US3765100A

US3765100A - Capacitive sensing dryer control

Info

Publication number: US3765100A
Application number: US00182580A
Authority: US
Inventors: D Heidtmann
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1971-09-22
Filing date: 1971-09-22
Publication date: 1973-10-16
Anticipated expiration: 1990-10-16
Also published as: BR7206526D0

Abstract

A dryer control including capacitive sensing means which makes contact with fabric within the dryer. The resistivity of the fabric is a function of the moisture retained therein and determines the level of alternating current which flows through the capacitive sensing means. Current is monitored by a control circuit including a multistate transistorized amplifier which serves to disable an SCR which is connected in series with a timing mechanism. As the fabrics dry and become less conductive, less alternating current is transmitted by the capacitive sensor, de-energizing the control circuit and enabling the SCR, which allows current to pass through the timing mechanism.

Description

United States Patent [1 1 Heidtmann CAPACITIVE SENSING DRYER CONTROL [75] Inventor: Donald S. Heidtmann, Louisville,

[73] Assignee: General Electric Company,

Louisville, Ky.

22 Filed: Sept. 22, 1971 21 Appl.No.: 182,580

[ Get. 16, 1973 Primary ExaminerMeyer Perlin Assistant Examiner-Ronald C. Capossela Attorney-Walter E. Rule et a1.

[5 7] ABSTRACT A dryer control including capacitive sensing means which makes contact with fabric within the dryer. The resistivity of the fabric is a function of the moisture retained therein and determines the level of alternating current which flows through the capacitive sensing means. Current is monitored by a control circuit including a multistate transistorized amplifier which serves to disable an SCR which is connected in series with a timing mechanism. As the fabrics dry andbecome less conductive, less alternating current is transmitted by the capacitive sensor, de-energizing the control circuit and enabling the SCR, which allows current to pass through the timing mechanism.

6 Claims, 2 Drawing Figures [52] U.S. Cl 34/45, 34/48, 73/3365 [51] Int. Cl. F26h 13/10 [58] Field of Search..; 236/44 E; 34/45, 34/48; 73/73, 336.5

[56] References Cited UNITED STATES PATENTS 3,222,798 12/1965 Thornberry et a1 34/45 2,904,765 9/1959 Seehof et a1. 73/336.5 X 3,192,642 7/1965 Hughes 34/45 3,411,219 11/1968 Bartholomew 34/45 2,930,016 3/1960 Weston et a1.. 73/336.5 X 2,962,897 12/1960 Muller.... 73/336.5 3,083,573 4/1963 Shaw 73/336.5

PAIENTEU um I a ma INVENTOR. DONALD s. HEiDTMANN H\S ATTORNEY 1 CAPACITIVE SENSING DRYER CONTROL BACKGROUND OF THE INVENTION The present invention relates to drying machines and, more particularly, to such machines having sensors placed therein for determining the moisture content of fabrics being dried.

In order to facilitate the operation of modern clothes dryers, such machines are commonly provided with a control for terminating the operation of the machine when the moisture contentof fabrics therein has been reduced to a predetermined level. In order to facilitate the operation of the control, it is necessary to provide sensing means for determining the moisture content of the fabric. One popular approach to this problem is to dispose a pair of sensing elements within the dryer such that they are directly contacted by the fabrics. The fabrics thus act to bridge the gap between the sensor elements, completing an electrical circuit and enabling the transmission of current therethrough. As the moisture content decreases the resistivity of the fabrics, and thus the resistivity of the electrical circuit portion constituted thereby, is increased. .This change in circuit parameters causes the control to provide an output signal whose level differs from that which was provided when the fabrics were moist, the new output signal being utilized to energize a timer which eventually terminates the operation of the machine.

SUMMARY OF THE INVENTION It is thus an object of the present invention to=provide new capacitive sensing means in a fabric drying machine.

It is another object of the present invention to provide a sensor in a fabric drying machine which does not make conductive contact with the fabrics therein.

A still further object of the present invention is to provide means for simultaneously sensing the moisture content of fabrics that do not directly contact the sensor cover.

In accordance with one aspect of this invention, there is provided a control for a dryer which includes a timer for terminating the operation of the dryer after some predetermined time has elapsed, a control for energizing the timer, and a capacitive sensing means for actuating the control. A preferred embodiment of the sensing means includes a housing or'dome of temperatureresisting dielectric material which protrudes into the chamber of a dryer containing fabrics to be dried. A

conductive coating disposed upon the inner surface of the housing constitutes one plate of a capacitor, the insulated housing comprising the capacitor dielectric. The conductive coating is resistively. coupled to a source of alternating potential, and the enclosure within which the fabrics are disposed coupled to neutral, or ground potential. When the fabrics are moist, they act to electrically couple the grounded enclosure within the outer surface of the insulated sensor housing, thus forming what may be considered to be the second "plate of the capacitor. A flow of alternating current to the conducting material inside the insulator housing is thus facilitated, and the voltage across the resistivecoupling resulting from the increased current flow is communicated to the control means.

In order not to affect the voltage drop across the resistive coupling, the initial stage of the control means comprises an amplifier having an extremely high input impedance. The amplifier serves to enable a final stage of the control which disables an SCR connected in series relationship with a timer motor. As the fabrics lose moisture current flow through the resistive coupling decreases, de-energizing the amplifier and facilitating the conduction of the SCR, thus energizing the terminating mechanism'or timer.

It will be seen that the capacitive sensing system which comprises the present invention has advantages not found in the prior art. In particular, the conductive path which couples the sensor to the grounded enclosure includes a substantial portion of the volume of fabric contained within the enclosure rather than merely a small segment of fabric which bridges a pair of closely-located electrical contacts as is the case with prior art devices. A more representative sampling of the conductivity, and thus moisture content, of the fabric is thus achieved.

BRIEF DESCRIPTION OF THE DRAWINGS matter which is regarded as the invention, it is believed that the invention will be better understood from the following description of the preferred embodiment, taken in connection with the accompanying drawings, in which:

FIG. 1 is a side elevational view of a fabric drying machine suitable for incorporation of the improved dryer control;

FIG. 2 is a schematic circuit diagram of one embodiment of the present invention.

DESCRIPTION OF TI-IE PREFERRED EMBODIMENT Referring now to FIG. 1, there is illustrated a fabric drying machine 10 of the domestic type which includes a'cabinet 11 provided with a front door 12 for access to the interior of an enclosure within. The enclosure, or drum, 14 is rotatably mounted within the cabinet and provides a means for retaining fabrics to be dried. An electric motor 16 is disposed generally adjacent to the drum 14. A pulley 17 at one end of the motor shaftis rotatably coupled to drum 14 by means of a drive belt 18. It will be understood by those skilled in the art that the speed or rotation of'drum 14 is determined by the speed of motor 16, and the relative diameters of pulley l7 and drum 14.

The rear end of the drum is supported within the cabinet upon a stub shaft 19 which is disposed along the axis of the drum, and is received in a stationary bearing means 20, affixed to the rear portion of the cabinet 11. The front end of the drum is rotatably supported upon a pair of rollers, or wheels, one of which is illustrated at 21. The wheels are rotatably mounted upon stub shafts, or axles, which are affixed to a front portion of cabinet 11, substantially parallel to the axis of rotation of drum 14. The drum is thus rotatably supported at one end by an axle shaft and at the other end by rollers, so that it may be rotated by motor 16 for causing fabrics contained therein to be agitated or tumbled in order to enhance the drying process. A plurality of clothes tumbling ribs 31 are provided upon the lateral wall of drum 14 to enable the fabrics to follow the lateral wall of the drum as it rotates upwardly, tumbling back down as the rib ascends further.

A flow of air for drying the fabrics within the drum is drawn from suitable air inlets, such as louvers 22 at the rear of the cabinet, and passed across

electrical heating elements

23 and 24 after which the heated air traverses openings 25 in the rear end of the drum and passes axially through the drum, and through the fabrics being agitated therein. The air leaves the fabric enclosing area by means of perforations 26 in an inner bulkhead of door 12 and passes, by means of aligned

openings

27 and 28, into a duct 29 disposed within the front walls of the cabinet. A blower 30, rotatably driven by motor 16, impels the air drawn from duct 29 through an exhaust duct and thence from the machine.

A sensor 32 as taught by the present invention is mounted upon the inner bulkhead of door 12. The sensor housing advantageously has a dome-like configuration, which will not snag or catch on fabrics as they pass across the outer surface thereof. The housing is preferably made of a heat-resistant plastic material such as Lexan, but may be made from any strong heat-resistant dielectric material.

Referring now to FIG. 2, a schematic circuit is shown which comprehends the sensor and associated control and time delay means which terminate operation of the dryer when moisture retained by the fabrics therein has declined to some predetermined level.

Terminals

40 and 41 are connected across a source of AC power which may, for instance, be 115 volt AC power as is normally available in a household. In this case, terminal 41 will represent neutral, or ground, potential. It will be recognized by those skilled in the art that in cases where 230 volt AC power is provided by means of a typical three-wire system, terminal 40 may be connected to one side of the 3-wire supply and terminal 41 to the neutral conductor. Coupled between buses L, and N, connected to

terminals

40 and 41 respectively, is a timing means. or motor 44 which acts in a manner to be described hereinafter. Also coupled between the buses, and thus across the AC power source, is a control which includes transistors 0,, Q Q and is supplied with rectified AC power by means ofa diode CR,. Sensor 32, includes, as schematically shown in FIG. 2, an insulated housing 45 and a conductive element 43 which is coupled by means of current limiting resistor 46 and sensing resistor 47 to bus L,. The insulated housing 45 preferably comprises a dome-shaped element made of a heat-resistant dielectrical material and the sensor is supported by a bulkhead so that the housing forms the inner surface of door 12. Disposed upon the inside of housing 45 is a conductive coating 43. Disposed upon the inside of housing 45 is a conductive coating 43. This coating may be a metallic paint or other metallic coating which has been deposited by a suitable process, such as vapor deposition. A conductor is electrically connected to the coating, and coupled by means of

resistors

46 and 47 to bus L,. Resistor 47 functions as a dropping resistor, providing a voltage which is representative of the current flowing through the resistor to or from the conductive coating 43 of the sensor element. Resistor 46, of a much higher value than the resistor 47, serves to limit current in the event that the sensor is damaged or short-circuited. Cable shielding means indicated at 48 is provided about the conductor which contacts resistor 46 to coating 43 and serves to shield the conductor from stray capacitance. To avoid undesirably high current flow in the event of damage to the sensor, the cable shield is coupled by means of current limiting resistor 42 to line voltage bus L,. A metal plate 33, interposed between the inner bulkhead of door 12 and sensing means 32, further shields the sensing means from stray capacitance which may be derived from the bulkhead.

Turning now to the control, a predetermined lower level of AC voltage is provided by means of a voltage divider 50, including

resistors

56 and 57, to the anode of diode CR, which operates with filter capacitor C, to provide filtered DC voltage to the control. Capacitor C integrates the voltage appearing across resistor 51 in a manner to be described below. A first, high-gain amplifier Q, is provided and advantageously takes the form of a pair of transistors coupled in a Darlington configuration. The emitter-collector circuit of the final, or second transistor of the Darlington circuit is resistively coupled between bus L, and resistor 51, while the base of the first transistor of the pair, comprising the input terminal of the amplifier Q, is coupled by means of conductor 52 to the intersection of the dropping resistor 47 and current limiting resistor 46, for detecting the signal developed by the sensor 32.

The gain of amplifier Q, is extremely large, being the product of the gains of the two transistors comprising Darlington circuit. A small change in current, and thus voltage drop, across resistor 47 will thus result in a significant change in the conductivity of the final transistor of the amplifier, and so of the voltage drop across resistor 51. It will be recognized by those skilled in the art that various forms of amplifiers may be substituted for the Darlington configuration shown in the preferred embodiment. The amplifier selected, however, must have a high gain so that current drawn by the amplifier will not be sufficient to disturb the voltage drop caused by current flowing through resistor 47 to the capacitive sensing means. It will thus be understood that integrated circuits, FETs, and other high input impedance, high-gain amplification devices may be utilized, with suitable changes in the associated control circuit.

A diode CR is provided between conductor 52 and line voltage bus L, to prevent wrong polarity voltages from damaging amplifier Q,. A second voltage divider 53 including

resistors

58 and 59 provides a predetermined level of DC voltage at the eimtter terminal of the final transistor of amplifier 0,, which determines the base voltage necessary to drive the final stage of 0, into conduction. A further transistor Q, is coupled to the collector of the output stage of 0,, and the base terminal ofa final transistor Q, is connected to the collector of Q Biasing resistor 60 determines the input impedance of transistor 0,. Resistor 61 preventssmall leakage currents (through transistor Q from being conducted into the base terminal of transistor Q The collector of transistor Q, is coupled to the positive side of the filtered DC voltage across capacitor C, by means of a resistor 54.

Connected in series between timer motor 44 and line voltage bus L, is the parallel combination of SCR,, diode CR and capacitor C As will be understood by those skilled in the art, diode CR conducts current in a direction opposite to that of SCR, and thus provides unidirectional current flow through timer motor 44 on alternating half cycles. In the preferred embodiment the timer motor may be of the induction type and will not operate on the half-wave current provided by CR, so that motor operation commences only when SCR, is rendered conductive. The gate terminal of SCR, is connected directly to the collector of transistor Q3, and thus coupled by means of resistor 54 to the positive side of the filtered DC voltage across capacitorC When the fabric drying machine is to be operated, a suitable AC voltage is impressed across

terminals

40 and 41, energizing the control circuit and sensor. Only half-wave AC is impressed across timer motor 44 since SCR, is assumed not to be in the conductive mode.

In order to illustrate the operation of the capacitive sensing device, it will first be assumed that damp fabrics are disposed within the dryer drum, and the control is energized. As will be recognized by those skilled in the art, the conductivity of the fabrics is directly related to the amount of moisture retained therein. The damp fabrics may thus be thought to comprise a conductive element, indicated by dotted lines at 55 in FIG. 2. Conductive coating 43 and insulating housing 45 may be considered to be the plate and dielectric, respectively, of a capacitor comprising the sensor 32. In order for the capacitor to conduct alternating current there must necessarily be a second conductor or plate disposed upon the outside of the dielectric 45, and means for coupling it to a source of neutral or ground potential. The damp fabrics constitute thisconductive element, the electrical connection provided thereby being a function of the degree of moisture retained by the fabrics. As the drum 14 rotates, the fabrics are tumbled about, passing near or against the outer surface of insulator 45 and forming a capacitor. The fabrics conduct the AC current from this capacitor to the surface of the drum, which is electrically connected to source of neutral or ground potential schematically represented by bus N. It will be recognized that drums provided in fabric drying machines are often coated with enamel, porcelain, or other insulating substances. However, the thinness of the coating, combined with the relatively large surface area, merely has the effect of introducing a very large capacitance in series between the damp fabrics and the body of the grounded drum. The impedance of this large capacitance to alternating current is very small with respect to other circuit parameters. Current is thus coupled by the damp fabrics from the insulated housing of the capacitive sensor to ground, facilitating current flow through dropping resistor 47. The voltage drop occurring across resistor 47 on alternate half cycles of the supply voltage will now be sufficient to bias amplifier 0. into conduction.

When Q conducts the current drawn through resistor 51 causes a voltage drop which tends to bias transistor Q into conduction. Capacitor C however, serves to integrate this voltage in order to prevent Q from being rendered conductive by occasional, spurious conduction of amplifier Q Similarly, when a suitable voltage has been present long enough to bias Q into conduction, occasional lapses of conduction of Q on negative-half-cycles,due to intermitten conduction by isolated segments of moisture fabric, will not deplete the bias on Q The voltage level at which conduction of the amplifier takes place may be determined by the relative size of the resistors in voltage divider 53, which determines the DC voltage level at the emitter terminal of the final transistor of amplifier Q On half cycles when line voltage on bus L is positive with respect to neutral bus N, current will flow through timer motor 44 only by means of diode CR bypassing SCR However, as hereinbefore noted, in the preferred embodiment timer motor 44 is of a type which cannot be energized by mere half-wave AC. When bus L becomes negative with respect to neutral bus N, however, charge flows from conductive coating 43 thr'ough limiting resistor 46, and thence through dropping resistor 47. The intersection of the two

resistors

46, 47 to which conductor 52 isconnected then becomes sufficiently positive with respect to bus L to energize amplifier 0,. Direct current is passed through diode CR and traverses resistor 51, biasing the base of transistor Q such that Q now conducts. The voltage level assumed by the collector of Q now becomes sufficiently positive with respect to bus L to bias final transistor 0;, into conduction, shunting the gate terminal of SCR, to the cathode thereof and disabling SCR, from conducting. It will thus be seen that so long as the fabrics within drum 14 retain enough moisture to conduct current to the dielectric housing 45 of the capacitive sensor the input terminal of amplifier 0 will be driven positive during negative half-cycles, disabling SCR, and preventing the timer motor 44 from operating and terminating the operation of the drying machine.

When sufficient moisture has been removed from the fabrics to render them relatively nonconductive, current flow to plate 43 of the capacitive sensor, and thus current flow through dropping resistor 47, becomes insufficient to bias amplifier Q into the conductive state on negative half-cycles. With amplifier Q non-conductive, insufficient current will flow through resistor 51 to bias transistor Q into conduction, thus precluding transistor Q from conducting and disabling SCR,. The resistivepath comprising resistor 57 of voltage divider 50, diode CR and resistor 54 thus provides a coupling from the gate terminal of SCR to apoint of neutral potential, energizing the SCR and allowing bidirectional current to flow through timing motor 44. Under these conditions, SCR, will continue to conduct alternately with diode CR passing full-wave alternating current through timing motor 44 and continuing the energization of the timing motor until the timing mechanism operated thereby terminates operation of the dryer.

I have found that a capacitive sensor and control circuit as illustrated in FIG. 2 for use with fabric dryers of the type described may advantageously be constructed with components having the following values:

Components Values Resistor 42 I00 K ohms Resistor 46 470 K ohms Resistor 47 2.2 M ohms Resistor 5] 270 K ohms Resistor 54 47 K ohms Resistor 56 3.3 K ohms Resistor 57 15 K ohms Resistor 58 4.7 K ohms Resistor 59 I5 K ohms Resistor 60 22 K ohms Resistor 61 47 K ohms SCR Cl06B (General Electric) CR CR FDH-6l5 (Fairchild) CR 1N-5059 (General Electric) C 10 F Q Dl3P4 (General Electric Eleflrir) D29A5 (General Electric) Q 2N5l72 (General Electric) It will be seen that it is possible to delete the timer motor from the circuit, using a switched element such as the SCR provided in the illustrated embodiment to energize other types of mechanisms for terminating the operation of the dryer. It is also possible to terminate the operation of the machine immediately upon energization of the SCR by switching means such as a relay. It will further be evident to those skilled in the art that other changes may be made in certain aspects of the invention without departing from the spirit of the present invention. It is therefore intended to include by the appended claims such modifications and variations as do not depart from the true spirit and scope of the invention.

What is claimed is:

1. In a fabric drying machine having an enclosure for receiving moist fabrics to be dried:

a. capacitive sensing means comprising:

i. insulating means having a first surface exposed to the interior of said enclosure for contacting said fabrics, and a second surface unexposed to the interior of said enclosure;

ii. first conductor means disposed upon said second surface of said insulating means to provide a first capacitor plate;

iii. second conductor means for coupling said first conductor means to one side ofa source of alternating potential;

iv. third conductor means for coupling said enclosure to a second side of said source of alternating potential, said moist fabrics when tumbled within said enclosure providing an electrical path between said enclosure and said insulating means to form a second capacitor plate with the conductivity of said path reflecting the degree of moisture retained by said fabrics; and

b. control means responsive to the level of current flow in said second conductor means for terminating the operation of said fabric drying machine.

2. In a fabric drying machine having an enclosure for receiving moist fabrics to be dried, said enclosure having an insulating material disposed on the inner surface thereof:

a. capacitive sensing means comprising;

i. a dielectric housing having an outer surface exposed to the interior of said enclosure for contacting said fabrics,

ii. conductive material disposed over the inner surface thereof to provide a first capacitor plate,

iii. means for resistively coupling said conductive material to a source of alternating potential,

iv. means for coupling said enclosure to a point of ground potential, said fabrics when tumbled within said enclosure providing an electricallyconductive path between said enclosure and said dielectric housing to form a second capacitor plate with the conductivity of said path reflecting the degree of moisture retained by said fabrics,

b. control means for sensing the voltage across said resistive coupling means and providing an output representative thereof; and

c. timer means coupled to said control means for terminating the operation of said fabric drying machine upon receiving said output from said control means.

3. The invention as recited by claim 2, wherein said control means includes an amplifier means having a high input impedance and a high gain, said resistive coupling means being coupled to the input of said amplifier, and said control means further including gated switching means, said switching means being adapted to be operated in response to an output signal derived from said amplifier means.

4. The invention as recited in claim 2, wherein said control means comprises amplifier means having a high input impedance and including at least one transistor, said resistive coupling means being connected to the input terminal of said amplifier, said transistor being operatively coupled to an output terminal of said amplifier means for disabling said switching means in response to a signal outputted by said amplifier means.

5. In a fabric drying machine, a capacitive sensor, comprising:

a. a rotatable enclosure for tumbling moist fabrics confined therein, said enclosure being adapted to be coupled to a first side of a source of alternating potential;

b. insulator means having a first and a second side,

said first side of said insulator means communicating with the interior of said enclosure for contacting said fabrics, said fabrics providing an electrically conductive path between said enclosure and said first side of said insulator means to form a capacitor plate; and

c. conductor means disposed upon said second side of said insulator means to provide a capacitor plate and coupled to a second means connecting said capacitive sensor in a control circuit for controlling the operation of the drying machine in response the moisture content of the fabric.

6. In a fabric drying machine having an enclosure for receiving moist fabrics to be dried:

a. capacitive sensing means comprising:

i. first conductive means being adapted to be connected to a first side ofa source of alternating potential and providing a first capacitor plate,

ii. insulator means having a first and second side,

said second side of said insulator means comm unicating with the interior of said enclosure and said first side disposed adjacent said first conductive means,

iii. second conductive means adapted to be connected to a second side of said source of alternating potential and communicating with said enclosure, said moist fabrics when tumbled within said enclosure providing an electrical path between said second conductive means and said insulator means to form a second capacitor plate with the conductivity of said path reflecting the degree of moisture retained by said fabrics; and

b. control means coupled to said capacitive sensing means for terminating the operation of said fabric drying machine in response to a predetermined change in the amount of current flowing through said capacitive sensing means.

Claims

1. In a fabric drying machine having an enclosure for receiving moist fabrics to be dried: a. capacitive sensing means comprising: i. insulating means having a first surface exposed to the interior of said enclosure for contacting said fabrics, and a second surface unexposed to the interior of said enclosure; ii. first conductor means disposed upon said second surface of said insulating means to provide a first cApacitor plate; iii. second conductor means for coupling said first conductor means to one side of a source of alternating potential; iv. third conductor means for coupling said enclosure to a second side of said source of alternating potential, said moist fabrics when tumbled within said enclosure providing an electrical path between said enclosure and said insulating means to form a second capacitor plate with the conductivity of said path reflecting the degree of moisture retained by said fabrics; and b. control means responsive to the level of current flow in said second conductor means for terminating the operation of said fabric drying machine.

2. In a fabric drying machine having an enclosure for receiving moist fabrics to be dried, said enclosure having an insulating material disposed on the inner surface thereof: a. capacitive sensing means comprising; i. a dielectric housing having an outer surface exposed to the interior of said enclosure for contacting said fabrics, ii. conductive material disposed over the inner surface thereof to provide a first capacitor plate, iii. means for resistively coupling said conductive material to a source of alternating potential, iv. means for coupling said enclosure to a point of ground potential, said fabrics when tumbled within said enclosure providing an electrically-conductive path between said enclosure and said dielectric housing to form a second capacitor plate with the conductivity of said path reflecting the degree of moisture retained by said fabrics, b. control means for sensing the voltage across said resistive coupling means and providing an output representative thereof; and c. timer means coupled to said control means for terminating the operation of said fabric drying machine upon receiving said output from said control means.

5. In a fabric drying machine, a capacitive sensor, comprising: a. a rotatable enclosure for tumbling moist fabrics confined therein, said enclosure being adapted to be coupled to a first side of a source of alternating potential; b. insulator means having a first and a second side, said first side of said insulator means communicating with the interior of said enclosure for contacting said fabrics, said fabrics providing an electrically conductive path between said enclosure and said first side of said insulator means to form a capacitor plate; and c. conductor means disposed upon said second side of said insulator means to provide a capacitor plate and coupled to a second means connecting said capacitive sensor in a control circuit for controlling the operation of the drying machine in response the moisture content of the fabric.

6. In a fabric drying machine having an enclosure for receiving moist fabrics to be dried: a. capacitive sensing means comprising: i. first conductive means being adapted to be connected to a first side of a source of alternating potential and providing a first capacitor plate, ii. insulator means having a first and second side, said second side of said insulator means communicating with the interior of said enclosure and said first side disposed adjaCent said first conductive means, iii. second conductive means adapted to be connected to a second side of said source of alternating potential and communicating with said enclosure, said moist fabrics when tumbled within said enclosure providing an electrical path between said second conductive means and said insulator means to form a second capacitor plate with the conductivity of said path reflecting the degree of moisture retained by said fabrics; and b. control means coupled to said capacitive sensing means for terminating the operation of said fabric drying machine in response to a predetermined change in the amount of current flowing through said capacitive sensing means.