referring now to fig1 a refrigerator 20 includes a cabinet 22 which may enclose a plurality of refrigerated compartments cooled by a forced air refrigeration system . a fresh food compartment door 24 in conjunction with the cabinet 22 and a divider wall 26 enclose a fresh food compartment 28 . a freezer compartment 30 is enclosed by the cabinet 22 , the divider wall 26 and a freezer door 32 . air is refrigerated as a result of being passed in heat exchange relationship with cooling means including an evaporator 38 and is forced by an evaporator fan 40 into the freezer compartment 30 through a discharge air duct 41 . an additional duct ( not shown ) is provided in the divider wall 26 to effect cooling of the fresh food compartment 28 . the cooling means also includes a conventional compressor 44 , a condensor 46 and an accumulator or header ( not shown ) interconnected through tubing to the evaporator 38 to effect the flow of refrigerant thereto . a condenser fan 50 circulates air through the condenser 46 and may be energized concurrently with the compressor 44 and the evaporator fan 40 . the evaporator 38 and the evaporator fan 40 are disposed within an evaporator compartment 52 which is enclosed by the cabinet 22 and a rear wall 54 of the freezer compartment 30 . referring also to fig2 disposed between the coils of the evaporator 38 in the form of a loop is a defrost heater 58 which is periodically energized during a defrost mode of operation by the control of the present invention to de - ice the evaporator 38 . the defrost heater 58 may be a conventional resistive heater that is energized directly from the ac line , as noted more specifically below . a control circuit box 60 is disposed within the fresh food compartment 28 which , as noted more specifically below , encloses a portion of the circuitry for implementing the control of the present invention . disposed within the evaporator compartment 52 is a housing 66 which encloses a first thermally - operated switch s1 , also referred to as a mode control switch , described in greater detail in connection with fig3 . referring now to fig3 there is illustrated a combined schematic and block diagram of the control according to the present invention . the control includes means for determining when a particular frost load has accumulated on a portion of the cooling means , in particular the evaporator 38 . such means comprises an optical frost sensor 70 , such as the altech ra3 optical frost sensor , fig2 which is disposed adjacent the evaporator 38 . the optical sensor 70 in turn controls a normally closed frost switch 72 shown in fig3 which is coupled to a resistive bias heater r1 . the resistive heater r1 is disposed in the housing 66 in heat - transfer relationship with the thermally - operated mode control switch s1 which is movable between first and second positions . in the first position , as illustrated in fig3 and also referred to as the cooling mode position , a wiper of the switch s1 connects a first power conductor or line l1 coupled to an ac source of power ( not shown ) to one power terminal of each of the evaporator fan 40 and the condenser fan 50 . the power conductor l1 is also coupled to one power terminal of the compressor 44 through a motor starting device 74 . the switch s1 , when in the second or defrost mode position , contacts a second terminal 104 which connects the line l1 to a first power terminal of the defrost heater 58 . a second power terminal of the defrost heater 58 is coupled to a first stationary contact 76 of a second switch s2 in the form of a power relay 78 . a second stationary contact 80 of the power relay 78 is coupled to a second power terminal of each of the compressor 44 , evaporator fan 40 and condenser fan 50 . the power relay includes a movable wiper 82 , the position of which is controlled by a relay coil 84 . in the position shown in fig3 the wiper 82 connects the seond power terminal of the defrost heater 58 to a second power conductor or line n coupled to the ac source of power . when in the position opposite to that shown in fig3 the wiper 82 connects the second power terminal of the compressor 44 , the evaporator fan 40 and the condensor fan 50 to the line n . the relay coil 84 is coupled to one terminal of a power resistor r2 , the other terminal of which is coupled to the power line n . it should be noted that the power resistor may be disposed outside of the cabinet 22 . the second power terminal of the defrost heater 58 is coupled through a resistive bias heater r3 to the power line n . the heater r3 , like the heater r1 , is disposed in the housing 66 and operates the thermally operated switch s1 in a manner described more specifically below . the energized / de - energized condition of the relay coil 84 is controlled by a control circuit 90 which is illustrated in greater detail in fig4 . the control circuit 90 includes first and second input lines 92 , 94 which are coupled to the power line l1 and to a first contact 95 of the mode control switch s1 , respectively . a pair of output lines 96 , 98 are connected across the relay coil 84 . also included is an evaporator temperature sensing means 100 which is responsive to the temperature of the evaporator 38 to produce an output signal when one of a pair of predetermined evaporator temperatures is detected . the control circuit 90 further includes a compartment temperature sensing means 102 which will produce an output signal during a cooling mode of operation when the temperature of the compartment 28 exceeds a predetermined compartment temperature . the evaporator temperature sensing means 100 comrpises a first thermistor r4 which , as seen in fig2 is disposed adjacent to , and preferably directly on the evaporator 38 . the thermistor r4 develops a first temperature signal thereacross representing the temperature of the evaporator 38 . the evaporator temperature sensing means 100 further includes , as seen in fig4 a first comparator in the form of an operational amplifier , or op amp u1 , having a first or noninverting input coupled to the thermistor r4 and a second or inverting input which receives a set point signal representing one of two predetermined evaporator temperatures . the inverting input of the operational amplifier u1 is coupled by a line 101 to the junction between three resistors r5 , r6 and r7 . the resistor r5 is , in turn , coupled to a positive dc voltage line 103 while the resistor r7 is coupled to the output line 98 . resistors r5 and r7 thus form a voltage divider between the lines 103 and 98 establishing a first set point signal on the line 101 which is coupled to the inverting input of operational amplifier u1 . the resistor r6 is connected in series with a resistor r8 and a diode d1 to the input line 94 . a junction 109 between the resistors r6 and r8 is additionally connected to the positive dc voltage line 103 through a diode d2 . when the mode control switch s1 is connected to the contact 104 when in the defrost mode position , there is no voltage present on the input line 94 . while in the defrost mode position , the series combination of resistors r6 and r8 and diode d1 is effectively removed from the circuit so that the voltage divider consisting of resistors r5 and r7 provides the first set point signal at a first signal level to the inverting input of operational amplifier u1 as previously described . thus , the evaporator temperature sensing means 100 energizes coil 84 of relay 78 to terminate a defrost operation , while the mode control switch s1 is in the defrost mode position , when the signal across the thermistor r4 exceeds the first level representing a first evaporator temperature , such as 70 ° f . when , however , the mode control switch s1 is connected to the contact 95 when in the cooling mode position , ac line voltage from the line l1 is connected to the input line 94 . the series circuit of diode d1 and resistors r8 , r6 and r7 causes a rectified ac voltage to be developed at the junction 109 . a capacitor c1 filters the voltage at the junction 109 . the diode d2 connected between the junction 109 and the line 103 is a clamping diode which prevents the voltage at the junction 109 from rising above the voltage on the line 103 . thus , when the mode control switch s1 is in the cooling mode position , the junction 109 is at the same approximate dc voltage level as the line 103 which effectively connects the resistor r6 in parallel with the resistor r5 . this parallel connection causes the first set point signal to change to a second signal level which is greater in magnitude than the first level but which corresponds to a lower temperature . hence , the line 94 , diodes d1 and d2 and capacitor c1 comprise means for connecting the resistor r6 in parallel with the resistor r5 so that the second level is developed at the junction 109 when the switch s1 is in the cooling mode position . consequently , during the time the switch s1 is in the cooling mode position , the evaporator temperature sensing means 100 energizes the coil 84 of the relay 78 to cause operation of the cooling means when the signal developed across the thermistor r4 exceeds the second level representing a second evaporator temperature , such as 20 ° f . such a condition may occur when the refrigerator is operated in low ambient temperatures , as noted more specifically below . the compartment temperature sensing means 102 comprises a second comparator implemented by an operational amplifier u2 having a noninverting input coupled to the junction between a second temperature sensor in the form of a thermistor r9 which develops a signal representing presenting the temperature of the fresh food compartment 28 and a resistor r10 . an inverting input line 111 connected to the inverting input of the operational amplifier u2 receives a second set point input signal from the wiper of a user - adjustable potentiometer r11 coupled in series with resistors r12 , r13 between the lines 103 and 98 . the user - adjustable potentiometer r11 includes an actuator knob 110 shown on the exterior of the control box 60 shown in fig1 . the inverting input line 111 is additionally connected by a diode d3 to the junction 109 . as previously noted , when the mode control switch s1 is in the cooling mode position , a dc voltage level is developed at the junction 109 which is clamped to the level of the voltage on the line 103 by the diode d2 . the positive voltage at the junction 109 back - biases the diode d3 because it is more positive than the voltage developed at the wiper of the user - adjustable potentiometer r11 . therefore , the voltage on inverting input line 111 comprising the set point signal for operational amplifier u2 , is established during the cooling mode at a first level ( or range of levels ) solely by adjustment of the user - adjustable potentiometer and is unaffected by the voltage at the junction 109 . however , when the mode control switch s1 is moved to the defrost mode position , the voltage on the line 94 drops to zero . this , in turn , causes the capacitor c1 to discharge through the resistors r6 and r7 . as the capacitor c1 discharges , the voltage at the junction 109 decreases causing the diode d3 to be forward biased into conduction . this lowers the voltage on the inverting input line 111 so that the second set point signal for the operational amplifier u2 is at a second level less than the first level ( or range of levels ) during the defrost mode which corresponds to a higher temperature . in fact , the second set point signal for the operational amplifier u2 during the defrost mode is forced to a level that corresponds to such a high temperature level that the compartment temperature sensing means 102 is essentially inoperative , or inhibited , during the defrost mode . the outputs of the operational amplifiers u1 , u2 are coupled through resistors r14 , r15 respectively , to a node 112 . the node 112 is coupled to the base of a pnp transistor q1 , the collector of which is coupled to the output line 96 . the operational amplifiers u1 , u2 are therefore connected in a logical or configuration whereby the op amps u1 , u2 jointly control the transistor q1 and hence the relay coil 84 of the mode switch 78 . it should be noted that the thermistor r9 , the power relay 78 and the circuitry shown in fig4 with the exception of the thermistor r4 are disposed in the control box 60 located in the fresh food compartment 28 . the operation of the control of the present invention will now be described with particular reference to fig3 and 4 . the position of the mode control switch s1 is controlled by the resistive bias heaters r1 , r3 , also shown in dotted outline form in fig2 inside the housing 66 . when current flows through either heater r1 or r3 , heat is built up within the housing 66 , in turn causing the wiper of the switch s1 to contact the terminal 95 . on the other hand , if neither of the resistive heaters r1 , r3 is conducting current , the temperature within the housing 66 falls , in turn causing the wiper of the switch s1 to contact the terminal 104 . during those times when the evaporator 38 is relatively frost free , the frost switch 72 of the optical sensor 70 is closed thereby causing heat to build up in the housing 66 . this heat in turn causes the wiper of the switch s1 to contact the terminal 95 . however , when a frost load of predetermined size has accumulated on the evaporator 38 , the frost switch 72 of the optical sensor 70 opens which interrupts the current through the resistor r1 , thereby causing the temperature within the housing 66 to drop , and eventually causing the wiper of the switch s1 to contact the terminal 104 . this , in turn , connects the line l1 to one power terminal of the defrost heater 58 , the second terminal of which is coupled through the contact 76 and wiper 82 of the switch s2 to the neutral line n . the defrost heater 58 is thereby energized and the cooling means are de - energized to initiate a defrost operation . during the defrost mode of operation , the input line 94 , fig4 is disconnected from the power line l1 . this , in turn , inhibits or disables the compartment temperature sensing means 102 and prevents this means from actuating the transistor q1 and hence the relay coil 84 . further , the set point signal coupled to the inverting input of the operational amplifier u1 of the evaporator temperature sensing means 100 is established at the first level representative of a first predetermined evaporator tempeature , which as previously noted is equal to 70 ° f . in other words , when the defrost heater 58 has melted the ice on the evaporator coils and the evaporator temperature reaches this 70 ° 0 f . threshold , a signal is produced by the operational amplifier u1 to turn on the transistor q1 which in turn energizes the relay coil 84 . the evaporator temperature sensing means 100 therefore comprises defrost termination means for producing a defrost termination signal upon a determination that the energization of the defrost means is no longer desired . referring again specifically to fig3 the relay coil 84 , when energized , causes the wiper 82 of the relay 78 to contact the terminal 80 , thereby disconnecting the power line n from the second power terminal of the defrost heater 58 and connecting the second power terminals of the compressor 44 , evaporator fan 40 and condensor fan 50 to the power line n . it should be kept in mind that immediately after the defrost operation has terminated , the wiper of the mode control switch s1 has not returned to the cooling mode position . this is due to the fact that the frsot switch 72 cannot supply current to the heater r1 during the defrost mode because the switch s1 is not in contact with the terminal 95 . hence , the cooling means comprising the compressor 44 , evaporator fan 40 and condenser fan 50 are not energized at this time . however , the movement of the wiper 82 out of contact with the terminal 76 causes a current to flow from the power line l1 through the wiper of the switch s1 , the defrost heater 58 and the resistor r3 to the power line n . this flow of current through the resistive heater r3 causes the temperature within the housing 66 to rise , in turn eventually causing the wiper of the mode control switch s1 to move into contact with the terminal 95 . this causes the power line l1 to be connected to the first power terminal of each of the compressor 44 , evaporator fan 40 and condenser fan 50 to energize same . the control is now operating in the cooling mode . during the cooling mode of operation , the input line 94 receives the ac voltage on the power line l1 . this ac voltage enables the compartment temperature sensing means 102 by back - biasing the diode d3 and permits the operational amplifier u2 to control the transistor q1 and the relay coil 84 to cycle the cooling means on and off in dependence upon whether the compartment temperature is greater than or less than a set point as determined by the setting of the user - adjustable potentiometer r11 . specifically , when the compartment temperature is greater than the set point , the relay coil 84 is energized to operate the compressor 44 , evaporator fan 40 and condenser fan 50 . when the compartment temperature is less than the set point , the relay coil 84 is de - energized to cause the wiper 82 to contact the terminal 76 and thereby disconnect the compressor 44 , the evaporator fan 40 and condenser fan 50 from the power line n . it should be noted that , when the mode control switch s1 is in the cooling mode position , the return of wiper 82 into contact with the terminal 76 will not cause the defrost heater 52 to energize since the switch s1 is not in contact with the terminal 104 . the ac voltage present on the line 94 during the cooling mode also causes the magnitude of the first set point signal coupled to the inverting input of the first operational amplifier u1 to change to a signal level representative of a second predetermined evaporator temperature , such as 20 ° f . during the cooling mode , therefore , the evaporator temperature sensing means 100 energizes the cooling means via the transistor q1 and the relay coil 84 whenever the evaporator temperature rises above 20 ° f . this is desirable since this typically occurs in a low ambient temperature condition wherein the refrigerator 20 is operating in a room with a low ambient temperature . in this case , the low ambient temperature may cause the temperature of the fresh food compartment 28 to remain at an abnormally low temperature . however , during this time , the freezer temperature may rise to undesirable levels due to the infrequent cycling of the cooling means . although the thermistor r4 is in proximity to the evaporator 38 and senses the temperature thereof , this temperature is a fairly accurate representation of the temperature within the freezer compartment 30 when the compressor and fans have been off for a long period , as occurs during a low ambient condition . it is therefore desirable to operate the cooling means when the control is in the cooling mode irrespective of the compartment temperature sensing means 102 when the evaporator temperature sensing means detects an evaporator temperature greater than the second predetermined temperature . the evaporator and compartment temperature sensing means 100 , 102 , therefore , together comprise temperature control means for producing a cooling signal upon a determination that energization of the compressor is desired when the control is in the cooling mode . if desired , resistors r16 and r17 , fig4 may be coupled between the output of each op amp u1 , u2 , and the noninverting input thereof to provide a hysteresis effect . this effect eliminates rapid cycling between high and low states in the op amp output when the signals at the inputs thereof would otherwise be approximately equal . in summary , during the defrost mode of operation , the compartment temperature sensing means 102 is disabled and the defrost operation is continued until the evaporator temperature reaches a first predetermined temperature , as detected by the evaporator temperature sensing means . during the cooling mode of operation , either of the temperature sensing means 100 , 102 can operate the cooling means via the transistor q1 and the relay coil 84 . specifically , cycling of the cooling means is normally accomplished by means of the compartment temperature sensing means 102 except when a low ambient temperature condition is encountered , in which case the cooling means may be energized by the evaporator temperature sensing means 100 . it should be noted that , in the preferred embodiment , the defrost initiate switch is thermally operated through the resistive heaters r1 , r3 and is located in the evaporator compartment 52 . this location is desirable since the defrost initiate switch s1 can terminate a defrost operation even if there is a failure in the components supplying current to the relay coil 84 or if there is a break in the relay coil 84 itself . in this event , even if the power relay 78 fails to energize at the predetermined temperature of 70 ° f ., the warm evaporator will eventually cause the defrost initiate switch s1 to remove power from the defrost heater 58 in turn preventing dangerous overheating . it should also be noted , however , that any switch capable of being set and reset in response to a condition other than temperature may be substituted for the thermally operated switch s1 , although a thermal overload switch would then be needed in series with the defrost heater 58 to serve as a backup defrost termination switch . it should also be noted that the power relay 78 may be replaced by a pair of solid state switches , if desired .