Abstract:
A smart dimmer for controlling the intensity of a lighting load from a source of AC power is operable to replace one of the three-way switches in a standard three-way lighting system without the need to replace the other three-way switch with a unique accessory switch. A simple rewiring is needed in the wallbox of the remaining three-way switch. In the resulting three-way lighting system, the smart dimmer is always coupled between the lighting load and the source and the remaining three-way switch is coupled between either of the load terminals of the dimmer and an accessory terminal of the dimmer. The remaining three-way switch acts to either couple or decouple an AC voltage from the accessory terminal. The smart dimmer is operable to detect a change in the state at the accessory terminal and toggle the lighting load on or off as a result of the change in state.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a wall mountable dimmer switch that can be wired for use with a three-way switch without the need to buy a special dimmer or switch. In particular, the present invention relates to a dimmer switch that can be substituted for either a line-side three-way switch or a load-side three-way switch in lighting circuit having two points of control, without the need to change or replace the other three-way switch.  
       BACKGROUND OF THE INVENTION  
       [0002]     Three-way switch systems for use in controlling loads in buildings, such as lighting loads, have long been known in the art. The switches used in these systems are wired to the building&#39;s alternating-current (AC) wiring system, are subjected to AC source voltage, and carry full load current, as opposed to low-voltage switch systems that operate at low voltage and low current and communicate digital commands (usually low-voltage logic levels) to a remote controller that controls the level of AC power delivered to the load in response to the commands. Thus, as used herein, the terms “three-way switch” and “three-way system” mean such switches and systems that are subjected to the AC source voltage and carry the full load current.  
         [0003]     In a three-way switch system, there are two three-way switches for controlling a single load, and each switch is fully operable to independently control the load irrespective of the status of the other switch. In such a system, one three-way switch must be wired at the AC source side of the system (sometimes called “line side”), and the other three-way switch must be wired at the load side of the system.  
         [0004]      FIG. 1A  shows a standard three-way switch system  100 , which includes two three-way switches  102 ,  104 . The switches  102 ,  104  are connected between an AC power source  106  and a lighting load  108 . When switches  102 ,  104  are both in position A (or both in position B), the circuit is complete and the lighting load  108  is energized. When switch  102  is in position A and switch  104  is in position B (or vise versa), the circuit is not complete and the lighting load  108  does not light up.  
         [0005]     Three-way dimmer switches that replace three-way switches are well known in the art. An example of a three-way dimmer switch system  150  including one prior art three-way dimmer switch  152  and one three-way switch  104  is shown in  FIG. 1B . The three-way dimmer switch  152  simply includes a dimmer circuit  152 A and a three-way switch  152 B. A typical, AC, phase-control dimmer  152  regulates the amount of energy supplied to the lighting load  108  by conducting for some portion of each half-cycle of the AC waveform, and not conducting for the remainder of the half-cycle. Because the dimmer switch  152  is in series with the lighting load  108 , the longer the dimmer  152  conducts, the more energy will be delivered to the lighting load  108 . Where the lighting load  108  is a lamp, the more energy delivered to the lighting load  108 , the greater the light intensity level of the lamp. In a typical dimming scenario, a user may adjust a control to set the light intensity level of the lamp to a desired light intensity level. The portion of each half-cycle for which the dimmer conducts is based on the selected light intensity level. Since two dimmer circuits cannot be wired in series, the three-way dimmer switch system  150  can only include one three-way dimmer switch  152 , which can be located on either the line side or the load side of the system.  
         [0006]     Three-way dimming systems that employ a “smart” dimmer switch and a specially designed auxiliary (remote) switch that permits the dimming level to be adjusted from multiple locations have been developed. A smart dimmer is one that includes a microcontroller or other processing means for allowing an advanced set of control features and feedback options to the end user. To power the microcontroller, smart dimmers include power supplies, which draw a small amount of leakage current through the lighting load each half-cycle when the FETs are non-conducting. The power supply uses this small amount of current to charge a capacitor and develop a direct-current (DC) voltage to power the microcontroller. An example of a multiple location lighting control system, including a wall mountable smart dimmer switch and wall mountable remote switches for wiring at all locations of a multiple location switch system is disclosed in commonly assigned U.S. Pat. No. 5,248,919, issued on Sep. 28, 1993, entitled “Lighting Control Device”, which is herein incorporated by reference in its entirety.  
         [0007]     Referring to the system  150  of  FIG. 1B , since no load current flows through the dimmer circuit  152 A of the three-way dimmer switch  152  when the circuit between the supply  106  and the lighting load  108  is broken by either three-way switch  152 B or  104 , the dimmer switch  152  is not able to include a power supply and a microcontroller. Thus, the dimmer switch  152  is not able to provide the advanced set of features of a smart dimmer to the end user.  
         [0008]      FIG. 2  shows an example multiple location lighting control system  200  including one wall mountable smart dimmer switch  202  and one wall mountable remote, or accessory, switch  204 . The dimmer switch  202  has a Hot (H) terminal, for receipt of an AC source voltage provided by an AC power supply  206 , and a Dimmed Hot (DH) terminal, for providing a dimmed-hot voltage to a lighting load  208 . The remote switch  204  is connected in series with the DH terminal of the dimmer switch  202  and the lighting load  208  and simply passes the dimmed-hot voltage through to the lighting load.  
         [0009]     The dimmer switch  202  and the remote switch  204  both have actuators to allow for raising, lowering, and toggling on/off the lighting load  208 . The dimmer switch  202  is responsive to actuation of any of these actuators to alter the dimming level (or power the lighting load  208  on/off) accordingly. In particular, actuation of an actuator at the remote switch  204  causes an AC control signal, or partially rectified AC control signal, to be communicated from that remote switch  204  to the dimmer switch  202  over the wiring between the Accessory Dimmer (AD) terminal of the remote switch  204  and the AD terminal of the dimmer switch  202 . The dimmer switch  202  is responsive to receipt of the control signal to alter the dimming level or toggle the load on/off. Thus, the load can be fully controlled from the remote switch  204 .  
         [0010]     The user interface of the dimmer switch  202  of the multiple location lighting control system  200  is shown in  FIG. 3 . As shown, the dimmer switch  202  may include a faceplate  310 , a bezel  312 , an intensity selection actuator  314  for selecting a desired level of light intensity of a lighting load  208  controlled by the dimmer switch  202 , and a control switch actuator  316 . Faceplate  310  need not be limited to any specific form, and is preferably of a type adapted to be mounted to a conventional wall box commonly used in the installation of lighting control devices. Likewise, bezel  312  and actuators  314  and  316  are not limited to any specific form, and may be of any suitable design that permits manual actuation by a user.  
         [0011]     Actuation of the upper portion  314 A of actuator  314  increases or raises the light intensity of lighting load  208 , while actuation of lower portion  314 B of actuator  314  decreases or lowers the light intensity. Actuator  314  may control a rocker switch, two separate push switches, or the like. Actuator  316  may control a push switch, though actuator  316  may be a touch-sensitive membrane or any other suitable type of actuator. Actuators  314  and  316  may be linked to the corresponding switches in any convenient manner. The switches controlled by actuators  314  and  316  may be directly wired into the control circuitry to be described below, or may be linked by an extended wired link, infrared link, radio frequency link, power line carrier link, or otherwise to the control circuitry.  
         [0012]     Dimmer switch  202  may also include an intensity level indicator in the form of a plurality of light sources  318 , such as light-emitting diodes (LEDs). Light sources  318  may be arranged in an array (such as a linear array as shown) representative of a range of light intensity levels of the lighting load being controlled. The intensity levels of the lighting load may range from a minimum intensity level, which is preferably the lowest visible intensity, but which may be zero, or “full off,” to a maximum intensity level, which is typically “full on.” Light intensity level is typically expressed as a percent of full intensity. Thus, when the lighting load is on, light intensity level may range from 1% to 100%.  
         [0013]     A simplified block diagram of the dimmer switch  202  and the remote switch  204  of the multiple location lighting control system  200  is shown in  FIG. 4A . The dimmer switch  202  employs a controllably conductive device, such as two field-effect transistors (FETs)  420 ,  422  provided in anti-serial connection between the Hot terminal H and the Dimmed Hot terminal DH, to control the current through, and thus the intensity of, the lighting load  208 . The first FET  420  conducts during the positive half-cycle of AC waveform and the second FET  422  conducts during the negative half-cycle of the AC waveform. The gates of FETs  420 ,  422  are connected to a gate drive circuit  424 , which provides control inputs to the FETs in response to command signals from a microcontroller  426 . Alternatively, the controllably conductive device could be implemented as another type of semiconductor switch, such as a triac or a silicon-controlled rectifier (SCR).  
         [0014]     Microcontroller  426  may be any suitable processing device, such as a programmable logic device (PLD), a microprocessor, or an application specific integrated circuit (ASIC). Microcontroller  426  generates command signals to a plurality of LEDs  418  for feedback to the user of the dimmer switch  202 . The microcontroller  426  receives inputs from a zero-crossing detector  430  and a signal detector  432 .  
         [0015]     A power supply  628  generates two DC output voltages V CC1  and V CC2 . The first output voltage V CC1  has a magnitude appropriate to power the microcontroller  626  and other low-voltage circuitry (such as 3.3 V DC  or 5 V DC ). The second output voltage V CC2  has a magnitude greater than V CC1  (approximately 8 V DC ) and is provided to the gate drive circuit  624  for driving the FETs  620 A,  620 B.  
         [0016]     The zero-crossing detector  430  determines the zero-crossing points of the input 120V, 60 Hz AC waveform from the AC power supply  206 . The zero-crossing information is provided as an input to microcontroller  426 . Microcontroller  426  provides the gate control signals to operate FETs  420 ,  422  to provide voltage from the AC power supply  206  to the lighting load  208  at predetermined times relative to the zero-crossing points of the AC waveform.  
         [0017]     Generally, two techniques are used for controlling the power supplied to the lighting load  208 : forward phase control dimming and reverse phase control dimming. In forward phase control dimming, the FETs  420 ,  422  are turned on at some point within each AC line voltage half-cycle and remains on until the next voltage zero-crossing. Forward phase control dimming is often used to control energy to a resistive or inductive load, which may include, for example, a magnetic low-voltage transformer or an incandescent lamp. In reverse phase control dimming, the FETs  420 ,  422  are turned on at the zero-crossing of the AC line voltage and turned off at some point within each half-cycle of the AC line voltage. Reverse phase control is often used to control energy to a capacitive load, which may include, for example, an electronic low-voltage transformer.  
         [0018]     Signal detector  432  has an input  440  for receiving switch closure signals from momentary switches designated T, R, and L. Switch T corresponds to a toggle switch controlled by switch actuator  316 , and switches R and L correspond to the raise and lower switches controlled by the upper portion  314 A and lower portion  314 B, respectively, of intensity selection actuator  314 .  
         [0019]     Closure of switch T will connect the input of the signal detector  432  to the DH terminal of the dimmer switch  202  when the FETs  420 ,  422  are non-conducting, and will allow both positive and negative half-cycles of the AC current to flow through the signal detector. Closure of switches R and L will also connect the input of the signal detector  432  to the DH terminal when the FETs  420 ,  422  are non-conducting. However, when switch R is closed, current can only flow through the signal detector  432  during the negative half-cycle of the AC power supply  406  because of a diode  434 . In similar manner, when switch L is closed, current can only flow through the signal detector  432  during the positive half-cycles because of a diode  436 . The duration of switch closures of switches T, R, and L are typically 100-200 milliseconds in length. The signal detector  432  detects when the switches T, R, and L are closed, and provides two separate output signals representative of the state of the switches as inputs to the microcontroller  426 . A signal on the first output of the signal detector  432  indicates a closure of switch R and a signal on the second output indicates a closure of switch L. Simultaneous signals on both outputs represent a closure of switch T. The microcontroller  426  determines the duration of closure in response to inputs from the signal detector  432 .  
         [0020]     The remote switch  204  provides a means for controlling the dimmer switch  202  from a remote location in a separate wall box. The remote switch  204  includes a further set of momentary switches T′, R′, and L′ and diodes  434 ′ and  436 ′. A wire connection is made between the AD terminal of the remote switch  204  and the AD terminal of the dimmer switch  202  to allow for the communication of actuator presses at the remote switch. The AD terminal is connected to the input  440  of the signal detector  432 . The action of switches T′, R′, and L′ in the remote switch  204  corresponds to the action of switches T, R, and L in the dimmer switch  202 .  
         [0021]     A schematic representation of the signal detector  432  is shown in  FIG. 4B . The input  440  if the signal detector  432  is received from the switches T, R, and L and the AD terminal. Two outputs  442  (AD_LOWER) and  444  (AD_RAISE) are provided to the microprocessor  426 . When the lower switch L is pressed, current will flow out of the input  440  through a diode D 1  and two resistors R 1 , R 2  of the signal detector  432  during the positive half-cycles of the AC power supply  406 . When the current flows, a bias voltage will develop across the resistor R 2 , which will cause a transistor Q 1  to begin conducting, thus pulling the output AD_LOWER up to the level of the voltage V CC2 . A resistor R 3  pulls the voltage at the output AD_LOWER down to circuit common during the negative half-cycles. Thus, an active-high control signal that consists of a pulse during each positive half-cycle will be generated at the output AD_LOWER when the switch L is pressed.  
         [0022]     When the raise switch R is pressed and the breakdown voltage of a zener diode Z 1  is exceeded, current will flow into the input  440  through a diode D 2 , the zener diode Z 1 , and two resistors R 4 , R 5  during the negative half-cycles. The zener diode Z 1  limits the voltage across the resistors R 4 , R 5  and thus the current through the resistors. A bias voltage produced across resistor R 5  when current flows will cause a transistor Q 2  to begin conducting and the output AD_RAISE will then be pulled down to circuit common. A resistor R 6  is provided to pull the voltage at the output AD_RAISE up to the voltage V CC1  during the positive half-cycles. In this case, an active-low control signal that consists of a pulse during each negative half-cycle will be generated at the output AD_RAISE when the switch R is pressed.  
         [0023]     When the toggle switch T is pressed, current will flow through the signal detector  632  during both half-cycles and both of the control signals as described above will be generated at the outputs AD_LOWER and AD_RAISE.  
         [0024]     When the switches T′, R′, and L′ are pressed on the remote switch  504 , the signal detector  432  functions the same as when the switches T, R, and L are pressed. Also, the signal detector  432  will function similarly if the remote switch  504  is located on the line side of the dimmer switch  502 . However, when switch L′ is pressed in this case, the diode D 1  will conduct during the negative half-cycles and the signal at the AD_LOWER output will have pulses during the negative half-cycles. Further, when the switch R′ is pressed, the diode D 2  will conduct during the positive half-cycles and the signal at the AD_RAISE output will have pulses during the positive half-cycles.  
         [0025]     Even though the multiple location lighting control system  200  allows for the use of a smart dimmer switch in a three-way system, it is necessary for the customer to purchase the remote switch  204  along with the smart dimmer switch  202 . Often, the typical customer is unaware that a remote switch is required when buying a smart dimmer switch for a three-way system until after the time of purchase when the smart dimmer switch is installed and it is discovered that the smart dimmer will not work properly with the existing three-way switch. Therefore, there exists a need for a smart three-way dimmer switch that may be installed in a three-way system without the need to purchase and install a special remote switch.  
       SUMMARY OF THE INVENTION  
       [0026]     In accordance with the present invention, a novel lighting control system for controlling the intensity of a lighting load from a source of AC power includes a load control device and a maintained switch. The dimmer switch includes two load terminals, a controllably conductive device coupled between the load terminals for carrying current to the load, and an accessory terminal. The load control device is operable to toggle the lighting load between an ON state when the lighting load is illuminated and an OFF state when the lighting load is not illuminated. The maintained switch is coupled between the accessory terminal and either one of the load terminals of the load control device. The maintained switch has a closed state in which the accessory terminal is coupled to either one of the load terminals and an open state in which the accessory terminal is not coupled to either one of the load terminals. The load control device is operable to toggle the lighting load when the maintained switch changes between the open state and the closed state.  
         [0027]     In another aspect, the present invention provides a method for controlling the intensity of a lighting load from a source of AC power in a lighting control system comprising a load control device and a maintained switch. The load control device includes two load terminals and an accessory terminal. The maintained switch has a closed state for coupling the accessory terminal to one of the two load terminals and an open state for decoupling the accessory terminal from the load terminal. The steps of the method comprise storing a previous state of the maintained switch, detecting the present state of the maintained switch, and comparing the present state with the previous state, and changing the intensity of the lighting load based on the step of comparing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1A  is a simplified block diagram of a standard three-way switch system;  
         [0029]      FIG. 1B  is a simplified block diagram of a prior art three-way dimmer switch system;  
         [0030]      FIG. 2  is a simplified block diagram of a typical prior art multiple location lighting control system;  
         [0031]      FIG. 3  is a user interface of the dimmer switch of the multiple location lighting control system of  FIG. 2 ;  
         [0032]      FIG. 4A  is a simplified block diagram of the dimmer switch and the remote switch of the multiple lighting control system of  FIG. 2 ;  
         [0033]      FIG. 4B  is a schematic representation of the signal detector of the dimmer switch of  FIG. 4A ;  
         [0034]      FIG. 5  is a simplified block diagram of the three-way dimmer switch system of the present invention; and  
         [0035]      FIG. 6  is a flowchart of the process for monitoring the AD terminal of the dimmer switch of the three-way dimmer switch system of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed.  
         [0037]     Referring to  FIG. 5 , there is shown a three-way dimmer switch system  500  in accordance with the present invention. The system  500  includes a novel smart three-way dimmer switch  502  and a standard maintained three-way switch  504 . There is no need for the installer to purchase a unique remote switch to replace the three-way switch  504 . The smart three-way dimmer switch  502  is wired in place of the line-side three-way switch  102  in  FIG. 1A  and is connected to an AC power source  506 . A simple rewiring  510  is required in the wallbox of the three-way switch  504  in order to disconnect the DH terminal of the smart three-way dimmer switch  502  from the first switch position of the three-way switch  504  (position A in  FIG. 5 ) and to connect the DH terminal to the lighting load  508 . The other switch position of the three-way switch (position B in  FIG. 5 ) is connected to the AD terminal of the smart three-way dimmer switch  502 .  
         [0038]     The result is that the smart three-way dimmer switch  502  is always connected between the AC power source  506  and the lighting load  508  independent of the position of three-way switch  504 . The three-way switch  504  now operates by either connecting the Dimmed Hot voltage to or disconnecting the Dimmed Hot voltage from the AD terminal on the smart three-way dimmer switch. The smart three-way dimmer switch  502  could also be wired to the load side of system  500  and operation of the three-way switch  504  would connect and disconnect the AC power source voltage to and from the AD terminal on the smart three-way dimmer switch. Also, a two-way switch could be used in place of three-way switch  504  since the first position A is not being used.  
         [0039]     The electrical schematic of the smart three-way dimmer switch  502  is exactly the same as the electrical schematic for the prior art smart three-way dimmer switch  202  shown in  FIG. 4A . Because of the operation of the three-way switch  504 , the signal detector  432  of the smart dimmer  502  of the present invention will either provide signals at both outputs AD_RAISE and AD_LOWER simultaneously or no signals at both outputs.  
         [0040]     The smart three-way dimmer switch  502  of the present invention has novel operating software running on microprocessor  426  in order to correctly function in system  500 . Rather than receiving a signal at the AD terminal that is a short pulse (100-200 milliseconds) representing a closure of one of the momentary switches T′, R′, L′ in remote switch  204 , the smart three-way dimmer switch  502  determines when the voltage at the AD terminal changes states (i.e., from an AC line voltage signal to zero volts, and vise versa). Based on this determination, the smart three-way dimmer switch  502  toggles the state of the lighting load  508 . The smart three-way dimmer switch  502  may also toggle the state of the lighting load  508  in response to an actuation of actuator  316  (or a similar actuator) on the user interface.  
         [0041]     A flowchart summarizing the novel method for monitoring the AD terminal of the smart three-way dimmer  502  is shown in  FIG. 6  and begins at step  610 . First, two variables N and PREV_AD_SAMPLE are initialized to zero and a third variable POWER_UP is initialized to TRUE at step  611 .  
         [0042]     Next, at step  612 , the microprocessor  426  samples the outputs of the signal detector  432  (that are representative of the voltage at the AD terminal) when the FETs  420 ,  422  are non-conducting. Preferably, the sampling should occur at or near the peak of the AC power source voltage in order to minimize the effect of noise on the sampling process. Often, AC power sources are influenced by sources of noise, which comprise a greater percentage of the AC power source voltage near the zero-crossings of the waveform, i.e., when the instantaneous voltage is small. Thus, the smart three-way dimmer attempts to sample the outputs of the signal detector near the peak of the AC power source voltage to avoid incorrect values being sampled.  
         [0043]     At step  612 , a determination is made as to whether the smart three-way dimmer is operating with forward-phase control dimming or reverse-phase control dimming. If the dimmer is operating with forward-phase control dimming (i.e., the FETS are non-conducting at the beginning of each half-cycle), the process moves to step  614 . If the firing angle of the dimmer is less than 50% (i.e., the FETs begin conducting before the peak of the line voltage), then the FETs are only non-conducting for a short period of time at the beginning of each half-cycle and the sampling occurs immediately before the firing angle at step  616 . If the firing angle of the of the dimmer is greater than or equal to 50% (at step  614 ), then the sampling occurs at the peak of the AC power source voltage at step  618 . If the determination is made at step  612  that the dimmer is operating with reverse-phase control dimming (i.e., the FETs are conducting at the beginning of each half-cycle), the process moves to step  620 . If the firing angle of the dimmer is greater than 50% (i.e., the FETs cease conduction after the peak of the line voltage), then the FETs are only non-conducting for a short period of time at the end of each half cycle and the outputs of the signal detector must be sampled immediately after the firing angle at step  620 . Otherwise, the sampling occurs at the peak of the line at step  618 . The result of the sampling process is stored in a variable AD_SAMPLE, which represents either one of the two states of the three-way switch  504 .  
         [0044]     Next, the microprocessor determines whether the variable AD_SAMPLE is different than the previous state of the AD terminal (PREV_AD_STATE). If MAX_SAMPLES consecutive samples are the same, and are different from the previous state of the AD terminal, then a valid change in the state of the connected three-way switch is detected. A counter N is used to repeatedly sample the AD terminal for a number of times equal to MAX_SAMPLES in order to minimize the effects of switch bouncing at the three-way switch  504  and noise in the AC power source voltage. At  624 , if the value of the counter N is zero, the process moves to step  626 . If the present sampled value, AD_SAMPLE, is equal to the previous state of the AD terminal, PREV_AD_STATE, then the process loops back to the beginning. If at step  626 , a change is detected at the AD terminal, the counter N is set to MAX_SAMPLES and a variable representing the previous sample of the AD terminal (PREV_AD_SAMPLE) is set to the value of the current AD sample at step  628 . The process loops back to the beginning to sample another value of the AD terminal.  
         [0045]     If at step  624 , the value of the counter N is not zero (meaning that a change had been detected at the AD terminal), a “debouncing” process begins. At step  630 , if the present sampled value is not equal to the previously sampled value, then MAX_SAMPLES consecutive AD samples did not have the same value and the counter N is set to zero at step  632  and the process loops back to the beginning. However, if the present sampled value is equal to the previous sampled value, then the counter N is decremented at step  634 .  
         [0046]     If at step  636 , the counter N is not equal to zero, meaning that the appropriate number of the same consecutive samples of the AD terminal have not been read, the process loops back to the beginning to sample another value of the AD terminal. On the other hand, if the counter is equal to zero at step  636 , then the appropriate number of the same consecutive samples have been read and a change in state of the AD terminal has been determined. The new state of the AD terminal is stored in the variable PREV_AD_STATE at step  638 . If the variable POWER_UP is FALSE at step  639 , the state of the dimmer and the lighting load (i.e., ON or OFF) must be toggled. If the dimmer is currently ON at step  641 , then the dimmer is turned OFF at step  642 . Otherwise, the dimmer is turned ON at step  644 . After either turning the dimmer OFF or ON, the process loops back to begin sampling again. If the variable POWER_UP is TRUE at step  639 , the dimmer has just powered up and the process loop of  FIG. 6  is executing for the first time. Thus, the variable POWER_UP is set to FALSE at step  640 , and the process loops back to the beginning without toggling the state of the dimmer.  
         [0047]     The dimmer switch  502  can operate in either the three-way dimmer switch system  500  of the current invention or the prior art multiple location lighting control system  200  of  FIG. 2 . The microprocessor is programmed in a novel manner to determine the nature of the signal at the AD terminal (momentary or maintained) and switch the operation between the two different modes. For example, if the dimmer switch  502  is operating in the manner of the present invention (i.e., a maintained mode of operation) and the microprocessor  426  receives a signal on only one of the two outputs of the signal detector  432  (indicating an actuation of the switch R′ or the switch L′ of a connected remote switch  204 ), the dimmer switch will change to a momentary mode operation. In the momentary mode, the dimmer switch will operate in a manner similar to the prior art system  200 , in which pulses at the AD terminal represent button presses on the remote switch  204 . However, if the dimmer switch  502  is operating in the momentary mode and the microprocessor  426  continues to receive signals at both outputs of the signal detector  432  for longer than a predetermined period of time, the dimmer switch  502  will switch to the maintained mode of operation in which changes in the state of the signal at the AD terminal cause the dimmer to toggle the state of the lighting load. Preferably, the predetermined period of time is approximately 10 seconds, which is appropriately longer than any special button presses that may occur at the user interface of the remote dimmer, such as a long hold for fade-to-off of the lighting load.  
         [0048]     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.