U.S. patent application number 11/037161 was filed with the patent office on 2005-07-07 for method and device for lighting an electronic or electromechanical apparatus.
This patent application is currently assigned to Asulab S.A.. Invention is credited to Cristoni, Romano, Grupp, Joachim, Poli, Gian-Carlo.
Application Number | 20050145775 11/037161 |
Document ID | / |
Family ID | 27768323 |
Filed Date | 2005-07-07 |
United States Patent
Application |
20050145775 |
Kind Code |
A1 |
Cristoni, Romano ; et
al. |
July 7, 2005 |
Method and device for lighting an electronic or electromechanical
apparatus
Abstract
The present invention is a lighting device for an electronic or
electromechanical apparatus such as a timepiece of the wristwatch
type. The invention includes 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.
Inventors: |
Cristoni, Romano; (Sonceboz,
CH) ; Poli, Gian-Carlo; (Les Geneveys-sur-Coffrane,
CH) ; Grupp, Joachim; (Enges, CH) |
Correspondence
Address: |
GRIFFIN & SZIPL, PC
SUITE PH-1
2300 NINTH STREET, SOUTH
ARLINGTON
VA
22204
US
|
Assignee: |
Asulab S.A.
Marin
CH
|
Family ID: |
27768323 |
Appl. No.: |
11/037161 |
Filed: |
January 19, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11037161 |
Jan 19, 2005 |
|
|
|
10378903 |
Mar 5, 2003 |
|
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6894440 |
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Current U.S.
Class: |
250/205 ;
362/23.18; 368/227; 368/67 |
Current CPC
Class: |
H05B 45/12 20200101 |
Class at
Publication: |
250/205 ;
362/029; 368/067; 368/227 |
International
Class: |
G01J 001/32 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 5, 2002 |
CH |
2002 0383/02 |
Claims
1. A lighting device for an electronic or electromechanical, the
lighting device including: a display device for displaying
time-related data or data not related to time, wherein the lighting
device further includes a light source arranged to light the
display device and measure intensity of ambient light; and a
measurement stage that provides an electric signal representative
of ambient lighting conditions measured by the light source.
2. The lighting device according to claim 1, wherein the light
intensity provided by the light source is a function of the
intensity of the measured ambient light.
3. The lighting device according to claim 1, wherein the light
source is arranged to measure the degree of ambient luminosity
intensity during periods in which the light source is not used for
lighting.
4-17. (canceled)
18. The lighting device according to claim 1, wherein the light
source is a light emitting diode.
19. The lighting device according to claim 1, wherein the display
device comprises an element selected from the group consisting of a
liquid crystal cell, a dial, and hands.
20. The lighting device according to claim 1, wherein the lighting
device is used in combination with optical elements used to
distribute light produced by the light source over the lighting
device.
21. A lighting method for a device displaying time-related data, or
other data not time-related, for an electronic or electromechanical
apparatus, wherein the method, includes the steps of: providing an
electronic or electromechanical apparatus having a display device
for displaying time-related data, or data not time-related, wherein
the apparatus includes: a light source arranged to light the
displace device and to measure intensity of ambient light, and a
measurement stage that provides an electric signal representative
of ambient lighting conditions measured by the light source;
lighting the display device with the light source, and, measuring
the intensity of ambient light using the light source.
22. The method according to claim 21, wherein the display device is
illuminated more or less intensely by the light source as a
function of the intensity of ambient light measured by the light
source.
23. The method according to claim 21, wherein the display device is
brightly lit by the light source when the display device is in an
illuminated environment.
24. The method according to claim 21, wherein, when the apparatus
is passed from a brightly lit environment to an environment in
semi-darkness or darkness, the light source lights the display
device with the same intensity as when the apparatus is in a lit
environment so as to light the display device before reaching a
moment at which the human eye has become accustomed to the
semi-darkness or darkness.
25. The method according to claim 21, wherein, when the apparatus
is passed from a brightly lit placed to a place in semi-darkness or
darkness, the light source weakly lights the display device after
having reached the moment when the human eye has become accustomed
to the dark is.
26. (canceled)
27. The method according to claim 21, wherein the display device is
a liquid crystal cell, wherein illumination by the light source is
synchronized with the liquid crystal cell display frequency, so as
to light said cell at a moment when cell contrast is optimum.
28. The lighting device for an electronic or electromechanical
apparatus according to claim 1, wherein the electronic or
electromechanical apparatus is a wristwatch.
29. A lighting device for an electronic or electromechanical
apparatus, the lighting device including a display device for
displaying time related data or data not related to time, wherein
the lighting device further includes a light source means for
lighting the display device and measuring the intensity of ambient
light.
Description
[0001] The present invention concerns a device for lighting a
portable electronic device such as a wristwatch in order to
facilitate the user's reading of the data provided by the device.
The present invention also concerns a method for implementing such
a device.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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'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's vision has
adjusted to the night vision conditions, the lighting device will
shine weakly.
[0011] 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.
[0012] 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:
[0013] FIG. 1 is a circuit diagram of the lighting device according
to the invention, and
[0014] 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.
[0015] 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.
[0016] 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.
[0017] Illumination of a wristwatch can occur using various means,
amongst which the following can be cited:
[0018] electroluminescent sheets on which designs are printed or
which are used in combination with a partially transparent
dial;
[0019] a light guide, for example a ring-shaped light guide, as
disclosed in European Patent No. EP-A-0 860 755;
[0020] a planar light guide that is either arranged on the dial
(front lighting), or above it when the dial is partially
transparent (back-lighting);
[0021] the hands of the watch as is disclosed, for example, in U.S.
Pat. No. 4,995,022;
[0022] lighting sources as disclosed in U.S. Pat. No.
6,106,127.
[0023] 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.
[0024] Reference will be made first of all to FIG. 1. During the
standby phase, light emitting diode D3, 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
D3, connected to the gate of a transistor T1, 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 D3. The current produced by
this current source passes through a resistor R1 which is connected
to the drain of transistor T1 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 D3, and
on the other hand of the actual value of resistor R1. As will be
better understood in the following part of the description, the
choice of the value of resistor R1 will allow a voltage threshold
to be fixed below which diode D3, when energised, will produce
intense lighting, and beyond which diode D3 will produce limited
lighting.
[0025] As can be seen in the circuit diagram, switching means
comprising a transistor T2 are connected to the common point
between transistor T1 and resistor R1. During the standby phase,
transistor T2 is still conductive and thus allows the voltage
present across the terminals of said resistor R1 to be quickly
applied to a capacitor C3. This capacitor C3 is mounted in parallel
with resistor R1 via a non-return diode D1, which prevents said
capacitor C3 from discharging through resistor R1. Likewise,
capacitor C3 is associated with a resistor R9 with which it forms
an RC circuit whose time constant determines the speed at which
capacitor C3 can discharge through resistor R9. 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 C3 and resistor R9 is adjusted as a function of
the time necessary for the human eye to adjust to modifications in
the ambient lighting.
[0026] 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 C3 via anti-return
diode D1, the latter having a very low resistance. The second time
constant defined by the elements described hereinbefore corresponds
to the time necessary for capacitor C3 to be discharged into
resistor R9. This time is longer than the time necessary to charge
capacitor C3 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 D3 and transistor
T1 associated therewith will very quickly charge capacitor C3. If
then, the device according to the invention passes into an
environment where the ambient lighting is weaker, capacitor C3 will
gradually be discharged through resistor R9. 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 R1 which is, it should
be recalled, proportional to the current produced by diode D3, is
less than the potential of capacitor C3. Consequently, the current
generator formed by said diode D3 and transistor T1 cannot recharge
capacitor C3. This will only be possible again at the moment when
capacitor C3 is sufficiently discharged and its potential becomes
less than the potential present across the terminals resistor R1.
Thus, the fluctuations in voltage present across the terminals of
capacitor C3 are the faithful reflection of the variations in
intensity of the ambient lighting.
[0027] As can be seen in FIG. 1 annexed to the present patent
application, RC circuit formed by capacitor C3 and resistor R9 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 PB1 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 PB1 is pressed at moment t1,
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 PB1 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 PB1 was
activated.
[0028] Output Q of timing circuit 2 is also connected to the gate
of transistor T2. As was already mentioned previously, transistor
T2 is conductive during the entire duration of the standby phase of
the device according to the invention, and capacitor C3 is
connected to the current generator formed by LED D3 and transistor
T1 via anti-return diode D1 and said transistor T2. However, as
soon as diode D3 is no longer being used as the ambient light
intensity sensor, but as a lighting source, the memory stage
(capacitor C3, resistor R9) must immediately be uncoupled from the
measurement or integration stage (diode D3, transistor T1, resistor
R1) to avoid falsifying the charge state of said capacitor C3. This
is the role of the signal generated at the output Q of timing
circuit 2 which has just made transistor T2 non-conductive.
[0029] When push-button PB1 is pressed to command LED D3 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 T2 is made
non-conductive to isolate capacitor C3 from the current generator
formed by diode D3 and transistor T1 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
T3 to make the latter conductive and allow diode D3 to be supplied
with electric energy. However, transistor T3 which controls
lighting is made conductive only at moment t2 (see FIG. 2 curve
"LED"), i.e. with a small time lag after moment t1 when push-button
PB1 is activated. This lag is introduced by an RC circuit formed of
a capacitor C1 and a resistor R3 arranged between timing circuit 2
and transistor T3. This deferred switching on of diode D3 allows
one to ensure that the state of the electric charge accumulated in
capacitor C3 is not modified.
[0030] 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 C3 at moment t1 when push-button PB1 is pressed. Indeed,
as long as diode D3 operates as a sensor, the voltage across the
terminals of capacitor C3 fluctuates as a function of variations in
the ambient light intensity (see FIG. 2, curve "Vlight"). At
instant t1 when push-button PB is pressed, the state of charge of
capacitor C3 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
C3 is discharged slowly in comparison with the duration of the
timing signal which corresponds to the period of time during which
LED D3 remains switched on. At the end of the timing, capacitor C3
is again powered and quickly finds a charge level corresponding to
the ambient lighting conditions.
[0031] The high or low logic level of output 0 of flip flop 1 is
applied to the gate of a transistor T6. Afterwards, if output 0 of
flip flop 1 is in the "0" logic state, transistor T6 remains open
and diode D3 is powered with a minimum current through two
resistors R4 and R5 series connected between said diode D3 and said
transistor T6. However, if output Q of flip-flop 1 is in logic
state "1", transistor T6 closes and diode D3 is then powered with a
maximum current through the single resistor R5. Indeed, when
transistor T6 is conductive, it practically short-circuits resistor
R4 insofar as the value of its internal resistance is very low
compared to that of R4.
[0032] At the end of the timing, the logic state of output Q of the
timing circuit passes to "0". Immediately, transistor T3 opens,
causing diode D3 to be switched off. Likewise, transistor T2 closes
again, such that capacitor C3 is again connected to the current
source stage formed by diode D3 and transistor T1 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).
[0033] 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 D3 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.
[0034] In the device's standby state, transistor T3 is
non-conductive since LED D3 does not have to be electrically
powered. Conversely, transistor T2 is closed and thus conductive
such that the voltage present across the terminals of resistor R1
can be applied to the terminals of capacitor C3 and thus allow the
latter to be charged. It will be recalled that the voltage across
the terminals of resistor R1 result from the current, which passes
through the latter, and which is produced by LED D3 and transistor
T1 operating as an ambient luminosity controlled current generator.
It will easily be understood that the charge state of capacitor C3
is a function of the potential drop at the common point between the
drain of transistor T1 and resistor R1. Thus, the value of resistor
R1 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 PB1 is activated, this will determine the
intensity of the electric current powering diode D3 and thus the
low or high intensity of the lighting produced by said diode
D3.
[0035] Since the device according to the invention is in the light,
let us assume that push-button PB1 is activated. The state of
charge of capacitor C3 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 PB1, 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 T2 to be non-conductive so that the
state of charge of capacitor C3 is not falsified by LED D3 being
switched on. Likewise, timing circuit 2 makes transistor T3 to be
conductive so that diode D3 can be supplied with electric current.
Diode D3 is however powered only a brief moment after push-button
PB1 has been activated, this timing being generated by an RC
circuit formed of a capacitor C1 and a resistor R3 and ensuring,
here too, that the state of charge of capacitor C3 will not be
modified by diode D3 being switched on. Finally, the high level "1"
of output Q of flip flop 2 is applied to the gate of transistor T6
in order to make it conductive, such that the current which will
power diode D3 is limited only by resistor R5. The lighting of
diode D3 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 D3 will shine sufficiently brightly for the client to be able
to see the lighting of said watch. After a certain operating time
of diode D3, that is determined by timing circuit 2, logic output Q
of said timing circuit 2 passes to zero. Immediately, transistor T3
is made non-conductive, causing diode D3 to be switched off, and
transistor T2 is made conductive, such that capacitor C3 gradually
returns to a state of charge corresponding to the ambient
luminosity.
[0036] 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
R1 which is a function of the degree of intensity of the ambient
luminosity, will be less than the potential of capacitor C3.
Consequently, the current generator formed by LED D3 and transistor
T1 cannot recharge capacitor C3 and the latter will start to
discharge gradually through resistor R9. The speed at which
capacitor C3 is discharged is fixed by the time constant of the
circuit formed by said capacitor C3 and resistor R9. This is a
parameter that can be adjusted as a function of the values of C3
and R9.
[0037] According to the invention, the value of the time constant
of circuit C3, R9 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 PB1 before the state of charge of capacitor C3 has
reached the potential drop value at the common point between
transistor T1 and resistor R1, the state of logic input D of flip
flop 1 will be high and diode D3 will shine brightly. If,
conversely, the user activates push-button PB1 while capacitor C3
has been discharged through resistor R9 and the voltage across its
terminals corresponds to the voltage across the terminals of
resistor R1, in this case the state of logic input D of flip flop 1
will be low and LED D3 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 PB1 shortly afterwards, LED D3 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 D3 will be weak.
Indeed, the user'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.
[0038] Finally the case in which one passes quickly from the
semi-darkness to the light has to be examined. In this case, via
transistor T2, the capacitor almost instantaneously reaches a state
of charge corresponding to strong illumination, such that if the
user activates push-button PB1, logic input D of flip flop 1 is at
a high level "1" corresponding to the case in which LED D3 provides
intense lighting.
[0039] 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.
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