Abstract:
An LED lamp adapted for use as a bicycle light includes a lamp/switch module and a power supply/control module. The control includes a microcontroller that performs both light operating functions and battery charging control functions. A low battery warning is provided as a non-repeating, short sequence of flashes of the lamp.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. provisional patent application No. 60/995,205, the entire disclosure of which is hereby incorporate by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to battery operated LED lamps, and more particularly to a high intensity, battery operated LED lamp, suitable for use as a bicycle light. 
     BACKGROUND OF THE INVENTION 
     Typical battery-powered bicycle headlamps have utilized incandescent lamps, although, with the introduction of high-intensity light emitting diodes (LEDs) efforts have been made to utilize LEDs in bicycle headlamps. A problem with bicycle headlamps, including LED headlamps, is that the beam intensity gradually diminishes as the battery drains. The beam intensity can gradually diminish to a very low level before the user is consciously aware that a dangerous condition has arisen. 
     In the case of an LED headlamp, it is desirable to provide for selectable power levels in order to conserve power and prolong battery life. Selection of power levels is preferably accomplished electronically by operating an LED, or a bank of LEDs, by current pulses, and varying the duty cycle of the pulses. Control of the LED or LED bank, therefore requires an electronic control circuit. 
     Lithium ion (Li-ion) power supplies are a reliable energy source for bicycle lights because they are reliable, they are rechargeable, they have a high power to weight ratio, and they have a long life, if operated properly. Control of the charging and operation of a Li-ion power supply is also preferably accomplished by means of an electronic control circuit that prevents excessive discharge, prevents overcharging, and, in the case of a Li-ion battery, maintains the cell voltages in balance. 
     SUMMARY OF THE INVENTION 
     This invention provides a light weight, versatile, Li-ion powered LED lamp that maintains a nearly constant illumination level, provides an automatic warning when the power source is approaching a discharged condition, and utilizes a single microcontroller to control LED operation as well as power supply charging functions. 
     More particularly, in a battery operated LED lamp and control in accordance with the invention comprises a light source comprising at least one light-emitting diode, a manually operable switch, an electrochemical source of direct current, and a control circuit connectable to the light source, the switch, and the direct current source. The control circuit includes a microcontroller, and is connectable to deliver operating current from the direct current source to the light source under the control of the microcontroller to operate the light source. 
     In accordance with a first aspect of the invention, the microcontroller is connected to sense the level of charge in the direct current source while the light source is being operated, and is responsive to a predetermined level of charge below a full charge, to effect multiple, sequential, interruptions in the delivery of operating current to the light source for a limited time interval. The interruptions occur at a rate sufficient to cause a visually perceptible flashing off and on of the light source, thereby providing a visual indication of a low level of charge in the direct current source. The user can then switch to a fresh power source, or take other appropriate measures. 
     In accordance with another aspect of the invention, the control circuit, which is connectable to the light source, is alternatively connectable to deliver charging current from an external supply to the direct current source. The microcontroller is programmed to effect delivery, by the control circuit, of pulses of current to the light source, at different duty cycles in response to successive operations of the switch. The microcontroller is also programmed to monitor the voltage across the direct current source, and to deliver a charging current from the external supply to the direct current source at a constant level when the monitored voltage is below a predetermined limit, and to regulate the charging current delivered from the external supply to the direct current source when the monitored voltage rises above the predetermined limit so that the charging current decreases as the direct current source approaches a full charge. 
     In accordance with still another aspect of the invention, the direct current source is a battery composed of two electrochemical cells connected in series, and the control circuit is connected to both of the electrochemical cells to monitor the difference in the voltage of the cells. The microcontroller is programmed to effect delivery, by the control circuit, of pulses of current to the light source, at different duty cycles in response to successive operations of the switch. The control circuit also including a resistance associated with each cell, the resistance being connectable across its associated cell, and an electronic switch circuit, responsive to the microcontroller during charging, for selectably connecting one or the other of these resistances into shunt relationship with its associated cell to apply a load thereto. The microcontroller is also programmed to cause the electronic switch circuit to switch a resistance into shunt relationship with the cell having the higher voltage when a difference in excess of a predetermined voltage difference is detected by the control circuit, whereby the voltages of the cells are maintained at a substantially equal level. 
     Other details and advantages of the invention will be apparent from the following detailed description when read in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the battery powered LED lamp and control circuit according to the invention; 
         FIG. 2  is a flow diagram illustrating operation of the control circuit in the lighting mode; and 
         FIG. 3  is a flow diagram illustrating operation of the control circuit in the charging mode. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The LED lamp in the battery operated LED lamp and control according to the invention can be composed of one or more light emitting diodes. In the embodiment illustrated in  FIG. 1 , the lamp is composed of a string of three LEDs,  10 ,  12  and  14  connected in series. The LED string receives pulses of current from transformer  16  through Schottky diode  18  at one end of the LED string, and the opposite end is connected through a very small, 0.05Ω, resistor  20  to a circuit ground. 
     LEDs tend to become open circuited if they fail. In the case of an LED string, failure of a single LED can be prevented from causing failure of the entire string of LEDs by shunting each LED with a silicon controlled rectifier (SCR) having its gate connected so that it is triggered into conduction if the voltage across the corresponding LED becomes sufficiently high as a result of an open circuit condition. This objective is accomplished by circuit  22  associated with the LED string, the circuit comprising an array of SCRs, Zener diodes, capacitors and resistors as shown. 
     The LED lamp and the above-described failure avoidance circuitry are preferably housed in a lamp/switch module along with a normally open momentary switch, the lamp/switch module being connectable by a cable to a power supply/control module containing the control circuitry and the electrochemical DC power source. Preferably, a cable connector that is used to connect the lamp/switch module to the power supply/control module is also used, alternatively, to connect the power supply/control module to a charging power supply. Thus, the lamp/switch module and the charging power supply cannot be connected to the power supply/control module at the same time; the power supply/control module is either in a lamp operating mode or in a charging mode, but cannot be in both modes. 
     Terminals  24  and  26  are terminals on the power supply/control module that are connectable to the lamp/switch module. Terminal  28 , which is a circuit ground terminal, and terminal  30  are terminals on the power supply that are also connectable to the lamp/switch module. Momentary, normally open, push button switch  32 , which is part of the lamp/switch module, is connectable to terminals  28  and  30  when the lamp circuit  22  is connected to terminals  24  and  26 , since terminals  24 ,  26 ,  28  and  30  are parts of the same cable connector on the power supply/control module. 
     The power supply control module also has a fifth terminal  33 , which, with circuit ground terminal, is connectable to an external charging power supply when the lamp/switch module is not connected to the power supply/control module. 
     Junction  34 , which is connected through a 4.7 KΩ resistor  36  to a five volt internal DC voltage supply derived from regulator  38 , is normally at a +5 volt level. The switch  32 , when closed, connects terminal  34  to the circuit ground through a 100Ω resistor  40 , shunting a Zener transient voltage suppressor  42 . 
     Junction  34  is connected to a “switch” terminal on microcontroller  44 , which, in this embodiment, is a PIC16F690 microcontroller, manufactured by Microchip Technology Inc. of 2355 West Chandler Blvd, Chandler, Ariz. 85224. 
     The power supply/control module includes an electrochemical direct current source, which, in the embodiment shown in  FIG. 1 , is a lithium ion (Li-ion) battery comprising two cells  46  and  48  connected in series. The positive side of the battery is connected through fuse  50  to line  52 , which leads to one end of the primary winding of transformer  16 , the opposite end being switchable to the circuit ground by FET  54 , which is controlled by AND gates  56  and  58 . These AND gates receive inputs from the microcontroller  44  through lines  60  and  62 . Pulses of current are delivered to the LED string from transformer  16  by operation of FET  54  under the control of the microcontroller. 
     The battery (cells  46  and  48 ) is also connected through fuse  50  to a charging line  64 , which is connected to the external power supply terminal  33  through a transistor  66 , controlled by an integrated circuit driver  68 , controlled in turn by the microcontroller  44  through AND gate  70 . AND gate  70  receives inputs from the microcontroller through line  62 , and from line  60  through an inverter  72 . 
     Current in the LED string is monitored by OP amp  74 , which receives an input from the junction between LED  14  and resistor  20 , and delivers an LED current terminal of the microcontroller  44 . 
     The voltage across the LED string is monitored by OP amp  75 , the output of which is connected to the microcontroller. 
     Battery voltage is monitored by the microcontroller through OP amp  76 , an input of which is connected through a resistor  78  to line  64 , and the output of which is connected through a resistor  80  to the microcontroller. The voltage of cell  48  is monitored by the microcontroller though OP amp  82 , the output of which is connected to the microcontroller. 
     Battery charging current is monitored by sampling the voltage across a 0.02Ω resistor  84  through a resistor  86  connected to an OP amp  88 . An output of OP amp  88 , representing the battery charging current, is connected to the microcontroller  44  through resistor  90 . 
     A comparator  94  also monitors the battery current by sampling the voltage across resistor  84 , and is connected to the microcontroller through line  96 . The purpose of this comparator is to detect spikes in the battery current, and to shut down the microcontroller if spikes occur. 
     FETs  98  and  100  are provided to maintain the cell voltages in balance during charging. As mentioned above, the microcontroller monitors both the overall battery voltage and the voltage of cell  48 . The gate of FET  100  is connected to a microcontroller output. The gate of FET  98  is controlled from the microcontroller through another FET  102 . The microcontroller detects difference between the cell voltages and causes one or the other of FETs  98  and  100  to switch a resistor across the cell having the higher voltage. Thus, if the voltage of cell  48  exceeds the voltage of cell  46  by an amount exceeding a predetermined amount, e.g., 25 millivolts, FET  100  is turned on, switching resistor  104  across cell  48 . If the voltage of cell  46  exceeds the voltage of cell  48 , FET  98  is turned on, switching resistor  106  across cell  46 . In this way, the cells are maintained in balance during charging. 
     Other components of the control circuit include an integrated circuit voltage regulator  108 , arranged to regulate the voltage in the charging line, a reset controller  110 , that resets the microcontroller if the external power supply voltage is out of a predetermined range, and charge-indicating LEDs  112 ,  114  and  116 , controlled by the microcontroller, for giving an indication of the state of charge of the battery. Transistor  118 , the base of which is connected through a resistor  120  to the microcontroller, and the emitter of which is connected to the five volt DC supply provided by the battery through regulator  38 , supplies operating current to various integrated circuits of the controller. When the lamp is off and the battery is charged, transistor  118  opens, turning off the integrated circuits to which it supplies operating current. The microcontroller also turns off its clock. As a result, current draw in the control is reduced to a low level. 
     The operation of the lamp/switch module, when connected to, and controlled by, the power supply/control module is depicted in  FIG. 2 . Initially, the system is in an idle mode. A first momentary depression of manually operable switch  32  activates the controller, and causes operating current, in the form of pulses, to be delivered to the LED string, causing the LEDs to light brightly. As the LEDs heat up, their current draw changes. LED current is monitored through OP amp  74  ( FIG. 1 ) and regulated by the microcontroller, which adjusts the duty cycle of the pulses to maintain an appropriate average current for a substantially constant level of bright illumination by the LEDs. 
     If the momentary switch  32  is depressed a second time, for a short interval, e.g., less than one second, the microcontroller proceeds to a medium brightness mode, in which the pulse duty cycle is lower than in the case of the bright mode. Here again, the proper average current need to maintain a constant level of intermediate brightness is maintained by monitoring the LED current and controlling the pulse duty cycle. 
     A third depression of the momentary switch  32  for a short interval causes the system to proceed to a low brightness mode, in which the pulse duty cycle is still lower, but again regulated for a constant level of low brightness by monitoring the LED current. 
     A fourth depression of the momentary switch causes the system to proceed to a “blink” mode, in which the microcontroller causes the LED string to flash on and off in a predetermined pattern, e.g., six flashes per second, or three flashes in one-half second, followed by a one-half second dark interval. The microcontroller can, of course, be programmed to produce almost any desired flash pattern. Here again, the LED current is monitored to maintain a constant brightness level 
     Still another depression of the momentary switch returns the system to the bright mode. 
     In any mode, holding the momentary switch  32  closed for a longer interval, e.g., for more than one second, will cause the microcontroller to proceed to its idle mode, turning off the lamp. 
     In charging, as depicted in  FIG. 3 , the battery voltage is monitored by the microcontroller through OP amp  76  ( FIG. 1 ) and the battery current is simultaneously monitored through OP amp  88 . If the voltage is below a preestablished limit, the microcontroller maintains a constant charging current in line  64  by controlling FET  66  through driver  68 . However, when the battery voltage reaches the preestablished limit, the microcontroller shifts to a voltage limiting mode, in which the charging current, which is monitored through OP amp  88 , is controlled so that the voltage does not exceed a preestablished limit. During charging in the voltage limiting mode, the voltage balance in the two cells of the battery is maintained as explained above by shunting resistor  104  across cell  48  or shunting resistor  106  across cell  46  under the control of the microcontroller. If the charging voltage is removed, e.g. by unplugging the external power supply (not shown) or by disconnecting the external power supply from terminals  28  and  32 , the microcontroller returns the control to an idle condition. 
     The microcontroller monitors the battery voltage through OP amp  76  not only during charging, but also during operation of the lamp. It thereby monitors the level of charge in the battery, and can sense when the battery charge has reached the point where it can no longer maintain the required brightness level. The microcontroller can be set, to sense that the battery has reached a 25% charge level, for example. When the battery charge reaches this point, a flag is st in the microcontroller, and the lamp is flashed a predetermined number of times, e.g., four times to indicate to the user that the battery charge is low. In a typical low battery warning, the lamp is turned off for ¼ second, each second. After the prescribed number of flashes has occurred, another flag is set in the microcontroller, and the first-mentioned flag is cleared. This prevents further flashing until after the battery is charged. The lamp will continue to operate, but the battery should be recharged as soon as possible. 
     In summary it will be seen from the foregoing that the invention comprises a novel system in which a single programmed microcontroller is used both to control lamp operation and charging of the battery. The microcontroller establishes several modes of lamp operation in response to the operation of a simple momentary switch, monitors battery charge during lamp operation and produces a low battery warning, controls battery charging, and controls battery cell balance. Charging and lamp operation are simplified, and operation of the lamp while the battery is charging, are prevented, by providing for alternative connection of the power supply/control module to an external charging power supply and to the lamp/switch module using a common cable connector. 
     Various modifications can be made to the apparatus described. For example, the lamp can consist of a single LED, or two or more LEDs. The operating mode depicted in  FIG. 2  can be varied, for example by providing for more or fewer brightness levels, by eliminating the blink mode, or by adding more flashing modes. The control can include various combinations of lamp operating features and battery charging features among those described, and can include additional features. 
     Still other modifications may be made to the apparatus and method described above without departing from the scope of the invention as defined in the following claims.