Abstract:
A method for powering two circuits in a portable light that are connected by a single conductive chassis so that each of the two circuits is able to have time shared access to a power source.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional patent application No. 61/626,663 filed Oct. 1, 2011 by the present inventor. 
     
    
     FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable 
       SEQUENCE LISTING OR PROGRAM 
       [0003]    Not Applicable 
       BACKGROUND 
     Prior Art 
       [0004]    The following tabulation is some prior art that presently appears relevant: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 US Patent Number 
                 US Patent Issue Date 
                 Patentee 
               
               
                   
                   
               
             
             
               
                   
                 7,594,735B2 
                 Sep. 29, 2009 
                 Kang et al. 
               
               
                   
                 7,652,216 
                 Jan. 26, 2010 
                 Sharrah et al. 
               
               
                   
                 6,841,941 
                 Jan. 11, 2005 
                 Kim et al. 
               
               
                   
                 7,393,120 
                 Jul. 1, 2008 
                 Kang et al. 
               
               
                   
                 7,771,077 
                 Aug. 10, 2010 
                 Miller 
               
               
                   
                 8,096,674 
                 Jan. 17, 2012 
                 Matthews et al. 
               
               
                   
                 7,517,109 
                 Apr. 14, 2009 
                 Kim et al. 
               
               
                   
                 7,880,100 
                 Feb. 1, 2011 
                 Sharrah et al. 
               
               
                   
                 7,722,209 
                 May 25, 2010 
                 Matthews et al. 
               
               
                   
                 7,540,625 
                 Jun. 2, 2009 
                 Matthews et al. 
               
               
                   
                 6,017,129 
                 Jan. 25, 2000 
                 Krietzman 
               
               
                   
                 7,997,756 
                 Aug. 16, 2011 
                 Rorick 
               
               
                   
                   
               
             
          
         
       
     
         [0005]    This application relates to powering electrical circuitry in a flashlight tail cap, while still also being able to power the light in the head of the flashlight, without requiring any additional electrical connections beyond the conductive metal body of the flashlight housing. As LED lights fill more and more applications sometimes additional functionality is required. This additional functionality then, in turn, sometimes requires circuitry in the tail cap of a flashlight. Consider LED flashlights. Traditionally they have had a simple electrical switch on either the side or the tail of the flashlight. Note that the end of the flashlight that emits light is often called the head and the opposite end is called the tail. A tail cap refers to the cap or lid that screws on the tail end of the flashlight. The tail cap is removable to allow batteries to be inserted. Note that some designs have the head of the flashlight unscrew to insert batteries instead of having the tail be removed. 
         [0006]    As flashlights have advanced through the years various tail caps have been used as noted in the prior art cited above. A brief summary of these tail cap switches is that the simplest of them are just open or closed switches. More advanced models introduced mechanical means of selecting different modes of operation including different dimming levels by having complicated mechanical switching paths built into the flashlight. These complicated switching designs required multiple electrical paths to exist between the electrical driver that provides electrical current to the LED and the tail cap switch or the selected setting in the tail cap. Having multiple electrical signals exist between the tail cap and the head is accomplished by several different methods in the prior art cited above including using multiple wires, using a flexible circuit, using a PCB, using a battery holder that provides connections to both sides, etc. to connect the electrical signals between the head and tail of the flashlight. These methods all increase the cost and complexity of assembly. 
       ADVANTAGES OVER PRIOR ART 
       [0007]    One thing is consistent with all of the prior art cited above: none of it solved the problem of how to have the tail cap provide more advanced levels of control of the flashlight without the burden of adding additional connections between the head and tail of the flashlight. While the methods cited in the prior art are certainly varied, they all take a fundamentally mechanical approach to the problem. The method disclosed in this patent takes a more electrical approach and, oddly enough, results in a much simpler mechanical design. This simpler mechanical design saves cost and increases reliability by reducing connectors. The method disclosed here also allows for more advanced circuits to be in the tail cap since power can now be provided. 
         [0008]    Another advantage over the prior art is that when a mechanical switch is used to open and close the circuit that switch must be able to withstand the maximum current load. This raises the cost and typically also the size of the switch, which in turn can limit the minimum size of the light. 
       SUMMARY 
       [0009]    This invention allows a flashlight to have power for the light on one end and power for the control circuit on the tail end with only a single electrical conductor connecting the two ends. 
     
    
     
       DRAWINGS 
       Figures 
         [0010]      FIG. 1 . Schematic that shows one embodiment of a powered tail cap circuit  FIG. 2 . Schematic that shows one embodiment for a flashlight driver circuit  FIG. 3 . Schematic that shows one embodiment for a flashlight battery contact circuit  FIG. 4 . Schematic that shows one embodiment for a flashlight LED circuit 
       
    
    
     DETAILED DESCRIPTION 
     FIG.  1   
       [0011]      FIG. 1  shows one embodiment of a flashlight tail cap circuit that can implement the method described in this patent. The circuit of  FIG. 1  is versatile and very easily adapted to a wide variety of operating voltages and current loads. For this embodiment the circuit of  FIG. 1  is located in the tail cap of the flashlight. 
       FIG.  2   
       [0012]      FIG. 2  shows one embodiment of a flashlight driver circuit. In this case the driver circuit was adapted to work with the other circuits shown in  FIG. 1 ,  FIG. 3 , and  FIG. 4  to form one complete working flashlight that implements the method described in this patent for having a powered tail cap. For this embodiment the circuit shown in  FIG. 2  is located in the head of the flashlight. 
       FIG.  3   
       [0013]      FIG. 3  shows one embodiment of a flashlight battery contact board. This board is designed to work with the other circuits shown in  FIG. 1 ,  FIG. 2 , and  FIG. 4  to implement a complete flashlight that has a powered tail cap as disclosed in this patent. For this embodiment the circuit shown in  FIG. 3  is located in the head of the flashlight. 
       FIG.  4   
       [0014]      FIG. 4  shows one embodiment of an LED board that is designed to work with the other circuits in the figures above to form one complete flashlight that has a powered tail cap. For this embodiment the circuit shown in  FIG. 4  is located in the head of the flashlight. 
       OPERATION 
     FIGS.  1 ,  2 ,  3 , and  4   
       [0015]    This circuit is designed to power the flashlight tail cap and, when desired, to also power the constant current circuit in the head of the flashlight. Note that all of this is accomplished with a single power source, which for this embodiment is a single rechargeable battery with a nominal voltage of 3.7 v. First the operation of the embodiment shown in the figures will be described from the moment that the battery is initially installed. After that the light on and light off cases will be described. 
         [0016]    When the flashlight embodiment shown in  FIGS. 1-4  is first powered up microcontroller  100  will be off. Pull down resistor  106  is at the gate of N-channel MOSFET  110 , so MOSFET  110  will be effectively an open circuit. This means that initially the only path for electrical current is through bypass resistor  200 , through the body of the flashlight which is indicated as Vchasis in the figures, through diode  104 , and finally charging capacitor  102 . Once the voltage on capacitor  102  has charged high enough to allow microcontroller  100  to operate, then the flashlight is ready to operate. For this embodiment the flashlight starts in the light off state. In the light off state the tail cap circuit of  FIG. 1  is powered but the circuits shown in  FIGS. 2-4  will be off, since when MOSFET  110  is not shorted to ground then capacitor  102  will rapidly charge to approximately the same voltage as the battery voltage. I say approximately because some voltage will be dropped across the diodes. Note that in the light off state microcontroller  100  will draw very little current since it can be put in a low power mode, thus not draining much electrical current from the battery. Since capacitor  102  will be approximately the full battery voltage then there is effectively no voltage left for the circuitry in the head of the flashlight. A very small voltage will be dropped across resistor  200 , however since microcontroller  100  draws so little current the voltage drop across resistor  200  is negligible and certainly is not enough to power the LED constant current circuit. Also note that microcontroller  100  is configured to have an internal pull up resistor so that if button  112  is pressed microcontroller  100  will be able to detect the pin going low. 
         [0017]    Microcontroller  100  would typically stay in the low power mode until an action happens. For this embodiment the action would be button  112  being pressed. When button  112  is pressed and the flashlight is in the light off state then microcontroller  100  would wake up from the low power mode and operate the light. Operating the light is accomplished by having microcontroller  100  apply a PWM signal to the gate of MOSFET  110 . When the PWM signal is on the high portion of the duty cycle then MOSFET  110  will become a very low resistance path to ground. When MOSFET  110  is acting as a low resistance path to ground then microcontroller  100  can remain powered by capacitor  102 . This allows the circuitry in the head of the flashlight, shown in  FIGS. 2-4 , to have the full voltage of the battery despite the tail cap circuit shown in  FIG. 1  being powered. Since capacitor  102  will start discharging while MOSFET  110  is on care must be taken to not have the period be too long nor to have the duty cycle go too close to 100% on. Given that the human eye will detect frequencies that are 100 Hz or above as being a continuous light, as opposed to a rapidly blinking light, the embodiment used a minimum frequency of 100 Hz. For the circuit values shown in  FIGS. 1-4  the maximum duty cycle can be as high as 95% while still retaining reasonable design margins for how much capacitor  102  will discharge. Since microcontroller  100  can turn MOSFET  110  on and off very quickly, all of these requirements are easily met. 
         [0018]    To control how bright the light is, the duty cycle of the PWN signal applied by microcontroller  100  to the gate of MOSFET  110  can vary the on time or high portion of the PWM signal. This is a standard technique well understood by those skilled in the art. The duty cycle can vary from 0-95% for the embodiment shown in  FIGS. 1-4 . A higher duty cycle could be achieved by lowering the value of resistor  200 . The lower the value of resistor  200  the faster capacitor  102  will charge. The faster capacitor  102  charges the greater the time that MOSFET  110  can be on, thus raising the maximum duty cycle. 
         [0019]    In addition to having the flashlight&#39;s LEDs be on in a constant method as described previously dimming and patterns can also be implemented. The beauty of this circuit is that it can implement dimming from 0 to 95% and any of the patterns commonly requested by the market such as strobe or SOS modes. 
       Operation 
     Alternate Embodiments 
       [0020]    There are several alternate embodiments that are readily apparent. For example, although the embodiment used as an example used a single battery for a power source, the circuit would with little modification work with multiple batteries in series. Other alternate embodiments are using a BJT transistor or a MOSFET for the switch. Instead of using diodes for the reverse polarity protection other circuit elements could be used instead. Although the example embodiment used a total of 4 PCB boards, this number could be readily changed. 
         [0021]    Since the tail cap already has power, a wide array of user interface options can now be put inside it including buttons, sensors, capacitive sensors, accelerometers, gyros, etc. 
         [0022]    The example embodiment showed the circuit that always had power as being on the high side however it doesn&#39;t have to always be that way. Any battery operated device that needs multiple circuits powered, with one or more on the “high side” and one or more on the “low side” could make use of this technique. 
         [0023]    Finally, a somewhat different approach would be to replace capacitor  102  with a small battery. Since very small batteries exist and since the current draw is low it would be possible to use a battery instead of a capacitor. 
       ADVANTAGES 
       [0024]    From the detailed description above a number of advantages over the prior art become evident. 
         [0000]    (a) This method allows circuits at the tail cap of a flashlight, where the user interface may reside, to remain powered without requiring any wires between the circuits at the head of the flashlight and the tail cap of the flashlight. Further, both circuits will have full battery voltage so a single power supply can be time shared by both circuits. Not requiring wires between the two circuits lowers cost greatly and also allows for a simpler mechanical solution. The simpler mechanical solution in turn allows for a wider array of form factors for the light since mechanical constraints that the wires impose are removed.
 
(b) By allowing the tail cap to remain powered simple buttons can be used instead of the latching buttons that are currently used. A latching button requires 2 clicks per mode change, which is more complicated and takes longer. Also, a latching button must be rated to handle the full current of the overall lighting circuit. The simple push button shown in  FIG. 1  is much lower cost and smaller because it does not have to handle the full current.
 
(c) Since the button is not being used to open and close power to the tail cap circuit, it can have a wider range of functions. For example, the present embodiment is able to operate either as a momentary on tactical flashlight or a standard multi-mode flashlight. In the prior art flashlights were not able to operate as both without additional electrical conductors as noted previously. With the present embodiment it can operate as both with no additional cost. This represents a clear advantage over the prior art.
 
         [0025]    Although the descriptions above contain many specificities, these should not be construed as limiting the scope of the embodiments but as merely providing illustrations of some of several embodiments. For example, I used a LED flashlight as an example but the same benefits and advantages of this method would apply to other LED lights such as LED headlamps, LED bike lights, etc. Thus the scope of the embodiments should be determined by the appended claims and their legal equivalents rather than by the examples given.