Abstract:
According to the present invention, a heating pad controller incorporating a discrete ASIC (Application Specific Integrated Circuit) is provided which varies the duty cycle characteristics of a periodic signal during which power is applied to a heating pad heating element during a portion of the signal (“on” time). An oscillator circuit is used to produce a controlled duty cycle control signal for controlling the power applied to the heating pad by varying the on-time of the duty cycle. User control of the length of the on-time of the duty cycle is provided by way of a user controlled switch, thereby providing for a plurality of controller operating modes (e.g., WARM, LOW, MEDIUM, HIGH, etc.). To configure the duty cycle for each heat setting the heating pad controller utilizes switchable electrical components of varying impedance connected to the ASIC. A heating pad controller according to the present invention can be configured for use with heating pads of varying sizes simply by installing electrical components with the appropriate impedance during manufacture of the circuit board.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to the field of heating system controllers. More specifically, the present invention relates to a controller for a heating pad. 
   BACKGROUND OF THE INVENTION 
   Heating pads are commonly used by individuals to provide controlled and localized heating to particular body parts or areas. The heating pads may be incorporated into an article of clothing, such as a glove, or may be provided as a stand alone article to be placed on an area which is desired to be heated. Heating pads typically include a heating element, such as a large resistive element, which is heated by the application of power. Heating pads also include a thermostat or other temperature control mechanism which allows a user to vary and control the amount of heat provided by the heating pad. 
   Heating pad temperature control may be achieved by controlling the amount of power delivered to the heating element within the heating pad. The amount of power is in turn controlled by altering either the amount of continuous power applied to the heating element, or intermittently applying power to thereby alter the amount of time during which power is applied to the heating element. This latter approach to temperature control is often referred to as “duty cycle” control, since it is the amount of on-time and off-time of the applied power that is being controlled. 
   Conventional heating pad controllers typically include a thermostat for sensing the heating pad temperature and turning off power to the heating element once the heating pad has reached a desired temperature. An additional “tickler” heater in thermal contact with the thermostat is selectively turned on to accelerate the turn-off of the thermostat, thus, shortening the on-time of the heating element and maintaining the heating element at a lower overall temperature. When a desired temperature setting is activated by a user controlled switch, current is supplied to a “tickler” heater. The added heat generated by the tickler heater in conjunction with the heat generated by the heating element causes the thermostat to reach its turn-off temperature sooner than it would without the application of the additional “tickler” heater. When the thermostat turns off, all power to the heating element and the tickler heater is also turned off. This results in a lower heating pad temperature setting since the heater on-time is shortened due to the quick turn-off of the thermostat. 
     FIG. 1  shows a conventional heating pad controller which includes a “tickler” heater H 1  for regulating the different heat settings. As shown in  FIG. 1 , thermostats T 1  and T 2  sense the temperature of the heating pad which is heated by heater H 3   
   Additionally, thermostat T 1  is in thermal contact with heater H 1 , a small “tickler” heater. User control is provided via switch S, which is a four position switch. In the high switch setting, contacts S 3  and S 4  are connected together; in the medium setting, contacts S 3  and S 4  are connected together and contacts S 2  and S 5  are connected together; in the low setting, contacts S 2  and S 5  are connected together; while in the off setting, contacts S 1  and S 6  are connected together. In the low setting, all the current flows through heater H 1 , which in turn heats thermostat T 1  causing it to prematurely turn off, thus maintaining primary heater H 3  at a lower overall temperature. The current also flows through heater H 3  causing it to warm up. In the medium setting, some of the current is diverted through heater or resistor H 2 , which is more thermally isolated from thermostats T 1  and T 2  than heater H 1 . This results in heater H 1  applying less heat to thermostat T 1  such that thermostat T 1  remains on for a relatively longer period of time, thus keeping heater H 3  at a medium temperature. In the high setting, no current flows through heater H 1 , and thus there is no additional or accelerated heating of thermostat T 1 . This results in heater H 3  being maintained at the highest temperature level limited only by thermostats T 1  and T 2  which are typically required in order to meet the prevailing safety codes for such devices. 
   SUMMARY OF THE INVENTION 
   According to the present invention, a heating pad controller incorporating a discrete ASIC (Application Specific Integrated Circuit) is provided which varies the duty cycle characteristics of a periodic signal during which power is applied to a heating pad heating element during a portion of the signal (“on” time). An oscillator circuit is used to produce a controlled duty cycle control signal for controlling the power applied to the heating pad by varying the on-time of the duty cycle. The timing of the oscillator circuit is primarily determined by the charging of a capacitor, which in turn is controlled by the resistance through which the capacitor charges. User control of the length of the on-time of the duty cycle is provided by way of a user controlled switch. The switch is used to selectively vary the resistance through which a capacitor in the oscillator circuit charges up. The larger the resistance selected by the switch, the longer the charging time of the capacitor, and the longer the on-time will be, or equivalently, the longer the time period between off-times of the duty cycle. 
   The output of the oscillator circuit, or more specifically the voltage across the capacitor, is input to a Schmidt trigger. When the voltage across the capacitor reaches a level sufficient to cause the Schmidt trigger to switch, the output of the Schmidt trigger changes state, dropping to a specific voltage inherent to the Schmidt trigger. The change in state of the Schmidt trigger turns on an open drain transistor which acts as a discharge path for the capacitor by supplying a ground connection to the positive terminal of the capacitor. When the discharging capacitor reaches a certain low voltage, the Schmidt trigger will once again change states, this time going from low to high and open circuiting the transistor, allowing the capacitor to begin charging again. The Schmidt trigger will continue to change states in this manner as long as a voltage equal to or greater than the Schmidt trigger&#39;s threshold voltage is applied across the capacitor. Throughout the continuous charging and discharging of the capacitor, the output of the Schmitt trigger is essentially a square wave. This square wave output is input to a counter which counts a predetermined number of voltage changes (oscillator cycles) before cutting off power to the heating element. Thus, a higher frequency of oscillation in the duty cycle will cause the counter to reach its predetermined count sooner, allowing the controller to cut off power to the heating element sooner. If a higher resistance value is selected by way of the user controlled switch, the capacitor will take longer to charge and the counter will have to wait longer to reach its predetermined count, thus, power to the heating element will remain on for a longer period of time. 
   Additionally, when the heating pad is first turned on or when the desired temperature setting is increased, continuous power, i.e., 100% duty cycle operation, is initiated in order to rapidly heat the heating pad to the desired temperature. Similarly, when the desired temperature setting is decreased, no power is applied to the heating element, i.e., 0% duty cycle operation. An automatic shut off feature is also provided, whereby the circuit shuts off power to the heating element whenever a predetermined amount of time passes with no user input. 
   The heating pad controller utilizes switchable electrical components of varying impedance connected to the ASIC to configure the duty cycle for each heat setting. In like manner, the warm up time for each heat setting is selected using a combination of impedances connected to the ASIC. The heating pad controller can be configured for use with heating pads of varying sizes simply by installing electrical components with the appropriate impedance during manufacture of the circuit board. 
   A plurality of controller operating modes (e.g., WARM, LOW, MEDIUM, HIGH, etc.) are provided by the present invention. Which operating modes are to be implemented in a given controller model is determined at the time of manufacture by installing an LED (light emitting diode) corresponding to each of the modes of operation to be included. On power-up the controller checks for the presence of each LED corresponding to an operation mode, and if an LED is omitted, the omission will be detected and the corresponding mode bypassed during operation. 
   Additionally, the heating pad controller can operate using different types of switches, by connecting an ASIC MODE pin to either ground or power. Thus, either a slide switch configuration or momentary pushbuttons can be used to select the heat setting. The controller can operate at AC frequencies of 50 Hz or 60 Hz, selectable via a logic signal applied to an ASIC pin. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the invention will be more clearly understood when taken together with the following detailed description of an embodiment which will be understood as being illustrative only, and the accompanying drawings reflecting aspects of that embodiment, in which: 
       FIG. 1  is a block diagram of a prior art heating pad control system; 
       FIG. 2  is a block diagram of a heating pad control system according to the present invention; 
       FIG. 3  is an electrical circuit schematic of a heating pad controller according to a first embodiment of the present invention; 
       FIG. 4  is an electrical circuit schematic of circuitry that is internal to the ASIC of a heating pad controller according to the present invention; 
       FIGS. 5   a – 5   b  are electrical circuit schematic diagrams for an oscillator circuit used in a heating pad controller according to the present invention; 
       FIG. 5   c  is a timing diagram showing capacitor, Schmidt trigger, and transistor voltages in an oscillator circuit of an embodiment of  FIGS. 5   a – 5   b;    
       FIG. 5   d  is a timing diagram showing the on/off time in which power is delivered to a heating element in relation to the predetermined count of a counter according to the present invention; 
       FIG. 5   e  is a series of timing diagrams of capacitor and Schmidt trigger voltages, and on/off time waveforms of power delivered to a heating element when the resistance of a resistor in an oscillator circuit of an embodiment of  FIGS. 5   a – 5   b  is varied. 
       FIG. 6  is a block diagram of circuitry that is internal to the ASIC of a heating pad controller according to the present invention; 
       FIG. 7  is an electrical circuit schematic of a heating pad controller according to a second embodiment of the present invention; 
       FIG. 8  is an electrical circuit schematic of circuitry that is internal to the ASIC of a heating pad controller according to the present invention; 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  is a block diagram illustrating a heating pad control system  10  according to the present invention. Although the present description is given in terms of a heating pad, it should be understood that the present invention is likewise applicable to the control of heating devices in general. Control system  10  includes a controller  20  which controls heating pad  30 . A power source  40  is supplied to both the controller  20  and the heating pad  30 . Essentially, controller  20  controls the power from power source  40  that is applied to heating pad  30 . Heating pad  30  includes a heating element (not shown) which converts the electrical energy from power source  40  into thermal energy to produce heat. The heating element may be a resistive element through which current is passed and heat generated therein. User interface  50  is connected to the controller  20  and allows the user to turn the system on/off and control the desired temperature of heating pad  30 . 
   First and second embodiments of controller  20  are shown in more detail in  FIGS. 3 and 7 . Referring now to  FIG. 3 , therein is shown controller  100  which is used to selectively provide power to a heating pad (not shown) which is connected across terminals  104  and  106 . 
   Controller  100  includes an oscillator circuit which is used to produce a controlled duty cycle control signal for controlling the power applied to the heating pad. The timing of the oscillator circuit is primarily determined by the charging and discharging of capacitor  116 . Specifically, since power is applied 100% of the time in the HIGH setting, only the MEDIUM, LOW, and WARM settings utilize programmable or adjustable duty cycles, and therefore, use the oscillator circuit to produce a controlled duty cycle. Charging of capacitor  116  is accomplished through duty cycle resistors  113 ,  114 , and  115 , corresponding to MEDIUM, LOW, and, WARM settings, respectively. Thus, for example, when the WARM setting is selected via switch  108 , the ASIC  109  applies a voltage via output pin D 3  to resistor  115 , thereby charging capacitor  116  through resistor  115 . Resistors  113  and  114 , corresponding to MEDIUM AND LOW settings respectively, are not used when controller  100  is set to WARM mode, thus ASIC  109  output pins D 1  and D 2  are open circuited preventing the application of voltage to these pins. 
   Warm-up mode resistors  110 ,  111  and  112  are connected to ASIC  109  pins W 1 , W 2  and W 3 , respectively, and are used for fast warm-up in heat modes MEDIUM, LOW AND WARM, respectively. During duty-cycle mode voltage is not supplied to these ASIC pins, since the resistors connected to these pins are used primarily in warm-up mode and are not used when the ASIC  109  enters duty cycle mode. As such, ASIC  109  turns output pins W 1 , W 2  and W 3  off, thereby ensuring that capacitor  116  is no longer being charged through warm-up resistors  110 ,  111 , or  112 . Turning off ASIC  109  output pins W 1 , W 2  and W 3  can be accomplished by open circuited these output pins as discussed below. 
   The capability of ASIC  109  to open-circuit certain output pins, preventing the application of voltage at such pins, can be achieved by a variety ways, for example, one such method uses open drain transistors with external pull-up resistors. When a heat setting is selected via switch  108  the open drain transistor connected to the corresponding ASIC pin requiring voltage is turned ON and a connection to the DC power supply is complete. In this condition, the ASIC  109  output pins not used to implement the selected heat setting are essentially open circuited by the high impedance created when the transistor is not active (OFF), or in other words, if an ASIC  109  output pin is not active (ON) it is open circuited. This is useful in that only the resistor being used to implement the selected heating mode is driven by the ASIC, thus the unused resistors will not reduce the resistance through which capacitor  116  charges by acting in parallel with the selected warm-up or duty-cycle resistor. Alternatively, turning off specific ASIC  109  output pins can be accomplished by connecting ASIC  109  output pins D 1 , D 2 , D 3 , W 1 , W 2  and W 3  internally to the output of open-drain AND gates in which case the ASIC  109  output pins are either in an ON condition at a logic high (5 Volt output) or in an OFF condition (open circuit). 
     FIG. 4  shows the internal circuitry of ASIC  109  responsible for controlling the duty cycle for heating pad controller  100 . ASIC  109  OSC 2  pin and LINE pin are inputs for AC signals which supply an oscillation frequency used to control the state of Heat ON signal  409 , responsible for providing power to the heating element of a heating pad. The oscillator frequency generated at the output of Schmidt trigger  402  is coupled to a Warm up/Duty cycle counter chain  423 . Warm up/Duty cycle counter chain  423  begins at 0 and counts oscillator cycles until the predetermined count required for duty cycle mode has been reached, at which time Warm up/Duty cycle counter chain  423  outputs a counter overflow signal  424  to the clock input pin of D flip-flop  406 . Since in duty cycle mode Warm Up signal  405  (input to OR gate  407 ) is held at a logic low by counter chain  423 , the output of OR gate  407  is controlled by the state on the Q-bar output of the D flip flop  406 . Thus, when Warm up/Duty cycle counter chain  423  overflows, Q-bar switches from a logic high to a logic low state, the output of OR gate  407  drops low causing the output of AND gate  408  to drop low and current flow to the heating pad is turned off. If the turn off of the heating pad due to the overflow of counter  423  occurs before AC input cycle counter chain  411  outputs reset signal  410 , the Heat On signal  409  will be a square wave with a duty cycle less than 100%. AC input cycle counter chain  411  counts a predetermined number of oscillator cycles and when it reaches its count it outputs a reset signal  410 , resetting D flip-flop  406  and Warm up/Duty cycle counter chain  423  and turning on current flow to the heating pad. Thus, if Warm up/Duty cycle counter chain  423  overflows before AC input cycle counter chain outputs reset signal  410 , current flow to the heating pad is turned off for a period of time prior to the output of reset signal  410  by AC input cycle counter chain  411 . However, if counter chain  423  does not reach its predetermined count prior to its reset by AC input cycle counter chain  411 , heat will remain on. The higher the frequency at the ASIC  109  OSC 2  pin, the faster Warm up/Duty cycle counter chain  423  will time out, with the result that the proportion of the heat-on time will be reduced. 
   Capacitor  116  ( FIG. 3 ) is connected to ASIC  109  at pin OSC 2 . As shown in  FIG. 4 , the OSC 2  pin is connected to a Schmidt trigger  402  as well as to an open drain transistor  404 .  FIGS. 5   a  and  5   b  show electrical circuit schematic diagrams of an oscillator circuit comprising capacitor  116  ( FIG. 3 ), any one of a plurality of duty cycles resistors, a supply voltage  105  ( FIG. 3 ), Schmidt trigger  402  ( FIG. 4 ), and transistor  404  ( FIG. 4 ).  FIG. 5   c  shows corresponding voltage and timing diagrams for capacitor  116 , Schmidt trigger  402 , and transistor  404  as capacitor  116  charges and discharges in the oscillator circuit of  FIGS. 5   a  and  5   b . Initially, the output of Schmidt trigger  402  is high and transistor  404  does not conduct, essentially, acting as an open circuit. Referring to  FIG. 5   c , when the voltage at the input of the Schmidt trigger  402  (point A; OSC 2  pin), i.e., the voltage across capacitor  116 , reaches a level sufficient to cause Schmidt trigger  402  to switch (high threshold voltage (Vth) of Schmidt trigger  402 ) the output of Schmidt trigger  402  goes from high to low. (The Schmidt trigger threshold voltage level is determined by the Schmidt trigger used and is an inherent characteristic of the part) The output of Schmidt trigger  402  is connected to the input of inverter  403  (point B) which inverts the signal output from Schmidt trigger  402  and applies this inverted output to the gate of transistor  404 , causing transistor  404  to conduct, grounding the positive terminal of capacitor  116  (point A; OSC 2  pin). 
   Transistor  404  turns on, creating a discharge path for capacitor  116 . The positive terminal of capacitor  116  (Point A; OSC 2  pin) is essentially grounded and capacitor  116  will now begin to discharge through transistor  404 . When the voltage level at the OSC 2  pin decays sufficiently, this causes the output of Schmidt trigger  402  to again change state, going from low to high. Schmidt trigger  402  will continue to change states in this manner as long as a constant voltage, equal to or greater than the Schmidt trigger threshold voltage, is applied to ASIC pin D 3  ( FIG. 3 ). 
   Referring to  FIG. 5   c , the voltage across capacitor  116  decays from Vth until it reaches the low switching voltage of Schmidt trigger  402  (Vtl), at which time Schmidt trigger  402  turns off transistor  404  and the capacitor  116  begins to charge. With a constant voltage applied to ASIC pin D 3  and the capacitance of capacitor  116  held constant, the charge time for capacitor  116  is controlled by the resistance through which it charges. Referring to  FIG. 5(   e ), the larger this resistance, the longer the charging time of the capacitor and the more time is needed for capacitor  116  to reach the high threshold voltage of Schmidt trigger  402 . Thus, the oscillator circuit has a frequency of oscillation which is determined by the selection of a particular resistor connected to capacitor  116  ( FIG. 3)  in conjunction with the voltages provided by ASIC  109  at pins D 1 , D 2 , and D 3  ( FIG. 3 ). The frequency of oscillation can be increased or decreased by decreasing or increasing, respectively, the resistance of the resistor through which capacitor  116  charges. It will be understood to those of skill in the art that the frequency of oscillation output by the oscillator circuit can be increased or decreased by varying the impedance of a plurality of electrical circuit components included in the oscillator circuit and is not limited to selectably varying the resistance of a resistor. In an alternative embodiment, the resistance of a resistor through which the capacitor  116  charges can be held constant and the capacitance of the capacitor  116  can be selectably varied, varying the charge time of capacitor  116 , resulting in a frequency of oscillation which is determined by the selection of a particular capacitor connected in the oscillator circuit. 
   Referring to  FIGS. 3 and 4 , an AC signal is applied to the LINE pin of ASIC  109  through resistor  107 . The ASIC LINE pin is clamped internally to VCC and GND by clamping diodes (not shown), which are well known to those of ordinary skill in the art. Referring now to  FIG. 4 , the LINE pin is connected to Schmidt trigger  412 , which takes the AC signal applied at its input and outputs a square wave. The square wave output of Schmidt trigger  412  is coupled to AC input cycle counter chain  411  which counts a predetermined number of oscillator cycles, and outputs a logic low reset signal  410  when it reaches its count. The logic low reset signal  410  is connected to the reset pin of D flip-flop  406  to reset the flip-flop, resulting in a logic high Q-bar output, each time AC input cycle counter chain  411  outputs a logic low reset signal  410 . The Q-bar output of D flip-flop  406  is coupled to AND gate  408  through OR gate  407  to produce a Heat ON signal  409  whenever the output of OR gate  407  and enable signal  422  are both a logic high. Thus, each time AC input cycle counter chain  411  outputs a logic low reset signal  410 , D flip-flop  406  is reset resulting in a logic high Q-bar output (input to OR gate  407 ) and the output of AND gate  408  (Heat On signal  409 ) changes from logic low to logic high. 
   AC input cycle counter chain  411  is preprogrammed to count a predetermined number of oscillator cycles before outputting a logic low reset signal  410 . For example, for an applied AC signal of 50 Hz and AC input cycle counter chain  411  set to count 160 oscillator cycles, counter chain  411  will output a logic low reset signal  410  every 3.2 seconds (160 cycles/50 cycles/sec=3.2 seconds). The logic low reset signal  410  is coupled to the reset pin of D flip-flop  406  to reset the flip-flop every 3.2 seconds, causing the Q-bar output of D-flip flop  406  to change from a logic low to a logic high, or, in the event that the Q-bar output is already a logic high, reset signal  410  is ignored by the D-flip flop  406  and the Q-bar output remains a logic high. The Q-bar output of D flip flop  406  is coupled to AND gate  408  through OR gate  407  to produce a Heat ON signal  409  whenever the output of OR gate  407  and enable signal  422  are both a logic 1. Thus, the Q-bar output of D flip-flop  406  is set at 3.2 second intervals by the logic low reset signal supplied by AC input cycle counter chain  411  and the heating pad is turned on every 3.2 seconds. Enable signal  422 , used to implement an auto shutoff feature as described below, is applied to AND gate  408  to turn heating off after the auto shutoff time has expired. 
   AC input cycle counter chain  411  is responsive to a signal at ASIC  109  input pin SEL 1  to adjust AC input cycle counter chain  411  to accommodate either 50 Hz or 60 Hz AC cycles. ASIC  109  pin SEL 1  insures that regardless of whether a 50 Hz or 60 Hz AC signal is applied to the LINE pin, the time at which AC input cycle counter chain  411  outputs a logic low reset signal  410  does not change. The logic low reset signal  410  is responsible for resetting D flip-flop  406  and Warm up/Duty cycle counter chain  423 , and ultimately, for turning on current flow to the heat pad, as described in more detail below. Thus, for example, if the predetermined count of AC input cycle counter chain  411  was not changed to reflect a change in the AC input signal applied to the LINE pin, changing the applied AC signal from 50 Hz to 60 Hz (common when using a heating pad controller in countries which provide AC power at a frequency of 60 Hz) would cause AC input cycle counter chain  411  to output a logic low reset signal  410  sooner than it would if counting oscillation cycles of a 50 Hz AC signal, resetting Warm up/Duty cycle counter chain  423  sooner, and ultimately causing power to the heating element to remain on for a longer period of time. 
   If ASIC  109  pin SEL 1  is left unconnected or connected to VCC, ASIC  109  is configured for 50 Hz operation, more specifically, AC input cycle counter chain  411  is set to count 160 oscillator cycles. If however, ASIC  109  pin SEL 1  is connected to ground, as shown in  FIGS. 3 and 7 , ASIC  109  is configured for 60 Hz operation and AC input cycle counter chain  411  is programmed to count 192 oscillator cycles before outputting logic low reset signal  410 . Thus, with an input AC signal of either 50 or 60 Hz, the time in which AC input cycle counter chain  411  outputs a logic low reset signal  410  will remain the same (i.e., 3.2 seconds in this example). 
   The oscillator frequency generated at the output of Schmidt trigger.  402  is coupled to Warm up/Duty Cycle counter chain  423 . In duty cycle mode, Warm up/Duty Cycle counter chain  423  is reset every 3.2 seconds by reset signal  410  as described above. Upon being reset, counter chain  423  begins at 0 and counts oscillator cycles until the predetermined count required for duty cycle mode has been reached, at which time warm up/duty cycle counter chain  423  outputs a counter overflow signal  424  (low-to-high/high-to-low pulse) to the clock input pin of D flip-flop  406 . The Q-bar output pin of D flip-flop  406  takes on the inverse of the state of the D input pin on the rising edge (low-to-high transition) of the clock signal and is an inherent characteristic of the D flip-flop. Thus, with the D input pin of D-flip flop  406  connected to VCC, the Q output pin will also be at VCC, resulting in a logic low at the Q-bar output of D flip-flop  406 . In Duty cycle mode, Warm Up signal  405  (input to OR gate  407 ) is a logic 0 and is used primarily in WARM-UP mode as discussed below. Thus, Heat-On signal  409  is controlled by the logic state on the Q-bar output of D flip-flop  406 . For example, when the Q-bar output of D flip-flop  406  is a logic 0, the output of OR gate  407  will also be a logic 0. The output of OR gate  407  is connected to the input of AND gate  408  making the output of AND gate  408  (Heat ON signal  409 ) logic 0 and heat will not be supplied to the heating pad. Thus, when counter chain  423  overflows resulting in a logic 0 on the Q-bar output of D flip-flop  406 , Heat On signal  409  switches to a logic 0 state, turning off current flow to the heating pad. Heat On signal  409  will remain in a logic 0 state until the end of the 3.2 second time interval set by AC Input cycle counter chain  411 , after which time warm up/duty cycle counter chain  423  and D flip-flop  406  are reset by reset signal  410  causing the Q-bar output of D-flip flop  406  to change from logic low to logic high and warm up/duty cycle counter chain  423  to begin its count from 0. In this manner, and with reference to  FIG. 5   e , if the overflow of counter chain  423  occurs before AC Input cycle counter chain  411  outputs reset signal  410 , the Heat On signal  409  will be a square wave with a duty cycle less than 100%. However, if the overflow of counter  423  does not occur before counter  411  outputs a reset signal, both Warm up/Duty Cycle counter chain  423  and D flip-flop  406  will be reset by reset signal  410 . Since Warm up/Duty Cycle counter chain  423  did not output count overflow signal  424  to drive the clock input pin of D flip flop  406 , the Q and Q-bar outputs of D flip flop  406  remain unchanged (logic low Q; logic high Q-bar), the reset signal  410  is ignored by D flip flop  406  since there is nothing to reset and heat will continue to be supplied to the heating pad (Logic high Heat On signal  409 ). The higher the frequency at the OSC 2  pin, the faster duty cycle counter  423  will time out, with the result that the proportion of time that the Heat On signal  409  is a logic high will be reduced. As shown earlier, the frequency at the OSC 2  pin is controlled by the resistance of the resistor across which capacitor  116  charges, thus, by decreasing this resistance, resulting in a higher frequency of oscillation at the OSC 2  pin, lower duty cycle can be achieved. 
   Referring to  FIG. 3 , controller  100  also includes a fast warm up circuit. When an operating mode is selected via switch S 1 , thereby turning on heating pad controller  100 , ASIC  109  places the controller in high power mode, 100% duty cycle, for a period of time herein referred to as the “warm up time”. This time varies with the heat setting and is set by external resistors  110 ,  111 , and  112 , which provide a selectable amount of current to charge up capacitor  116 . Resistors  110 ,  111 , and  112  are not limited to any specific resistance value, although typically the resistance of resistor  112  will be greater than the resistance of resistor  111  and the resistance of resistor  111  will be greater than the resistance of resistor  110 . The increase in resistance causes a lower frequency of oscillation as discussed above, and results in Warm up/Duty cycle counter chain  423  taking longer to reach its predetermined count and heating pad controller  100  remaining in high power mode, 100% duty cycle, for a longer period of time. 
   Current to warm-up resistors  110 ,  111 , and  112  is provided by ASIC  109  pins W 1 , W 2  AND W 3 , respectively, thereby providing for the charging of capacitor  116  and setting the oscillator frequency at the OSC 2  pin in a manner analogous to that described for setting the duty cycle time frequency. As mentioned above, the timing of the oscillator circuit is primarily determined by the charging of capacitor  116 , which in turn is controlled by the resistance through which the capacitor charges. During warm-up mode, Warm up/Duty cycle counter chain  423  ( FIG. 4 ) counts a predetermined number of oscillator cycles and, unlike duty cycle mode, when the predetermined count has been reached, power to the heating pad is maintained “on” and Warm-up/Duty cycle counter chain  423  switches from warm up mode to duty cycle mode. Thus, in warm up mode, resistors  110 ,  111 , and  112  set a timeout value after which Warm Up/Duty cycle counter chain  423  switches from Warm Up mode to duty cycle operating mode. 
   Referring to  FIG. 4 , during Warm up mode, the Warm up/Duty cycle counter chain  423  provides a logic high Warm Up output signal  405  to OR gate  407 . The output of OR gate  407  is applied to AND gate  408  to enable full power to be applied to the heating pad. The Warm up/Duty cycle counter chain  423  counts a predetermined number of oscillator cycles and when the predetermined count has been reached, Warm Up signal  405  is reset (changed from logic high to a logic low) and Warm Up/Duty cycle counter chain  423  switches from Warm Up mode to duty cycle operating mode. Warm Up signal  405  is also connected to the input of open-drain AND gates  424 – 429  and is responsible for controlling whether voltage is to be supplied to warm-up resistors while the ASIC is operating in Warm Up mode or duty-cycle resistors when the ASIC switches to Duty Cycle mode. For example, while in Warm Up mode, logic high Warm Up signal  405  input to open-drain AND gates  427 – 429  will allow a selected one of ASIC output pins W 1 , W 2  or W 3  to be active (ON). Which of ASIC output pins W 1 , W 2  and W 3  is active (ON) will depend on which heating mode is selected as represented by mode signal  507 . The inverted output of warm up signal  405  (logic low), output of inverter  430 , is connected to the input of open-drain AND gates  424 – 426 . With a logic low input, the output of open-drain AND gates  424 – 426  will be open circuited as discussed above and the ASIC output pins D 1 , D 2  and D 3  corresponding to duty cycles resistors  113 – 115  will not be active (open circuit). Accordingly, when Warm Up/Duty cycle counter chain  423  switches from Warm Up mode to duty cycle operating mode, Warm Up signal  405  is reset, switching from logic high to logic low and ASIC output pins W 1 , W 2  or W 3  are turned off (open circuit) having a logic low warm up signal  405  input to open-drain AND gates  427 – 429  and a selected one of ASIC output pins D 1 , D 2  and D 3  will be active (ON). Which of ASIC  109  output pins D 1 , D 2  or D 3  is active (ON) will depend on which heating mode is selected as represented by mode signal  507 . Mode signal  507  will be discussed in detail below. 
   In duty cycle mode, the predetermined count at which Warm up/Duty Cycle counter  423  will output a signal indicating that the required number of counts has been reached is lowered. To achieve fast warm up, the counter chain must be capable of counting oscillator cycles for a time period on the order of minutes and therefore must be a relatively long counter chain. The counter chain required for counting in the duty cycle mode is on the order of seconds; hence the need to utilize a different predetermined count value in duty cycle mode than is needed in Warm-up mode. 
   Referring to  FIG. 3 , after the quick warm-up period has expired with Warm up/Duty cycle counter chain  423  reaching its predetermined count of oscillator cycles, ASIC  109  turns outputs W 1 , W 2  and W 3  off, thereby ensuring that capacitor  116  is no longer being charged through resistors  110 ,  111 , or  112 . Instead, charging is accomplished through duty cycle resistors  113 ,  114 , and  115  subject to the voltage levels appearing at ASIC  109  pins D 1 , D 2 , and D 3  as described above. 
   During duty cycle mode, warm up signal  405  will remain logic low until a higher operating mode (heat setting) of heating controller  100  is selected via switch S 1 , at which time, Warm up request signal  431  is reset causing Warm up/Duty cycle counter chain  423  to switch back into warm up, mode. Entering warm up mode, warm up signal  405  switches from logic low to logic high and constant power (100% duty cycle) is delivered to the heating pad for the duration of the warm up period defined for the particular heat mode. 
   Controller  100  can operate at AC frequencies of 50 Hz or 60 Hz selectable via a logic level applied to ASIC  109  pin SEL 1 . Referring to  FIG. 3 , if selection pin SEL 1  is left unconnected or connected to VCC, ASIC  109  is configured for 50 Hz operation. If, however selection pin SEL 1  is connected to GND as shown, ASIC  109  is configured for 60 Hz operation. 
   Controller  100  also provides for direct drive of LEDS  118 ,  119 ,  120 , and  121 . The heat setting modes available for a particular controller model are selected during manufacture of the controller by connecting an LED corresponding to each available mode. Referring to  FIG. 8 , LED pin  305  corresponds to any one of a plurality of ASIC  109  pins assigned to an LED (i.e., LED 1 , LED 2 , LED 3 , etc) and representing an operation mode (heat setting) of heating pad controller  100 . On power-up ASIC  109  checks for the presence of each LED corresponding to an operational mode by outputting a logic low LED drive signal  301  to the Gate of open drain transistor  302 . If an LED is not present on a particular pin, essentially leaving the LED pin unconnected (opened), the voltage at LED pin  305  (Source of transistor  302 ) will approach VCC. However, if an LED is connected to pin  305 , the voltage at pin  305  will be significantly lower than VCC due to the voltage drop across the LED. A Schmidt trigger  303  connected to LED Pin  305  produces an output signal  304 , indicative of whether an LED is connected to pin  305 . For example, if an LED is not present on ASIC pin  305 , the voltage at LED pin  305  will approach VCC, reaching the threshold voltage of Schmidt trigger  303 , causing the output of Schmidt trigger  303  to drop low. However, if an LED is present on ASIC pin  305 , the voltage at pin  305  will not reach the switching voltage of Schmidt Trigger  303 , keeping the output of Schmidt trigger  303  unchanged (logic high). The output of Schmidt Trigger  303  is latched by a skip latch  306  which effectively records whether an LED is present on an LED Pin by monitoring the high or low output voltage of Schmitt Trigger  303 . Skip latch signal  307 , along with the skip latch signals of the other ASIC pins assigned to LEDS, are used by ASIC  109  to determine which operating modes (if any) should be skipped. For example, if a logic high Schmidt trigger output signal  304  is input to Skip latch  306 , indicative of the presence of an LED connected to LED pin  305 , Skip latch  306  will output a Skip latch signal  307  allowing the operational mode assigned to the specific LED pin. However, if a logic low Schmidt trigger output signal  304  is input to Skip latch  306 , indicative of the absence of an LED at LED pin  305 , Skip latch  306  will output a Skip latch signal  307  preventing the operational mode assigned to that specific LED pin. In this manner, the heat modes available for heating pad controller  100  are selected by the connection of an LED, or absence thereof, corresponding to each available mode. 
   According to an alternative embodiment, in the event that an operational mode (heat setting) is desired in heating pad controller  100  and an LED is not desired for that particular heat mode the corresponding LED Pin can be shorted to ground. With the LED pin  305  shorted to ground, there is effectively a zero voltage at the input of Schmitt trigger  303 , thus, Schmidt trigger  303  will not switch its output from high to low and ASIC  109  will allow the operational mode while an LED is not present at the LED pin. The level detector (Schmidt Trigger  303 ) and Skip Latch  306  records the fact that the operational mode is desired as discussed above, while an LED is not present at the pin. 
   The information from the skip latch  306  is used during operation to control whether a heating mode is skipped or implemented in the heating pad controller. For example, referring to  FIG. 3 , if the LED  120  were omitted by leaving ASIC  109  pin LED 3  open, the omission would be detected on power up, and the skip latch  306  corresponding to the LOW mode would be reset. Therefore, the pushbutton or slide switch corresponding to the LOW mode can be omitted if that setting is not desired for a particular heater control module. Thus, for example, in a second embodiment of a heating pad controller using a two-button switch configuration according to  FIG. 7 , if LED  120  is omitted by leaving ASIC  109  pin LED 3  open; when a user presses the UP key  202  while in the WARM mode, the mode will change from WARM to MEDIUM, thereby bypassing the LOW mode. 
     FIG. 6  is a simplified block diagram of the LED drive and pin monitor circuit  502  internal to ASIC  109 .  FIG. 6  also shows a simplified block diagram of the PB/key decode circuit  504 . RESET CIRCUIT  501  is responsive to the power supply  105  ( FIG. 3 ) voltage applied to ASIC  109  (VCC and GND) to set the ASIC circuitry to a predetermined initialization state when voltage is first applied to the ASIC, or upon removal and reapplication of voltage to the ASIC. Upon detecting a voltage from the power supply a reset condition is induced and RESET CIRCUIT  501  enables LED DRIVE AND PIN MONITOR CIRCUIT  502  to initiate a pin monitoring function as previously described, resulting in the setting or clearing of a skip latch for each of the ASIC  109  pins assigned to an LED. The skip latch signals  503 , resulting from the detection of LEDS by LED DRIVE AND PIN MONITOR CIRCUIT  502  shortly after reset, are communicated as logic level signals to PB/KEY DECODE CIRCUIT  504 , which uses the signals to determine which operating modes (if any) should be skipped. PB/KEY DECODE CIRCUIT  504  is responsive to a logic level at the SEL 2  pin as previously described to enable the ASIC to be configured for use with either a pushbutton/slide switch arrangement or two-button, “increment mode”, switch configuration. PB/KEY DECODE CIRCUIT  504  decodes key inputs  506  and outputs mode signal  507  to HEAT CONTROL  508 . 
   As shown in  FIG. 4 , Mode signal  507  instructs ASIC  109  to supply voltage to one of ASIC output pins W 1 , W 2 , W 3 , D 1 , D 2  or D 3 , driving a specific warm-up or duty cycle resistor used by heating pad controller  100  to implement a selected heat mode. This signal will change as the ASIC switches from warm-up mode to duty-cycle mode, turning off the ASIC  109  output pin voltage connected to the warm-up resistor used in warm-up mode and turning on the ASIC  109  pin voltage connected to the duty-cycle resistor which will be used for duty-cycle mode. 
   Mode signal  507  is input to HEAT CONTROL  508 . When power to the heating element of a heating pad is required, HEAT CONTROL  508  outputs a logic high Heat ON signal  514 . Heat on Signal  514  is input to SCR/TRIAC DRIVE CIRCUIT  515 . An AC signal  516  applied to the ASIC  109  LINE input pin is provided to SCR/TRIAC DRIVE CIRCUIT  515  so that SCR/TRIAC DRIVE CIRCUIT  515  can output an SCR/TRIAC signal  521  coincident with zero crossings in a manner well know in the art. AC signal  516  is also applied to PB/KEY DECODE CIRCUIT  504  and HEAT CONTROL  508  which uses the signal as a time base for counting operations. 
   PB/KEY DECODE CIRCUIT  504  also outputs LED control signals  509  to LED DRIVE AND PIN MONITOR CIRCUIT  502  to turn LEDs  510  on or off appropriately depending upon the current operating mode. 
   Referring to  FIG. 3 , controller  100  can operate using one of two switch input configurations, selectable by connecting ASIC  109  pin SEL 2  to either ground or power. If selection pin SEL 2  is connected to GND, the ASIC  109  is configured to operate utilizing switch  108 . Switch  108  is of either a slide or momentary pushbutton switch arrangement configured such that one of a plurality of ASIC pins is grounded. The switch positions represent the heat settings OFF, WARM, LOW, MEDIUM, and HIGH and correspond to ASIC  109  input pins OFF, KEY 1 , KEY 2 , KEY 3 , AND KEY 4 , respectively. Internal to ASIC  109 , each input KEY pin is connected to an open drain transistor with an external pull-up resistor (not shown). Initially, the transistors connected to each KEY pin are off. When switch  108  is positioned over one of ASIC  109  pins KEY 1 , KEY 2  OR KEY 3  (e.g. KEY 1 ), PB/Key Decode circuit  504  ( FIG. 6 ) outputs mode signal  507  to heat control  508 , responsible for supplying voltage to warm-up resistor  112  through ASIC  109  output pin W 3  as described a above with reference to  FIG. 4 . 
   An alternative embodiment of a heating pad controller  100  as well as a second switch configuration is shown by controller  200  in  FIG. 7 . Here, ASIC  109  pin SEL 2  is connected to VCC rather than GND. In this configuration, called increment mode, only the ASIC  109  pins corresponding to the Down key  201  and the Up key  202  are active. ASIC  109  pins OFF, KEY 3 , and KEY 4 , which correspond to OFF, MEDIUM, AND HIGH, in the embodiment of  FIG. 3  are now grounded, as they will not be used in increment mode. On power-up, the first heat setting defaults to OFF and each push of the UP key  202  increments the heat setting through the available settings, such as WARM, LOW, MEDIUM, HIGH and back to OFF. The Down key  201  decrements the heat settings, terminating with the heat setting OFF. 
   Controller  200  includes a user safety feature designed to minimize and preferably eliminate any potential hazard due to a user inadvertently leaving the heating pad on. This feature includes an automatic shut off feature which turns off power to the heating pad when no user control, i.e., switch activation, is detected for a predetermined period of time, for example, 60 minutes. This is based on the premise that when no user control is detected for a sufficiently long period of time, this is a good indicator that the user has inadvertently left the heating pad on. 
   The Auto shutoff feature ensures that if a key is not pressed or a keyswitch setting remains unchanged for a predetermined period of time, the Heating pad will be turned off. Referring to  FIG. 7 , capacitor  204  and resistor  203  set an oscillator frequency in a manner analogous to that described previously with regard to the ASIC  109  OSC 2  pin. Referring to  FIG. 4 , the OSC 1  pin of the ASIC  109  ( FIG. 3 ,  FIG. 7 ) is coupled to schmidt trigger  417  resulting in an OSC 1  signal  419  being applied to Auto shutoff counter chain  420 . Auto shutoff Counter chain  420  counts OSC 1   419  cycles, eventually reaching its predetermined count and timing out, producing a logic low timeout signal  422 . Timeout signal  422  is applied to AND gate  408  to turn heating off after the Auto shutoff time has expired. When a key is pressed, key detect signal  421  resets Auto shutoff counter chain  420  causing the counter  420  to begin counting again at 0, and sets signal  422  to a logic 1, turning power to the heating pad back on. Thus, when a change in key state is detected, Key detect signal  421  resets Auto shutoff counter chain  420 , heating is again enabled if it was previously disabled, and the auto shutoff counter begins counting from the beginning again. Additionally, when signal  422  is a logic 0, an LED flashes indicating to the user that the heating pad controller has timed-out. If a button corresponding to a heat setting is pushed or the slide selector moved, the timer is reset, the LED stops flashing and heat is applied to the pad. If ASIC  109  is operating in increment mode, the first push of a heat setting selection button returns the heating pad to the heat setting set prior to timing out. Also, if a heating pad controller according to any of the above mentioned embodiments is off due to time-out or is turned off for a period of less than 3.2 minutes, quick warm-up is suspended and the unit goes directly to the selected duty cycle mode. 
   While in the embodiment of  FIG. 7 , ASIC  109  OSC 1  pin is connected to enable the oscillator to operate, in  FIG. 3 , the ASIC  109  OSC 1  pin is connected to GND thereby disabling auto shutoff. 
   In an alternative embodiment of heating pad controller  200 , if the ASIC  109  OSC 1  pin ( FIG. 4 ) is tied to VCC, ASIC  109  can be configured to set a customizable timeout time for the heating pad controller. In this embodiment, capacitor  204  and resistor  203  no longer set an oscillation frequency (signal  419 ) to drive auto shutoff counter chain  420 , instead, reset signal  410  is input to Auto shutoff counter chain  420  and the counter is set to a predetermined number of counts. For example, ASIC  109  sets the timeout to be 60 minutes by selecting reset signal  410  to be input to counter chain  420  in lieu of signal  419  (OSC 1  pin tied to VCC) and setting the auto shutoff counter chain  420  to 1125 counts (1125 counts/timeout*3.2 seconds/count=3600 seconds/timeout=60 minutes/timeout). 
   As shown in  FIG. 6 , Auto Shutoff circuit  511  operates as previously described and is reset upon receipt of a Key Detect signal  512  from PB/KEY DECODE CIRCUIT  504 . Upon the Auto Shutoff circuit  511  timing out, timeout signal  513  is applied to heat control  508 . Upon receipt of timeout signal  513 , heat control  508  resets Heat ON signal  514 , thereby ensuring that SCR/TRIAC DRIVE CIRCUIT  515  does not generate the output necessary to turn the heating pad on. Heat control  508  also generates a shutoff signal  520 . This signal is applied to LED DRIVE AND PIN MONITOR CIRCUIT  502  which uses the signal to cause one or more LEDs to flash when a timeout has occurred. 
   While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.