CA1095152A - Program modification circuit for electronic appliance programmer - Google Patents

Abstract

PROGRAM MODIFICATION CIRCUIT
FOR ELECTRONIC APPLIANCE PROGRAMMER
ABSTRACT OF THE DISCLOSURE
An appliance programmer, in particular an automatic washer programmer, employs a memory circuit and a sequence control for establishing a predetermined program. Included in the program are a program modifi-cation circuit and a rapid advance circuit whose functions are respectively to automatically modify the program inherent in the programmer and to rapidly step the pro-grammer through its operational program. The memory and sequence control of the programmer are designed so that, absent the program modification circuit, the programmer can effect only a single predetermined program. For this reason the program modification circuit has been designed to enable the programmer to effect other programs. The program modification circuit receives inputs from selected function display lights to indicate that a certain function is being performed and from the sensors to indicate that a function has been completed or a condition has been satisfied in the machine. The program modification circuit then causes several of the frequency dividers in the frequency divider chain of the clock to be bypassed. This results in the shortening of the next predetermined time period which the clock is programmed to establish. There-fore, the operation which is then being performed is shortened in duration so that the function appears to be curtailed thus causing a modification of the predetermined program. In addition, a rapid advance circuit is provided which allows for manually speeding up the functions of the programmer by stepping the programmer through its sequence of operations at a much faster rate than normal. This also allows the programmer to be tested rapidly for ease in servicing.

Description

BACKGROUND OF THE INVENT ION
Field of the Invention _ This invention relates to an appliance programmer and is more particularly concerned with the provision of a circuit for modifying a predetermined program of an appliance.
Descri~tion of the Prior Art Integrated circuit programmers for controlling the operation of appliances are well known in the art. In addition, integrated circuit programmers having program modification circuitry are known in the art. One such cir-cuit is shown in United States Patent 3,840,752 issued October 8, 1974 to Kamran Eshraghian which discloses a pro-grammer for an automatic washer wherein a bistable circuit causes the programmer to advance rapidly through preselected program steps in response to actuation of a manual switch and information derived from a shift register. In U.S.
-Pat~nt No. 4,001,599 issued January 4, 1977 to Joseph Karklys, a circuit is shown which, in response to the electrical potential on input terminals, alters the dura~ion of selected program operations by selecting differènt clock output pulses.
SU~ARY OF THE INVENTION
In the manufacture of integrated circuit programmers, economies of scale can be achieved if a basic integrated circuit design can be used in a large number of programmers.
Therefore, it is advantageous to use the same design for the integrated circuit in many different appliance models.

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Since these models may require di~fcrent operating programs, circuits have been designed which can modify the basic pro-gram of such an integrated circuit programmer. One such circuit is described in the above mentioned U.S. Patent 4,001,599.
An object of the presen-t invention is to provide a novel circuit for an appliance programmer whereby the basic program may be modified.
To this end the invention COIlsists of an automatic wash-ing appliance including an electronic control for steppingthe appliance through an operating program comprising a sequence of operations, a clock for supplying electrical pulses at discrete timer intervals, a sequence control for establishing said sequence of operations, a memory circuit connected to said clock and said sequence control for gen-erating a sequence of output signals, a plurality of output circuits connected to said memory circuit for performing said operations in response to said output signals, and sensing means connected to said memory circuit for generating a sensing signal indicating a condition of the appliance, an improved program modification means connected to said output circuits, said senslng means and said clock for modifying the length of at least one of said discrete time intervals, comprising: switching means and a gate and responsive to said sensing signal for modifying said operating program, said sensing means comprising a water level sensing switch for generating a signal indicative of the water level in said washing appliance.
In a more specific form the invention consists of an automatic washing appliance including a wash tub and an electronic control for stepping the appliance through an :, ~
..
.: . - ~ ' ' . - : ' operating program comprising a sequcnce of operations, a clock for supplying electrical pulses at prede-termined dis-crete time intervals, a sequence con~rol for establishing said sequence of appliance operations, a memory circuit connected to said clock and said sequence control ~or generating a se~uence of output sig.nals, a plurality of output circuits connected to said memory circuit for per-forming said operations in response to said output signals, water level sensing means connected to said memory circuit 10 for generating a signal indicative of the water level in the tub, an indicator circuit connected to said memory circuit for providing an indication of the operation which is being performed by the appliance, an improved program modification means connected to at least one of said output clrcuits comprising: switching means and a gate and - responsive to said sensing signal whereby said operating program may be modified by accelerating at least one of said operations in said sequence, said switching means com-prising a PNP transistor, the collector of which is connected 20 to an input of the gate and the base of which is connected to the water level sensing means and the emitter of which is connected to a fixed source of reference potential, and said clock comprising an oscillator and a chain of frequency dividing stages, and wherein said gate receives an input from the water level sensing means and the oscillator and wherein the output of the gate is fed to one of the frequency dividers.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features of embodiments of the invention, thelr organization and operation will be best understood from the following detailed description taken in conjunction ,~ ~

' with the accompanying drawings oE whic}l:

FI~. 1 is a perspective eutaway view of an auto-matie washer which may embody the programmer of the present invention;
FIG. 2 is a schematie b:loek diagram of a pro-grammer eonstrueted in aeeordanee with the present inven-tion;
FIG. 3 is a schematie logie diagram of an oper-ator eontrolled program modifieation eireuit which may be employed in the circuit of FIG. 2;
FIG. ~ is a schematie logie diagram of the program modifieation eireuit and rapid advanee eireuit whieh may be employed in the eireuit of FIG. 2;
FIG. S is a program chart illustrating a pre-determined program and program modification in aeeordanee with the prineiples of the present invention; and FIG. 6 is another er,~bodiment of the eloek and , ,. ~ , ' . ' " .. ,. . . ~ ~ ' a portion o~ the program modification circui-t.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The au-tomatic washer, clS illustra-ted in FIG. 1, comprises a cabinet 10 including a cabinet top 12 having a lid 14 and a console 16. The console ]6 is provided with a programmer~l8, a plurality of operative controls 20, and a plurality of indicator Lamps 22 to control and monitor the operation of the washer. The operations controlled by the programmer include a washing operation in which -the clothes are agitated to remove soil, a draining operation in which the washing liquid is removed frorn the machine, and a spinning or extraction opera-tion in which the clothes are centrifuged to remove more liquid from the fabric.
A tub 24 has a concentric perforate basket 26 therein for receiving clothes to be washed. A tub ring 28 is provided to prevent splash from the washing action from flowing over the top of the tub. The tub ring 28 is pro-vided with an opening 30 through which clothes are received into the basket 26. An agitator 32 is provided within the basket 26 to impart a washing action to the clothes.
The tub 24 is supported by a baseplate 34 which is suspended from the cabinet 10 by three suspension rods, rod 36 being the only such rod illustrated. The rod 36 is connected to the baseplate 34 by a resilient member 39 and to the cabinet 10 by a resilient member 40.
A motor 42 and a transmission 44 are suspended ; from the baseplate 34 and provide a means for driving the agitator 32 and the basket 26. A valve pump 46 is provided to circulate the washing liquid during the washing oper-ation and to remove the liquid from the tub 24 during the . ~

P~-4910-1-AW-*~ t~ 2 draining and spinning operations. A clu tch and b~ake assembly indicated at ~8 allows for the engagement of the agitator 32 for an oscillatory mo-tion during the washing operation, while holding the basket 26 stationary, and for disengagement of -the agitator 32 and engagement of the basket 26 for spinning of the basket during the spinning operation. The transmission 44, the pump 46 and the clutch and brake assembly 48 are driven by the motor 42 by means of a drive belt 50.
- Water flows into the machine through a programmer controlled solenoid valve 52 through a conduit 54 and an antisyphon device 56 into the tub 24. During the washing operation, washing liquid is circulated from the tub 25 by suction from the valve to pump 46 through a button trap 58 and a conduit 60 and is pumped through a conduit 62 to a filter 64. The water flows through the filter where particles such as lint are removed and the filtered water flows through the discharge end of the filter into the basket 24 through the tub ring opening 30. The filter is -mounted through a vertical flange 66 of the top 12 so as -to overhang the opening 30. During draining of the tub 24, the pump valve is operated and wash liquid flows from the tub 24 through the button trap 58 and the conduit 60 to the pump 46. The wash liquid is then pumped through a conduit 68, a check valve 70 which allows flow of liquid in this direction only and a conduit 72 to a drain (not shown).

Referring to FIG. 2, a schematic block diagram of the programmer 18 is generally illustrated as comprising a power supply 80 which ls connecte. to a ~ommercial, , .

PA-~910-1-AW-electrical supply 82, such as 125 volts AC,60 HZ. The individual power connections to t:he individual circuits have been omitted for the sake of clarity. It is, however, well known in the art to generate a DC vol-tage fro~ a com-mercial line supply. Input switches 84 are connected to an input memory which retains in storage -the information entered with the input swi-tches. The input swi-tches give the operator several options to select from such as the wash bath tempera-ture and the agitation and spin speeds.
The input memory is connected to an input encoder 88 which provides speciEied outputs for certain combinations of inputs. As an example, the input encoder may comprise a number of AN~ gates. The information from the input encoder is provided to a read only memory 90 which, by way of example, may comprise a matrix. The matrix, provides specified outputs only for predetermined combinations of inputs as is well known in the art. Therefore, for example, when a specific input switch has been depressed and the sequence control 108 provides a certain ou-tput and the clock 92 provides a clock pulse, the read only memory 90 will provide a specified output to an output decoder 9~.
The output decoder 92 is connected by way of output buffers 96 and output amplifiers 98 to a plurality of power circuits 100, hereinafter called output circuits, which perform various operatlons of the program including, by way of example, the opening and closing of the valves for filling and draining.
The sequence control 108 is provided for estab-lishin~ the sequence of steps in the operating cycle of the appliance as lllustrated in FIG. 5. The sequence control PA-fi910 l-AW--can be cons-tructed similar to that illustra-ted in the aforementioned United States Patent No. 3,662,186. The clock 92 ~enerates electrical output pulses at discrete time intervals from a pulse source such as a commercial line supply of 120 volts, 60 HZ. The clock cons-truc-tion is well known~in the art and may comprise the circuit des-cribed in United States Patent No. 3,639,8~4.
The Read Only Memory also receives inputs from sensors 102 such as, for e~ample, a water level switch which indicates that a desired water level has been established within the tub 24. As mentioned above, the Read Only Memory provides an output to an output decoder which decodes the information provided by the Read Only Memory and feeds the same to output buffers from which the information is routed to the output amplifiers and the power circuits which oper-ate the appliance.
The automatic washer is also provided with function display lamps 106 which indicate the progression of the washing program. The function display lights are driven by display amplifiers 104 and receive their input signals from the input memory and display drivers circuit 86. The ; program modification circuit 105 is shown connected to the function display lights, the sensor circuit and the clock 92.
As descrlbed in greater detall below, the program modifica-: tion circuit automatically alters the basic cycle so that a modified cycle is generated.
In the embodiment illustrated in FIG. 2, function display lights 106 are energized by display amplifier 104 which receives its input from the display drivers associated 30 with the input:memory 86. The display lights 106 may advan-~

g _ PA-4910-1-~W-LS~

tageously comprise light emitting diodes, al-though o-ther types of display ligh-ts may also be used. The function of the display lights 106 is to indicate which machine functions have been selected by the operator by means of the input switches, and also to indicate the progression of the washing program.
Referring now to FIG. 3, apparatus for operator con-trolled manual alteration of the program at the beginning of the cycle is illustrated. The purpose of this structure is to allow the operator a specified amount of time in which to modify the basic program and exercise available program options. Once the specified amount of time elapses, the programmer will lock out any further options selected by - the operator. A timer is provided and each time the oper-ator selects another option within the specified time, the timer is reset and begins counting anew toward the specified amount of time.
In FIG. 3 the input switches 84 have been illus-- trated ln detail as comprising a pair of switches 112 and - 20 114 which may be inexpensive momentary switches and which may be depressed by the operator to select specific options.
By way of example, the operator may elect to skip specific - steps in the program. By depressing the switches 112 and 114, respective inputs are applied to a pair of AND gates 116 and 118. The other inputs to these AND gates are connected to a flip-flop 126 and, until a reset pulse lS
received by a flip-flop 126, an output will appear which~
is fed to the gates 116 and 118. Therefore, the gates 116 and 118 will produce an output which is fed to a pair of 30 latches 130 and 132. These latches represent the input memory 86 of F~IG. 2 and include a "clear" input 134 which is ener~ized at the beginning o:E the program cycle. This may be a reset pulse from the clock such as by way of a single shot flip-flop. The outputs of the latches are provided to two A~D gates 136 and 138 which also receive inputs from the reset side of the flip-flop 1.36 by way of a connection 128. The switches 112 and 114 are also connec-ted to an OR
gate 112 which provides a pulse throuyh a timer 124 whenever one of the switches 112, 114 is actuated. Therefore, the timer is reset each time one of the switches is actua-ted. If the timer is not reset for a period of 8 seconds, for example, the timer will provide an output pulse which will reset the flip-flop 126 and will disable AND gates 116 and 118 so that any,further actuation of the switches 112 and 114 is locked out. At this time, however, the AND gates 136 and 138 are enabled by the signal on the rese-t output 128 of the flip-flop 126 to provide respective output pulses to the input encoder 88. The input encoder 88 processes this . information to alter the basic predetermined program of the appliance. The flip-flop.126 also provides inputs for a pair of AND gates 140 and 142 which receive respective inputs from the output decoder 94. The gates 140.and 1.42 respectively drive the light emitting diodes 148 and 150 by way of amplifiers 144 and 146 to indicate the progression of the program,. It should-be noted that the diodes 148 and 150 are energized by way of time shared circuit con-- nections to the integrated circuit. These connec-tions include portions A and B which are time shared with the connections to the operator switches 112 and 114.
Referring now to FIG. 4, the clock 92 has been shown ,, , : ' PA-4910-1-~W-in greater detail as comprising an oscillator 171 which receives its inpu-t from line voltaye supply 82. Oscil-lator 171 may also comprise a pulse shaping circuit which uses the AC line frequency as the oscillator frequency.
The output of the oscillator is fed to a chain of flip-flops 172 ~hich divide the oscillator frequency. In addi.tion, the output from the oscillator is fed to the input of a NAND
gate 170. The output of flip-flop 172 provides an input for exclusive OR gate 173. The other input for exclusive OR ga-te 173 is derived from NAND gate 170. The outpu-t of gate 173 is fed to a set of flip-flops 174 which further divide the frequency of -the pulses. This divided frequency becomes the clock output so that puIses are provided at predetermined time intervals. By means of internal circuit connections between the flip-flops any desired time interval can be provided. Thus, any number of clock output pulses are available to time the various operations as illustrated in FIG. 5.. A clock of this type is illustrated in United States Patent 3,639,844 as mentioned above.
. Three output buffers 167, 168 and 201 are shown which comprise the circuitry illustrated in block form by output buffer block 96 in FIG. 2. Output buffers 167, 168 and 201 comprise MOS field effect transistors or MOSFETS.
Buffer 167 drives output amplifier 185 by means of resistor 97. Output amplifier 185 is typical of the output amplifiers labeled 98 in FIG. 2 and comprises an.NPN transistor 99 -together with a resistor 101 which biases the base thereof.
The junction between the resistor 101 and the emitter of the - transistor is connected to a negative source of voltage which is derived from the power supply 80. The output 5~

amplifier 185 drives a water fill valve coil 158 which is connected in parallel with a commutating diode 161. A
single pole double throw swi-tch 160 is shown with its switchblade connected to a fixed source of reference potential 190, here illustrated as grouncl, and with one of its fixed contacts 155 connected to the solenoid 158.
The other fixed contact 157 of switch 160 is shown con~
nected to the anode of a diode 154. The ca-thode of the diode 154 is shown connected to the cathode of a diode 152 whose anode is connected to a resistor 153. The other end of the resistor 153 is connected to ground. The junction of resistor 153 and the anode of diode 152 is connected to the anode of a light emitting diode 148 whose cathode is connected to a display amplifier 104. The display ampli-fier 104 derives its input signal from the input memory 86 as explained above.
It should also be noted that the circuit de-energizes the water fill valve solenoid in two ways. The first disconnect function is accomplished by the water level switch 160. The second disconnect function is accomplished by the programmer when it stops sendlng an energization signal to the output buffer for the fill valve solenoid.
Therefore, if either the programmer or the water level switch malfunctions, the fill valve solenoid will still be de-energized to prevent flooding of the automatic washer.
The -junction of the cathodes of diodes 152 and 154 is connected to a negative source of potential by means of resistor 15'3. This junction is also connected to one cathode of the series eonneeted diodes 162 and 163. The -30 anode of diode 163 is eonnectecl to the base of transistor P~-~910-1-AW-165. The base of the transistor is also connec-ted to ground by means of a resistor 164. The transistor is of the PNP -type and has its emitter connected -to ground as well. The collector of the -transistor is connected to the source of the main relay output buffer 168. The source oE
the main relay buffer 168 is also connected to an output amplifier 187 by means of a resistor 179. The output amplifier 187 is similar in construction to ou-tput ampli-~ier 185. The output of amplifier 187 is used to drive the main relay solenoid coil 182 which is connected in parallel with a commutating diode 183. The parallel com-bination of the solenoid and the diode is connected to ground.
The source of field effect transistor 168 is also connected to ground by means of a switch 180 and to the anode of diode 176 whose cathode is connected to the input of the output amplifier 186 which drives the spin solenoid 178.
The output amplifier 186 is similar in con-struction to amplifier 185 and is driven from output buffer201 by means of a resistor 175. Spin solenoid 178 is connected in parallel with a commutating diode 177 and the combination is conneated to ground. It should be noted that solenoids 158, 178 and 182 could be replaced by a/c operated solenoids. If this were done they could be driven by means of bi-directional semiconductor devices such as triacs The circuit operates as follows. With reference to FIG. 5 it can be seen that during Step No. 5 the tub is filled, during Step No. 6 agi-tation takes place, during I

~ffl5~

Step No. 7 the tub is drained and during Step No. 8 the basket is spun to centrifugally spin the remaining water from the clothes load. During steps 5, 6, 7 and 8, the function display light 148 iS energized thus indicating to the operator that the prewash function is ta~ing place.
When the wash tub is filled with water, switchblade 160 touches contact 157. Since the anode of diode 154 is now connected to ground and its cathode is connected, by way of resistor 159, to a source of negative potential, the diode 154 will conduct. Point B will therefore be virtually at ground potential. Function display light 148 iS energized and point A will therefore be below ground potential. Diode 152 is therefore back biased, and cannot conduct. Diodes 162 and 163 cannot conduet as the voltage drop across diodes 162, 163 and resistor 164 would exceed the voltage drop across diode 154. Therefore, transistor 165 cannot eonduet and the potential on line 200 will be established by the other components in the eireuit. However, during Step 7 the tub is dralned and at a certain point the switch 160 will reset and the switchblade will contact fixed eontaet 155. Diode 154 Will therefore no longer conduct and diode 152 will begin to eonduet as the potential at point B drops to a negative value. If function display light 148 were not energized, however, the voltage at point - A would be higher at this time, namely slightly below ground, so that point B would also be at a voltage slightly below ground due to the eonduction of diode 152. ~nder those eonditions, diodes 162 and 163 eould not conduct.

However, if funetion display light 148 iS energized, the voltage at point B will be mueh lower because the voltage .

PA-~9].0-l~AW-~95~

at point A is ne~ative. Therefore, the -two diodes 162 and 163 conduct thereby turning on transis-tor 165 and lowering the voltage on line 200. The drop in voltage on line 200 is inverted by inverter 169 so that a high input appears on one of the inputs of NAND gate 170. Therefore, the pulses from oscillator 171 will cause the output of ~AND
gate 170 to oscillate at the 60 ~IZ rate, thereby bypassing the pulse train emanating from flip-flops 172 and speeding up the clock output pulses. This, in turn, will cause the programmer to ~uickly step through the remainder of the drain period and the spin function as well. Therefore, the modified cycle will appear as shown in FIG. 5 with the time N being dependent upon the rate at which the tub is drained and the level to which the tub was filled.
The voltage drop across MOSFET 168 is insufficient to cause the gate 170 to toggle. It should also be noted that resistor 153 is of small enough value, typically 1000 OHMS, so that the potential at point A is more positive than the potential at point B when display light 148 con-20 . ducts.
It can therefore be seen that by virtue of the program modification circuit, the predetermined program is automatically altered so that the tub is not completely drained of water ln Step 7 and the spin operation.of Step 8 is skipped. T~is occurs in response to the information : from the funct.ion display light 148 and the water level switch. The circuit therefore automatically modifies the program cycle, without any operator control. The cir-cuit is extremlely simple in that only a single transistor and some diodes are used. The circuit can easily be in-~ . . . . .. _ . , . . . . , , .. . _ . .. . .. _ . _ . .

PA-~9l0-1-AW-corporated in-to the integrated circuit or can be retained as an add-on external circuit.
~ lanual switch 180 is provided so -tha~ -the poten-tial on line 200 can be grounded manually and, therefore, the entire program of the programmer can be stepped through rapidly. This is especially useful when the pro~rammer needs servicing or during checkout of the automatic washer.
Diode 176 is provided so that when line 200 is grounded, diode 176 conducts thereby taking away the drive from output amplifier 186. The spin solenoid, therefore, cannot be actuated and will not chatter while the programmer quickly steps through the spin function. Diode 176 may not be needed if the accelerated spin period is sufficiently short or if the spin solenoid 178 is sufficiently slow in reacting to activation voltages. The reason for selecting the input line 200 for connection to the switching transistor 165 and switch 180 is that during any operation of the programmer, this line will be energized and available as the main relay needs to be energized.
FIG. 6 shows another embodiment of the clock 92A
wherein the exclusive OR gate 173 has been replaced by two NAND gates 202 and 203. The circuit 92A connects to the circuit of FIG. 5 at point 204.
Circuit 92A operates as follows: When no rapid advance is desi.red, point 204 is high. Gate 179, thereforé, has a low input and will have a continuous high output.
When rapid advance is desired and point 204 goes low, the output of gate 170 will oscillate between high and low at the oscillator rate of 60 HZ. Due to the low voltage at 30 point 204, gate 203 will produce a continuous high outpu-t.
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' ' ~ ' PA-4910~ W-The output of gate 202 wlll therefore oscillate at the 60 HZ rate, thereby rapidly skipping throu~h the undesired functions.
Although I have described my invention by reference -to a particular lllustrative embodiment, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention.
I, therefore, intend to include within the Patent all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.

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Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An automatic washing appliance including an electronic control for stepping the appliance through an operating program comprising a sequence of operations, a clock for supplying electrical pulses at discrete timer intervals, a sequence control for establishing said sequence of operations, a memory circuit connected to said clock and said sequence control for generating a sequence of output signals, a plurality of output circuits connected to said memory circuit for performing said operations in response to said output signals, and sensing means connected to said memory circuit for generating a sensing signal indicating a condition of the appliance, an improved program modification means connected to said output circuits, said sensing means and said clock for modifying the length of at least one of said discrete time intervals, comprising:
switching means and a gate and responsive to said sensing signal for modifying said operating program, said sensing means comprising a water level sensing switch for generating a signal indicative of the water level in said washing appliance.

2. An appliance according to claim 1, wherein one of said output circuits comprises a plurality of appliance function indicators and said program modification means is responsive to the condition of one of the function display indicators.

3. An appliance according to claim 1, wherein the clock comprises an oscillator and a chain of frequency dividing stages and wherein said modification means causes several of the dividing stages to be bypassed.

4. An appliance according to claim 1 and including a manual speed-up circuit comprising a switch which, in its activated position, enables a programmer to step through its entire operating program at high speed.

5. An automatic washing appliance including a wash tub and an electronic control for stepping the appliance through an operating program comprising a sequence of operations, a clock for supplying electrical pulses at predetermined discrete time intervals, a sequence control for establishing said sequence of appliance operations, a memory circuit connected to said clock and said sequence control for generating a sequence of output signals, a plurality of output circuits connected to said memory circuit for performing said operations in response to said output signals, water level sensing means connected to said memory circuit for generating a signal indicative of the water level in the tub, an indicator circuit connected to said memory circuit for providing an indication of the operation which is being performed by the appliance, an improved program modification means connected to at least one of said output circuits comprising: switching means and a gate and responsive to said sensing signal whereby said operating program may be modified by accelerating at least one of said operations in said sequence, said switching means comprising a PNP

transistor, the collector of which is connected to an input of the gate and the base of which is connected to the water level sensing means and the emitter of which is connected to a fixed source of reference potential, and said clock comprising an oscillator and a chain of frequency dividing stages, and wherein said gate receives an input from the water level sensing means and the oscillator and wherein the output of the gate is fed to one of the frequency dividers.

6. The appliance according to claim 5, wherein the switch-ing means is bypassed by a manually operated switch.

7. The appliance according to claim 5, wherein the clock comprises an oscillator, a chain of frequency dividers and two NAND gates, said gate being connected to receive an input from the water level sensing means and the oscillator, and said NAND gates being connected to allow at least one fre-quency divider to be bypassed when an input is received from said water level sensing means during a predetermined operation.