Abstract:
In a receiver circuit module ( 22 ) having a microcontroller (U 1 ), an output line (FILTER_LED) connectable to a source circuit module ( 20 ) is also used as an input. A switch (SW 2 ) disposed in the source circuit module is closed to change the wave form which is read by the microcontroller as an input signal to drive a 5V signal to the gate of a solid state switch (Q 1 ) turning it on to thereby energize an output device in the source circuit module via the same output line (FILTER_LED). By means of the dual function of the output line the control can notify a user of an HVAC system, for example, of system problems with a blinking light and/or an audible alarm as well as serving to notify the user at a remote location that selected maintenance is due, such as a need to change filters without additional control lines. The source circuit module can be mounted near the thermostat of the HVAC system or on the central heating and cooling unit.

Description:
This application claims benefit of application 60/172,876 filed Dec. 20, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to heating and cooling systems for buildings and the like and more particularly to thermostat circuits used with a microcontroller based controller for such systems. 
     BACKGROUND OF THE INVENTION 
     In present residential HVAC (heating, ventilating and air conditioning) systems a 24 volt AC signal is sent from a wall thermostat to a receiving control. The control either reads the signal as ON or the signal is used to directly turn on a 24 VAC electromechanical device. The signal is read as ON or OFF depending on the presence of the 24 VAC with respect to ground. For each function added, e.g., each electromechanical device added, typically at least one new wire is provided. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to add functions while minimizing the wires needed. Another object is the provision of a control in which functions are added without adding wires. 
     Briefly described, the invention uses the same 24 VAC signal and substantially the same wiring as in conventional controls but reads the signal in a different way. A receiving control is employed using a full wave power supply which provides a logic ground which is different from earth ground in the 24 VAC control. The logic ground is at a different potential from earth ground (24 VAC ground) and is a different potential from 24 VAC. The invention takes advantage of the ability to read the 24 VAC signal as potentially four different states by using a microcontroller. 
     Since the voltages are compatible with 24 VAC and 24 VDC components, the circuit on this line is capable of driving these loads as well as reading the status of switches. Extra wiring is avoided by using one wire in a dual function, i.e., as both an input as well as an output. The phasing and the circuit is used so that when a switch is pressed, the microcontroller uses a line normally used as an output as an input. In the preferred embodiment, this is used in a diagnostic circuit to alert a user of the HVAC unit of system problems with a blinking light and/or an audible alarm. The indicator light and alarm can be mounted near the thermostat or on the HVAC unit, as desired. 
     Additional objects and features of the invention will be set forth in part in the description which follows and in part will be obvious from the description. The objects and advantages of the invention may be realized by means of the methods, means of instrumentalities and combinations, particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a is a schematic diagram of a typical prior art 24 VAC control system; 
     FIG. 2 is a diagram showing signals with respect to logic ground; 
     FIG. 3 is a diagram showing 24 VAC referenced to earth ground; 
     FIG. 4 is a diagram showing a microprocessor input wave form; 
     FIG. 5 is a diagram showing output wave forms for several circuit configurations; 
     FIG. 6 is a diagram of a source circuit used in a control system made in accordance with the invention; 
     FIG. 7 is a diagram of a receiver circuit used in a control system made in accordance with the invention; and 
     FIGS. 8 and 9 are flow charts of the main routine used in practicing the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a typical 24 VAC control system is shown comprising a thermostat  10  depicted in dashed lines and receiver control  12  coupled to a transformer T 1 . In presently existing residential control techniques, a 24 volt AC signal is sent from wall thermostat  10  to the receiving control  12 . The control reads the signal as ON when one of S 1 , S 2  switches are closed or OFF when the switches are open, that is, as shown depending on the presence of the 24 VAC with respect to earth ground. Alternatively, the signal can be used to directly turn on a 24 VAC electromechanical driver (not shown). 
     A control made in accordance with the invention can use the same 24 VAC signal and substantially the same wiring but reads the signal in a different way. The receiving control made in, accordance with the invention uses a full wave power supply. This power supply results in a logic ground that is different from the earth ground used in the FIG. 1 24 VAC control. The logic ground is at a different potential from earth ground (24 VAC ground) and is a different potential from 24 VAC. With respect to logic ground such signals are shown in FIG. 2 showing signal a, no connection at 0 VAC; signal b, common only; signal c, 24 VAC only; and signal d, both 24 VAC and common. 
     A control made according to the present invention takes advantage of the ability to read the 24 VAC signal as potentially 4 different states. As will be explained in greater detail below, the receiving control has 24 VAC supplied to it from a common 24 VAC system transformer. The control has a “reference” supplied to it from the connection to 24 VAC power and earth ground. Using either 24 VAC or earth ground as a reference, the control can use the reference to decipher which phase input is being sent to the control. The ability to read these inputs during each phase is made possible and feasible by use of a microcontroller. The microcontroller has its own oscillator clock. The clock is used to time the AC wave form and to take readings during each quarter wave point, i.e., 180° apart. In this connection, reference may be had to FIG. 3 which shows a 60 Hz, 24 VAC wave e referenced to earth ground, and FIG. 4 which illustrates the points f read by the microcontroller each quarter cycle of an input wave form g comprising both 24 VAC and common. Generation of these signals are obtained by adding discrete components in the form of diodes to the control as illustrated in FIG. 5 in which a diode in the 24 VAC line provides an output wave form c; a diode connected to common provides an output wave form b; and a diode in both the 24 VAC line and in common provides an output wave form g. 
     In addition to the multiple 24 VAC signals on the same signal line, a diagnostic control made using the present invention also uses one of these signals as a power source to drive an LED indicator. On a source or generator module such as shown in FIG. 6, an LED is turned ON and OFF by the driver on the receiver board. During the OFF cycle, the receiver control looks at the control line as an input. It uses this period to determine if the switch is being pushed. When the switch is pushed, the signal changes from a half wave Common (C) input, to a Full wave R and C input. The use of these 24 VAC control lines as multiple inputs and outputs on a single line makes this useful for the residential HVAC market, for example. It is a very low cost method to communicate without adding wires. This is a requirement to retrofit existing systems. 
     With reference to FIGS. 6 and 7, the diagnostic control can be broken down into two parts: the source circuit  20 , FIG.  6  and the receiver circuit  22 , FIG.  7 . There is a dynamic two-way communication between the two modules. The source circuit provides an interface to the end customer and the receiver control is the main system board. 
     The source circuit has four terminals connected to it. The R-C terminals are the low voltage 24 VAC power supply terminals and ALARM_OUTPUT and FILTER_LED are connected to the receiver board. The 24 VAC is rectified through four diodes, CR 1 , CR 2 , CR 3 , CR 4 , to create a full-bridge rectification. The full-bridge rectification with a 24 VAC input creates a DC voltage power supply that drives the piezo-ceramic buzzer BZ 1 . A 2.0 K Ohm resistance R 3 ′ is a current limiting device to ensure the correct load across the buzzer. The DC voltage power supply also supplies power to a red diode LED 1 ′ which is also current limited by a 10 K Ohm resistance R 1 ′. Because the source circuit is an interface to the end customer, a switch SW 1  is provided to disable the buzzer. The low or ground side of the buzzer has a connection point ALARM_OUTPUT to the receiver board. Whenever the buzzer and LED 1 ′ are to be enabled, the receiver module switches these outputs on through the “ALARM_OUTPUT” line. Thus, the outputs of the receiver circuit at QC 1 , “ALARM_OUTPUT”, and QC 2 , “FILTER_LED”, to be discussed, are connected to the source circuit. These outputs are respectively driven by Q 3  at pin  12  and Q 1  at pin  13  of microcontroller U 1 . 
     The receiver control  22  is supplied with 24 VDC through diode, D 6 . A +5 VDC provided to U 1  is sustained through the 5 V power supply circuit of zener diode Z 3 , capacitor C 5 , resistor R 27  and capacitor C 6 . The negative half-wave, C, is read as ON and OFF into pin  14  of microcontroller U 1  by zener diode Z 4 , resistors R 22  and R 23  every 16.7 ms or 60 cycles per second. This interrupt is used to calibrate all timings and read all other inputs. The +5VDC power supply also provides power to read the external sensors, SUPPLY SENSOR QC 4 , QC 5  and RETURN SENSOR QC 6 , QC 3 , through resistors R 29  and R 14 , pin  1  of microcontroller U 1 , and through resistors R 17  and R 13 , pin  2  of microcontroller U 1 , respectively. When the temperature of these sensors change, the resistance changes causing the voltage to change at these pins. The voltage change is an input to U 1  for the built-in analog-to-digital converter. 
     The other inputs read into microcontroller U 1  come from the wall thermostat. These inputs are “O” at pin  9 , “Y” at pin  11 , and “W 1 ” at pin  10  of microcontroller U 1 . 
     The diode logic of diodes CR 1  and CR 2  provides phasing communication between the source and receiver boards. The “FILTER_LED” line is utilized to read either one or both phases of the alternating current, AC, waveform. The receiver circuit determines whether the switch, SW 2 , is being closed because switch closing sends the positive phase, R, to the control. The 2.0 K Ohm resistance, R 2 ′, is a current limiting device for the yellow diode, LED 2 ′, labeled as “FILTER_LED”. The 10 K Ohm resistance, R 4 ′, provides a voltage divider to ensure the correct voltage across LED 2 . 
     The “FILTER_LED” connection point from the source to the receiver control is also used as an input. The U 1  microcontroller at pin  13  uses this line to turn on light emitting diode LED 2 ′ but also, the phasing communications allows this same connection point to be an input. The switch SW 2  is read into pin  8  of microcontroller U 1 . The 300 K Ohm, resistance R 3 , and 100 K Ohm, resistance R 19 , provide the voltage divider input to microcontroller U 1 . The software algorithm in the microcontroller allows these dual capabilities. 
     With respect to FIGS. 8 and 9, the main routine of the system&#39;s algorithm starts at  100  and initializes the analog/digital multiplexer and serial port interface (SI 10 PV) at  102 . After initialization, the system checks the integrity of the two sensors. If either sensor has failed, open or shorted, the receiver circuit toggles the FILTER_LED at  104 , half second ON, half second OFF. This provides a user interface to ensure that sensors are operating correctly. Microcontroller U 1 , at step  106 , sends out 25 bytes of data on pin  5  (serial data out) and pin  7  (serial clock output). 
     Decision step  108  determines whether a half second flag has been generated and, if not, the routine cycles through step  108  until the flag is set. Once this occurs, decision step  110  determines if the system is in the manufacturing mode and, if so, the receiver control runs a speed-up automated manufacturing test code at step  112 , then going to step  114  “GOTORAT” to calculate the ratio. If the system is not in the manufacturing mode, step  116  detects whether there is a “Y” or a “W” input signal from the wall thermostat. If either of these inputs is ON, then a 20 day timer is incremented and decision step  118  determines whether the 20 day timer has expired. Going back to decision step  116 , if the “Y” or the “W” is not ON and an alarm condition exists, step  120 , then the output driver is prepared for the “ALARM” at step  124 , and the routine goes to step  130  (steps  122 ,  126 ) which checks to see if “W” is on. When the 20 day timer has expired at step  118  then, at step  128 , the “FILTER_LED” on pin  13  of microcontroller U 1  is enabled and the routine goes back to decision step  120 . With a negative response at decision step  118 , the routine goes to decision step  130 . At step  130  (CHK_W) a “W” input ON will clear any alarm conditions and all timers at step  132  before going to “GOTORAT”, step  114 . If the decision at step  130  is negative then decision step  134  determines whether the “Y” signal is ON and if so a 7 minute timer begins and upon expiration at step  136 , decision step  138  determines whether the “O” signal is ON. If the “Y” signal is not ON at step  134  then the alarm and timers are cleared at step  140  with the routine going to “GOTORAT”, step  114 . If the 7 minute timer has not expired in decision step  136 , the routine goes to “GOTORAT” at step  114 . 
     Decision step  138  determines whether the system is in the cooling mode (“O” ON) or the heating mode (“O” OFF). If in the heating mode, “O” not ON, then the system will enable the alarm at step  144  if the delta temperature, which is the difference between the supply and return sensors, is less then 5° F. (step  142 ). If the system is in the cooling mode, “O” ON, then the system will enable the alarm if the delta temperature is not greater than 12° F. or is equal to or greater than 30° F. steps  146 ,  148 . Following step  144  the routine goes to “GOTORAT”, step  114 . 
     Following negative decisions at steps  142 ,  148  the routine goes to process step  150  which clears the alarm indicating the system is performing correctly. The ratiometric numbers of the supply and return sensor to the reference resistance are calculated at step  114 . Once the ratiometric numbers are computed, the value must go to a look-up table for translation into degrees Fahrenheit, steps  152 ,  154 . The number translated from the look-up table is used to compute the delta temperature at  156 . 
     Using phasing for communication occurs when at step  158 , switch SW 2  is closed. Microcontroller U 1  always turns off Q 1  to read whether the positive phase of the 24 VAC is present on pin  8 . The system utilizes the “FILTER_LED” line to read SW 2  and to also drive Q 1 . Once this check is complete, the algorithm goes to 160 and then back to 100 to complete the cycle again. 
     Source and receiver circuit modules  20 ,  22  respectively, made in accordance with the invention included the following components: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 Microcontroller U1 
                 MC68HC705JJ7 
                 Resistor R1′ 10K, ¼  W 
               
               
                 Diode CR1-CR4 
                 IN4007 
                 Resistor R2′ 2.0K, ¼  W 
               
               
                 Diode D6 
                 IN4007 
                 Resistor R3′ 2.0K, ¼  W 
               
               
                 Light Emitting 
                 100K, ¼  W 
                 Resistor R4′ 10.0K, ⅛  W 
               
               
                 Diode LED1 
               
               
                 Light Emitting 
                 100K, ¼  W 
                 Zener Diode Z1, 12 V 
               
               
                 Diode LED1′ 
               
               
                 Light Emitting 
                 300K, ¼  W 
                 Zener Diode Z3, 5.1 V 
               
               
                 Diode LED2′ 
               
               
                 Resistor R1 
                 10K, ⅛  W 
                 Zener Diode Z4, 5.1 V 
               
               
                 Resistor R2 
                 2K, ⅛  W 
                 Capacitor C2 - .47 uF, 
               
               
                   
                   
                 100 V 
               
               
                 Resistor R3 
                 2K, ⅛  W 
                 Capacitor C4 - .01 uF, 
               
               
                   
                   
                 50 V, 10% 
               
               
                 Resistor R4 
                 1.5K, ⅛  W 
                 Capacitor C5 - 1 uF, 50 V, 
               
               
                   
                   
                 10% 
               
               
                 Resistor R5 
                 10K, ⅛  W 
                 Oscillator OSC, OSC caps 
               
               
                 Resistor R9 
                 1K, ¼  W 
                 Transistor Q1, MP SA 06 
               
               
                 Resistor R10 
                 1K, ¼  W 
                 Transistor Q3, MP SA 06 
               
               
                 Resistor R12 
                 10K, ¼  W 
                 Buzzer BZ1, SPB 14 
               
               
                 Resistor R13 
                 10K, ¼  W 
                 Switch SW1, 
               
               
                   
                   
                 065T-SPDT-A 
               
               
                 Resistor R14 
                 1K, ¼  W 
                 Switch SW2, 
               
               
                   
                   
                 TL59DF100Q 
               
               
                 Resistor R15 
                 1M, ⅛  W (Optional) 
               
               
                 Resistor R16 
                 100K, ⅛  W 
               
               
                 Resistor R17 
                 1.5K, 2 W 
               
               
                 Resistor R18 
                 100K, ¼  W 
               
               
                 Resistor R19 
                 100K, ⅛  W 
               
               
                 Resistor R20 
                 100K, ⅛  W 
               
               
                 Resistor R21 
                 2K, ¼  W 
               
               
                 Resistor R22 
                 2K, ¼  W 
               
               
                 Resistor R23 
                 2K, ¼  W 
               
               
                 Resistor R24 
                 2K, ⅛  W 
               
               
                 Resistor R25 
                 2K, ⅛  W 
               
               
                 Resistor R26 
                 1K, ¼  W 
               
               
                 Resistor R27 
                 C5 1 uF, 50 V, 10% 
               
               
                 Resistor R28 
                 C6 10 uF, 16 V 
               
               
                 Resistor R29 
                 C7 1 uF, 50 V, 10% 
               
               
                   
               
             
          
         
       
     
     Although the invention has been described with respect to a specific preferred embodiment thereof, variations and modifications will become apparent to those skilled in the art. It is, therefore, the intention that appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.