q and transistor T 2  is made non-conductive to isolate capacitor C 3  from the current generator formed by diode D 3  and transistor T 1  and controlled by the degree of ambient luminosity. At the same time, the high level of logic output Q of timing circuit  2  is applied to the gate of a transistor T 3  to make the latter conductive and allow diode D 3  to be supplied with electric energy. However, transistor T 3  which controls lighting is made conductive only at moment t 2  (see  FIG. 2  curve “LED”), i.e. with a small time lag after moment t 1  when push-button PB 1  is activated. This lag is introduced by an RC circuit formed of a capacitor C 1  and a resistor R 3  arranged between timing circuit  2  and transistor T 3 . This deferred switching on of diode D 3  allows one to ensure that the state of the electric charge accumulated in capacitor C 3  is not modified. 
   The high or low logic level of output Q of flip-flop  1  is shown in the “lighting” curve of FIG.  2 . Two bold horizontal lines indicate the logic state “0” or “1” of output Q of flip-flop  2 . The logic state of output Q is a function of the charge state of capacitor C 3  at moment t 1  when push-button PB 1  is pressed. Indeed, as long as diode D 3  operates as a sensor, the voltage across the terminals of capacitor C 3  fluctuates as a function of variations in the ambient light intensity (see  FIG. 2 , curve “Vlight”). At instant t 1  when push-button PB is pressed, the state of charge of capacitor C 3  is fixed and remains substantially the same for the entire duration of the timing, although the lighting conditions may continue to fluctuate as is indicated in dotted lines on curve “Vlight” of FIG.  2 . Indeed, because of its time constant, capacitor C 3  is discharged slowly in comparison with the duration of the timing signal which corresponds to the period of time during which LED D 3  remains switched on. At the end of the timing, capacitor C 3  is again powered and quickly finds a charge level corresponding to the ambient lighting conditions. 
   The high or low logic level of output Q of flip flop  1  is applied to the gate of a transistor T 6 . Afterwards, if output Q of flip flop  1  is in the “0” logic state, transistor T 6  remains open and diode D 3  is powered with a minimum current through two resistors R 4  and R 5  series connected between said diode D 3  and said transistor T 6 . However, if output Q of flip-flop  1  is in logic state “1”, transistor T 6  closes and diode D 3  is then powered with a maximum current through the single resistor R 5 . Indeed, when transistor T 6  is conductive, it practically short-circuits resistor R 4  insofar as the value of its internal resistance is very low compared to that of R 4 . 
   At the end of the timing, the logic state of output Q of the timing circuit passes to “0”. Immediately, transistor T 3  opens, causing diode D 3  to be switched off. Likewise, transistor T 2  closes again, such that capacitor C 3  is again connected to the current source stage formed by diode D 3  and transistor T 1  and its charge state gradually finds a level corresponding to the ambient light intensity. Finally, a second timer  4  resets logic output Q of flip-flop  1  to zero (see  FIG. 2 , “reset” curve). 
   An operating cycle of the device according to the invention will now be examined. It is assumed, to start, that the device is in the standby state, i.e. in a state where LED D 3  is not illuminated but is only used to detect the degree of ambient light intensity. It is further assumed that at the beginning of this operating cycle, the device is in the light. 
   In the device&#39;s standby state, transistor T 3  is non-conductive since LED D 3  does not have to be electrically powered. Conversely, transistor T 2  is closed and thus conductive such that the voltage present across the terminals of resistor R 1  can be applied to the terminals of capacitor C 3  and thus allow the latter to be charged. It will be recalled that the voltage across the terminals of resistor R 1  result from the current, which passes through the latter, and which is produced by LED D 3  and transistor T 1  operating as an ambient luminosity controlled current generator. It will easily be understood that the charge state of capacitor C 3  is a function of the potential drop at the common point between the drain of transistor T 1  and resistor R 1 . Thus, the value of resistor R 1  will determine the value of the voltage applied to logic input D of flip flop  1  and allow the latter to decide whether its logic input D is at high level “1” or low level “0”. Depending upon whether logic input D of flip flop  1  is at “0” or “1” at the moment when push-button PB 1  is activated, this will determine the intensity of the electric current powering diode D 3  and thus the low or high intensity of the lighting produced by said diode D 3 . 
   Since the device according to the invention is in the light, let us assume that push-button PB 1  is activated. The state of charge of capacitor C 3  is at a high level, such that the logic state of input D of flip flop  1  is at its high level “1”. Via the effect of activation of push-button PB 1 , output Q of timing circuit  2  passes to “1” and orders the transfer of logic state “1” of input D of flip-flop  1  to output Q of the latter. Simultaneously, timing circuit  2  makes transistor T 2  to be non-conductive so that the state of charge of capacitor C 3  is not falsified by LED D 3  being switched on. Likewise, timing circuit  2  makes transistor T 3  to be conductive so that diode D 3  can be supplied with electric current. Diode D 3  is however powered only a brief moment after push-button PB 1  has been activated, this timing being generated by an RC circuit formed of a capacitor C 1  and a resistor R 3  and ensuring, here too, that the state of charge of capacitor C 3  will not be modified by diode D 3  being switched on. Finally, the high level “1” of output Q of flip flop  2  is applied to the gate of transistor T 6  in order to make it conductive, such that the current which will power diode D 3  is limited only by resistor R 5 . The lighting of diode D 3  will therefore be maximal. This functions is especially useful when a salesperson in a jewellery shop wishes to show a client the appearance that the watch has when lit in the semi-darkness. Indeed, despite the brightness of the sales point, diode D 3  will shine sufficiently brightly for the client to be able to see the lighting of said watch. After a certain operating time of diode D 3 , that is determined by timing circuit  2 , logic output Q of said timing circuit  2  passes to zero. Immediately, transistor T 3  is made non-conductive, causing diode D 3  to be switched off, and transistor T 2  is made conductive, such that capacitor C 3  gradually returns to a state of charge corresponding to the ambient luminosity. 
   Let us assume now that the user passes suddenly from a light environment to a semi-dark environment and that he wishes to consult his watch. On passing from a brightly lit place to a place that is less well lit, the device according to the invention will be in a situation in which the potential drop created by resistor R 1  which is a function of the degree of intensity of the ambient luminosity, will be less than the potential of capacitor C 3 . Consequently, the current generator formed by LED D 3  and transistor T 1  cannot recharge capacitor C 3  and the latter will start to discharge gradually through resistor R 9 . The speed at which capacitor C 3  is discharged is fixed by the time constant of the circuit formed by said capacitor C 3  and resistor R 9 . This is a parameter that can be adjusted as a function of the values of C 3  and R 9 . 
   According to the invention, the value of the time constant of circuit C 3 , R 9  will be of the order of several minutes. It is in fact a period of time that corresponds to the mean time necessary for the human eye to become accustomed to the dark when the person comes from a brightly lit environment. Thus, if the user activates push-button PB 1  before the state of charge of capacitor C 3  has reached the potential drop value at the common point between transistor T 1  and resistor R 1 , the state of logic input D of flip flop  1  will be high and diode D 3  will shine brightly. If, conversely, the user activates push-button PB 1  while capacitor C 3  has been discharged through resistor R 9  and the voltage across its terminals corresponds to the voltage across the terminals of resistor R 1 , in this case the state of logic input D of flip flop  1  will be low and LED D 3  will shine weakly. This feature of the invention advantageously enables the user to read the indications provided by his watch in all circumstances. Thus, if the user passes abruptly from the light to the semi-darkness, and he activates push-button PB 1  shortly afterwards, LED D 3  will shine brightly to allow him to read the data displayed by his watch since his vision will not yet be fully used to the dark. Conversely, if a longer period of time elapses between the moment when the user enters the darkness and the moment when he wishes to consult his watch, the lighting intensity provided by diode D 3  will be weak. Indeed, the user&#39;s eyes will have had time to become accustomed to night vision and it will thus no longer be necessary to light the watch brightly. This has a double advantage: on the one hand the user is not dazzled when he consults his watch, for example at night, and on the other hand substantial energy savings are made. 
   Finally the case in which one passes quickly from the semi-darkness to the light has to be examined. In this case, via transistor T 2 , the capacitor almost instantaneously reaches a state of charge corresponding to strong illumination, such that if the user activates push-button PB 1 , logic input D of flip flop  1  is at a high level “1” corresponding to the case in which LED D 3  provides intense lighting. 
   It goes without saying that the present invention is not limited to the embodiments that have just been described, and that various simple modifications and variants can be envisaged without departing from the scope of the present invention. In particular, one could envisage that the resistor values could be programmed by the user in order to adjust the capacitor charge and discharge time and thus the lighting adjustment times to day and nighttime vision conditions. Another advantageous embodiment consists in using an electronic circuit which illuminates a liquid crystal cell in a pulsed manner in order to improve the display legibility and contrast. Indeed, if one measures the display contrast of a liquid crystal cell integrated in a watch, a beat of this contrast synchronised with the electrode addressing signal of said cell will be noted? Thus, if the pulsed illumination is synchronised with the liquid crystal cell in an optimum manner, the observer will only see the maximum display contrast.