Abstract:
An optoelectronic circuit for receiving a variable voltage containing alternating increasing and decreasing phases, the optoelectronic circuit including a plurality of groups of light-emitting diodes and a switching device for allowing or interrupting the circulation of a current through each group, the switching device also being suitable for detecting whether said variable voltage is supplied by a dimmer.

Description:
[0001]    The present patent application claims the priority benefit of French patent application FR14/63416 which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    The present description relates to an optoelectronic circuit, particularly to an optoelectronic circuit comprising light-emitting diodes. 
       DISCUSSION OF THE RELATED ART 
       [0003]    An optoelectronic circuit, especially used to form lighting, may be connected to a source of an AC voltage, for example, the sinusoidal voltage of the mains. To modify the luminous power supplied by the lighting circuit, it is known to place a dimmer between the source of the sinusoidal voltage and the optoelectronic circuit. There exist several types of dimmers, particularly leading edge dimmers and trailing edge dimmers. 
         [0004]    It may be desirable to use a lighting circuit comprising light-emitting diodes. A disadvantage is that dimmers have generally been designed to operate with incandescent lamp lighting circuits and may not operate properly when they are connected to an optoelectronic circuit comprising light-emitting diodes. 
       SUMMARY 
       [0005]    An object of an embodiment is to overcome all or part of the disadvantages of the previously-described optoelectronic circuits comprising light-emitting diodes powered with an AC voltage. 
         [0006]    Another object of an embodiment is to allow a proper operation of a dimmer placed between the AC voltage source and the optoelectronic circuit. 
         [0007]    Thus, an embodiment provides an optoelectronic circuit intended to receive a variable voltage containing an alternation of rising and falling phases, the optoelectronic circuit comprising a plurality of light-emitting diode assemblies and a switching device capable of allowing or of interrupting the flowing of a current in each assembly, the switching device being further capable of detecting whether said variable voltage is supplied by a dimmer. 
         [0008]    According to an embodiment, the switching device is capable of connecting the assemblies of light-emitting diodes according to a plurality of connection configurations successively according to a first order during each rising phase of the variable voltage in the absence of a dimmer and a second order during each falling phase of the variable voltage in the absence of a dimmer, the switching device being further capable of detecting the presence of the dimmer when the duration of at least one connection configuration is shorter than a duration threshold and/or when at least two connection configurations follow each other according to a third order different from the first order and from the second order. 
         [0009]    According to an embodiment, the duration threshold depends on said connection configuration. 
         [0010]    According to an embodiment, the switching device comprises at least one switch for each assembly of light-emitting diodes, the switching device being capable of transmitting binary control signals for the turning off or the turning on of the switches according to said connection configurations, the switching device being, further, capable of determining whether the duration between the successive switching times of one of at least two control signals of two successive connection configurations is shorter than said duration threshold. 
         [0011]    According to an embodiment, the switching device comprises, for each assembly, a comparison unit capable of comparing the voltage at one of the terminals of the assembly, and/or a voltage depending on said voltage at one of the terminals of the assembly, with at least one first voltage threshold and possibly with a second voltage threshold and a control unit connected to the comparison units and capable, during each rising phase, of interrupting the flowing of a current in each assembly from among certain assemblies of the plurality of assemblies when said voltage of said assembly rises above the second voltage threshold or when said voltage of the assembly, adjacent to said assembly and conducting the current, rises above the first voltage threshold and, during each falling phase, of controlling the flowing of a current in each assembly from among certain assemblies of the plurality of assemblies when said voltage of the assembly, adjacent to said assembly and conducting the current, decreases below the first voltage threshold. 
         [0012]    According to an embodiment, the switching device is capable of detecting the presence of the dimmer when, for at least two assemblies, the voltages associated with the two assemblies rise above the first voltage threshold or the second voltage threshold or fall below the first voltage threshold within a duration shorter than said duration threshold. 
         [0013]    According to an embodiment, the optoelectronic circuit comprises a current source and, for each assembly, a switch connecting the current source to said terminal of said assembly, the control unit being capable, for each assembly from among certain assemblies of the plurality of assemblies, of controlling the turning on of the switch associated with said assembly when said voltage of the assembly, adjacent to said assembly and conducting the current, falls below the first voltage threshold in each falling phase. 
         [0014]    According to an embodiment, the switching device is further capable of detecting whether the variable voltage is supplied by a leading edge dimmer or a trailing edge dimmer. 
         [0015]    According to an embodiment, the switching device is further capable of determining that the variable voltage is supplied by a leading edge dimmer when the duration of at least one connection configuration is shorter than a duration threshold during at least a rising phase of the variable voltage and/or when at least two connection configurations follow each other according to a fourth order different from the first order during at least a rising phase of the variable voltage and the switching device is, further, capable of determining that the variable voltage is supplied by a trailing edge dimmer when the duration of at least one connection configuration is shorter than a duration threshold during at least one falling phase of the variable voltage and/or when at least two configuration connections follow each other according to a fifth order different from the second order during at least one falling phase of the variable voltage. 
         [0016]    According to an embodiment, the switching device is capable of at least temporarily decreasing the input impedance of the optoelectronic circuit when a dimmer is detected. 
         [0017]    According to an embodiment, the switching device is capable of having a constant current flow through the optoelectronic circuit when a dimmer is detected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of dedicated embodiments in connection with the accompanying drawings, among which: 
           [0019]      FIG. 1  is an electric diagram of an example of an optoelectronic circuit connected to a source of a sinusoidal voltage by a dimmer; 
           [0020]      FIGS. 2 and 3  are timing diagrams of the voltage supplied by the dimmer of  FIG. 1  respectively in the case of a leading edge dimmer and of a trailing edge dimmer; 
           [0021]      FIG. 4  is an electric diagram of an example of an optoelectronic circuit comprising light-emitting diodes capable of being connected to a source of a sinusoidal voltage; 
           [0022]      FIG. 5  is a timing diagram of the power supply current and voltage of the light-emitting diodes of the optoelectronic circuit of  FIG. 4 ; 
           [0023]      FIG. 6  is an electric diagram of an example of an optoelectronic circuit comprising light-emitting diodes comprising a light-emitting diode switching device; 
           [0024]      FIG. 7  is a timing diagram of signals of the optoelectronic circuit of  FIG. 6 ; 
           [0025]      FIGS. 8 and 9  are timing diagrams of signals of the optoelectronic circuit of  FIG. 6  when it is connected to a leading edge and to a trailing edge dimmer; 
           [0026]      FIG. 10  shows, in the form of a block diagram, an embodiment of a method of detecting the presence or the absence of a dimmer; 
           [0027]      FIG. 11  partially and schematically shows an embodiment of a unit for detecting the presence or the absence of a dimmer; 
           [0028]      FIG. 12  is an electric diagram of another example of an optoelectronic circuit comprising light-emitting diodes comprising a light-emitting diode switching device; 
           [0029]      FIG. 13  schematically shows an embodiment of a control unit of a light-emitting diode switching device; 
           [0030]      FIG. 14  shows, in the form of a block diagram, an embodiment of a method of controlling a light-emitting diode switching device; 
           [0031]      FIG. 15  is an electric diagram of an embodiment of an optoelectronic circuit comprising light-emitting diodes, comprising a dimmer detection device; 
           [0032]      FIG. 16  shows a more detailed embodiment of an optoelectronic circuit comprising light-emitting diodes, comprising a dimmer detection device; 
           [0033]      FIGS. 17 and 18  are more detailed electric diagrams of embodiments of portions of the optoelectronic circuit of  FIG. 16 ; 
           [0034]      FIG. 19  is a timing diagram of voltages of the optoelectronic circuit of  FIG. 16 ; 
           [0035]      FIG. 20  shows an electric diagram of another embodiment of an optoelectronic circuit comprising light-emitting diodes comprising a dimmer detection device; and 
           [0036]      FIGS. 21 and 22  are drawings respectively similar to  FIGS. 17 and 18  and show electric diagrams of more detailed embodiments of portions of the optoelectronic circuit of  FIG. 20 . 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. In the following description, unless otherwise indicated, terms “substantially”, “approximately”, and “in the order of” mean to within 10%, preferably to within 5%. 
         [0038]      FIG. 1  very schematically shows an electronic system  1  comprising a source  2  of an AC voltage V SOURCE,  for example, a sinusoidal voltage, a dimmer  5  receiving AC voltage V SOURCE  and supplying a modified AC voltage V IN , and an optoelectronic circuit  10  comprising input terminals IN 1  and IN 2  having AC voltage V IN  applied therebetween. As an example, input voltage V SOURCE  may be a sinusoidal voltage having a frequency, for example, in the range from 10 Hz to 1 MHz. Voltage V SOURCE  for example corresponds to the mains voltage. 
         [0039]    Optoelectronic circuit  10  is capable of supplying a light signal having its luminous power depending, in particular, on voltage V IN . Dimmer  5  may be a phase cut dimmer comprising an electronic switch having a conduction time limited to a fraction of period T of voltage V SOURCE . 
         [0040]      FIG. 2  shows an example of a curve of the variation of voltage V IN  when source voltage V SOURCE  is sinusoidal with a period T and when dimmer  5  is a leading edge dimmer. Voltage V IN  follows signal V SOURCE  except for a time period T′ at the beginning of each positive and negative sine wave arc during which voltage V IN  is substantially zero. Leading edge dimmers may be formed with triacs. 
         [0041]      FIG. 3  shows an example of a curve of the variation of voltage V IN  when source voltage V SOURCE  is sinusoidal with a period T and when dimmer  5  is a trailing edge dimmer. Voltage V IN  follows signal V SOURCE  except for a time period T″ at the end of each positive and negative sine wave arc during which voltage V IN  is substantially zero. Trailing edge dimmers may be formed with MOS transistors. 
         [0042]    Ratio a of time period T′ or T″ to half-period T/2 of sinusoidal signal V SOURCE  is called firing angle of dimmer  5 . A leading edge or trailing edge dimmer may comprise a variable resistance, which enables to modify firing angle α. 
         [0043]    It may be desirable to use light-emitting diodes to form optoelectronic circuit  10 . 
         [0044]      FIG. 4  shows an example of an optoelectronic circuit  10  comprising light-emitting diodes. Optoelectronic circuit  10  comprises a rectifying circuit  12  comprising a diode bridge  14 , receiving voltage V IN  and supplying a rectified voltage V ALIM  which powers light-emitting diodes  16 , for example, series-assembled with a resistor  15 . Call I ALIM  the current flowing through light-emitting diodes  16 . 
         [0045]      FIG. 5  is a timing diagram of power supply voltage V ALIM  and of power supply current I ALIM  for an example where AC voltage V IN  corresponds to a sinusoidal voltage. When voltage V ALIM  is greater than the sum of the threshold voltages of light-emitting diodes  16 , light-emitting diodes  16  become conductive. Power supply current I ALIM  then follows power supply voltage V ALIM . There thus is an alternation of phases OFF without light emission and of light-emission phases ON. 
         [0046]    A disadvantage is that dimmers  5  available for sale have generally been designed to operate with incandescent lamp illumination circuits and may not operate properly when they are connected to an optoelectronic circuit comprising light-emitting diodes. As an example, the proper operation of certain dimmers may require for the input impedance of optoelectronic circuit  10  seen by dimmer  5  to be low when voltage V IN  is close to 0 V. However, in phases OFF when no light is emitted, light-emitting diodes  16  are non-conductive and optoelectronic circuit  10  then has a high input impedance, which may disturb the operation of dimmer  5 . 
         [0047]    According to an embodiment, the optoelectronic circuit comprises a device for detecting the presence or the absence of a dimmer connected to the input terminals of the optoelectronic circuit. According to an embodiment, the optoelectronic circuit further comprises a device capable of modifying certain properties of the optoelectronic circuit when a dimmer is detected, particularly to decrease the input impedance seen by the dimmer when the optoelectronic circuit is powered with a low voltage, to avoid disturbing the operation of the dimmer. 
         [0048]    There exist optoelectronic circuits comprising a light-emitting diode switching circuit capable of progressively increasing the number of light-emitting diodes receiving power supply voltage V ALIM  during a rising phase of the power supply voltage and of progressively decreasing the number of light-emitting diodes receiving power supply voltage V ALIM  during a falling phase of the power supply voltage. The switching circuit is generally capable of short-circuiting a variable number of light-emitting diodes according to the variation of voltage V ALIM . This enables to decrease the duration of each phase OFF with no light emission. 
         [0049]      FIG. 6  shows an electric diagram of an example of an optoelectronic circuit  20  comprising a light-emitting diode switching device. The elements of optoelectronic circuit  20  common with optoelectronic circuit  10  are designated with the same reference numerals. In particular, the optoelectronic circuit comprises rectifying circuit  12  receiving power supply voltage V IN  between terminals IN 1  and IN 2  and supplying rectified voltage V ALIM  between nodes A 1  and A 2 . As a variation, circuit  20  may directly receive a rectified voltage, and it is then possible for the rectifying circuit not to be present. 
         [0050]    Optoelectronic circuit  20  comprises N series-connected assemblies of elementary light-emitting diodes, called general light-emitting diodes D i  in the following description, where i is an integer in the range from 1 to N and where N is an integer in the range from 2 to  200 . Each general light-emitting diode D 1  to D N  comprises at least one elementary light-emitting diode and is preferably formed of the series and/or parallel assembly of at least two elementary light-emitting diodes. In the present example, the N general light-emitting diodes D i  are series-connected, the cathode of general light-emitting diode D i  being coupled to the anode of general light-emitting diode D i+1 , for i varying from 1 to N−1. The anode of general light-emitting diode D 1  is coupled to node A 1 . General light-emitting diodes D i , with i varying from 1 to N, may comprise the same number of elementary light-emitting diodes or different numbers of elementary light-emitting diodes. 
         [0051]    Optoelectronic circuit  20  comprises a current source  22  having a terminal connected to node A 2  and having its other terminal connected to a node A 3 . Current source  22  may correspond to a resistor. Circuit  20  comprises a light-emitting diode switching device  24 . As an example, device  24  comprises N controllable switches SW 1  to SW N . Each switch SW i , with i varying from 1 to N, is assembled between node A 3  and the cathode of general light-emitting diode D i . Each switch SW i , with i varying from 1 to N, is controlled by a signal S i  supplied by a control unit  26 . Control unit  26  may be totally or partly formed by a dedicated circuit or may comprise a microprocessor or a microcontroller capable of executing a series of instructions stored in a memory. As an example, signal S i  is a binary signal and switch SW i  is off when signal S i  is in a first state, for example, the low state, and switch SW i  is on when signal S i  is in a second state, for example, the high state. 
         [0052]    Optoelectronic circuit  20  comprises one or a plurality of sensors connected to control unit  26 . It may be a single sensor, for example, a sensor capable of measuring voltage V ALIM  or the current flowing between terminals IN 1  and IN 2 , or a plurality of sensors, where each sensor may be associated with a general light-emitting diode D i . As an example, a single sensor  28  has been shown in  FIG. 6 . 
         [0053]    Control unit  26  is capable of controlling the turning on or the turning off of switches SW i , with i varying from 1 to N−1, according to the value of voltage V ALIM  according to a sequence based on the measurement of a physical parameter, for example, at least one current or one voltage. As an example, the turning off and the turning on of switches SW i  may be controlled by control unit  26  based on the signals supplied by sensor  28  or the sensors. As a variation, the turning off and the turning on of switch SW i  may be controlled from the measurement of the voltage at the cathode of each general light-emitting diode D i . As a variation, the turning off and the turning on of switch SW i  may be controlled from the measurement of voltage V ALIM  or the measurement of the voltage at the cathode of each general light-emitting diode D i . The number of switches SW 1  to SW N  may vary according to the turn-off and turn-on sequence implemented by control unit  26 . As an example, switch SW N  may not be present. 
         [0054]      FIG. 7  shows curves of the variation of signals S i , with i varying from 1 to N−1, N being equal to 4 during a cycle of voltage V ALIM  in the case where voltage V IN  is a sinusoidal voltage for an example of a switching method implemented by switching device  24 . As an example, at the beginning of a rising phase of voltage V ALIM , signals S i , with i varying from 1 to N−1, are initially at “1” so that switches SW i  are on. Switches SW 1 , SW 2 , and SW 3  are successively turned off at times t 1 , t 2 , and t 3  along the rise of voltage V ALIM  so that general light-emitting diodes D 2 , D 3 , and D 4  are successively powered with current. During a falling phase of voltage V ALIM , switches SW 3 , SW 2 , and SW 1  are successively turned on at times t′ 3 , t′ 2 , and t′ 1  to successively short-circuit general light-emitting diodes D 4 , D 3 , and D 2 . 
         [0055]    According to an embodiment, the light-emitting diode switching device is, further, capable of detecting the presence or the absence of a dimmer connected to terminals IN 1  and IN 2 . The function of detecting the presence or the absence of a dimmer may advantageously be implemented with the light-emitting diode switching device already equipping certain optoelectronic circuits comprising light-emitting diodes with few modifications. An embodiment of a method of detecting the presence or the absence of a dimmer will be described with an optoelectronic circuit comprising light-emitting diodes  20 , comprising a light-emitting diode switching device  24  having the structure shown in  FIG. 6 . It should however be clear that the method of detecting the presence or the absence of a dimmer may be implemented with other structures of light-emitting diode switching devices, particularly light-emitting diode switching devices described in patent applications US 2012/0056559, US 2008/0211421, US 2011/0273102, and U.S. Pat. No. 7,081,722. 
         [0056]    According to an embodiment, control unit  26  is capable of comparing at least some of switching times t i , that is, of turning on and/or off, switches SW i , with i varying from 1 to N, during a rising phase of voltage V ALIM  and at least some of switching times t′ i  of switches SW i , with i varying from 1 to N, during a falling phase of voltage V ALIM  In the absence of a dimmer, voltage V ALIM  varies progressively during each cycle, switching times t i , with i varying from 1 to N, being then different during each cycle and switching times t′ i  being also different during each cycle. When a dimmer is present, each cycle comprises a first phase, at the beginning or at the end of a cycle, during which voltage V ALIM  is substantially at 0 V, and a second phase during which voltage V ALIM  substantially follows voltage V IN , shifted by the threshold voltages of diodes  14  of rectifying bridge  12 . There thus is, during each cycle, an abrupt increase or an abrupt decrease of voltage V ALIM  at the transition between the first and second phases. This causes the simultaneous switching of at least two switches during each cycle. 
         [0057]    In the previously-described embodiment, a switching time t i  or t′ i  corresponds to a time when a switch is turned off or on, that is, at a time when a binary signal S i  for controlling a switch SW i  switches. More generally, a switching time corresponds to a change in the configuration of the connection of light-emitting diode assemblies D i , which causes a modification of the electric path followed by the current between terminals IN 1  and IN 2 . A switching time can then correspond to a switching time of a binary signal supplied by a sensor to control unit  36  and/or to a switching time of a binary signal supplied by control unit  26  to a switch. In particular, when the switching times correspond to the switchings of signals supplied by sensors, according to the control method implemented by control unit  26 , the fact for switching times to be simultaneous may not cause the simultaneous turning on of a plurality of switches or the simultaneous turning off of a plurality of switches. In the following description, turn-off time t i  will designate a switching time in a rising phase of voltage V ALIM  and turn-on time t′ i  will designate a switching time in a falling phase of voltage V ALIM . 
         [0058]      FIGS. 8 and 9  illustrate the principle of the detection of the presence or of the absence of a dimmer. 
         [0059]      FIG. 8  is a drawing similar to  FIG. 7  in the case where optoelectronic circuit  20  is connected to a leading edge dimmer with a firing angle α equal to 0.5. As appears in this drawing, turn-off times t 1 , t 2 , and t 3  of switches SW 1 , SW 2 , and SW 3  are substantially simultaneous. When firing angle α is different from 0.5, the number of switches which are simultaneously turned off may be smaller than N−1. 
         [0060]      FIG. 9  is a drawing similar to  FIG. 7  in the case where optoelectronic circuit  20  is connected to a trailing edge dimmer with a firing angle α equal to 0.5. As appears in this drawing, turn-on times t′ 1 , t′ 2 , and t′ 3  of switches SW 1 , SW 2 , and SW 3  are substantially simultaneous. When firing angle α is different from 0.5, the number of switches which are simultaneously turned on may be smaller than N−1. 
         [0061]      FIG. 10  shows, in the form of a block diagram, an embodiment of a method of detecting the presence or the absence of a dimmer which may be used by control unit  26 . 
         [0062]    At step  40 , control unit  26  determines switching times t i , t′ i  of switching device  24  during a cycle of voltage V ALIM . The method carries on at step  42 . 
         [0063]    At step  42 , control unit  26  compares with one another at least some of turn-off times t i  of switches SW i  and compares with one another at least some of turn-off times t′ i  of switches SW i . As an example, control unit  26  may compare turn-off times t i  and t i+1  and turn-on times t′ i  and t′ i+1 . Control unit  26  may further compare at least some of turn-off times t i  with at least some of turn-on times t′ i . The method carries on at step  44 . 
         [0064]    At step  44 , according to the result of the comparison at step  42 , control unit  26  determines whether a dimmer is present. If at least two turn-off times t i  are close or substantially simultaneous or if at least two turn-on times t′ i  are close or substantially simultaneous, this means that a dimmer is present. “Close” means that the duration between the two switching times t i  and t i+1  or t′ i  and t′ i+1  is smaller than a duration threshold which may depend on the considered time t i  or t′ i . In the case where turn-off times t i  are not close or simultaneous and where, further, turn-on times t i  are not close or simultaneous, this means that there is no dimmer. Steps  40 ,  42 , and  44  may be at last partly carried out. 
         [0065]    According to an embodiment, at step  44 , control unit  26  is further capable of determining whether the detected dimmer is a leading edge dimmer or a trailing edge dimmer according to whether the close or simultaneous switching times are switch turn-on times or switch turn-off times. In the example of a switching method illustrated in  FIGS. 8 and 9 , a leading edge dimmer is detected when the simultaneous switching times are turn-off times and a trailing edge dimmer is detected when the simultaneous switching times are turn-on times. 
         [0066]    According to an embodiment, at step  44 , control unit  26  is further capable of determining firing angle α of the dimmer. This may in particular be performed from the determination of the time period, during a cycle of voltage V ALIM , between the switching time (turn-on or turn-off) of a switch, which simultaneously occurs with other switching times in a rising or falling phase of voltage V ALIM , and the switching time of this same switch which occurs in the other phase, falling or rising, of voltage V ALIM . 
         [0067]      FIG. 11  shows an embodiment of a unit  45  for detecting the presence or the absence of a dimmer capable of implementing the method previously described in relation with  FIG. 10 . Detection unit  45  may be part of control unit  26 . 
         [0068]    According to the present embodiment, unit  45  receives at least N signals Qen i , with i varying from 1 to N, each signal Qen i  being representative of a change of configuration of switching device  24  during a rising phase of signal V ALIM . According to an embodiment, signal Qen i  may correspond to the complementary of control signal S i  of switch SW i . Unit  45  further comprises N−1 timers  46   i  (Timer), with i varying from 1 to N−1. Each timer  46   i  receives signal Qen i  and is activated when signal Qen i  switches from “0” to “1”. Each timer  46   i  supplies a binary signal Een i  which switches state when a predetermined duration is reached after the activation of timer  46   i . The predetermined duration may depend on the considered timer  46   i . Unit  45  further comprises N−1 “AND” logic gates  47   i , with i varying from 1 to N−1. Each logic gate  47   i  receives signal Een i  and signal Qen i+1  and supplies a binary signal LEdetect i  at “1” when signal Een i  and Qen i+1  are simultaneously at “1”. Unit  45  further comprises an “OR” logic gate  48  receiving signals LEdetect i , with i varying from 1 to N−1, and supplying a binary signal LEdetect which, for example, is set to “1” when at least one of signals LEdetect i , with i varying from 1 to N−1, is at “1” and which is set to “0” when all signals LEdetect i , with i varying from 1 to N−1, are at “0”. 
         [0069]    When the duration between at least two turn-off times t i  and t i+1  is shorter than the duration measured by timer  46   i , signal LEdetect i  is set to “1” and signal LEdetect is set to “1”. This means the detection of a leading edge dimmer. 
         [0070]    According to the present embodiment, unit  45  receives at least N signals Qdis i , with i varying from 1 to N, each signal Qdis i  being representative of a change of configuration of switching device  24  during a falling phase of signal V ALIM  According to an embodiment, signal Qdis i  may correspond to control signal S i  of switch SW i . Unit  45  further comprises N−1 timers  49   i , with i varying from 2 to N. Each timer  49   i  receives signal Qdis i  and is activated when signal Qdis i  switches from “0” to “1”. Each timer  49   i  supplies a signal Edis i  which switches state when a predetermined duration is reached after the activation of timer  49   i . The predetermined duration may depend on the considered timer  49   i . Unit  45  further comprises N−1 “AND” logic gates  50   i , with i varying from 2 to N. Each logic gate  50   i  receives signal Edis i  and signal Qdis i−1  and supplies a binary signal TEdetect i  at “1” when signal E i  and Qdis i+1  are simultaneously at “1”. Unit  45  further comprises an “OR” logic gate  51  receiving signals TEdetect i , with i varying from 2 to N, and supplying a binary signal TEdetect which, for example, is set to “1” when at least one of signals TEdetect i , with i varying from 2 to N, is at “1”, and which is set to “0” when all signals TEdetect i , with i varying from 2 to N, are at “0”. A trailing edge dimmer is detected when signal TEdetect is at “1”. 
         [0071]    When the duration between at least two turn-on times t′ i  and t′ i+1  is shorter than the duration measured by timer  49   i+1 , signal TEdetect i+1  is set to “1” and signal TEdetect is set to “1”. This means the detection of a trailing edge dimmer. 
         [0072]    According to an embodiment, the duration measured by each timer  46   i  or  49   i  is the same for each timer  46   i  or  49   i . According to an embodiment, the duration measured by each timer  46   i  or  49   i  depends on the timer  46   i  or  49   i . According to an embodiment, the duration measured by each timer  46   i  or  49   i  is smaller than the theoretical duration expected between times t i  and t i+1  or between times t′ i  and t′ i+1  in the absence of a dimmer. The theoretical duration may be determined from the knowledge of the maximum amplitude and frequency of signal V ALIM  and on the number of light-emitting diodes of each light-emitting diode assembly D i . 
         [0073]    The embodiment shown in  FIG. 11  may advantageously be achieved by a digital circuit or an analog circuit. In the case of a digital circuit, timer  46   i ,  49   i  may be rated by a clock signal. In the case of an analog circuit, timer  46   i ,  49   i  may comprise a capacitor charged at constant current. 
         [0074]    According to another embodiment, the unit for detecting the presence or the absence of a dimmer is capable of storing the successive times t i  and t′ i . This enables, advantageously at previously-described step  44 , to compare more than two turn-off times t i , more than two turn-on times t′ i  and/or turn-off times t i  with turn-on times t′ i . To store successive times t i  and t′ i , a timer which is for example activated at switching time t 1  and stopped at switching time t′ 1  may be used. The duration between times t 1  and t′ 1  is representative, in the absence of a dimmer, of the period of voltage V ALIM . 
         [0075]    Advantageously, the previously-described embodiments of methods of detecting the presence or the absence of a dimmer may be implemented with known optoelectronic circuits comprising a light-emitting diode switching device, with no other modification than the addition of the detection unit. 
         [0076]      FIG. 12  corresponds to  FIG. 5  of U.S. Pat. No. 7,081,722 which is herein incorporated by reference.  FIG. 12  shows an embodiment of an optoelectronic circuit comprising a light-emitting diode switching device with which the previously-described embodiments of the method of detecting the presence or the absence of a dimmer may be implemented. Indeed, signals Qen i , previously described in relation with  FIG. 11 , may correspond to the complementaries of the signals for controlling the gates of MOS transistors Qi, with i varying from 1 to 4, of  FIG. 12  and signals Qdis i , previously-described in relation with  FIG. 11 , may correspond to the signals for controlling the gates of MOS transistors Qi, with i varying from 1 to 4. 
         [0077]      FIG. 13  shows an embodiment of control unit  26  of optoelectronic circuit  20 . Control unit  26  comprises a processing unit  52  and a unit  53  for detecting the presence or the absence of a dimmer. Processing unit  52  receives signals and/or signals Qen i  and is capable of supplying control signals S i , with i varying from 1 to N. Detection unit  53  receives signals Qen i  and/or signals Qdis i  and supplies signals LEdetect and TEdetect to processing unit  52 . 
         [0078]      FIG. 14  shows, in the form of a block diagram, where an embodiment of a method of controlling a light-emitting diode switching device capable of being implemented by control unit  26  shown in  FIG. 13 . The control method comprises previously-described steps  40 ,  42 ,  44 . At step  44 , if detection unit  53  has detected the presence of a dimmer, the method carries on at step  54 . At step  44 , if detection unit  53  has not detected the presence of a dimmer, the method carries on at step  55 . 
         [0079]    At step  54 , processing unit  52  may control a first operating mode adapted to the presence of a dimmer. According to an embodiment, the first operating mode comprises decreasing the input impedance of the optoelectronic circuit seen by the dimmer when no light-emitting diode is conducting. According to an embodiment, a first operating mode comprises maintaining a current flowing between terminals IN 1  and IN 2  permanently above a current threshold which may be adapted to the proper operation of the dimmer. According to an embodiment, a first operating mode comprises permanently maintaining a constant current between terminals IN 1  and IN 2 . The method carries on at step  40 . 
         [0080]    At step  55 , control unit  26  may control a second adapted operating mode when a dimmer is not present, which for example corresponds to the normal operating mode of switching device  22 . The method carries on at step  40 . 
         [0081]    Steps  40  to  55  may be implemented for each cycle of voltage V ALIM ,one cycle out of two, one cycle out of ten, etc. 
         [0082]    According to an embodiment, at the start, the optoelectronic circuit may operate according to the first operating mode before the first implementation of the method of detecting the presence or the absence of a dimmer. Thereby, if the presence of a dimmer is confirmed at step  44 , the optoelectronic circuit is already in the first operating mode. Risks of a poor operation of the dimmer at the start are thus advantageously avoided. 
         [0083]    The first operating mode implemented at step  54  may depend on the type of detected dimmer. As an example, in the first operating mode, when a current flowing between terminals IN 1  and IN 2  is permanently maintained above a current threshold, the current threshold may depend on the type of detected dimmer. 
         [0084]    The first operating mode implemented at step  54  may depend on the determined firing angle α. As an example, in the first embodiment, when a constant current is maintained between terminals IN 1  and IN 2 , the current level may depend on the determined firing angle α. 
         [0085]      FIG. 15  shows an embodiment of an optoelectronic circuit  56  comprising a light-emitting diode switching device  57  capable of detecting the presence or the absence of a dimmer connected to terminals IN 1  and IN 2  and further capable, in the first operating mode, of decreasing the input impedance of optoelectronic circuit  56  seen by the dimmer. Optoelectronic circuit  56  comprises all the elements of optoelectronic circuit  20  shown in  FIG. 6  and further comprises an additional switch SW 0  connecting nodes A 1  and A 3 , controlled by a binary signal S 0  supplied by control unit  26 . According to an embodiment, at previously-described step  55 , in the second operating mode, in the absence of detection of a dimmer, switch SW 0  is left permanently off. At previously-described step  54 , in the first operating mode, when a dimmer is detected, switch SW 0  is turned on at the beginning and at the end of each cycle of voltage V ALIM . According to an embodiment, unit  26  may control, at the beginning of a cycle of voltage V ALIM , the turning off of switch SW 0  when the signal measured by sensor  28  exceeds a threshold, and control, at the end of a cycle of voltage V ALIM , the turning on of switch SW 0 , while switch SW 1  is on, when the signal measured by sensor  28  decreases below a threshold. When switch SW 0  is turned on at the beginning and at the end of the cycle of voltage V ALIM , a current may flow between input terminals IN 1  and IN 2  as soon as voltage V ALIM  is different from zero. Optoelectronic circuit  56  thus has a low input impedance between input terminals IN 1  and IN 2  at the beginning and at the end of the cycle of voltage V ALIM . A dimmer connected to input terminals IN 1  and IN 2  can then operate properly. 
         [0086]    According to an embodiment, current source  22  is a controllable current source and control unit  26  supplies a control signal COM to current source  22  in order to control the current source to modify the current supplied by current source  22  in the first operating mode. According to an embodiment, current source  22  may be controlled to supply a constant current for each cycle of voltage V ALIM  while a dimmer is detected. 
         [0087]      FIG. 16  shows a more detailed electric diagram of an embodiment of an optoelectronic circuit  60 . The elements common between optoelectronic circuit  60  and optoelectronic circuit  20  are designated with the same reference numerals. 
         [0088]    Call V CS  the voltage across current source  22  and I CS  the current supplied by current source  22 . Optoelectronic circuit  60  may comprise a circuit, not shown, for supplying a reference voltage to power current source  22 , possibly obtained from voltage V ALIM  . For i varying from 1 to N, call V Ci  the voltage between the cathode of general light-emitting diode D i  and node A 2 . Further, voltage V ALIM  is also called V C0 . In the following description, unless otherwise mentioned, the voltages are referenced to node A 2 . 
         [0089]    Optoelectronic circuit  60  further comprises N+1 comparison units COMP i , with i varying from 0 to N, capable of each receiving voltage V Ci  and of supplying a signal H i  and a signal L i . Control unit  26  receives signals L 0  to L N  and Ho to H N  and supplies signals S 0  to S N  for controlling switches SW 0  to SW N . 
         [0090]    The elementary light-emitting diodes of each general light-emitting diode D i , with i varying from 1 to N are, for example, planar light-emitting diodes, each comprising a stack of layers resting on a planar surface, having at least one active layer capable of emitting light. The elementary light-emitting diodes are, for example, planar light emitting diodes or light-emitting diodes formed from three-dimensional semiconductor elements, particularly microwires, nanowires, or pyramids, for example comprising a semiconductor material based on a compound mainly comprising at least one group-III element and one group-V element (for example, gallium nitride GaN), called III-V compound hereafter, or mainly comprising at least one group-II element and one group-VI element (for example, zinc oxide ZnO), called II-VI compound hereafter. Each three-dimensional semiconductor element is covered with at least one active layer capable of emitting light. 
         [0091]    For i varying from 0 to N, switch SW i  is, for example, a switch based on at least one transistor, particularly a field-effect metal-oxide gate transistor or enrichment (normally on) or depletion (normally off) MOS transistor. 
         [0092]    In the present embodiment, control unit  26  is capable of controlling the turning on or off of switches SW i , with i varying from 0 to N, according to the value of voltage V Ci . To achieve this, each comparison unit COMP i , with i varying from 0 to N, is capable of comparing voltage V Ci  with at least two thresholds Vhigh i  and Vlow i . As an example, signal L i  is a binary signal which is in a first state when voltage V Ci  is smaller than threshold Vlow i  and which is in a second state when voltage V Ci  is greater than threshold Vlow i . As an example, signal H i  is a binary signal which is in a first state when voltage V Ci  is smaller than threshold Vhigh i  and which is in a second state when voltage V Ci  is greater than threshold Vhigh i . The first states of binary signals H i  and L i  may be equal or different and the second states of binary signals H i  and L i  may be equal or different. 
         [0093]      FIG. 17  shows an electric diagram of a more detailed embodiment of a portion of electronic circuit  60 . According to the present embodiment, each comparator COMP i  comprises a first operational amplifier  62 , operating as a comparator. The inverting input (−) of operational amplifier  62  is connected to the cathode of general light-emitting diode D i , for i varying from 1 to N and to node A l  for comparator COMP 0 . The non-inverting input (+) of operational amplifier  62  receives voltage threshold Vhigh i  which is supplied by a unit  64 , which may comprise a memory. Operational amplifier  62  supplies signal H i . Each comparator COMP i  further comprises a second operational amplifier  66  operating as a comparator. The inverting input (−) of operational amplifier  66  is connected to the cathode of general light-emitting diode D i , for i varying from 1 to N and to node A 1  for comparator COMP 0 . The non-inverting input (+) of operational amplifier  66  receives voltage threshold Vlow i  which is supplied by a unit  68 , which may comprise a memory. Operational amplifier  66  supplies signal L i . 
         [0094]      FIG. 18  shows an electric diagram of a more detailed embodiment of current source  22  and of switch SW i . In the present embodiment, current source  22  comprises an ideal current source  70  having a terminal connected to a first source of a reference voltage VREF. The other terminal of current source  70  is connected to the drain of a diode-assembled N-channel MOS transistor  72 . The source of MOS transistor  72  is connected to node A 2 . The gate of MOS transistor  72  is connected to the drain of MOS transistor  72 . Reference potential VREF may be supplied from voltage V ALIM . It may be constant or vary according to voltage V ALIM . The intensity of the current supplied by current source  22  may be constant or be variable, for example, vary according to voltage V ALIM . 
         [0095]    For each general light-emitting diode D i , current source  22  comprises an N-channel MOS transistor  74  having its gate connected to the gate of transistor  72  and having its source connected to node A 2 . MOS transistors  72  and  74  form a current mirror, current I CS  supplied by current source  70  being copied, possibly with a multiplication factor. 
         [0096]    According to the present embodiment, switch SW i  comprises an N-channel MOS transistor  76  having its drain connected to the cathode of general light-emitting diode D i  and having its source connected to the drain of transistor  74 . The voltage applied to the gate of transistor  76  corresponds to previously-described signal S i . 
         [0097]      FIG. 19  shows timing diagrams of power supply voltage V ALIM,  equal to voltage V C0 , and of the voltages V Ci  measured by each comparator COMP i , with i varying from 1 to N, illustrating the operation of optoelectronic circuit  60  according to the embodiment shown in  FIG. 16  in the case where N is equal to 4 and in the case where each general light-emitting diode D i  comprises the same number of elementary light-emitting diodes arranged in the same configuration, and thus has the same threshold voltage Vled. As an example, voltage V ALIM  supplied by rectifying bridge  12  is a rectified sinusoidal voltage comprising a succession of cycles having voltage V ALIM  increasing from the zero value, crossing a maximum value, and decreasing to the zero value, in each of them. As an example, two successive cycles of voltage V ALIM  are shown in  FIG. 19 . 
         [0098]    An embodiment will now be described for the second embodiment in the absence of detection of a dimmer. Call t 0  to t 20  successive times. 
         [0099]    At time t 0 , at the beginning of a cycle when a dimmer is not detected, switch SW 1  is turned on and all switches SW i , with i varying from 2 to N, are turned off. Voltage V ALIM  rises from the zero value and distributes between general light-emitting diode D 1 , switch SW 1 , and current source  22 . Voltage V ALIM  being smaller than threshold voltage Vled of general light-emitting diode D 1 , there is no light emission (phase Po) and voltage V C1  remains substantially equal to zero. 
         [0100]    At time t 1 , when the voltage across general light-emitting diode D 1  exceeds threshold voltage Vled, general light-emitting diode D 1  becomes conductive (phase P 1 ). The voltage across general light-emitting diode D 1  then remains substantially constant and voltage V C1  keeps on increasing along with voltage V ALIM . As soon as power supply voltage V C1  is sufficiently high to allow the activation of current source  22 , current I CS  flows through the general light-emitting diode D 1  which emits light. As an example, voltage V CS , when current source  22  is in operation, is preferably substantially constant. 
         [0101]    At time t 2 , when voltage V C1  exceeds threshold Vhigh i , unit  26  successively controls the turning on of switch SW 2  and then the turning off of switch SW 1 . Voltage V ALIM  then distributes between general light-emitting diodes D 1  and D 2 , switch SW 2 , and current source  22 . Preferably, threshold Vhigh i  is substantially equal to the sum of the threshold voltage of general light-emitting diode D 2  and of operating voltage V CS  of current source  22  so that, at the turning on of switch SW 2 , general light-emitting diode D 2  conducts current I CS  and emits light. The fact for switch SW 2  to be turned on before the turning off of switch SW i  ensures that there will be no interruption of the current flow in general light-emitting diode D 1 . Phase P 2  corresponds to a phase of light emission by general light-emitting diodes D 1  and D 2 . 
         [0102]    Generally, when a dimmer is not detected, during a rising phase of power supply voltage V ALIM , for i varying from 1 to N−1, while switch SW i  is on and the other switches are off, unit  26  successively controls the turning on of switch SW i+1  and the turning off of switch SW i  when voltage V Ci  exceeds threshold Vhigh i . Voltage V ALIM then distributes between general light-emitting diodes D 1  to D i+1 , switch SW i+1 , and current source  22 . Preferably, threshold Vhigh i  is substantially equal to the sum of the threshold voltage of general light-emitting diode D i+1  and of operating voltage V CS  of current source  22  so that, at the turning on of switch SW i+i , general light-emitting diode D i+1  conducts current I CS  and emits light. Phase P i+1  corresponds to the emission of light by general light-emitting diodes D 1  à D i+1 . The fact for switch SW i+1  to be turned on before the turning off of switch SW i  ensures that there will be no interruption of the current flow in general light-emitting diodes D 1  to D i . 
         [0103]    Thus, at time t 3 , unit  26  controls the turning on of switch SW 3  and the turning off of switch SW 2 . Phase P 3  corresponds to the emission of light by general light-emitting diodes D 1 , D 2 , and D 3 . At time t 4 , unit  26  controls the turning on of switch SW 4  and the turning off of switch SW 3 . Phase P 4  corresponds to the emission of light by general light-emitting diodes D 1 , D 2 , D 3 , and D 4 . 
         [0104]    Power supply voltage V ALIM  reaches its maximum value at time t 5  during phase P 4  in  FIG. 19  and starts a falling phase. 
         [0105]    At time t 6 , when voltage V 4  decreases below threshold Vlow 4 , unit  26  successively controls the turning on of switch SW 3  and the turning off of switch SW 4 . Voltage V ALIM  then distributes between general light-emitting diodes D 1 , D 2 , and D 3 , switch SW 3 , and current source  22 . Preferably, threshold Vlow 4  is selected to be substantially equal to the sum of operating voltage V CS  of current source  22  and of the minimum operating voltage of switch SW 4  so that, at the turning on of switch SW 3 , there is no interruption of the current flow. 
         [0106]    Generally, during a falling phase of power supply voltage V ALIM , when a dimmer is not detected, for i varying from 2 to N, when voltage V Ci  decreases below threshold Vlow i , unit  26  successively controls the turning on of switch SW i−1  and the turning off of switch SW i . Voltage V ALIM  then distributes between general light-emitting diodes D 1  to D i−1 , switch SW i−1 , and current source  22 . Preferably, threshold Vlow i  is selected to be substantially equal to the sum of operating voltage V CS  of current source  22  and of the minimum operating voltage of switch SW i  so that, at the turning on of switch SW i−1 , there is no interruption of the current flow. 
         [0107]    Thus, at time t 7 , unit  26  controls the turning on of switch SW 2  and the turning off of switch SW 3 . At time t 8 , unit  26  controls the turning on of switch SW 2  and the turning off of switch SW  1 . At time t 9 , voltage V C1  becomes zero so that general light-emitting diode D 1  is no longer conductive and current source  22  is off. At time t 10 , voltage V ALIM  becomes zero and a new cycle starts. Times t 11  to t 20  are respectively similar to times t 1  to t 10 . In the present embodiment, comparator COMP  1  may have a simpler structure than comparators COMP i , with i varying from 2 to N, since threshold Vlow i  is not used. 
         [0108]    In the case where a leading edge dimmer is present, voltage V ALIM  is zero at the beginning of a cycle and then abruptly increases. During such an abrupt increase, at least two comparators COMP i  and COMP simultaneously switch signals H i  and H j . If k is the highest index of comparator COMP k  which switches signal H k , control unit  26  successively controls the turning on of switch SW k+1  and then the turning off of all switches SW 0  to SW k . 
         [0109]    In the case where a trailing edge dimmer is present, voltage V ALIM  abruptly decreases during a cycle and then remains substantially zero until the end of the cycle. During such an abrupt decrease, at least two comparators COMP i  and COMP simultaneously switch signals L i  and L j . If k is the highest index of comparator COMP k  which switches signal L k , control unit  26  successively controls the turning on of switch SW k−1  and then the turning off of all switches SW k+1  to SW N . 
         [0110]    In the first operating mode, when a dimmer is detected, switch SW 0  is turned on at the end and at the beginning of each cycle, for example, as long as the voltage across general light-emitting diode D 1  is smaller than threshold voltage Vled, the other switches being off. 
         [0111]    In the first embodiment, control unit  26  may further control current source  22  as previously described. 
         [0112]    According to another embodiment of optoelectronic circuit  60 , each comparator COMP i  of optoelectronic circuit  60  only supplies signal L i . An advantage of this embodiment is that the structure of comparator COMP i  can be simplified. Indeed, it is possible for comparator COMP i  not to comprise operational amplifier  62 . 
         [0113]    The operation of the optoelectronic circuit according to this other embodiment is then identical to what has been previously described, with the difference that switches SW i , with i varying from 0 to N−1 in the first embodiment, with i varying from 1 to N−1 in the second embodiment, are initially on and that, in a rising phase of power supply voltage V ALIM , switch SW i−1  is turned off when voltage V Ci  becomes greater than threshold Vlow i . Indeed, this means that current starts flowing through switch SW i . 
         [0114]    More specifically, in a rising phase of power supply voltage V ALIM,  while light-emitting diodes D 1  to D i−1  are conductive and light-emitting diodes D i  to D N  are off, when voltage V Ci  falls below threshold Vlow i , unit  26  controls the turning off of SW i−l . Indeed, a rise in voltage V Ci  means that the voltage across light-emitting diode D i  becomes greater than the threshold voltage of light-emitting diode D i  and that the latter becomes conductive. 
         [0115]    The operation of the optoelectronic circuit according to this other embodiment in a falling phase of power supply voltage V ALIM  may be identical to that which has been previously described for optoelectronic circuit  60 . 
         [0116]      FIG. 20  shows an electric diagram of another embodiment of an optoelectronic circuit  90 . All the elements common with optoelectronic circuit  60  are designated with the same reference numerals. Unlike optoelectronic circuit  60 , optoelectronic circuit  90  does not comprise switch SW N . Further, unlike optoelectronic circuit  60 , for i varying from 1 to N−1, optoelectronic circuit  90  comprises a resistor R i  provided between node A 3  and switch SW i , and optoelectronic circuit  90  comprises a resistor R N  provided between node A 3  and the cathode of general light-emitting diode D N . Call B i  a node between resistor R i  and switch SW i , for i varying from 1 to N−1, and B N  a node between resistor R N  and the cathode of general light-emitting diode D N . Further, each comparator COMP i , with i varying from 1 to N, further receives the voltage at node B i . Signal H i  then is a binary signal which is in a first state when the voltage at node B i  is smaller than a threshold MIN i  and which is in a second state when the voltage at node B i  is greater than threshold MIN i . A resistor R 0  may be provided in series with switch SW 0 . 
         [0117]    Comparator COMP i  and resistor R i  may be replaced with any device capable of determining whether a current greater than a current threshold flows through the branch comprising switch SW i . According to an embodiment, a current mirror is arranged on the branch comprising SW i  to copy the current flowing through switch SW i . The copied current can then be compared with a current threshold. 
         [0118]      FIG. 21  shows an electric diagram of a more detailed embodiment of a portion of optoelectronic circuit  90 . In the present embodiment, comparator COMP i  comprises all the elements of comparator COMP i  shown in  FIG. 17 , with the difference that operational amplifier  66  is replaced with a hysteresis comparator  92  receiving the voltage across resistor R i  and supplying signal H i . 
         [0119]      FIG. 22  shows an electric diagram of a more detailed embodiment of current source  22  and of switch SW i  for optoelectronic circuit  90 . Current source  22  comprises all the elements of the current source shown in  FIG. 18 . Resistor R i  is interposed between MOS transistor  74  and node B i , a terminal of resistor R i  being connected to the drain of transistor  74  and the other terminal of resistor R i  being connected to node B i . 
         [0120]    The operation of optoelectronic circuit  90  may be identical to the operation of previously-described optoelectronic circuit  60  with the difference that, in a rising phase of power supply voltage V ALIM , switch SW i  is turned off when current starts flowing through resistor R i+1 . 
         [0121]    More specifically, switches SW i , with i varying from 1 to N−1, are initially on, switch SW 0  being off in the second operating mode when a dimmer is not detected and being on in the first operating mode when a dimmer is detected. In a rising phase of power supply voltage V ALIM , for i varying from 1 to N−1, while light-emitting diodes D 1  to D i−1  are conductive and light-emitting diodes D i  to D N  are off, when the voltage across light-emitting diode D i  becomes greater than the threshold voltage of light-emitting diode D i , the latter becomes conductive and a current starts flowing through resistor R i . This results in a rise in the voltage at node B i . As soon as the voltage at node B i  rises above threshold MIN i , unit  26  controls the turning on of switch SW i−l . 
         [0122]    The operation of optoelectronic circuit  90  in a falling phase of power supply voltage V ALIM  may be identical to that which has been previously described for optoelectronic circuit  60 . 
         [0123]    Optoelectronic circuit  90  has the advantage that thresholds MIN i  and Vlow i  can be independent from the characteristics of light-emitting diodes D i . In particular, they do not depend on the threshold voltage of each light-emitting diode D i . 
         [0124]    Various embodiments with various variations have been described hereabove. It should be noted that those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step.