in the exemplary embodiment of the invention as disclosed in the drawing , a vacuum cleaner generally designated 10 is shown to comprise a housing 11 including a base portion 12 and a cover 13 cooperatively enclosing a dirt - collecting bag 14 . air is drawn into the bag through an inlet 15 in base 12 by means of a blower , or fan , 16 driven by a suitable electric drive motor 17 mounted on a support wall 18 in the base . the filtered air is discharged around the drive motor 17 and outwardly through an outlet 19 in the rear wall 20 of the base . as discussed above , clogging of the dirt - collecting bag 14 may provide a back pressure to the suction means tending to overheat drive motor 17 when the bag reaches a substantially clogged condition . the present invention comprehends an improved control generally designated 21 comprising an indicating means for indicating to the user the substantially clogged condition of the dirt - collecting bag . as shown in fig1 an indicator illustratively comprising an indicator lamp device 22 may be mounted in the cover 13 . operation of lamp 22 is effected when the dirt - collecting bag 14 becomes substantially clogged so as to indicate to the user the need for removing the dirt laden bag before overheating the drive motor 17 . control 21 is disposed in the outlet chamber 23 between motor 17 and outlet opening 19 in the path of air flow therebetween so as to be responsive to the temperature and velocity of the exhaust air . as best seen in fig3 and 4 , control 21 , in the illustrated embodiment , comprises a thick film device including a thermally conductive substrate 24 carrying a positive temperature coefficient resistor chip 25 closely adjacent a heater 26 so as to be in thermal conductive association therewith . also carried on the substrate element 24 are a ballast resistor 27 and a stabilizing resistor 28 . as shown in fig3 at least one of the heater 26 , variable resistor 25 , and substrate 24 are disposed in thermal transfer association with the flowing air , and as shown , the entire control 21 is so disposed . the operation of control 21 is best seen with reference to fig4 wherein the variable resistor chip 25 is connected in series with the stabilizing resistor 28 and the ballast resistor 27 across power supply leads l1 and l2 when the on - off switch 29 of the vacuum cleaner is closed . as shown , the drive motor and two - speed switch 30 are connected in series with the on - off switch 29 for driving the suction means 16 in the operation of the vacuum cleaner selectively at lower or higher speeds . in the illustrated embodiment , the indicating lamp 22 comprises a conventional neon lamp which is connected from switch 29 to between resistors 27 and 28 . heater 26 is connected between switch 29 and power supply lead l2 to complete the control circuitry . variable resistor 25 illustratively may comprise a sprague no . 3042709 ptcr chip having a switching temperature of approximately 105 ° c . adapted to change in resistance at that temperature from a cool resistance of 3 . 3k ohms to a hot resistance of approximately 1 megohm . stabilizing resistor 28 may comprise a 17 k ohm resistor and ballast resistor 27 may comprise 21 . 1k ohm resistor to provide selective operation of the neon lamp 22 when the temperature sensed by the variable resistor 25 reaches approximately 105 ° c . as indicated , the substrate 24 comprises a thermally conductive substrate so as to conduct heat from heater 26 to the variable resistor 25 for improved sensitivity . air flowing in heat transfer association therewith maintains the resistor 25 below the resistance change - over temperature as long as the dirt - collecting bag 14 remains substantially unclogged . however , when the dirt - collecting bag becomes substantially clogged , the dissipation of heat from heater 26 is sufficiently lowered to permit variable resistor 25 to sense a temperature in excess of approximately 105 ° c ., and thereby switch to its high resistance condition providing firing voltage to the neon lamp 22 and thereby causing an indication to the user of the clogged bag condition . the ballast resistor 27 effectively limits the current through the fired neon lamp to the necessary safe value . the stabilizing resistor 28 effectively pre - biases the circuit so that the voltage applied to neon lamp 22 when the variable resistor 25 is in the low resistance state is only slightly less than the firing voltage . thus , a relatively small change in the resistance of the resistor 25 immediately fires the lamp 22 to provide the desired clogged bag indication . stabilizing resistor 28 further minimizes the effect of variation in the firing voltages of different neon lamps and effectively eliminates flickering of the lamp once it is turned on . as will be obvious to those skilled in the art , the specific heating capacity of heater 26 may be correlated with different power drive motors 17 to provide optimum indicating functioning . preferably , the heater 26 is selected so as to permit maximum power to be developed by the motor 17 with a normal maximum restriction at the inlet 15 , such as may be caused by the use of a crevice tool connected to the inlet , without causing the indicating lamp 22 to light . thus , control 21 maximizes sensitivity while yet minimizes possibility of nuisance cycling as may be caused by a substantial restriction to the inlet such as by the use of a crevice tool . it has been found that the use of control 21 , including heater 26 for controlling the resistance of variable resistance 25 and thus the operation of neon lamp 22 , provides a substantially greater sensitivity than that obtained with conventional pressure switch indicators . as will be obvious to those skilled in the art , maximum sensitivity may be obtained by adjusting the resistors 27 and 28 for accurate specific correlation with the other parameters of the specific circuit . the resistors , as discussed above , may comprise thick film resistors on substrate 24 and , thus , may be easily and economically adjusted to accuracies of ± 1 / 2 % by using a standard laser trimming operation . control 21 may be adjusted for use with any suitable size vacuum cleaner motor . illustratively , highly accurate control of such motors has been obtained in actual practice where the motors have been rated at 1hp . and 2 . 8 hp . with heater resistors of approximately 2 . 0k ohms and 3 . 3k ohms , respectively , by suitable selection of the control circuit parameters . similarly , other size motor drives may be similarly controlled . control 21 functions equally well when the motor is operated on either high or low speed . the ptc resistor 25 switching temperature and heater resistor 26 resistance value may be selected such that the net cooling effect of the higher speed , higher temperature air is substantially equal to that of the lower speed , lower temperature air when the dust bag is full . control 21 eliminates the need for any moving parts in controlling the operation of the indicating lamp 22 and as the air flowed in heat transfer association therewith is substantially completely unobstructed , as illustrated in fig1 and 2 , no clogging of the control portion of the air flow system occurs so as to maintain the indicating means highly accurate with minimum servicing requirements . changes in the suction - producing ability of the air moving means of the vacuum cleaner do not affect the functioning of the control 21 so that desired indication of the clogged bag condition is maintained throughout the life of the vacuum cleaner . the foregoing disclosure of specific embodiments is illustrative of the broad inventive concepts comprehended by the invention .
a vacuum cleaner having an indicator for indicating an undesirable clogged condition of a dirt - collecting structure thereof . the indicator includes a control having a heater and a thermally responsive device responding to heat transferred from the heater to operate an observable indicator carried by the vacuum cleaner when a substantially clogged condition of the dirt - collecting means occurs . operation of the indicator by the thermally responsive device is prevented as long as the dirt - collecting structure remains substantially unclogged as a result of the cooling action of air being flowed through the vacuum cleaner .