Abstract:
An infrared LED of an IrDA transceiver module is usable to transmit IrDA signals as well as RC control signals. When making an IrDA transmission, the IrDA LED is driven with a lower amount of current. When making an RC transmission, the IrDA LED is driven with an increased amount of current such that infrared emissions received by an RC receiver are of adequate power to be received as RC control signals. A current-limiting circuit allows more LED current to flow the longer current is allowed to flow through the IrDA LED. By controlling the durations of infrared bursts in the RC transmission, the average LED current during infrared bursts of RC transmissions is controlled. Using this technique allows the IrDA module to be used to transmit RC signals at different transmission power settings. To reduce power consumption, the minimum transmission power necessary to engage in RC communications is used.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, and claims priority under 35 U.S.C. §120 from, nonprovisional U.S. patent application Ser. No. 11/437,884 entitled “Controlling Transmission Power in an IrDA/RC Transmitter Circuit,” filed on May 19, 2006, the subject matter of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to controlling transmission power in infrared communications. 
     BACKGROUND INFORMATION 
     Portable electronic devices such as personal digital assistants (PDAs), cell phones, digital cameras, MP3 players, and laptop computers often use a type of infrared transceiver called an IrDA (Infrared Data Association) transceiver to transfer information. Each IrDA transceiver has an infrared light emitting diode (LED) that emits infrared radiation having a center wavelength somewhere in the range of from 850 nm to 900 nm (for example, 875 nm). Each IrDA transceiver also has an IR receiver (for example, a PIN diode) for receiving infrared signals of this wavelength. If, for example, each of two such portable electronic devices has an IrDA transceiver, then one device can transmit data to the other device across an IrDA link using infrared signals that comply with an IrDA standard. IrDA is, for example, often employed to communicate files of digital information between portable devices having such IrDA transceivers. 
     Electronic consumer devices in the home such as televisions, VCRs, DVD players, DVRs, CD players, stereo equipment, home theatre equipment, and so forth are typically controlled by remote control (RC) devices that also transmit infrared signals. These ordinary infrared signals, referred to here as “RC” (or remote control) infrared signals, typically have a center wavelength somewhere in the range of from 900 nm to 950 nm (for example, 940 nm). To turn the power on to a television, for example, a user may press a power key on a remote control device. The remote control device has an infrared LED that emits an RC infrared signal to the television. An infrared RC receiver circuit in the television receives the RC infrared signal, decodes the signal, and responds by turning the television on. 
     It has been recognized that it would be desirable to be able to use a portable electronic device to control an electronic consumer device that is designed to respond to RC infrared signals. If, for example, a user had a cellular telephone in his/her hand, then the user could use the cellular telephone to control a television. 
     It has been recognized that an IrDA transmitter LED within an IrDA transceiver module can be used as an infrared transmitter both for IrDA and RC applications. Although the peak wavelength of the transmitter LED in the IrDA transceiver is at a wavelength that is different from the wavelength of peak sensitivity of the RC infrared receiver in the electronic consumer device, some radiation transmitted by the infrared transmitter is nevertheless received by the RC receiver. The IrDA transmitter transmits energy in a band and the RC receiver receives energy in a band. The two bands overlap. Due to the mismatch of the center transmitter and receiver wavelengths, however, it may be necessary for the IrDA transmitter LED power to be increased in order for enough energy to be received at the RC receiver for the communication to work properly. An application note from Vishay Semiconductors (Vishay Semiconductor Application Note entitled “Utilizing a Vishay IrDA Transceiver for Remote Control”, document number 82606, 14 pages, Feb. 20, 2004) discloses reducing the value of a current-limiting resistor so as to increase IrDA LED transmitter peak current and thereby to increase emission intensity of the IrDA LED transmitter. 
     Improvements and enhancements to a system employing an IrDA transceiver to transmit infrared signals to an RC receiver are sought. 
     SUMMARY 
     The same infrared light emitting diode (LED) of an (Infrared Data Association) IrDA transceiver module is usable both to transmit IrDA communication signals as well as to transmit infrared signals that are usable as remote control (RC) signals to control an electronic consumer device. When making an IrDA transmission, the IrDA LED is driven with a lower amount of current appropriate for IrDA communication. When making an RC transmission, the IrDA LED is driven with an increased amount of current such that infrared emissions received by an RC receiver within the electronic consumer device are of adequate power to be received as RC signals. A current-limiting circuit allows more LED current to flow the longer current is allowed to flow through the IrDA LED. By controlling the durations of infrared bursts in the RC transmission, the average LED current during infrared bursts of the RC transmission is controlled. Longer infrared bursts have larger average LED currents. Shorter infrared bursts have smaller average LED currents. Using this technique of controlling the durations of infrared bursts to control transmission power allows the IrDA module to be used to transmit RC signals at different power settings. RC signals having a higher power setting involve infrared bursts of longer duration. RC signals having a lower power setting involve infrared bursts of shorter duration. To reduce power consumption in the RC mode, the transmission power setting of the minimum transmission power necessary to engage in RC communications is used. 
     In one novel aspect, a portable electronic device (for example, a cell phone, a PDA, an MP3 player, a digital camera, or a laptop computer) includes the IrDA transceiver module and the current-limiting circuit described above. The IrDA LED of the IrDA transceiver module within the portable electronic device is usable to transmit RC signals to an electronic consumer device (for example, a television) of a user, but the user does not have codeset information for generating appropriate RC control signals for the particular electronic consumer device. The user therefore takes the portable electronic device to a location that dispenses RC codeset information. The location may, for example, be an electronic store that disseminates RC codeset information. The IrDA transceiver module of the portable electronic device is then used to engage in IrDA communications to download RC codeset information from another IrDA device maintained by the store. The RC codeset information is codeset information for controlling the user&#39;s electronic consumer device. Once the RC codeset information has been downloaded into the portable electronic device, the user takes the portable electronic device home and uses the IrDA LED of the transceiver in the portable electronic device in combination with the current-limiting circuit to transmit RC signals (such an RC signal is sometimes referred to as an “RC operational signal”) from the portable device to the electronic consumer device. The RC signals are generated using the downloaded RC codeset information and therefore are usable to control the electronic consumer device. The transmission power of the RC signals is selected or adjusted or controlled as described above by selecting, adjusting or controlling the durations of the infrared bursts within the RC signals. Where, for example, the remote control device involves a microcontroller that drives a digital control signal TX to the IrDA transceiver module top turn off and on a switch within the IrDA module, the microcontroller sets the durations of the pulses (pulse widths) of the control signal TX so that the resulting RC signal that is transmitted from the IrDA transceiver module is of the appropriate power setting. By changing the power setting of the RC transmissions, the transmission range of the remote control device can be changed. By maintaining the power setting of the RC transmissions at as low a setting as is necessary for proper communication, power consumption of the remote control device is reduced. 
     Other embodiments and advantages and considerations and methods are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
         FIG. 1  is a diagram of an IrDA (Infrared Data Association) circuit  1  within a portable electronic device in accordance with one novel aspect. The duration of an infrared burst determines the average LED drive current during the burst, and therefore also determines the average transmission power during the burst. 
         FIG. 2  is a perspective view of the IrDA transceiver module in circuit  1  of  FIG. 1 . 
         FIG. 3  is cross-sectional side view of the IrDA transceiver module of  FIG. 2 . 
         FIG. 4  is a cross-sectional top-down view of the IrDA transceiver module of  FIG. 2 . 
         FIG. 5  is a graph that shows a spectral emission distribution curve of an infrared LED within an IrDA transceiver module as well as a composite spectral sensitivity curve for numerous typical IR remote control (RC) receivers. 
         FIG. 6  is a diagram that shows how increased current flow through infrared LED of an IrDA transceiver module will cause the LED to output enough energy at 900 nm for RC receiver operation. 
         FIG. 7  shows waveforms of signals that can be output from the IrDA transceiver module of  FIG. 1 . 
         FIG. 8  is a waveform diagram that illustrates how the average LED drive current through the infrared LED of the IrDA transceiver module of  FIG. 1  can be controlled in the RC mode to change the average LED drive current flowing through the LED during an infrared burst during generation of an RC control signal. The smaller average LED drive current on the left side of  FIG. 8  represents a lower transmission power setting. The higher average LED drive current on the right side of  FIG. 8  represents a higher transmission power setting. 
         FIG. 9  (Prior Art) is a diagram of a conventional LED drive circuit that is part of an ordinary held-held RC remote control device of the type used to control consumer electronic appliances in the home. 
         FIG. 10  is a diagram of a current-limiting circuit in an RC remote control device in accordance with a second novel aspect. 
         FIG. 11  is a flowchart of a method in accordance with another novel aspect. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 1  is a diagram of an IrDA (Infrared Data Association) circuit  1  within a portable electronic device in accordance with one novel aspect. Circuit  1  includes an IrDA transceiver module  2 , a passive LED drive current-limiting circuit  3 , a central processing unit (CPU)  4 , and an operating system  5 . The operating system has access to IR remote control (RC) codeset information  6 . IrDA transceiver module  2  is disposed on a printed circuit board. In the present example, the portable electronic device of which the IrDA transceiver module  2  is a part is a personal digital assistant (PDA) and the printed circuit board is the printed circuit board of the PDA. Passive LED drive current-limiting circuit  3  includes a first resistor  7  that is coupled in parallel with a series combination of an inductor  8  and a second resistor  9 . First resistor  7 , inductor  8 , and second resistor  9  are all discrete components disposed on the printed circuit board. 
       FIG. 2  is a perspective view of IrDA transceiver module  2 .  FIG. 3  is a cross-sectional side view of IrDA transceiver module  2  of  FIG. 2 .  FIG. 4  is a top-down view of a cross-section of IrDA transceiver module  2  of  FIG. 1 . IrDA transceiver module  2  may be an IrDA module of conventional construction such as an IrDA module available from Zilog, Inc. of San Jose, Calif. The IrDA module  2  includes three dice: 1) an IR transmitter diode die  10 , 2) an IR receiver PIN diode die  11 , and 3) a controller die  12 . The three dice are mounted on a very small printed circuit board  13 . Molded plastic  14  covers the printed circuit board and die assembly and forms two semi-spherical lenses  15  and  16 . Lens  15  is the receiver lens that focuses incoming IR radiation  17  onto the PIN receiver diode  11  with the module. Lens  16  redirects the radiation emitted from IR transmitter LED die  10  into a beam  18 . 
     Returning to  FIG. 1 , when an IrDA transmission is to occur, the 875 nm transmitter LED  10  is controlled in the conventional IrDA manner. In one example, an IrDA TX signal is supplied to the IrDA module via an IrDA/RC TX terminal  19  on the IrDA module, and an IrDA driver switch portion  20  of controller die  12  switches current through LED  10  in conventional fashion. IrDA driver switch portion  20  in this example is a field effect transistor. The amount of LED drive current initially when current starts to flow through LED  10  is limited primarily by the first resistor  7  in the left leg of the current-limiting circuit. The resistance of first resistor  7  is approximately one ohm. The IrDA TX signal stops drawing current through LED  10  before significant current begins to flow through inductor  8  in the right leg of the current-limiting circuit. The average LED drive current is therefore approximately 265 mA (over one IR burst duration), and the resulting intensity of radiation emitted from IrDA module  2  is approximately 40 mW/sr at 875 nm. 
     When a remote control (RC) transmission is to occur, however, a digital RC control signal TX is generated by software executing on the CPU  4  using the codeset information  6 . For additional detail on codesets, on how codesets can be stored, and on an exemplary way to generate an RC transmission from codeset information, see: U.S. patent application Ser. No. 10/777,023, filed Feb. 10, 2004, now U.S. Pat. No. 7,259,696; and U.S. Pat. No. 7,339,513, filed Aug. 27, 2004 (the entire subject matter of these two documents is incorporated herein by reference). No microcontroller that stores codeset information need be provided where the IrDA circuit  1  is part of a personal digital assistant (PDA). Rather, codeset information  6  is stored in main memory on the PDA such that software executing on the CPU  4  of the PDA can access the codeset information and use it to drive the IrDA/RC TX terminal  19  with the RC TX signal. This RC TX signal is provided to IrDA module  2  via the same IrDA/RC TX terminal  19  that is used to supply the IrDA TX signal. Although an RC infrared signal is to be transmitted, transmitter LED  10  has a transmission peak at 875 nm. An IR remote control receiver (not shown) on an electronic consumer device (not shown) that is to be controlled by the RC transmission has a peak spectral sensitivity at a wavelength of approximately 940 nm. It is recognized that most IR remote control receivers in electronic consumer devices will work satisfactorily if they receive 40 mW/steradian at 940 nm. 
       FIG. 5  is a graph that shows the spectral emission distribution curve  21  of 875 nm IrDA transmitting LED  10  as well as a composite spectral sensitivity curve  22  for numerous typical IR remote control (RC) receivers. Note that the amount of radiation emitted from the 875 nm transmitting diode  10  drops from its normalized high at 875 as wavelength increases to the right of its peak. The amount of radiation emitted is down to sixty percent of its normalized high at 900 nm. Also note that the spectral sensitivity of the composite IR remote control (RC) receiver curve is at its normalized high at 940 nm, but that it is down to seventy percent of its high at 900 nm to the left of its peak. The curve  21  of the IrDA transmitter LED overlaps the response curve  22  of the RC receiver. 
     Although IrDA uses an infrared signal of approximately 870 nm and RC remote control devices use an infrared signal of approximately 960 nm, the IrDA transmitter LED within module  2  actually emits radiation over a range of wavelengths. By increasing the transmission power of the IrDA LED  10 , the amount of energy transmitted at 960 nm can be made sufficiently large that the IrDA transmitter LED is usable to make RC transmissions to an RC receiver. See United States Publication 2005/0185962, published Aug. 25, 2005, and United States Publication 2006/0018662, published Jan. 26, 2006 (the entire subject matter of these two publications is incorporated herein by reference). 
       FIG. 6  is a diagram that shows how increased current flow through the IrDA 875 nm transmitter diode  10  in the RC transmitter mode will cause IrDA module  2  to output enough energy at 900 nm for RC receiver operation. 
     The circuit of  FIG. 1  works by taking advantage of the fact that RC signals are transmitted with longer bursts of IR energy (approximately 10 microseconds) whereas the IrDA signals are transmitted with shorter bursts of IR energy (approximately 1.6 microseconds). During a short IrDA burst, inductor  8  of the passive LED drive current-limiting circuit  3  of  FIG. 1  is not conducting a significant amount of current. Current flow through the passive LED drive current-limiting circuit  3  is limited primarily by the current-limiting first resistor  7  in the left leg of the current-limiting circuit  3 . The value of the first resistor  7  is selected to limit the current flow into the LED  10  to a relatively low current level suitable for the IrDA transmission. 
     During a relatively longer RC burst, inductor  8  of the current-limiting circuit  3  of  FIG. 1  conducts larger and larger amounts of current. If the magnetic field were to build up in inductor  8  until the core of inductor  8  were to saturate, then current flow through the current-limiting circuit  3  would level off at a current determined by the equivalent parallel resistance of first resistor  7  and second resistor  9  in the right leg of the current-limiting circuit  3 . This peak amount of current would be the current flow into the LED  10  during the RC transmission. 
     In accordance with one novel aspect, an amount of IR energy transmitted by LED  10  per RC infrared burst is selected, adjusted and/or controlled by CPU  4  of the PDA.  FIG. 7  is a diagram that shows waveforms of signals in accordance this novel aspect. In the IrDA mode (illustrated in the portions of the waveforms on the left side of the page), the IR burst duration is so short that inductor  8  in the right leg of the current-limiting circuit  3  (see  FIG. 1 ) has not begun to conduct significant current by the time the end of the burst occurs. Current flow through LED  10  is therefore limited by the first resistor  7  in the left leg of the current-limiting circuit as set forth above in connection with  FIG. 1 . The IrDA transmission is not modulated with a sub-carrier. The average current that flows into and through LED  10  is approximately 195 mA average during an IR burst duration in the IrDA mode. The current flows from the supply voltage conductor, through current-limiting circuit  3 , through LED  10 , through the switch in IrDA driver  20 , and to a ground potential conductor. 
     RC mode operation is illustrated in the portion of the waveforms on the right side of  FIG. 7 . The 1.2 millisecond mark times of the RC signal illustrated in the upper waveform are modulated with a sub-carrier (sometimes referred to as the “carrier”). The second row of waveforms shows this modulation of the RC signal in greater detail. Individual bursts of IR are approximately 8 microseconds long. There are 32 microseconds between successive bursts. Due to the longer duration of an IR burst in the RC mode (in the illustrated example, eight microseconds), the inductor  8  begins to conduct current during a burst period. LED drive current rapidly rises until the end of the IR burst as illustrated. Inductor  8  is not yet saturated at the time that the IR burst ends in the RC example at the right of  FIG. 7 . Accordingly, the LED current of the waveform second from the bottom continues to rise through the duration of the IR burst. The average LED current is approximately 400 mA during an IR burst in the RC example at the right of  FIG. 7 . 
       FIG. 8  illustrates how the average LED drive current through LED  10  during an IR burst is set, controlled and/or controlled in the RC mode in the novel aspect. The average current flowing into LED  10  is increased by increasing the duty cycle of the RC sub-carrier. This is done by increasing the duty cycle of the controlling TX signal on IrDA/RC TX terminal  19 . The resulting IR burst on the left side of the diagram of  FIG. 8  is the same eight microsecond burst illustrated in  FIG. 7 . It was generated by a TX control signal of a duty cycle of 8 to 32, where the period of the sub-carrier is the 40 microseconds. The average LED drive current is 400 mA during the burst time. 
     The burst on the right side of  FIG. 8  is, however, generated by a TX control signal of a duty cycle of 18 to 22, where the period of the sub-carrier is the same 40 microseconds. Rather than stopping the IR burst at the time when the current flowing through the current-limiting circuit  3  has risen to 600 mA as in the burst on the left side of the diagram of  FIG. 8 , the IR burst is extended such that the LED drive current flowing through the current-limiting circuit  3  continues to rise. The duration of the IR burst in the example on the right side of  FIG. 8  is 18 microseconds. The average LED drive current is 550 mA during the burst. It is therefore seen that increasing the duty cycle of the TX control signal results in a higher average LED drive current flowing through LED  10  during a burst time. It also results in a higher average LED drive current flowing through the LED  10  when the entire mark time is considered. 
     Changing the duty cycle of the sub-carrier of the TX control signal during the RC mode does not generally disrupt receipt of the RC signal by typical receivers in typical electronic consumer devices. RC receivers are generally tuned to the frequency of the sub-carrier, not to the duty cycle of sub-carrier. Note that in both the low average transmission power situation at the left of  FIG. 8  and in the high average transmission power situation at the right of  FIG. 8 , that the period of the sub-carrier is the same 40 microseconds. 
     The use of current-limiting circuit  3  involving inductor  8  and the changing of the duty cycle of the sub-carrier to increase RC LED drive current during IR bursts is not limited to use with an IrDA transceiver module or with IrDA.  FIG. 9  (Prior Art) is a diagram of a conventional 960 nm LED drive circuit that is part of an ordinary hand-held RC remote control device of the type used to control consumer electronic appliances in the home. The LED drive current  100  flowing through 960 nm LED  101  is fixed by the resistance of the current-limiting resistor  102 . 
       FIG. 10  is a diagram of a current-limiting circuit  200  in an RC remote control device in accordance with a second novel aspect. Average LED drive current  201  flowing through a 960 nm LED  202  during an IR burst of an RC signal is controlled by controlling the duty cycle of the RC sub-carrier signal. IR bursts of longer duration have a higher average LED drive current. The microcontroller of the RC remote control device that typically drives the TX control signal to the LED  202  can, with minimal additional complexity, be made to set and/or change and/or control the duty cycle of the sub-carrier of the IR bursts in order to increase and/or decrease the average LED drive current  201  during a burst. The RC remote control device may, for example, have two or more different LED transmission power settings. When the first setting is used, the IR bursts are longer and are therefore of a higher first average transmission power. When the second setting is used, the IR bursts are shorter and are therefore of a lower second average transmission power. In one example, the remote control device of the second novel aspect of  FIG. 10  is an RC remote control device that does not involve an IrDA transceiver module and that does not and cannot transmit IrDA communication signals. 
     In a first method, a low transmission power setting is attempted initially. The user of an RC remote control device may, for example, initiate this condition by putting the remote control device into a programming mode, and then pressing an appropriate key on the remote control and holding it down. A microcontroller of the remote control device then increases the transmission power setting step by step until the user notices that the RC communication link starts to operate in a satisfactory manner. The microcontroller does this by increasing the duty cycle of the sub-carrier of the RC signal. Once satisfactory RC communication is achieved, then the increasing of power setting is halted and higher transmission power settings are not attempted. The user may, for example, stop the power setting from increasing by releasing the key. In this way, if a higher average power transmission setting is not required, then the high transmission power setting is not used and power is saved by using the lowest power setting that functions acceptably. 
     In a second method, a high transmission power setting is used initially. The user of the RC remote control device can initiate this condition by putting the remote control device into a programming mode, and then pressing an appropriate key and holding it down. The microcontroller of the remote control device then reduces the transmission power setting step by step until the user notices that the RC communication link has stopped functioning satisfactorily. In one example, proper functioning can involve turning the electronic consumer device off and then back on. The microcontroller reduces the transmission power setting by reducing the duty cycle of the RC signal&#39;s sub-carrier. The user may indicate that the RC communication link has stopped functioning by releasing the key on the RC remote control device. Releasing the key causes the RC remote control device to increase its transmission power setting one increment and then to use this power setting for subsequent operation. Regardless of how the transmission power setting is set, the transmission power setting is selected to be the setting of the least transmission power that works satisfactorily. As compared with a conventional RC remote control device that has only one power setting and consequently may use more transmission power that is required in a given circumstance, the novel RC remote control device uses the smallest amount of LED drive current required that results in satisfactory communication between the RC remote control device and the electronic consumer device to be controlled. By reducing the amount of transmission power used, battery life of the remote control device is extended. This control of RC transmission power sees use in RC remote control devices that do not have an IrDA communication capability as well as in electronic devices that employ IrDA transceivers to transmit RC infrared signals. 
     In a third method, a remote control device uses a particular first transmission power setting (step  300 ). If the user presses a key on the remote for less than a predetermined normal duration, then the first transmission power setting is used for the duration of the key press. If, however, the user presses a key for more than the predetermined duration (step  301 ), then the microcontroller automatically increases the transmission power setting (step  302 ). The transmission power setting may, for example, be increased to the maximum transmission power setting for the entire remainder of the key press duration or for subsequent short time periodic intervals during the remainder of the key press duration. The feature of automatically increasing the transmission power setting during unusually long key presses may be usable to prevent users from becoming irritated with a remote control device whose power setting is so low that communication is sometimes marginally functional and sometimes, depending on the orientation of the remote control device and the electronic consumer device, is nonfunctional at the low power setting. 
     In an embodiment where the portable electronic device (for example, a cell phone, a PDA, an MP3 player, a digital camera, a laptop computer) includes an IrDA transceiver module that is usable to send RC control signals to an electronic consumer device, (for example, a television), the user may not have codeset information for generating control signals for the user&#39;s electronic consumer device. In one novel aspect, the user takes the portable electronic device to a location that dispenses codeset information. The location may, for example, be an electronic store that disseminates codeset information. The IrDA transceiver module of the portable electronic device is then used to download codeset information from another IrDA device maintained by the store. The codeset information is codeset information for controlling the user&#39;s electronic consumer device. The user may test usage of the RC codeset at the location. Once the codeset information has been downloaded into the portable electronic device, the user takes the portable electronic device home and uses the IrDA LED of the transceiver in the portable electronic device in combination with the current-limiting circuit described above to transmit RC control signals from the portable device to the electronic consumer device. The RC control signals are generated using the downloaded codeset information and therefore control the electronic consumer device. 
     In one embodiment, a microcontroller of a remote control device monitors the voltage level of the LED supply voltage. The LED supply voltage is usually a battery voltage VBAT and the remote control device is powered by the battery. Ordinarily, the transmission power of the LED of the remote control device would decrease with decreasing supply voltage. In this embodiment, however, the microcontroller detects the decreasing LED supply voltage, and increases the duty cycle of the carrier to compensate such that LED transmission power is higher than it otherwise would be. In one example, the compensation is adequate to maintain RC transmission power at a substantially constant level despite a decrease in LED supply voltage as the battery of the remote control device ages. 
     Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. A current-limiting circuit can be placed in the current supply path of another type of radiation emitting device (for example, an LED of a flashlight or light fixture or another type of light), and the burst duration can be controlled to increase or decrease the average drive current flowing through the emitting device during the on-time of the emitting device. The burst duration can be controlled by supplying a control signal onto a control terminal of a transistor that draws current through the emitting device. A microcontroller or other suitable signal generating device is usable to control and/or change the duty cycle of the control signal, thereby controlling and/or changing the duty cycle of the drive current flowing through the emitting device. Increasing the duty cycle increases the average drive current flowing through the emitting device. Decreasing the duty cycle decreases the average drive current flowing through the emitting device. The emitting device need not be a light transmitting device but rather can be a transmitter of another type of energy such as, for example, an RF transmitter. The technique of increasing transmitting power of an emitter typically used in a first communication band such that side skirt emissions of the emitter are of adequate power to be received as an intelligence signal in another communication band is not limited to using an IrDA LED to transmit RC control signals. The particular circuitry illustrated in  FIG. 1  for the current-limiting circuit is only exemplary. The series resistance of the right leg need not be provided by a separate component but rather can be the series resistance of inductor  8 . Other circuit configurations for accomplishing the function of the current-limiting circuit can be employed. A current-limiting circuit can involve a transistor or other mechanism for switching in different current limiting resistances. For example, the series resistance of the right leg of the current-limiting circuit  3  can have controlled resistance that is a function of the burst duration setting. The cost of providing a remote control device having multiple selectable transmission settings is low because the capability can be provided without the addition of a significant amount of complexity or expensive active circuitry. The feature can, for example, be provided by adding only a few discrete passive components (the passive current-limiting circuit  3  of  FIG. 1 ) and by programming the microcontroller or CPU of the remote control device to change the pulse width of the digital RC control signals output to the infrared LED. 
     Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.