Patent Publication Number: US-2018035504-A1

Title: Signal transmitter and lighting system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit and priority of Japanese Patent Application No. 2016-150045, filed on Jul. 29, 2016, the entire contents of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a signal transmitter and a lighting system. 
     BACKGROUND ART 
     Conventionally, an LED lighting system having a dimmer, a power supply and a light source (a light fixture) has been proposed (see JP 2010-287372 A (hereinafter referred to as “Document 1”)). 
     The power supply of the LED lighting system described in Document 1 includes an AC/DC converter and a dimming interface. The dimming interface is configured to superpose a dimming signal from the dimmer on a DC voltage obtained from the AC/DC converter. The light source includes a current controller and an LED light source. The current controller is configured to receive a voltage (a signal superposed voltage) from the dimming interface to light the LED light source by the signal superposed voltage as a power supply while controlling the light output of the LED light source based on the signal superposed voltage. 
     In the lighting system described in Document 1, a high frequency dimming signal (a transmission signal) is superposed on the DC voltage through the dimming interface. In this case, there is a possibility that electromagnetic waves may be radiated as noise through the wiring to the light fixture as an antenna or that the dimming signal may leak out as noise (to adjacent houses) through the wiring. 
     SUMMARY 
     It is an object of the present disclosure to provide a signal transmitter and a lighting system, capable of lighting a light fixture by DC power and decrease transmission noise of a transmission signal and leakage of the transmission signal. 
     A signal transmitter according to an aspect of the present disclosure includes a power supply, an external signal receiver and a communication controller. The power supply is configured to receive DC power to supply a DC output voltage to a light fixture. The external signal receiver is configured to receive an external instruction signal that represents a lighting state of the light fixture based on an output of a sensor configured to detect a state of a lighting space illuminated by the light fixture. The communication controller is configured to control signal transmission to the light fixture by controlling the power supply. The communication controller is configured to change an output voltage level of the power supply to transmit, to the light fixture, a transmission signal for setting the light fixture to the lighting state represented by at least the external instruction signal. 
     A lighting system includes signal transmitters, one AC/DC converter and light fixture sets. The AC/DC converter is configured to receive AC power to supply DC power to each of the signal transmitters. The light fixture sets are electrically connected to the signal transmitters, respectively. Each of the light fixture sets includes at least a light fixture. Each of the signal transmitters includes a power supply, an external signal receiver and a communication controller. The power supply is configured to receive the DC power to supply a DC output voltage to a corresponding light fixture set. The external signal receiver is configured to receive an external instruction signal that represents a lighting state of the corresponding light fixture set based on an output of a sensor configured to detect a state of a lighting space illuminated by the corresponding light fixture set. The communication controller is configured to control signal transmission to the corresponding light fixture set by controlling the power supply. The communication controller is configured to change an output voltage level of the power supply to transmit, to the corresponding light fixture set, a transmission signal for setting the corresponding light fixture set to the lighting state represented by at least the external instruction signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures depict one or more implementations in accordance with the present teaching, by way of example only, not by way of limitation. In the figures, like reference numerals refer to the same or similar elements where: 
         FIG. 1  is a block diagram of a lighting system in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a front view of an operation unit in a signal transmitter of the lighting system; 
         FIG. 3  is a timing chart illustrating an operation of the signal transmitter; 
         FIG. 4  is a timing chart illustrating another operation of the signal transmitter; 
         FIG. 5  is a timing chart illustrating a normal judgement operation and an abnormal judgement operation by the signal transmitter; 
         FIG. 6  is a waveform chart representing an output current and a dimming rate of the signal transmitter; 
         FIG. 7  is another waveform chart representing an output current and a dimming rate of the signal transmitter; 
         FIG. 8  is a correlation diagram of an excess ratio of the output current and a first dimming ratio of the signal transmitter; 
         FIG. 9  is a correlation diagram of an excess ratio of the output current and a time period of the second dimming ratio of the signal transmitter; 
         FIG. 10  is a block diagram of a first modified example of the lighting system; and 
         FIG. 11  is a block diagram of a second modified example of the lighting system. 
     
    
    
     DETAILED DESCRIPTION 
     The following embodiments generally relate to signal transmitters and lighting systems and, more particularly, to a signal transmitter configured to transmit a transmission signal and a lighting system including the signal transmitter, a light fixture and an AC/DC converter. 
     The signal transmitter and the lighting system in the present embodiment may be mainly applied to an office, a factory, a store or the like. The signal transmitter and the lighting system in the present embodiment may also be applied to a dwelling such as a detached house or a condominium. 
     The embodiment will be explained with reference to the drawings. 
     As shown in  FIG. 1 , a lighting system  10  in the embodiment preferably includes an AC/DC converter  1 , a signal transmitter  2  and one or more light fixtures  3 . 
     Preferably, the AC/DC converter  1  is configured to receive commercial power of a commercial power supply  9  to output a DC voltage V 1 . The AC/DC converter  1  may adjust the DC voltage V 1  (a voltage value thereof) to a prescribed value. 
     Preferably, the signal transmitter  2  is configured to receive the DC voltage V 1  via two power supply lines E 11  and E 12  from the AC/DC converter  1  to output a DC output voltage (hereinafter simply referred to as an “output voltage”) V 2  via two power supply lines E 21  and E 22 . Light fixtures  3  may be electrically connected between the two power supply lines E 21  and E 22 . In the embodiment, each light fixture  3  is configured to be activated by the output voltage V 2  as a power supply. That is, it is preferable that the power to be supplied to each light fixture  3  via the signal transmitter  2  from the AC/DC converter  1  be DC power and that the power distribution to each light fixture  3  from the AC/DC converter  1  be DC power distribution. Hereinafter, the DC voltage V 1  is referred to as an “input voltage V 1 ”. 
     Preferably, each light fixture  3  is configured to emit light by the DC power supplied via the two power supply lines E 21  and E 22  to illuminate a space A 1  (a lighting space A 1 ) as an lighting object. 
     Hereinafter, the signal transmitter  2  of the embodiment will be explained. As shown in  FIG. 1 , the signal transmitter  2  may include a power supply  21 , a main controller  22 , a wire connector  23 , an operation unit (a console)  24 , a wireless receiver  25  and a detector  26 . 
     The power supply  21  may include an input unit  211 , an output unit  212  and a step-down circuit  213 . 
     Preferably, the input unit  211  is configured to receive the DC input voltage (hereinafter simply referred to as an “input voltage”) V 1 . The input unit  211  may have a first input terminal  21 A and a second input terminal  21 B. The second input terminal  21 B may be electrically connected to ground (ground of the signal transmitter  2 ). In this case, the first input terminal  21 A is an input terminal on a high potential side, while the second input terminal  21 B is an input terminal on a low potential side. Here, “electrically connected” means direct or indirect electrical connection. 
     Preferably, the input unit  211  is electrically connected to the AC/DC converter  1  via the two power supply lines E 11  and E 12 . In this case, the first input terminal  21 A is electrically connected to one end of the power supply line E 11 , while the second input terminal  21 B is electrically connected to one end of the power supply line E 12 . Respective other ends of the two power supply lines E 11  and E 12  are electrically connected to the AC/DC converter  1 . In the embodiment, the input unit  211  is to receive the input voltage V 1  via the two power supply lines E 11  and E 12 . That is, it is preferable that the AC/DC converter  1  be configured to apply the input voltage V 1  to the input unit  211 . 
     Preferably, the output unit  212  is configured to output the output voltage V 2 . The output unit  212  may have a first output terminal  21 C and a second output terminal  21 D. In the example of  FIG. 1 , the output unit  212  is electrically connected to each light fixture  3  via the two power supply lines E 21  and E 22 . In this example, the first output terminal  21 C is electrically connected to one end of the power supply line E 21 , while the second output terminal  21 D is electrically connected to one end of the power supply line E 22 . Respective other ends of the two power supply lines E 21  and E 22  are electrically connected to the light fixtures  3 . 
     Note that each of the input voltage V 1  and the output voltage V 2  (each voltage value thereof) is preferably 50V or less. In the embodiment, a maximum of each voltage value is 36V. 
     Preferably, the step-down circuit  213  is configured to controllably adjust the output voltage V 2  by stepping down the input voltage V 1 . As shown in  FIG. 1 , the step-down circuit  213  may include two capacitors C 1  and C 2 , an inductor L 1  and two switches Q 1  and Q 2 . Each of the two switches Q 1  and Q 2  may be a normally-off MOSFET (Metal Oxide Semiconductor Field Effect Transistor). In the embodiment, the switch Q 1  corresponds to a first switch, while the switch Q 2  corresponds to a second switch. Accordingly, the switch Q 1  and the switch Q 2  are hereinafter referred to as a “first switch Q 1 ” and a “second switch Q 2 ”, respectively. 
     The capacitor C 1  may be an input capacitor that is electrically connected in parallel with the input unit  211 . That is, the capacitor C 1  may be electrically connected between the first and second input terminals  21 A and  21 B. A terminal, on a high potential side, of the capacitor C 1  may be electrically connected to a first end (a drain) of the first switch Q 1 . A control end (a gate) of the first switch Q 1  may be electrically connected to the main controller  22 . A second end (a source) of the first switch Q 1  may be electrically connected to a first end (a drain) of the switch Q 2 . A control end (a gate) of the second switch Q 2  may be electrically connected to the main controller  22 . A second end (a source) of the second switch Q 2  may be electrically connected to a terminal, on a low potential side, of the capacitor C 1 . 
     In the example of  FIG. 1 , a first diode D 1  is a body diode of the first switch Q 1 . That is, the first diode D 1  is electrically connected in anti-parallel with the first switch Q 1 . Specifically, a first end (a cathode) of the first diode D 1  is electrically connected to a terminal, on a high potential side, of the first switch Q 1  (the drain of the first switch Q 1 ), while a second end (an anode) of the first diode D 1  is electrically connected to a terminal, on a low potential side, of the first switch Q 1  (the source of the first switch Q 1 ). 
     In the example of  FIG. 1 , a second diode D 2  is a body diode of the second switch Q 2 . That is, the second diode D 2  is electrically connected in anti-parallel with the second switch Q 2 . Specifically, a first end (a cathode) of the second diode D 2  is electrically connected to a terminal, on a high potential side, of the second switch Q 2  (the drain of the second switch Q 2 ), while a second end (an anode) of the second diode D 2  is electrically connected to a terminal, on a low potential side, of the second switch Q 2  (the source of the second switch Q 2 ). 
     In the example, the drain of the first switch Q 1  is electrically connected to a terminal, on a high potential side, of the capacitor C 2 . A terminal, on a low potential side, of the capacitor C 2  is electrically connected to the source of the first switch Q 1  via the inductor L 1 . That is, one end of the inductor L 1  is electrically connected to the source of the first switch Q 1 , while another end of the inductor L 1  is electrically connected to the terminal, on the low potential side, of the capacitor C 2 . 
     The capacitor C 2  may be a smoothing capacitor that is provided between the output terminals, and electrically connected in parallel with the output unit  212 . That is, the capacitor C 2  may be electrically connected between the first and second output terminals  21 C and  21 D. 
     Preferably, the main controller  22  includes an external signal receiver  221 , an internal signal receiver  222 , a signal selector  223 , a judgement controller  224  and a communication controller  225 . The main controller  22  may further have a function for controlling an operation of the power supply  21 . 
     In the embodiment, the external signal receiver  221  is electrically connected to the wire connector  23 . For example, a communication cable W 1  is connected to the wire connector  23 , and allows a signal from a controller (an external controller)  4  to pass through. The communication cable W 1  may be any of a twisted pair cable, an exclusive communication cable or a LAN (Local Area Network) cable, but is not limited to thereto. In this case, the external signal receiver  221  is configured to receive a signal from the controller  4  via the wire connector  23 . 
     Preferably, one or more sensors  5  (e.g., sensors  51 - 53 ) is provided in or around the lighting space A 1 , and the controller  4  is configured to receive a detection signal from the sensor(s)  5 . Each sensor  5  may be activated by the output voltage V 2  between the two power supply lines E 21  and E 22 , like the light fixtures  3 . In the example of  FIG. 1 , sensors  5  such as an illuminance sensor  51 , a motion sensor  52  and a fire sensor  53  are provided. The illuminance sensor  51  may have a function that detects illuminance in the lighting space A 1 . The motion sensor  52  may have a function that detects human presence in the lighting space A 1 . The fire sensor  53  may detect, by heat, smoke or the like, the occurrence of fire in or around the lighting space A 1 . Note that the communication between the controller  4  and the sensors  5  may be any of wire communication and wireless communication. The object to be detected by each sensor  5  is not limited to a particular event. 
     Preferably, the controller  4  is configured to generate an external instruction signal (S 1 , described later) that represents a lighting state of each light fixture  3  based on the detection signals of the sensors  5 . The controller  4  may transmit the external instruction signal S 1  to the signal transmitter  2  via the communication cable W 1 . Preferably, the external signal receiver  221  is configured to receive the external instruction signal  51  from the controller  4  via the wire connector  23 . 
     The internal signal receiver  222  may be electrically connected to the operation unit  24  and the wireless receiver  25 . 
       FIG. 2  depicts a configuration example of the operation unit  24 . In this example, the operation unit  24  has buttons  242  and  243  and a display  244  that are arranged on a front surface of a housing  241  shaped like a rectangular case. The button  242  may be pushed by a user in order to increase each dimming rate of the light fixtures  3 . The button  243  may be pushed by the user in order to decrease each dimming rate of the light fixtures  3 . The display  244  may be composed of a level meter that displays the dimming rate by, for example the numbers of bars in order to visually display the dimming rate that is set with the buttons  242  and  243 . The operation unit  24  may be any of; a configuration in which the light fixtures  3  are provided with their own operation units  24 , or one each; and a configuration in which the light fixtures  3  share an operation unit  24 . In the case of sharing an operation unit  24 , the operation unit  24  may further include a button for selecting a light fixture  3  as a setting target of the dimming rate from the light fixtures  3 . 
     Preferably, the operation unit  24  is configured to generate an internal instruction signal S 21  that represents a lighting state of each light fixture  3  according to the operations of the buttons  242  and  243 , and then supply the internal instruction signal S 21  to the internal signal receiver  222 . Preferably, the internal signal receiver  222  is configured to receive the internal instruction signal S 21  from the operation unit  24 . 
     The wireless receiver  25  may be configured to receive a wireless signal from a remote control device  6  to be operated by a user in the lighting space A 1 . Examples of the wireless signal from the remote control device  6  include radio wave, near infrared light, visible light and the like. The remote control device  6  may include buttons and a display like the operation unit  24  and be configured to generate an internal instruction signal S 22 , which represents a lighting state of each light fixture  3 , based on according to user&#39;s operation. The remote control device  6  may be configured to wirelessly transmit the internal instruction signal S 22  to the wireless receiver  25 . Preferably, the wireless receiver  25  is configured to receive the internal instruction signal S 22  from the remote control device  6  to supply the internal instruction signal S 22  to the internal signal receiver  222 . In this case, the internal signal receiver  222  is configured to receive the internal instruction signal S 22  from the wireless receiver  25 . 
     Hereinafter, an internal instruction signal S 2  will be used when the internal instruction signals S 21  and S 22  are not distinguished from each other. 
     Preferably, the signal selector  223  is configured to receive the external instruction signal S 1  from the external signal receiver  221  and the internal instruction signal S 2  from the internal signal receiver  222  to select any one instruction signal from the external and internal instruction signals S 1  and S 2 . 
     The signal selector  223  is further configured to output, as an instruction signal S 0 , the instruction signal that is selected from the external and internal instruction signals S 1  and S 2 . 
     In this case, the instruction signal S 0  is to be supplied from the signal selector  223  to the communication controller  225 . Receiving the instruction signal S 0 , the communication controller  225  can recognize a lighting state of a light fixture(s)  3  instructed by the instruction signal S 0 . The communication controller  225  is therefore configured to control the step-down circuit  213  based on the instruction signal S 0  to change the output voltage V 2 , specifically an output voltage level of the power supply  21 , thereby transmitting a transmission signal, for controlling the light fixtures  3 , to the light fixtures  3 . 
     Each light fixture  3  may include a signal receiver  31 , a constant current circuit  32  and a light source  33 . 
     Preferably, the signal receiver  31  is configured to monitor the output voltage V 2  between the two power supply lines E 21  and E 22  to demodulate the transmission signal by comparing the output voltage V 2  with a threshold. That is, the signal receiver  31  may detect a change in the output voltage level to acquire the transmission signal. The signal receiver  31  may be configured to supply the constant current circuit  32  with a PWM (Pulse Width Modulation) signal based on the transmission signal. The signal receiver  31  is, for example a control IC (Integrated Circuit). The control IC preferably includes a buffer memory. 
     For example, the signal receiver  31  is configured to: judge that a data element of the transmission signal is “0” when the output voltage V 2  is greater than or equal to the threshold; and judge that the data element of the transmission signal is “1” when the output voltage V 2  is less than the threshold. Preferably, the signal receiver  31  is configured to recognize the instructed lighting state based on the data of the demodulated transmission signal to control the constant current circuit  32  so that the light source  33  is adjusted to the instructed lighting state. 
     The embodiment controls the dimming rate as the lighting state of the light source  33 . In this case, the signal receiver  31  is configured to recognize the instructed dimming rate based on the data of the demodulated transmission signal to supply the constant current circuit  32  with a PWM signal that is set to a duty corresponding to the instructed dimming rate. For example, the signal receiver  31  may: set the duty to 100% when the dimming rate (a dimming ratio) is 100 [%]; set the duty to 0% when the dimming rate is 0 [%]; and set the duty to 50% when the dimming rate is 50 [%]. That is, the signal receiver  31  may control the constant current circuit  32  so that the light source  33  is adjusted to the instructed dimming rate. 
     Preferably, the constant current circuit  32  is configured to receive the output voltage V 2  from the two power supply lines E 21  and E 22  to supply a load current to the light source  33 , thereby lighting the light source  33 . The constant current circuit  32  may be configured to increase or decrease the load current flowing through the light source  33  (a current value thereof), thereby adjusting the dimming rate of the light source  33 . The constant current circuit  32  is, for example a step-down switching converter. 
     Preferably, the light source  33  has solid light emitting devices  331 . Each of the solid light emitting devices  331  is, for example an LED (Light Emitting Diode). The solid light emitting devices  331  may be connected in series. 
     An example of a transmission process of the transmission signal by the signal transmitter  2  will be explained in detail. 
     Examples of the operation mode of the communication controller  225  include a steady mode, a communication mode and a stop mode. 
     When the signal transmitter  2  is activated, the communication controller  225  sets the operation mode to the steady mode. The communication controller  225  under the steady mode turns and keeps the first and second switches Q 1  and Q 2  off and on (nonconductive and conductive states), respectively, thereby adjusting so that the output voltage V 2  is equal to the input voltage V 1  (the output voltage V 2  becoming the same state as that during a period of time T 1  in  FIG. 3 ). The communication controller  225  under the steady mode transmits no transmission signal to the light fixtures  3 . 
     Note that “the output voltage V 2  is equal to the input voltage V 1 ” means not only the difference between the input voltage V 1  and the output voltage V 2  being zero but also the difference between the input voltage V 1  and the output voltage V 2  being a small value that can be regarded as substantially zero. For example, the output voltage V 2  may have a smaller value than the input voltage V 1  by a voltage drop caused by electronic components and the like constituting the step-down circuit  213 . 
     Then, if one or more signals of the external instruction signal S 1 , the internal instruction signal S 21  and the internal instruction signal S 22  are supplied to the signal transmitter  2 , an instruction signal S 0  is supplied to the communication controller  225 . The communication controller  225  supplied with the instruction signal S 0  sets the operation mode to the communication mode. The communication controller  225  under the communication mode controls the first and second switches Q 1  and Q 2  as shown in  FIG. 3 , thereby changing the output voltage level to transmit a transmission signal to the light fixtures  3 . Note that Vg 1  and Vg 2  in  FIG. 3  are gate voltages of the first and second switches Q 1  and Q 2 , respectively. 
     Specifically, the communication controller  225  under the communication mode controls the first and second switches Q 1  and Q 2  so that the output voltage level is switched between a first voltage level V 21  and a second voltage level V 22 . The first voltage level V 21  has, for example the same as the voltage value of the input voltage V 1 . For example, the second voltage level V 22  has a value obtained by stepping down the input voltage V 1  that has a smaller value than the first voltage level V 21 . In a specific example, the signal transmitter  2  includes a voltage detector circuit configured to detect the output voltage V 2  to obtain a detection result. For example, the voltage detector circuit has a series circuit of resistors connected between both ends of the capacitor C 2  (see  FIG. 1 ), and is configured to divide the output voltage V 2  by the resistors to obtain a divided voltage as the detection result. The communication controller  225  is configured to adjust the output voltage level to the first voltage level V 21  by controlling the first and second switches Q 1  and Q 2  so that the detection result of the output voltage V 2  accords with a voltage of a first target value. The communication controller  225  is also configured to adjust the output voltage level to the second voltage level V 22  by controlling the first and second switches Q 1  and Q 2  so that the detection result of the output voltage V 2  accords with a voltage of a second target value. Note that the voltage detector circuit is not indispensable, but may be replaced with another configuration as long as it can detect the output voltage V 2  to obtain a detection result. For example, it may be an electric conductor (e.g., a conductive pattern) for detecting the output voltage V 2  to provide the communication controller  225  with a detection result, which is electrically connected between a positive output terminal of the power supply  21  and the communication controller  225 . 
     The transmission signal contains information for changing the lighting state of the light sources  33  that is information for changing the dimming rates of the light sources  33  in this example. The dimming rate means a level (a degree) of a light output of the light source  33 , and the dimming rate (dimming ratio) is defined as 100[%] when the light source  33  is fully lit (lit at a rated output). The transmission signal contains a bit stream composed of a plurality of bits. The bit stream contains at least, object identification data that is identification data uniquely assigned to a light fixture  3  as a control object, and control data that represent the lighting state (dimming rate) of the light fixture  3  as the control object. 
     When a bit value of the transmission signal is “1”, the communication controller  225  under the communication mode periodically repeats turning on and off the first switch Q 1  (preferably at a constant cycle) with the second switch Q 2  kept in an Off state, thereby decreasing the output voltage V 2  from the first voltage level V 21 . As a result, an electric charge of the capacitor  2  is discharged, and the output voltage V 2  decreases from the first voltage level V 21  to the second voltage level V 22  (a period of time T 2  in  FIG. 3 ). 
     Specifically, when the first and second switches Q 1  and Q 2  are in On and Off states, respectively, the electric charge stored in the capacitor  2  is discharged through a path (a discharge path) of the terminal on the high potential side of the capacitor C 2 , the first switch Q 1 , the inductor L 1  and the terminal on the low potential side of the capacitor C 2 . In this case, the signal transmitter  2  has a possibility that a current flowing through the inductor L 1  (an inductor current) will increase and the inductor current will exceed the rated current of the inductor L 1 . Therefore, in order to prevent the inductor current from exceeding the rated current, the communication controller  225  turns the first switch Q 1  on with the second switch Q 2  kept in an Off state, and turns the first switch Q 1  off with the second switch Q 2  kept in an Off state, when a (first) prescribed time elapses. The prescribed time is, for example 0.1 [ms]. When the first switch Q 1  is turned off with the second switch Q 2  kept in the Off state, the discharge path is changed to a path of the terminal on the high potential side of the capacitor C 2 , the capacitor C 1 , the second diode D 2  of the second switch Q 2 , the inductor L 1  and the terminal on the low potential side of the capacitor C 2 . The inductor current also decreases when the first switch Q 1  is turned off with the second switch Q 2  kept in the Off state. 
     When a (second) prescribed time elapses after the first switch Q 1  is turned off with the second switch Q 2  kept in the Off state, the communication controller  225  turns the first switch Q 1  on with second switch Q 2  kept in the Off state. The second prescribed time is, for example 0.1 [ms]. 
     That is, when the bit value of the transmission signal changes from “0” to “1”, the communication controller  225  repeatedly alternates between first control in which the first switch is turned on with the second switch Q 2  kept in the Off state and second control in which the first switch is turned off with the second switch Q 2  kept in the Off state. It is accordingly possible to discharge the electric change stored in the capacitor C 2  while preventing the inductor current from exceeding the rated current. 
     If the output voltage V 2  decreases and then reaches the second voltage level V 22 , the communication controller  225  periodically repeats turning on and off the second switch Q 2  (preferably at a fixed cycle) with the first switch Q 1  kept in an Off state (see a period of time T 3  in  FIG. 3 ). The communication controller  225  can accordingly keep the output voltage V 2  at the second voltage level V 22  (see the period of time T 3  in  FIG. 3 ). 
     The communication controller  225  then transmit each bit information of the transmission signal at a cycle T 0 . That is, the sum of the period of time T 2  and the period of time T 3  (T 2 +T 3 ) equals the cycle T 0 . Note that the cycle T 0  is set to be, for example 5 [ms]. 
     In the example of  FIG. 3 , a subsequent bit value of the transmission signal is “0”. The communication controller  225  under the communication mode repeatedly turns the second switch Q 2  on and off with the first switch Q 1  kept in the Off state (a period of time T 4  in  FIG. 3 ). The communication controller  225  can therefore increase the output voltage V 2  from the second voltage level V 22  to the first voltage level V 21 . 
     If the output voltage V 2  increases and then reaches the first voltage level V 21 , the communication controller  225  keeps the second switch Q 2  in an On state with the first switch Q 1  kept in the Off state so that the output voltage V 2  is kept at the first voltage level V 21  (a period of time T 1  in  FIG. 3 ). A sum of the period of time T 1  and the period of time T 4  (T 1 +T 4 ) equals the cycle T 0 . 
     The communication controller  225  under the communication mode then repeats the abovementioned operation in accordance with a bit value of the transmission signal. Note that when the bit value “0” is continuously transmitted by the transmission signal, the operation during the period of time T 1  is repeated even in a subsequent cycle T 0 . In addition, when the bit value “1” is continuously transmitted by the transmission signal, the operation during the period of time T 3  is repeated even in a subsequent cycle T 0 . 
     Preferably, the communication controller  225  under the communication mode makes a ratio of the period of time T 3  to the cycle T 0  (T 3 /T 0 ) as short as possible when a bit value of the transmission signal is “1”. In this case, it is possible to suppress variations in electric power to be supplied to the light fixtures  3  and the flicker of the light sources  33  during the communication. 
     Preferably, the communication controller  225  under the communication mode is provided with start and stop bits at the start and end of the transmission signal, respectively. The start bit is a bit or a bit stream, for representing the start of the transmission signal. The stop bit is a bit or a bit stream, for representing the end of the transmission signal. For example, the start bit is a bit stream of “111”, and the stop bit is a bit stream of “000”. However, when the transmission signal has a signal length of a fixed length, each light fixture  3  can judge the end of the transmission signal even if the stop bit is not transmitted. 
     The communication controller  225  has completed transmitting the transmission signal, and then changes the operation mode from the communication mode to the steady mode to keep the output voltage level of the power supply  21  at the first voltage level V 21 . 
     The communication controller  225  further has the stop mode as the operation mode. The stop mode is a mode for stopping the output of the step-down circuit  213 . For example, when an abnormality (a malfunction) occurs in the signal transmitter  2 , the communication controller  225  selects the stop mode as the operation mode. The communication controller  225  under the stop mode keeps each of the first and second switches Q 1  and Q 2  in an Off state. The communication controller  225  can accordingly stop the power supply to the light fixtures  3  because the output voltage V 2  becomes zero. Note that the communication controller  225  under the stop mode may keep only the second switch Q 2  in an Off state. That is, the communication controller  225  may be configured to keep at least the second switch Q 2  in an Off state when the operation mode is the stop mode. 
     In the above example, the signal transmitter  2  transmits the transmission signal by changing the output voltage V 2 , specifically the output voltage level of the power supply  21 . The signal transmitter  2  can therefore reduce noise caused by the transmission of the transmission signal and a leakage of the transmission signal in comparison with the configuration in which a high frequency transmission signal is output with the transmission signal superposed on a DC voltage. 
     In the example, the signal transmitter  2  transmits the transmission signal by changing the output voltage level. It is accordingly possible to suppress the attenuation of the transmission signal, caused by the inductance of the two power supply lines E 21  and E 22 . The signal transmitter  2  can therefore stabilize signal transmission and lengthen the transmission distance in comparison with the configuration in which a high frequency transmission signal is output with the transmission signal superposed on a DC voltage. 
     In the example, the signal transmitter  2  transmits the transmission signal by controlling the first and second switches Q 1  and Q 2  to change the output voltage V 2  (the output voltage level), as stated above. The signal transmitter  2  can therefore transmit the transmission signal with a comparatively simple configuration (circuit configuration). For example, the signal transmitter  2  can comparatively decrease the number of switches constituting the transmitter and decrease a conduction loss of the switches. That is, the signal transmitter  2  can reduce the transmission loss. 
       FIG. 4  shows an example of an operation of the signal transmitter  2  when the operation mode of the communication controller  225  is switched from the steady mode to the communication mode. Note that, in  FIG. 4 , P 0  represents a dimming rate instructed by the instruction signal S 0  (an instructed dimming rate) and Pa represents an actual dimming rate of a light source  33  (an actual dimming rate). 
     The communication controller  225  is supplied with the instruction signal S 0  for changing the instructed dimming rate P 0  to 50% (a time t 1  in  FIG. 4 ). In  FIG. 4 , a period of time between the time t 1  and a time t 2  is a period of time that the communication controller  225  needs to recognize that the instructed dimming rate P 0  changes from 100% to 50%. After receiving the instruction signal S 0 , the communication controller  225  switches the operation mode from the steady mode to the communication mode. Note that, in  FIG. 4 , a period of time T 21  is a period of time during which the operation mode is the steady mode and a period of time T 22  is a period of time during which the operation mode is the communication mode. 
     If a predetermined waiting time elapses from the time t 2  (a time t 3  in  FIG. 4 ), the communication controller  225  starts transmitting the transmission signal (a first transmission). The transmission signal to be transmitted from the communication controller  225  contains the object identification data and the control data. The object identification data to be contained in the transmission signal represent identification data of a light fixture  3  as a control object, and the control data to be contained in the transmission signal represent the instructed dimming rate P 0 . In  FIG. 4 , a period of time T 11  is a period of time during which the object identification data are transmitted, and a period of time T 12  is a period of time during which the control data are transmitted. 
     The communication controller  225  retransmits the same transmission signal when a period of time T 13  elapses from the first transmission of the transmission signal (a second transmission). In  FIG. 4 , a period of time T 14  is a period of time during which the object identification data are retransmitted, and a period of time T 15  is a period of time during which the control data are retransmitted. 
     Each signal receiver  31  of the light fixtures  3  stores identification data assigned to its own light fixture  3  in advance. If the object identification data contained in the received transmission signal accord with the identification data of the light fixture  3 , the signal receiver  31  judges that the control object is the light fixture  3 , and then reads the control date contained in the transmission signal. If the object identification data contained in the received transmission signal do not accord with the identification data of the light fixture  3 , the signal receiver  31  discards the transmission signal. The signal receiver  31  of the light fixture  3  as the control object controls the constant current circuit  32  so that the actual dimming rate Pa of the light source  33  accords with the instructed dimming rate P 0 . In the light fixture  3  as the control object shown in the example of  FIG. 4 , the actual dimming rate Pa changes from 100% to 50% after a period of time T 16  elapses from the first reception of the transmission signal. If the second transmission process is finished, the communication controller  225  switches the operation mode from the communication mode to the steady mode. Note that the data volume of the identification data is, for example about 3 to 10 bits. The data volume of the control data is, for example about 8 to 10 bits. 
     When the light fixtures  3  constitute one group and group identification data is assigned to the group, the communication controller  225  may set the group identification data to the object identification data. In this case, the communication controller  225  can control the actual dimming rate Pa of the light fixtures  3  in the group in a lump. 
     For example, when the number of light fixtures  3  is five, the five light fixtures  3  are assigned respective identification data of “1” to “5”. In this example, when the five light fixtures  3  are dealt as a first group, group identification data of the first group are assigned “11”. When the light fixtures  3  with identification data of “1” to “3” are dealt as a second group, group identification data of the second group are assigned “12”. When the light fixtures  3  with identification data of “4” and “5” are dealt as a third group, group identification data of the third group are assigned “13”. 
     In this example, the communication controller  225  sets any one of “1” to “5” to the object identification data contained in the transmission signal in order to individually control the five light fixtures  3 . The communication controller  225  also sets “11” to the object identification data contained in the transmission signal in order to control the light fixtures  3  of the first group in a lump. The communication controller  225  also sets “12” to the object identification data contained in the transmission signal in order to control the light fixtures  3  of the second group in a lump. The communication controller  225  also sets “13” to the object identification data contained in the transmission signal in order to control the light fixtures  3  of the third group in a lump. 
     The lighting state to be represented by the transmission signal is not limited to the dimming rate, but the transmission signal may contain a lighting state such as a color temperature (color adjustment), lighting, extinguishing or flashing. 
     An example of an operation of the signal selector  223  in the signal transmitter  2  will be explained. 
     The controller  4  generates the external instruction signal S 1  according to detection results of sensors  5  (the illuminance sensor  51 , the motion sensor  52 , the fire sensor  53  and the like). For example, when the illuminance in the lighting space A 1  detected with the illuminance sensor  51  is lower than the threshold, the controller  4  outputs the external instruction signal S 1  for increasing the actual dimming rate Pa of the light fixtures  3 . When the illuminance in the lighting space A 1  detected with the illuminance sensor  51  is higher than the threshold, the controller  4  outputs the external instruction signal S 1  for decreasing the actual dimming rate Pa of the light fixtures  3 . When the motion sensor  52  detects human presence in the lighting space A 1 , the controller  4  outputs the external instruction signal S 1  for turning the light fixtures  3  on. When the motion sensor  52  does not detect human presence in the lighting space A 1 , the controller  4  outputs the external instruction signal S 1  for turning the light fixtures  3  off. 
     On the other hand, the internal instruction signal S 21  is produced as a result of a user operating the operation unit  24 . The internal instruction signal S 22  is produced as a result of a user operating the remote control device  6 . 
     The signal selector  223  preferentially selects any one of the external instruction signal S 1  and the internal instruction signal S 21  according to a preset selection rule when receiving both of the external instruction signal S 1  and the internal instruction signal S 21 . 
     In a first example, the signal selector  223  operates according to a first selection rule for preferentially selecting an instruction signal containing the lowest instructed dimming rate. 
     In the example, P 1  represents the instructed dimming rate contained in the external instruction signal S 1 , and P 2  represents the instructed dimming rate contained in the internal instruction signal S 2 . In this case, the signal selector  223  selects a lower instructed dimming rate from the external instruction signal S 1  and the internal instruction signal S 2 . If the instructed dimming rate P 1  is lower than the instructed dimming rate P 2 , the signal selector  223  selects the external instruction signal S 1 . If the instructed dimming rate P 2  is lower than the instructed dimming rate P 1 , the signal selector  223  selects the internal instruction signal S 2 . 
     As another example, the signal selector  223  preferentially selects any one of the internal instruction signals S 21  and S 22  according to the first selection rule when receiving both of the internal instruction signals S 21  and S 22 . 
     In this example, P 21  represents the instructed dimming rate contained in the internal instruction signal S 21 , and P 22  represents the instructed dimming rate contained in the internal instruction signal S 22 . In this case, the signal selector  223  selects a lower instructed dimming rate from the internal instruction signals S 2  land S 22 . If the instructed dimming rate P 21  is lower than the instructed dimming rate P 22 , the signal selector  223  selects the internal instruction signal S 21 . If the instructed dimming rate P 22  is lower than the instructed dimming rate P 21 , the signal selector  223  selects the internal instruction signal S 22 . 
     That is, the signal selector  223  preferably selects, as the instruction signal S 0 , an instruction signal containing the lowest instructed dimming rate of the external instruction signal S 1  and the internal instruction signals S 21  and S 22 . It is accordingly possible to suppress power consumption of the light sources  33  to improve energy saving. 
     In a second example, the signal selector  223  operates according to a second selection rule for preferentially selecting an instruction signal containing the lowest color temperature to be adjusted. In this case, the signal selector  223  selects, as the instruction signal S 0 , an instruction signal containing the lowest color temperature to be adjusted, from the external instruction signal S 1  and the internal instruction signals S 21  and S 22 . 
     In a third example, the signal selector  223  operates according to a third selection rule for preferentially selecting an instruction signal containing the lowest dimming rate when each of the instruction signals contains both of a dimming rate and a color temperature. In this case, the signal selector  223  selects, as the instruction signal S 0 , an instruction signal containing the lowest instructed dimming rate from the external instruction signal S 1  and the internal instruction signals S 21  and S 22 . 
     The signal selector  223  may operate according to a selection rule for preferentially selecting any one of the external instruction signal S 1  and the internal instruction signals S 2  according to a state of the lighting space A 1 . 
     Examples of the state of the lighting space A 1  include “normal” and “abnormal” states. In this case, the signal selector  223  operates according to a fourth selection rule for preferentially selecting the external instruction signal S 1  in the case of the normal state while preferentially selecting the internal instruction signals S 2  in the case of the abnormal state, and for preferentially selecting any one internal instruction signal containing a higher instructed dimming rate from the internal instruction signals S 21  and S 22  when receiving both of the internal instruction signals S 21  and S 22  in the case of the abnormal state. 
     The controller  4  can judge whether the lighting space A 1  is in the normal state or the abnormal state based on a detection result(s) by the sensors  5 . For example, the controller  4  judges that the lighting space A 1  is in the abnormal state, in case the fire sensor  53  detects the occurrence of fire in the lighting space A 1 . The controller  4  judges that the lighting space A 1  is in the normal state, in case the fire sensor  53  does not detect the occurrence of fire. The controller  4  judges that the lighting space A 1  is in the abnormal state, in case the motion sensor  52  detects intrusion (intruder) into the lighting space A 1 . The controller  4  judges that the lighting space A 1  is in the normal state, in case the motion sensor  52  detects no intrusion into the lighting space A 1 . The state of the lighting space A 1  is not limited to the occurrence of fire or the detection of intrusion, but examples of the state of the lighting space A 1  may further include an operation state of a machine in the lighting space A 1  and the occurrence of earthquakes. 
     The controller  4  transmits an abnormal signal S 3  to the signal transmitter  2  via the communication cable W 1  when judging that the lighting space A 1  is in the abnormal state. The external signal receiver  221  receives the abnormal signal S 3  from the controller  4  via the wire connector  23 , and then supplies the abnormal signal S 3  to the signal selector  223 . The signal selector  223  can recognize that the lighting space A 1  is in any of the normal state and the abnormal state based on the abnormal signal S 3 . Note that in  FIG. 1  the external instruction signal S 1  and the abnormal signal S 3  are transmitted to the signal selector  223  via the same path from the external signal receiver  221 . However, the external instruction signal S 1  and the abnormal signal S 3  may be transmitted to the signal selector  223  via individual paths from the external signal receiver  221 . 
       FIG. 5  shows an example of an operation of the signal selector  223  according to the fourth selection rule. In  FIG. 5 , T 31  represents a normal time period during which no abnormal signal S 3  occurs, and T 32  represents an abnormal time period during which the abnormal signal S 3  occurs. 
     In the example of  FIG. 5 , during the normal time period T 31 , the instructed dimming rate P 1  of the external instruction signal S 1  is 50%, and an instructed dimming rate P 2  of the internal instruction signal S 2  (an internal instruction signal S 21  or S 22 ) is 100%. In this case, the lighting space A 1  is in the normal state, and therefore the signal selector  223  outputs the external instruction signal  51  as the instruction signal S 0 . That is, the instructed dimming rate P 0  contained in the instruction signal S 0  equals the instructed dimming rate P 1  (=50%) contained in the external instruction signal  51 . Therefore, in the light fixture(s)  3  as the control object, the actual dimming rate Pa of the light source(s)  33  is adjusted to 50%. 
     The abnormal state occurs at a time t 11 , and the signal selector  223  outputs the internal instruction signal S 2  as the instruction signal S 0  during an abnormal time period T 32  from the time t 11 . That is, the instructed dimming rate P 0  contained in the instruction signal S 0  equals the instructed dimming rate P 2  (=100%) contained in the internal instruction signal S 2 . Therefore, in the light fixture(s)  3  as the control object, the actual dimming rate Pa of the light source(s)  33  is adjusted to 100%. 
     Thus, it is possible to improve safety when the lighting space A 1  is in the abnormal state, by prioritizing an operational instruction of the remote control device  6  and the operation unit  24  in the lighting space A 1  in order to prioritize a person&#39;s safety in the lighting space A 1  when the lighting space A 1  is in the abnormal state. The abnormal state may be detected in response to the user&#39;s operation of the operation unit  24  or the like. 
     Preferably, the signal transmitter  2  has an overload inhibitory function for preventing the failure by overload. The overload inhibitory function will be explained with reference to  FIG. 6 . 
     Preferably, the detector  26  is configured to detect (measure) a current value of a current I 1  flowing between a negative electrode of the capacitor C 1  and the source of the second switch Q 2  (a detection current I 1 ). The detector  26  may include a resistor R 1  and an amplifier circuit  261 . 
     A first end of the resistor R 1  may be electrically connected to the negative electrode of the capacitor C 1 . A second end of the resistor R 1  may be electrically connected to the source of the second switch Q 2 . The amplifier circuit  261  may be configured to amplify a voltage between both ends of the resistor R 1  by the detection current I 1  to supply the judgement controller  224  with the amplified voltage as a detection value Vs. 
     Preferably, the judgement controller  224  is configured to monitor an output current Ia of the signal transmitter  2 . In the embodiment, the judgement controller  224  is configured to not directly monitor the output current Ia but indirectly monitor the output current Ia based on the detection value Vs of the detection current I 1 . In this case, the judgement controller  224  is configured to receive the detection value Vs from the detector  26  to monitor the output current Ia as a monitor value based on the detection value Vs. The judgement controller  224  is further configured to judge whether the output current Ia as the monitor value is greater than or equal to a first prescribed value It 1  by judging whether the detection value Vs is greater than or equal to a prescribed value. That is, the judgement controller  224  is to judge that an overload state in which the monitor value is excessive when the output current Ia is greater than or equal to the first prescribed value It 1 . 
     The first prescribed value It 1  is, for example a maximum allowed current of the signal transmitter  2 . The maximum allowed current means a limit (a maximum value) of a load current that is allowed to continuously flow through a load without hindrance for practical purposes. The maximum allowed current of the signal transmitter  2  has a current value that is, for example 80% of the rerated current of the signal transmitter. The maximum allowed current of the signal transmitter  2  also corresponds to the number of the light fixtures  3  that are allowed to be electrically connected to the signal transmitter  2  (connection allowable number). In the embodiment, the connection allowable number of the signal transmitter  2  is represented as “N”. Note that the maximum allowed current of the signal transmitter  2  may be a value obtained by considering a usage environment temperature (an ambient temperature) of the signal transmitter  2 . 
     Preferably, the judgement controller  224  is configured to notify the judgement result to the communication controller  225 . Preferably, the communication controller  225  is configured to send out a transmission signal that represents a dimming rate to make the output current Ia smaller than the first prescribed value It 1 , when the judgement controller  224  judges that the output current Ia is greater than or equal to the first prescribed value It 1 . 
     Specifically, when the judgement controller  224  judges that the output current Ia is greater than or equal to the first prescribed value It 1 , the communication controller  225  may change a dimming rate Ps to be instructed by the transmission signal to a first dimming rate M 1  in order to make the output current Ia smaller than the first prescribed value It 1 . The first dimming rate M 1  is, for example smaller than a dimming rate (in  FIG. 6 , 100%) before the judgement controller  224  judges that the output current Ia is greater than or equal to the first prescribed value It 1 . The communication controller  225  can decrease the output current Ia of the signal transmitter  2  to a value that is less than the prescribed value (in  FIG. 6 , It 1 ). The signal transmitter  2  can accordingly suppress the occurrence of failure caused by overload. 
     Preferably, the communication controller  225  is configured to transmit the transmission signal while alternately switching the dimming rate Ps between the first dimming rate M 1  and a second dimming rate M 2 , after changing the dimming rate Ps to the first dimming rate M 1 . The second dimming rate M 2  is, for example smaller than a dimming rate when the output current Ia equals the first prescribed value It 1 . The second dimming rate M 2  is smaller than the first dimming rate M 1 . In this case, the communication controller  225  may set each of a period of time T 41  and a period of time T 42  to a period of time during which a change in light output of the light source  33  is visible to the naked eye, where the period of time T 41  is a period of time during which the dimming rate Ps is the first dimming rate M 1 , and the period of time T 42  is a period of time during which the dimming rate Ps is the first dimming rate M 2 . Each of the period of time T 41  and the period of time T 42  is, for example about several seconds. 
     Hereinafter, an operation of the signal transmitter  2  when a new light fixture  3  is added to N light fixtures  3  will be explained. Note that in the example below the new light fixture  3  is added when all the N light fixtures  3  are turned off, and then all the light fixtures  3  are fully lit. The dimming rate of each light source  33  in the light fixtures  3  when they are fully lit is, for example 100%. 
     In the signal transmitter  2 , when the new light fixture  3  is added to the N light fixtures  3 , the number of the light fixtures  3  exceeds the connection allowable number and therefore the output current I 1  exceeds the maximum allowed current. That is, with the signal transmitter  2 , when the new light fixture  3  is added to the N light fixtures  3 , the output current Ia becomes the first prescribed value It 1  or more. 
     Preferably, when the judgement controller  224  judges that the output current Ia is greater than or equal to the first prescribed value It 1  (in  FIG. 6 , a period of time T 40 ), the communication controller  225  changes the dimming rate Ps to be instructed by the transmission signal to the first dimming rate M 1 . As a result, the output current Ia is decreased to below the first prescribed value It 1 . In the example of  FIG. 6 , after the dimming rate Ps to be instructed by the transmission signal is changed from 100% to the first dimming rate M 1 , the communication controller  225  changes the dimming rate Ps so that the first dimming rate M 1  during the period of time T 41  and the second dimming rate M 2  during the period of time T 42  are alternately repeated. In  FIG. 6 , I 11  represents a value of the output current Ia when the dimming rate Ps is the first dimming rate M 1 , and  112  represents a value of the output current Ia when the dimming rate Ps is the first dimming rate M 2 . In  FIG. 6 , T 40  represents a period of time from when the output current Ia is greater than or equal to the first prescribed value It 1  to when the dimming rate Ps is change from 100% to the first dimming rate M 1  (detection time). The period of time T 40  is, for example about several milliseconds. The second dimming rate M 2  is, for example a low dimming rate by which a change in light output of each light source  33  is visible to the naked eye (a person such as a contractor). However, the second dimming rate M 2  is not limited to the low dimming rate by which a change in light output of each light source  33  is visible to the naked eye. The second dimming rate M 2  may be a dimming rate for turning the light sources  33  off. 
     As stated above, when the new light fixture  3  is added to the N light fixtures  3 , the signal transmitter  2  decreases the dimming rate Ps to be instructed by the transmission signal so that the output current Ia is less than the first prescribed value It 1 . The signal transmitter  2  can accordingly make the output current Ia smaller than the maximum allowed current. That is, when the number of light fixtures  3  exceeds the connection allowable number N, the signal transmitter  2  decreases respective light outputs of the light sources  33  of the light fixtures  3  and dims the light sources  33  of all the light fixtures  3 . The signal transmitter  2  can therefore suppress the occurrence of failure caused by overload. 
     In addition, the signal transmitter  2  transmits the transmission signal so that the first dimming rate M 1  and the second dimming rate M 2  are alternately repeated after the dimming rate Ps to be instructed by the transmission signal is changed to the first dimming rate M 1 , in order to make the output current Ia less than the first prescribed value It 1 . As a result, the signal transmitter  2  can notify the overload to the contractor or the like. 
     Therefore, the signal transmitter  2  according to the embodiment can suppress the occurrence of failure caused by overload and notify users (e.g., contractor or the like) of the overload by dimming or turning off the light fixtures  3 . 
     Note that the second dimming rate M 2  may be greater than the first dimming rate M 1 . It is however necessary to make the second dimming rate M 2  smaller than a dimming rate when the output current Ia equals the first prescribed value It 1 . In this case, the second dimming rate M 2  is greater than the first dimming rate M 1 , and smaller than the dimming rate when the output current Ia equals the first prescribed value It 1 . 
       FIG. 6  illustrates the operation of the signal transmitter  2  when a new light fixture  3  is added to the N light fixtures  3  when they are unlit, and then all the light fixtures  3  are fully lit. However, like operation may be performed even if the new light fixture  3  is added to the N light fixtures  3  when they are lit. 
     As shown in  FIG. 7 , it is preferable that the communication controller  225  be configured to make the first dimming rate lower as a difference between a value of the output current Ia and the first prescribed value It 1  is greater, when the judgement controller  224  judges that the value of the output current Ia is greater than or equal to the first prescribed value It 1 . 
     Specifically, the judgement controller  224  has, as the prescribed value, the first prescribed value It 1  and a second prescribed value It 2  and the communication controller  225  has, as the first dimming rate, first dimming rates M 1  and M 11 . In the example of  FIG. 7 , the second prescribed value It 2  is greater than the first prescribed value It 1 , and the first dimming rate M 11  is lower than the first dimming rate M 1 . When the judgement controller  224  judges that the value of the output current Ia is greater than or equal to the first prescribed value It 1 , and less than the second prescribed value It 2 , the communication controller  225  changes the dimming rate Ps to be instructed by the transmission signal to the first dimming rate M 1  (as illustrated in  FIG. 6 ). When the judgement controller  224  judges that the value of the output current Ia is greater than or equal to the second prescribed value It 2 , the communication controller  225  changes the dimming rate Ps to be instructed by the transmission signal to the first dimming rate M 11  (as illustrated in  FIG. 7 ). 
     That is, the first dimming rate M 11  when the value of the output current Ia is greater than or equal to the second prescribed value It 2  is made smaller than the first dimming rate M 1  when the value of the output current Ia is greater than or equal to the first prescribed value It 1  and less than the second prescribed value It 2 . In the embodiment, data of the first prescribed value It 1  and the second prescribed value It 2  and data of the first dimming rates M 1  and M 11  are stored in a memory of the main controller  22 . 
     Therefore, when the value of the output current Ia is greater than or equal to the second prescribed value It 2 , the signal transmitter  2  can further suppress the occurrence of failure caused by overload by making the first dimming rate smaller than that when the value of the output current Ia is greater than or equal to the first prescribed value It 1  and less than the second prescribed value It 2 . Note that for example, the number of new light fixtures  3  added to the N light fixtures  3  is two or more, when the value of the output current Ia is greater than or equal to the second prescribed value It 2 . 
     Preferably, the communication controller  225  makes the first dimming rate smaller as the number of new light fixtures  3  added to the N light fixtures  3  more increases. Specifically, the communication controller  225  may make the first dimming rate smaller as a difference between the value of the output current Ia and the first prescribed value It 1  is greater. That is, the communication controller  225  may make a first dimming rate Mx smaller as an excess rate a of the output current Ia is greater as shown in  FIG. 8 . The excess rate a of the output current Ia and the first dimming rate Mx are respectively given by 
         a ={(Value of Output Current  Ia )/First Prescribed Value  It 1}−1, and  Mx={ 1/(1+ a )}×100,
 
     where 0≦Mx≦100. 
     Correlation characteristics in  FIG. 8  show that the first dimming rate Mx linearly decreases as the excess rate a of the output current Ia increases, but is not limited to this. The present embodiment may have correlation characteristics showing that the first dimming rate Mx nonlinearly decreases (e.g., along a curved line) as the excess rate a increases or that the first dimming rate Mx stepwise decreases as the excess rate a increases. 
     Preferably, a period of time T 43  of the first dimming rate M 11  (see  FIG. 7 ) is shorter than the period of time T 41  of the first dimming rate M 1  (see  FIG. 6 ). That is, the period of time T 43  of the first dimming rate M 11  to be set when the output current Ia is greater than or equal to the second prescribed value It 2  is shorter than the period of time T 41  of the first dimming rate M 1  to be set when the output current Ia is greater than or equal to the first prescribed value It 1  and less than the second prescribed value It 2 . 
     Preferably, a period of time T 44  of the second dimming rate M 2  (see  FIG. 7 ) is longer than the period of time T 42  (see  FIG. 6 ). That is, the period of time T 44  of the second dimming rate M 2  to be set when the value of the output current Ia is greater than or equal to the second prescribed value It 2  is longer than the period of time T 42  of the second dimming rate M 2  to be set when the value of the output current Ia is greater than or equal to the first prescribed value It 1  and less than the second prescribed value It 2 . 
     Therefore, the signal transmitter  2  can quickly start changing respective light outputs of the light sources  33  to quickly notify overload to a person such as a contractor. The signal transmitter  2  also facilitates recognition of a change in light output of each light source  33  by the person such as the contractor. In the embodiment, data of the period of time T 41  to the period of time T 44  are stored in the memory of the main controller  22 . 
     It is preferable that a sum of the period of time of the first dimming rate and the period of time of the second dimming rate be constant. In this case, T 41 +T 42 =T 43 +T 44 . However, the sum of the period of time of the first dimming rate and the period of time of the second dimming rate is not necessarily constant. 
     The period of time T 43  is not necessarily shorter than the period of time T 41 , but may be the same as the period of time T 41 . 
     Preferably, the communication controller  225  makes the period of time of the second dimming rate M 2  longer as the number of new light fixtures  3  to be added to the N light fixtures  3  increases. Specifically, the communication controller  225  may make the period of time of the second dimming rate M 2  longer as the difference between the value of the output current Ia and the first prescribed value It 1  is greater. That is, the communication controller  225  may make the period of time of the second dimming rate M 2  (length of time Tx) longer as the excess rate a of the output current Ia becomes greater as shown in  FIG. 9 . The period of time of the second dimming rate (length of time Tx) is given by 
     
       
      
       Tx=β×α,  
      
     
     where β is an arbitrary value (e.g., 10). 
     Correlation characteristics in  FIG. 9  show that the period of time of the second dimming rate (length of time Tx) linearly decreases as the excess rate a of the output current Ia becomes greater, but is not limited to this. The present embodiment may have correlation characteristics showing that the length of time Tx nonlinearly increases (e.g., along a curved line) as the excess rate a of the output current Ia becomes greater or that the length of time Tx stepwise increases as the excess rate a becomes greater. 
     Preferably, the communication controller  225  is configured to set the dimming rate Ps to be instructed by the transmission signal based on the instruction signal S 0  from the signal selector  223  and the judgement result by the judgement controller  224 . For example, when the judgement controller  224  judges that the output current Ia is greater than or equal to the first prescribed value It 1  after the communication controller  225  receives the instruction signal S 0  from the signal selector  223 , the communication controller  225  makes the dimming rate Ps to be instructed by the transmission signal smaller than the instructed dimming rate P 0  of the instruction signal S 0 . The signal transmitter  2  can accordingly suppress the occurrence of failure caused by overload. 
     The detector  26  is configured to detect (measure), as the monitor value, the value of the output current Ia of the signal transmitter  2 , but not limited thereto. The detector  26  may be configured to detect (measure), as the monitor value, a value of the output voltage V 2  of the signal transmitter  2 . In this case, the output current of the signal transmitter  2  is controlled constant. 
     The detector  26  may be configured to detect (measure), as the monitor value, one of an input current or input voltage V 1  of the signal transmitter  2 . The detector  26  may be configured to detect (measure), as the monitor value, one of output power or input power of the signal transmitter  2 . 
     Even if the monitor value is any of the input current, the output current Ia, the input voltage V 1 , the output voltage V 2 , the input power and the output current, the signal transmitter  2  can notify users that the monitor value is greater than or equal to the prescribed value. 
     When transmitting a transmission signal that represents a dimming rate and a color temperature, the communication controller  225  preferably operates based on a judgment result by the judgement controller  224  as stated below. 
     For example, when the judgement controller  224  judges that the output current Ia is greater than or equal to the first prescribed value It 1 , the communication controller  225  transmits a transmission signal for changing only the dimming rate of the abovementioned dimming rate and the color temperature. Thus, the communication controller  225  changes only the dimming rate, thereby enabling a person such as contractor to easily recognize a change in light output of the light sources  33  in comparison with a case where both the dimming rate and the color temperature are changed. 
     Alternatively, the communication controller  225  may identify the light fixtures  3 , thereby blinking only a light fixture(s)  3  that is(are) newly added and exceeds the connection allowable number N, or blinking not all the light fixtures  3  but only part thereof. Thus, blinking only the light fixture(s)  3  that is(are) newly added and exceeds the connection allowable number N, or only light fixtures  3  in the vicinity thereof enables the contractor or the like to more easily recognize the occurrence of overload. It is also possible to operate other light fixtures  3  at steady lighting to keep the light environment in normal state. 
     Note that when the judgement controller  224  judges that the monitor value is greater than or equal to the first prescribed value, the communication controller  225  changes the dimming rate Ps to be instructed by the transmission signal to the first dimming rate M 1 , but is not limited thereto. The communication controller  225  may change the dimming rate Ps to be instructed by the transmission signal to the first dimming rate M 1 , when the judgement controller  224  judges that the monitor value is greater than the first prescribed value. 
     In the lighting system  10 , the main controller  22  may be configured to transmit a transmission signal containing data of the dimming rate Ps to the light fixtures  3  via a signal cable. In this case, the output unit  212  further includes signal terminals that allow the signal cable to be electrically connected to. The lighting system  10  further includes the signal cable for transmitting the transmission signal in addition to the two power supply lines E 21  and E 22 . 
     The main controller  22  may be configured to transmit the transmission signal directly to the light fixtures  3 . In this case, the main controller  22  is configured to transmit the transmission signal to the light fixtures  3  over wireless communication media such as radio wave, near infrared light or visible light, for example. 
     The lighting system  10  includes the N light fixtures. The N may be one or more. That is, the lighting system  10  may include one or more light fixtures  3 . 
     Each of the input unit  211  and the output unit  212  is not limited to a connector, but may be a socket or the like. The AC/DC converter  1  is not limited to a step-up chopper circuit having a power-factor correction function, but may be, for example a rectifier circuit including rectifier diodes, or the like. Each of the first and second switches Q 1  and Q 2  is not limited to an N-channel enhancement MOSFET, but may be, for example a P-channel enhancement MOSFET or the like. A driver circuit for the first and second switches Q 1  and Q 2  is provided in the main controller  22  of the signal transmitter  2 , but may be provided outside the main controller  22 . 
     The main controller  22  is not limited to a microcomputer, but may be, for example an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), a control IC (Integrated Circuit) or the like. Preferably, the main controller  22  includes at least a processor. The main controller  22  may include a memory which stores a program executable by the processor so as to carry out the control described herein. 
     The detector  26  includes the amplifier circuit  261 , but need not include the amplifier circuit  261 . In this case, the resistor R 1  is electrically connected directly to the judgement controller  224 . 
     Each of the solid light emitting devices  331  of each light source  33  is not limited to an LED, but may be a solid light emitting device other than the LED, such as an organic Electro Luminescence (OEL), a Laser Diode (LD) or the like. The number of solid light emitting devices  331  is not limited to two or more, but may be one. The solid light emitting devices  331  are connected in series, but not limited thereto. The solid light emitting devices  331  may be connected in parallel or in series-parallel. 
     Each light fixture  3  is not limited to a downlight, but may be, for example a spotlight or the like. The constant current circuit  32  of the light fixture  3  is not limited to a step-down chopper circuit, but may be, for example a step-up chopper circuit or a step-up/down chopper circuit. Each constant current circuit  32  is configured to control a light output of a corresponding light source  33  by PWM, but not limited thereto. For example, each constant current circuit  32  may be configured to control a light output of a corresponding light source  33  by amplitude modulation, or by PWM and amplitude modulation. 
     Hereinafter, a first modified example of the embodiment will be explained with reference to  FIG. 10 . 
     In the example of  FIG. 10 , a controller  4 , light fixtures  7  and a sensor  8  are electrically connected to a first system  91  configured to be supplied with commercial power from the commercial power supply  9 . The light fixtures  7  and the sensor  8  are disposed outside the lighting space A 1  stated above. 
     The lighting system  10  stated above is connected to a second system  92  configured to be supplied with commercial power from the commercial power supply  9 . 
     In the modified example, detection signals of the sensors  5  and  8  are transmitted to the controller  4  by wireless communication. The controller  4  is configured to communicate with the signal transmitter  2  via the communication cable W 1 . The controller  4  can therefore control the lighting states of the light fixtures  3  based on not only the detection signal of the sensor  5  but also a detection signal of the sensor  8 . 
     Based on respective instruction signals from the controller  4 , the remote control device  6  and the operation unit  24 , the signal transmitter  2  can transmit the transmission signal to the light fixtures  3  to control the lighting states of the light fixtures  3 . 
     A second modified example of the embodiment will be explained with reference to  FIG. 11 . 
     In a lighting system  10 B shown in the example of  FIG. 11 , signal transmitters  2  are electrically connected to one AC/DC converter  1 . Each of the signal transmitters  2  is configured to supply an output voltage V 2  to its own light fixtures  3  and sensor  5  to perform lighting control like the abovementioned embodiment. Note that in  FIG. 11  the signal transmitters  2  are electrically connected with light fixture sets  3 A,  3 B, and the like, respectively. That is, a lighting system  10 B includes the light fixture sets  3 A,  3 B, and the like as the control object, and the signal transmitters  2  correspond one-to-one to the light fixture sets  3 A,  3 B and the like. 
     In the modified example, the one AC/DC converter  1  supplies DC power to the signal transmitters  2 , and therefore the system configuration can be simplified in comparison with the case where the signal transmitters  2  are electrically connected one-to-one to AC/DC converters  1 . 
     As stated above, a signal transmitter  2  according to a first aspect of the embodiment includes a power supply  21 , an external signal receiver  221  and a communication controller  225 . The power supply  21  is configured to receive DC power to supply a DC output voltage V 2  to a light fixture(s)  3 . The external signal receiver  221  is configured to receive an external instruction signal S 1  that represents a lighting state of the light fixture  3  based on an output (signal) of a sensor (set)  5  configured to detect a state(s) of a lighting space A 1  illuminated by the light fixture(s)  3 . The communication controller  225  is configured to control signal transmission to the light fixture  3  by controlling the power supply  21 . The communication controller  225  is configured to change an output voltage level of the power supply  21  to transmit, to the light fixture(s)  3 , a transmission signal for setting the light fixture(s)  3  to the lighting state represented by at least the external instruction signal S 1 . As an example, the signal transmitter  2  may be configured to output the output voltage V 2  to the light fixtures  3  and transmit the transmission signal to the light fixtures  3 . 
     The signal transmitter  2  can therefore decrease transmission noise of the transmission signal and leakage of the transmission signal by changing the output voltage level to transmit the transmission signal in comparison with the case where a high frequency transmission signal is superposed on a DC voltage to be sent out. That is, the signal transmitter  2  can light the light fixture  3  by supplying DC power thereto and decrease the transmission noise of the transmission signal and the leakage of the transmission signal. 
     The signal transmitter  2  can change the output voltage V 2 , specifically the output voltage level of the power supply  21 , thereby transmitting the transmission signal. The signal transmitter  2  can stabilize signal transmission and lengthen the transmission distance in comparison with the configuration in which a high frequency transmission signal is output with the transmission signal superposed on a DC voltage because it is possible to suppress the attenuation of the transmission signal, caused by the inductance of the two power supply lines E 21  and E 22 . 
     In the first aspect, it is preferable that the transmission signal contain identification data uniquely assigned to the light fixture  3 , and control data representing the lighting state to be instructed to the light fixture  3  (hereinafter referred to as a “second aspect”). 
     The signal transmitter  2  can therefore perform individual control and group control of light fixtures  3 . 
     In a first or second aspect, the power supply  21  includes two input terminals  21 A and  21 B, a series circuit of a first switch Q 1  and a second switch Q 2 , a first diode D 1 , a second diode D 2 , a series circuit of a capacitor C 2  and an inductor L 1 , and two output terminals  21 C and  21 D. The two input terminals  21 A and  21 B are configured to receive the DC power. The series circuit of the first and second switches Q 1  and Q 2  is electrically connected between the two input terminals  21 A and  21 B. The first diode D 1  is anti-parallel connected to the first switch Q 1 . The second diode D 2  is anti-parallel connected to the second switch Q 2 . The series circuit of the capacitor C 2  and the inductor L 1  is connected between both ends of the first switch Q 1 . The output terminals  21 C and  21 D are electrically connected to high and low potential sides of the capacitor C 2 , respectively. The two output terminals  21 C and  21 D are configured to output the DC output voltage V 2 . The communication controller  225  is configured to control the first and second switches Q 1  and Q 2  to change the output voltage level. Hereinafter, the configuration is referred to as a “third aspect”. 
     The signal transmitter  2  can therefore transmit the transmission signal through the comparative simple configuration (circuit configuration). For example, the signal transmitter  2  can comparatively decrease the number of switches constituting the transmitter and a conduction loss of the switches. That is, the signal transmitter  2  can reduce the transmission loss. 
     In any one of the first to third aspects, it is preferable that the signal transmitter  2  further include an internal signal receiver  222  and a signal selector  223 . The internal signal receiver  222  is configured to receive an internal instruction signal S 2  that is generated according to user&#39;s operation and that represents the lighting state of the light fixture  3 . The signal selector  223  is configured to select either the external instruction signal S 1  or the internal instruction signal S 2  to be supplied to the communication controller  225 . The communication controller  225  is configured to transmit the transmission signal for setting the light fixture  3  to a lighting state represented by either the external instruction signal S 1  or the internal instruction signal S 2 , from the signal selector  223 . Hereinafter, the configuration is referred to as a “fourth aspect”. 
     The signal transmitter  2  can therefore select any of: the external instruction signal S 1  based on the output of the sensor  5  configured to detect the state of the lighting space A 1 ; and the internal instruction signal S 2  based on the user&#39;s operation. As a result, the signal transmitter  2  can switch between automatic control based on the output of the sensor  5  and manual control by the user&#39;s operation. 
     In the fourth aspect, preferably the signal selector  223  is configured to select either the external instruction signal S 1  or the internal instruction signal S 2  according to the state of the lighting space A 1  (hereinafter, referred to as a “fifth aspect”). 
     The signal transmitter  2  can therefore switch between the automatic control based on the output of the sensor  5  and the manual control based on the user&#39;s operation, in accordance with the state of the lighting space A 1 . 
     In the fifth aspect, preferably the signal selector  223  is configured to: select the external instruction signal S 1  when the state of the lighting space A 1  is in a normal state; and select the internal instruction signal S 2  when the state of the lighting space A 1  is in an abnormal state (hereinafter, referred to as a “sixth aspect”). 
     It is possible to improve safety when the lighting space A 1  is in the abnormal state because the signal transmitter  2  prioritizes an operational instruction of the remote control device  6  and the operation unit  24  in the lighting space A 1  in order to prioritize the person&#39;s safety in the lighting space A 1  when the lighting space A 1  is in the abnormal state. 
     In any one of the first to sixth aspects, preferably the signal transmitter  2  further includes a judgement controller  224 . The judgement controller  224  is configured to monitor any one or more of an output current Ia, the output voltage V 2 , output power, an input current, an input voltage V 1  and input power of the power supply  21  as a monitor value and judge whether the monitor value is greater than or equal to a prescribed value. The communication controller  225  is configured to transmit the transmission signal for changing a dimming rate of the light fixture  3  so that the monitor value is smaller than the prescribed value, when the judgement controller  224  judges that the monitor value is greater than or equal to the prescribed value. Hereinafter, the configuration is referred to as a “seventh aspect”. 
     When the number of light fixtures  3  exceeds the connection allowable number N, the signal transmitter  2  can therefore suppress the occurrence of failure caused by overload by decreasing the light output of the light source  33  per light fixture  3  to light respective light sources  33  of all the light fixtures  3 . 
     In the seventh aspect, the communication controller  225  is configured to, when the judgement controller  224  judges that the monitor value is greater than or equal to the prescribed value, change the dimming rate of the light fixture  3  to a first dimming rate M 1  so that the monitor value is less than the prescribed value. Preferably, the communication controller  225  then transmits the transmission signal for changing the dimming rate by repeatedly alternating between the first dimming rate M 1  and a second dimming rate M 2  that is lower than the dimming rate when the monitor value is equal to the prescribed value. Hereinafter, the configuration is referred to as an “eighth aspect”. 
     The signal transmitter  2  can therefore notify persons of overload by transmitting the transmission signal for changing the dimming rate so that the first dimming rate M 1  and the second dimming rate M 2  are alternately repeated, when the monitor value is greater than or equal to the prescribed value. 
     In the eighth aspect, preferably the communication controller  225  is configured to, when the judgement controller  224  judges that the monitor value is greater than or equal to the prescribed value, make the first dimming rate lower as a difference between the monitor value and the prescribed value is greater (hereinafter, referred to as a “ninth aspect”). 
     The signal transmitter  2  can therefore further suppress the occurrence of failure caused by overload when the connection number of light fixtures  3  exceeds the connection allowable number N. 
     In an eighth or ninth aspect, preferably the communication controller  225  is configured to, when the judgement controller  224  judges that the monitor value is greater than or equal to the prescribed value, make a period of time during which the dimming rate is the second dimming rate M 2  longer as a difference between the monitor value and the prescribed value is greater (hereinafter, referred to as a “tenth aspect”). 
     The signal transmitter  2  can comparatively lengthen a period of time during which the light source  33  is dimmed with the light output thereof decreased, and therefore facilitates the recognition of a change in light output of the light source  33  by the person such as the contractor. 
     In any one of the seventh to tenth aspects, the communication controller  225  is configured to transmit the transmission signal for changing the dimming rate and a color temperature of the light fixture  3 . Preferably, the communication controller  225  is configured to change only the dimming rate so that the monitor value is less than the prescribed value, when the judgement controller  224  judges that the monitor value is greater than or equal to the prescribed value. Hereinafter, the configuration is referred to as an “eleventh aspect”. 
     The signal transmitter  2  facilitates the recognition of a change in light output of the light source  33  by a person such as contractor by changing only the dimming rate in comparison with the case where both the dimming rate and the color temperature are changed. 
     In any one of the first to eleventh aspects, the communication controller  225  is configured to change the output voltage level between a first voltage level V 21  and a second voltage level V 22  to transmit the transmission signal. Hereinafter, the configuration is referred to as a “twelfth aspect”. 
     In any one of the first to twelfth aspects, the lighting state represents at least one of a color temperature and a dimming amount (dimming rate) of the lighting fixture  3 . Hereinafter, the configuration is referred to as a “thirteenth aspect”. 
     A signal transmitter  2  according to a fourteenth aspect of the embodiment includes a power supply  21 , an external signal receiver  221  and a communication controller  225 . The power supply  21  is configured to receive DC power to supply a DC output voltage V 2  to a light fixture  3 . The external signal receiver  221  is configured to receive an external instruction signal S 1  that represents a lighting state of the light fixture  3  based on an output of a sensor  5  configured to detect a state of a lighting space A 1  illuminated by the light fixture  3 . The communication controller  225  is configured to transmit, to the light fixture  3 , a transmission signal for setting the light fixture  3  to the lighting state represented by at least the external instruction signal S 1 . 
     The signal transmitter  2  can decrease transmission noise of the transmission signal and leakage of the transmission signal. 
     In the fourteenth aspect, the communication controller  225  controls the power supply  21  to change the DC output voltage V 2  of the power supply  21  to transmit the transmission signal. The DC output voltage V 2  is for providing operating current to a light source  33  included in the light fixture  3 . Hereinafter, the configuration is referred to as a “fifteenth aspect”. 
     In a fourteenth or fifteenth aspect, a signal transmitter  2  includes at least one of a signal cable through which the communication controller  225  transmits the transmission signal to the light fixture  3 , or a wireless communication interface through which the communication controller  225  transmits the transmission signal to the light fixture  3  wirelessly. Hereinafter, the configuration is referred to as a “sixteenth aspect”. 
     In any one of the first to sixteenth aspects, the communication controller  225  is configured to: adjust the output voltage level to a first voltage level V 21  by making a detection result of the DC output voltage V 2  accord with a voltage of a first target value; and adjust the output voltage level to a second voltage level V 22  by making the detection result of the DC output voltage V 2  accord with a voltage of a second target value. Hereinafter, the configuration is referred to as a “seventeenth aspect”. In an example, the signal transmitter  2  includes a voltage detector circuit configured to detect the output voltage V 2  to obtain the detection result. In another example, the communication controller  225  is provided with the detection result from an electric conductor for detecting the output voltage V 2  to provide the communication controller  225  with the detection result, which is electrically connected between the positive output terminal of the power supply  21  and the communication controller  225 . 
     Thus, the communication controller  225  adjusts the output voltage level to the first and second voltage levels V 21  and V 22  and can thereby change the output voltage level to transmit the transmission signal to the light fixture  3 . The communication controller  225  can therefore decrease transmission noise of the transmission signal and leakage of the transmission signal. 
     A lighting system  10 ,  10 A or  10 B according to an eighteenth aspect of the embodiment includes a signal transmitter  2  of any one of the first to seventeenth aspects, an AC/DC converter  1  configured to receive AC power to supply the DC power to the signal transmitter  2 , and the light fixture  3 . 
     By including the signal transmitter  2 , a lighting system  10 ,  10 A or  10 B can light the light fixture  3  by supplying DC power thereto and decrease the transmission noise and the leakage of the transmission signal. 
     A lighting system  10 B according to a nineteenth aspect of the embodiment includes signal transmitters  2 , one AC/DC converter  1  and light fixture sets  3 A,  3 B and the like. The AC/DC converter  1  is configured to receive AC power to supply DC power to each of the signal transmitters  2 . 
     The light fixture sets  3 A,  3 B and the like are electrically connected to the signal transmitters  2 , respectively. Each of the light fixture sets  3 A,  3 B and the like includes at least a light fixture  3 . Each of the signal transmitters  2  includes a power supply  21 , an external signal receiver  221  and a communication controller  225 . The power supply  21  is configured to receive the DC power to supply a DC output voltage V 2  to a corresponding light fixture set. The external signal receiver  221  is configured to receive an external instruction signal S 1  that represents a lighting state of the corresponding light fixture set based on an output of a sensor  5  configured to detect a state of a lighting space A 1  illuminated by the corresponding light fixture set. The communication controller  225  is configured to control signal transmission to the corresponding light fixture set by controlling the power supply  21 . The communication controller  225  is configured to change an output voltage level of the power supply  21  to transmit, to the corresponding light fixture set, a transmission signal for setting the corresponding light fixture set to the lighting state represented by at least the external instruction signal  51 . As an example, each signal transmitter  2  may be configured to supply the output voltage V 2  to its own light fixtures  3  and transmit the transmission signal to the light fixtures  3 . 
     The system configuration of the lighting system  10 B can be simplified in comparison with the case where the signal transmitters  2  are electrically connected one-to-one to AC/DC converters  1 . 
     In the nineteenth aspect, the transmission signal contains identification data uniquely assigned to the light fixture  3 , and control data representing the lighting state for the light fixture  3 . Hereinafter, the configuration is referred to as a “twentieth aspect”. 
     A light fixture  3  according to a twenty-first aspect of the embodiment includes a light source  33 , a constant current circuit  32  and a signal receiver  31 . The constant current circuit  32  is configured to receive a DC output voltage V 2  to supply a load current to the light source  33 . The DC output voltage V 2  is input via two power supply lines E 21  and E 22  from the abovementioned signal transmitter  2  of the first aspect. The signal receiver  31  is configured to monitor the DC output voltage V 2 . The signal receiver  31  is configured to detect a change in the output voltage level to acquire a transmission signal and adjust a lighting state of the light fixture  33  by controlling the constant current circuit  32  based on data of the transmission signal. 
     The light fixture  3  receives the DC voltage V 2  from the signal transmitter  2 , and can therefore light the light source  33  by the DC power and decrease transmission noise and leakage of the transmission signal. 
     While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that they may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all modifications and variations that fall within the true scope of the present teachings.