A thermostat can be used, as part of a temperature control system, to control heating units and air conditioning units.  The electronic thermostat of the present invention accommodates heating units and air conditioning units in order to maintain comfort and economy levels of a predetermined area. 
Known electronic thermostats have a day set temperature for heating, e.g., 70 degrees Fahrenheit, a night set back temperature for heating, e.g., 62 degrees Fahrenheit, an actual time readout, a time for set back to start, a time for set back to end, and overrides to raise or lower temperature as desired. 
Known electronic thermostats also have a day set temperature for cooling, e.g., 72 degrees Fahrenheit, and a night set back temperature for cooling, e.g., 76 degrees Fahrenheit.  A switch, which controls the operation of a fan, may be set on "ON" for continuous operation or on "AUTO" for automatic (cyclical) operation.  Further, the thermostat has a "OFF"/"HEAT"/"COOL"/"AUTO" switch (system state switch). 
For heating, the system state switch is set to "HEAT."  For cooling, the system state switch is set to "COOL." 
Sometimes, such as in the spring or fall seasons, one may desire to control both the air conditioner and furnace with the thermostat due to the wide range of outside temperatures that may occur during these times.  This can be done by setting the system state switch to "AUTO."  In this state, the thermostat can turn on the air conditioner on a hot spring day and then turn on the furnace that night as it gets cooler.  This enables the operator to keep an area within a desired range of temperatures.  The present invention functions similarly.  A thermostat is a device which is capable of keeping a predetermined area within a range of preselected temperatures when the device is coupled to both an air conditioning unit and a heating unit.  The electronic thermostat controls the state (i.e. on or off) of both the air conditioning unit and the heating unit in order to keep a predetermined area within a range of preselected temperatures.  Essentially, if the temperature is greater than the maximum preselected temperature one desires, the electronic thermostat turns on the air conditioning unit.  On the other hand, if the temperature is less than the minimum preselected temperature one desires, the electronic thermostat turns on the heating unit. 
Currently, thermostats are provided which require programming for a variety of operating modes.  Programming is often difficult, especially for the elderly who are less apt to be familiar with electronic gadgetry.  For instance, a programming error can result by attempting to set the maximum desirable temperature lower than the minimum desireable temperature.  If this programming error were allowed to go undetected, the electronic thermostat would seek to turn on both the heating unit and the air conditioning unit, wasting a great amount of energy.  Previous electronic thermostats deal with this programming error by taking this possible error into account in their software, hardware and/or "firmware."  However, even if the electronic thermostat executes a program which prevents turning on both the air conditioning and heating units, in addition to warning the operator of the error, the operator is still forced to reprogram the electronic thermostat, wasting valuable time.  Further, until the operator recognizes the warning, the predetermined area may not be heated or cooled, resulting in discomfort or even damage to the predetermined area if the operator is not present.  Therefore, it is much more desireable to have an electronic thermostat which is impossible to incorrectly program. 
Prior to the present invention, electronic thermostats also required maintenance in the form of periodically checking and/or replacing batteries to ensure the electronic thermostat will function in the event of a power failure.  However, the present invention requires no periodic maintenance because a super capacitor is used to retain the memory of the electronic thermostat in the event of a temporary power failure.  Also, known electronic thermostats require the operator to remember to check and/or replace a filter, increasing the efficiency of air flow and energy use.  However, the present invention flashes the words "CHECK FILTER" once a fan has accumulated a predetermined running time.  for example 360 hours or more of running time.  Further, known electronic thermostats could not display the local (outdoor) temperature and the high and low local temperatures of the day.  Lastly, known electronic thermostats do not contain diagnostic information for the furnace and compressors within the unit itself. 
Accordingly, it is an object of the present invention to provide an electronic thermostat which can be programmed simply and intuitively by almost any user.  Another object of the present invention is to provide an electronic thermostat which is "user friendly" to the operator, especially elderly operators (i.e., the group of people who are home most often and need to adjust their thermostats most frequently). 
Still another object of the present invention is to provide notification to the operator to check for a dirty filter when the fan has accumulated a predetermined running time. 
A further object of the present invention is to provide a structure for providing the current outside temperature, and the high and low temperature of the day. 
Yet a further object of the present invention is to provide a structure and a method for easily displaying diagnostic information on a temperature control system. 
Yet another object of the present invention is to provide an electronic thermostat capable of storing preselected temperatures, in addition to other information, in its memory in the event of a power failure for a preselected period of time, without requiring periodic maintenance. 
A further object of the present invention is to provide an electronic thermostat capable of simultaneously displaying the maximum and minimum temperatures for both the first and second time intervals, in addition to other information, with the simple touch of a button. 
Still another object of the present invention is to provide an electronic thermostat which includes intuitively operated slides for setting control temperatures for both day and night set back operation. 
Another object of the present invention is to provide an electronic thermostat with a digital readout enlarged in a scale that eliminates the need for "back lighting" the display, which is commonly done on known electronic thermostats with smaller displays to facilitate their viewing in dimly lit areas, for example hallways. 
Another object of the present invention is to provide a simple means to change from day to night operation by pressing a button having an integral LED that glows during night operation, whereby the glowing LED makes it easy to locate this button and effect the change in dimly lit areas, for example hallways. 
Another object of the present invention is to provide an electronic thermostat with a digital readout coordinated with slide controls for easily setting the temperature operating program for both day and night operation. 
A further object of the present invention is to provide a thermostat cover that can be manipulated to display, in addition to other information, the settings of the first pair of slide means, the second pair of slide means, the third pair of slide means, and the current temperature. 
Yet another object of the present invention is to provide a method for intuitively and simply controlling the temperature of a predetermined area via an electronic thermostat. 
Other objects and advantages of this invention will become apparent hereinafter.