Patent Publication Number: US-2021172772-A1

Title: Flowmeter Comprising an Electronic Magnetic Sensor with Buffered Power Supply

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of European Patent Application No. 19214926.8, filed on Dec. 10, 2019, the entire disclosure of which is incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of flow meters, and in particular to home appliances such as dishwashers or washing machines having flow meters for determining and controlling the flow of a fluid. More particularly, the invention relates to home appliances comprising a flow meter with a magneto-sensitive sensor which provides digital pulse signals indicating a flow of a fluid. 
     BACKGROUND ART 
     Flow meters which generate digital pulse signals according to the flow of a fluid are well-known and widely used in home appliances such as dishwashers and washing machines. Such flow meters usually comprise a movable element such as an impeller or vane wheel which is rotated by the flow of a fluid to be measured, so that the rotating speed of the impeller is substantially proportional to the rate of flow. Commonly, the impeller or vane wheel comprises a magnetic element such as a permanent magnet rotating with the wheel, wherein a magnetically sensitive reed contact is arranged adjacent to the rotating impeller (usually attached to the outside of an impeller casing) and opens and closes its contact depending on the rotational position of the impeller due to the varying magnetic flux or field applied by the rotating magnetic element to the reed contact. Commonly, one terminal of the reed contact is connected to ground, and the other terminal is connected to a digital input terminal of an evaluating circuit or control unit, wherein this digital input terminal is further connected to a supply voltage via a so called pull-up resistor, in order to provide a well-defined digital HIGH signal (according to TTL logic levels, for example) when the reed contact is open and a LOW signal when the reed contact is closed. The evaluation circuit or control unit detects the consecutive HIGH and LOW level signals at the digital input terminal and determines the rate of flow (i.e. volume and/or volumetric flow rate) depending on the pulse rate (e.g. the rate of HIGH-LOW transitions) generated at the digital input terminal. 
     With reference to  FIG. 5 , a flow meter arrangement in a home appliance according to the prior art comprises a control unit or evaluation circuit  10  having a first power supply terminal  16  (providing a first power supply voltage or potential Vs 1 ), a digital input terminal  14  which is connected to a second power supply terminal  18  (providing a second power supply voltage or potential Vs 2 ) via a resistor  12  (so called pull-up resistor). A reed contact  50  is connected between input terminal  14  and first power supply terminal  16 . Usually, the first power supply voltage or potential Vs 1  corresponds to ground, while the second power supply voltage or potential Vs 2  corresponds to a positive supply voltage such as +3.3 V or +5 V usually used in digital circuits such as TTL or CMOS logic. The digital input terminal  14  is thus configured to receive a digital voltage signal (Vd). In other words, digital input terminal  14  detects a HIGH level signal (e.g. close to +3.3 V) when reed contact  50  is in an open state, and a LOW level signal (e.g. 0.0 V or ground) when reed contact  50  is in a closed state. A movable permanent magnet attached an impeller (both not shown in the drawings but well-known to the skilled person) is arranged such that it applies a varying or alternating magnetic field or flux to reed contact  50  when the impeller or vane wheel of the flow meter is rotated by the flow of a fluid to be measured. Thus, the reed contact is repeatedly exposed to a magnetic field of flux and opens and closes intermittently, so that a pulse sequence of HIGH level signals and LOW level signals is generated at digital input terminal  14 , the frequency of which is an indication of the flow rate of the fluid and the number of pulses is an indication of the volume (amount) of the conveyed fluid. 
     While this common digital flow meter arrangement is simple and cost efficient, in particular reed contact  50  entails problems. For example, reed contacts are mechanical switches which generate so called chatter or contact bounce when closing, so that in particular the transition from LOW level to HIGH level voltage may generate several pulses which must be suppressed by a filtering circuit or by a corresponding processing of the input signal within evaluation circuit or control unit  10 . Moreover, reed contact  50  is subject to mechanical wear or fatigue, so that reed contact  50  and thus the flow meter may fail during a longer operation time. Such fail of the reed contact due to mechanical wear or fatigue is a common problem in home appliances which are designed for long operational life time. 
     There were some attempts to use electronic sensors or electronic switching elements, respectively, such as Hall effect sensor arrangements, in order to avoid the problems accompanying the reed contacts. These Hall sensor arrangements comprise electronic switches such as transistors which are not subject to mechanical wear or fatigue and do not exhibit contact bounce. 
     However, reed contact  50  having only two terminals (or leads), as shown in  FIG. 5 , cannot be easily replaced by a common Hall sensor chip, for example, since such Hall sensor chips require a dedicated power supply via a third terminal (or lead). Even Hall sensor chips with only two terminals and without a separate power supply terminal cannot be used to replace reed contact  50 , since they cannot provide a short circuit or a very low resistance between their terminals, which is necessary for detecting a LOW level signal at common control units of home appliances. For a reliable operation of such two terminal Hall sensor chips, the voltage between their terminals must not fall below a relatively high voltage level, so that no LOW level signal can be generated at a simple digital input terminal such as shown in  FIG. 5 . Therefore, further adaptions and amendments to the control unit or evaluation unit  10  of the home appliance were necessary in order to use Hall sensor chips in flow meter arrangements. 
     U.S. Pat. No. 4,645,950 shows a two-lead Hall sensor providing a digital current signal. This Hall sensor is adapted to generate a first current strength appearing at its output terminal, indicating the sensing of a magnetic field, and a different second current strength, indicating a weak or absent magnetic field. A current measuring means is provided to evaluate the change of current strength. 
     However, there is still a need for a flow meter having a magneto-sensitive electronic unit which can replace a reed contact without increasing the complexity and costs of an evaluation circuit or control unit of a home appliance due to an additional electric current evaluation circuit, for example. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to overcome the disadvantages of the prior art, and to provide a flow meter having a magneto-sensitive electronic unit (a magneto-sensitive electronic switching unit) which can replace a reed contact without requiring any amendments to the evaluation circuit or control unit in a home appliance such as a dishwasher or a washing machine. 
     This object is solved by a flow meter for a home appliance according to independent claim  1 . The dependent claims are directed to advantageous further developments of the invention. 
     In a first aspect of the invention, a flow meter for a home appliance, in particular a dishwasher or a washing machine, comprises a control unit having a first power supply terminal, a second power supply terminal and a digital input terminal which is connected to the second power supply terminal via a limiting element, wherein the digital input terminal is configured to receive digital voltage signals; a movable element which is moved by a flow of a fluid and which comprises at least one movable magnetic element; and a two-lead magneto-sensitive unit which is arranged to detect a magnetic field or flux applied by the movable magnetic element, and which comprises a signal terminal configured to be connected to the digital input terminal of the control unit and a reference terminal configured to be connected to the first power supply terminal of the control unit; wherein the magneto-sensitive unit comprises a magnetic sensor element having two output terminals connected to the signal terminal and the reference terminal, respectively, of the magneto-sensitive unit and a power supply terminal; and a buffered power supply unit connected to the signal terminal and the reference terminal of the magneto-sensitive unit, and configured to generate a continuous auxiliary voltage from the signal terminal and the reference terminal of the magneto-sensitive unit and to provide the continuous auxiliary voltage at the power supply terminal of the magnetic sensor element. 
     According to a further embodiment of the invention, the buffered power supply unit comprises a buffering element connected between the reference terminal of the magneto-sensitive unit and the power supply terminal of the magnetic sensor element, and a rectifying element connected between the signal terminal of the magneto-sensitive unit and the power supply terminal of the magnetic sensor element. The rectifying element may preferably comprise a diode, preferably a Schottky diode. Alternatively, the rectifying element may comprise an active rectifier. 
     In an independent embodiment of the invention, the magnetic sensor element comprises a Hall effect sensor which comprises a Hall effect element and an electronic switching element, preferably a field effect transistor (FET). Alternatively, the magnetic sensor element comprises a magneto-resistive sensor which comprises a magneto-resistive element and an electronic switching element, preferably a field effect transistor (FET). In a further alternative embodiment, the magnetic sensor element comprises an inductive sensor which comprises an induction coil and an electronic switching element, preferably a field effect transistor (FET). Additional modules like an amplifier or a comparator, which are not show in detail here, may be integrated in the magnetic sensor element to interface magnetic sensor element and electronic switching element. 
     According to another further development of the embodiments of the invention, the movable magnetic element comprises a permanent magnet., Alternatively, the movable magnetic element comprises a magnetic flux conducting element. 
     According to another embodiment of the invention, the magneto-sensitive unit is configured to assume a first state providing a high resistance between the signal terminal and the reference terminal to generate a first digital signal level at the digital input terminal when the magnetic field or flux applied by the movable magnetic element corresponds to a first condition, and to assume a second state providing a low resistance or short circuit between the signal terminal and the reference terminal to generate a second digital signal level at the digital input terminal when the magnetic field or flux applied by the movable magnetic element corresponds to a second condition. 
     In a further development of the preceding embodiment, the magneto-sensitive unit is configured to assume and maintain a third state providing a medium resistance between the signal terminal and the reference terminal when the movable element is stopped so that the magnetic field or flux applied by the movable magnetic element continuously corresponds to the second condition, wherein the medium resistance is sufficiently low to prevent the generation the first digital signal level at the digital input terminal of the control unit. In this case, the magneto-sensitive unit may optionally further comprise an adapting unit connected to the magnetic sensor element and configured to adapt the magnetic sensor element so that the magneto-sensitive unit assumes and maintains the third state when the magnetic field or flux applied by the magnetic element continuously corresponds to the second condition. 
     According to another embodiment of the invention, the magneto-sensitive unit is configured to oscillate between a fourth and a fifth state with a predetermined frequency and/or a predetermined pulse duty factor when the movable element is stopped so that the magnetic field or flux applied by the magnetic element continuously corresponds to the second condition, wherein the fourth state provides a resistance between the signal terminal and the reference terminal generating the first digital signal level at the digital input terminal and the fifth state provides a resistance between the signal terminal and the reference terminal generating the second digital signal level at the digital input terminal, and wherein the predetermined frequency and/or the predetermined pulse duty factor is detected by the control unit to determine that the fluid flow has stopped. In this case, the magneto-sensitive unit may optionally further comprise an adapting unit connected to the magnetic sensor element and configured to adapt the magnetic sensor element so that the magneto-sensitive unit oscillates between the fourth and fifth states with the predetermined frequency and/or the predetermined pulse duty factor when the magnetic field or flux applied by the magnetic element continuously corresponds to the second condition. 
     In a second aspect of the invention, a two-lead magneto-sensitive unit for use with the above mentioned flow meter of the first aspect comprises a signal terminal and a reference terminal; a magnetic sensor element having two output terminals connected to the signal terminal and the reference terminal, respectively, of the magneto-sensitive unit and a power supply terminal; and a buffered power supply unit connected to the signal terminal and the reference terminal of the magneto-sensitive unit, and configured to generate a continuous auxiliary voltage from the signal terminal and the reference terminal of the magneto-sensitive unit and to provide the continuous auxiliary voltage at the power supply terminal of the magnetic sensor element. 
     Further advantages and preferred embodiments of the present invention will be described in the following together with the drawings listed below. The expressions “left”, “right”, “below” and “above” used in the following description are referred to the drawings in an alignment such that the reference numbers and the notation of the figures used can be read in normal orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a flow meter according to the present invention; 
         FIG. 2  is a partial circuit diagram showing the internal structure of a buffered power supply unit of a flow meter according to a first embodiment of the present invention; 
         FIG. 3  is a partial circuit diagram showing the internal structure of a buffered power supply unit of a flow meter according to a second embodiment of the present invention; 
         FIG. 4  is a partial circuit diagram of a flow meter according to a third embodiment of the present invention; and 
         FIG. 5  is a circuit diagram of a flow meter according to prior art comprising a reed contact. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a flow meter according to a first embodiment of the present invention will be described. The general arrangement of a flow meter according to the invention is similar to that of a conventional flow meter as described above in connection with  FIG. 5 . In particular, an impeller or vane wheel (not shown) is rotated by a fluid flow to be measured and comprises at least one movable magnetic element (not shown) which is rotated together with the impeller, for example. The movable magnetic element may be a permanent magnet, for example. According to the present invention, a two-lead magneto-sensitive unit  100  is arranged adjacent to the impeller (preferably outside the fluid, e.g. positioned at the outside of an impeller casing) so that the movable magnetic element applies a magnetic field or flux to magneto-sensitive unit  100 , wherein the applied magnetic field or flux is varied when the movable magnetic element rotates. The flow meter arrangement preferably comprises a sensor socket (not shown) for receiving magneto-sensitive unit  100 . The sensor socket preferably comprises two contact pins which are connected to a first power supply terminal  16  which provides a first supply voltage or potential Vs 1  and a digital input terminal  14  of a control unit (evaluation circuit)  10  of the home appliance. Digital input terminal  14  of control unit  10  is further connected via a limiting element  12  (preferably a pull-up resistor) to a second power supply terminal  18  which provides a second supply voltage or potential Vs 2 , which may be +3.3 V or +5.0 V or any other common digital supply voltage for digital logic circuits such as TTL or CMOS. The term “terminal” must not be construed in a limiting way, but encompasses a contact pin of a plug or socket, or an internal contact pin of an electric circuit or unit, as well as any point of a respective electric circuit (also across the boundaries of physical units or modules), such as a trace of a printed circuit board (PCB), carrying the corresponding potential or signal described. For example, limiting element  12  may be attached to contact pins of control unit  10 , or may be integrated into control unit  10  and connected to corresponding points (e.g. PCB traces) of its electric circuitry. 
     For the sake of clarity, in following description of preferred embodiments of the invention, it is assumed that the first power supply voltage Vs 1  is ground (i.e. 0.0 V) and the second power supply voltage Vs 2  is a positive supply voltage, such as the above mentioned supply voltages of common digital logic circuits of +3.3 or +5.0 V. Thus, digital input terminal  14  is configured to receive digital voltage signals (Vd) provided by a sensor arrangement by either presenting a high resistance or open state (resulting in a digital HIGH level signal close to Vs 2 ) or a low resistance or short circuit state (resulting in a digital LOW level signal close to Vs 1 ) between terminals  14  and  16 . However, it falls within the meaning of the present invention, if the first and second power supply voltages Vs 1  and Vs 2  are swapped, i.e. Vs 1 =+3.3 V and Vs 2 =0 V (ground). Alternatively, it is also conceivable that Vs 2  is a negative power supply voltage. All these amendments correspond to a simple inversion of the digital logic levels HIGH and LOW, and a skilled person is able to adapt or invert the circuit diagrams and the construction of the flow meter of the present invention accordingly. 
     The sensor socket may also receive a common two-lead reed contact unit, such as reed contact  50  of  FIG. 5 , instead of the magneto-sensitive unit  100  of the invention. Thus, the two-lead magneto-sensitive unit  100  of the invention and a common reed contact unit are interchangeable with each other. Magneto-sensitive unit  100  preferably comprises a plug (not shown) which is configured to match with the sensor socket and which comprises a signal terminal  114  to be connected to the digital input terminal  14 , as well as a reference terminal  116  to be connected to the first power supply terminal  16 . The sensor socket is preferably protected against polarity reversal. 
     Magneto-sensitive unit  100  further comprises a magnetic sensor element  120  having three terminals, namely two output terminals  126 ,  127  as well as a power supply terminal  128 , as shown in  FIG. 1 . Magnetic sensor element  120  may be a commercially available magneto-sensitive switching element, such as a Hall effect sensor or a magneto-resistive sensor or the like which may manufactured in the form of a unit or an integrated chip, and usually comprises, for example, a magneto-sensitive component  122  such as a Hall element or a different magneto-resistive element which is manipulated by an applied magnetic field or flux, and an electronic switching element  124  such as a transistor, preferably a FET (field effect transistor), which is connected between the output terminals  126 ,  127  and is controlled or switched in accordance with the strength of the magnetic field applied to magneto-sensitive component  122 . Electronic switching element  124  is preferably configured to switch the connection between output terminals  126 ,  127 , and thus between signal terminal  114  and reference terminal  116  and also between digital input terminal  14  and first power supply terminal  16 , to a first state having a high resistance (electronic switching element  124  in its non-conducting or open state) or to a second state having a low resistance or short circuit (electronic switching element  124  in its conducting or closed state). In other words, magneto-sensitive unit  100  is configured to assume a first state providing a high resistance between signal terminal  114  and reference terminal  116  to generate a first digital signal level (HIGH) at digital input terminal  14  when the magnetic field or flux applied by the movable magnetic element corresponds to a first condition (weak or absent magnetic field or flux), and to assume a second state providing a low resistance or short circuit between signal terminal  114  and reference terminal  116  to generate a second digital signal level (LOW) at digital input terminal  14  when the magnetic field or flux applied by the movable magnetic element corresponds to a second condition (strong or present magnetic field or flux). The internal structure of magnetic sensor element  120  is not limited to the structure described above, and other designs are feasible. For example, magnetic sensor element  120  may further comprise a control circuitry such as an operational amplifier or a comparator for receiving a signal from magneto-sensitive component  122  and providing a suitable control signal to electronic switching element  124 . Magnetic sensor element  120  may also be based on induction and may comprise an induction coil. In this case, instead of the strength of the magnetic field or flux, the rate of change of the magnetic flux at the magnetic sensor element  120  is detected, wherein an increasing magnetic field or flux as a first condition and a decreasing magnetic field or flux as a second condition are detected and converted into a signal that controls electronic switching element  124 . Independent from the type of magnetic sensor element  120 , the movable magnetic element may comprise a permanent magnet or a magnetic flux conducting element made of a magnetically soft material. If the movable magnetic element comprises a magnetically soft material, magneto-sensitive unit  100  preferably further comprises a permanent magnet and a magnetic circuit which is affected by the movement of the movable magnetic element such that the magnetic field or flux at magneto-sensitive component  122  is varied between the first and second conditions. In other words, the moving magnetic element varies the magnetically effective air gap, and thus the reluctance and the magnetic flux. 
     Magneto-sensitive unit  100  further comprises a buffered power supply unit  130  which has a first terminal  132  directly connected to reference terminal  116  and a second terminal  134  directly connected to signal terminal  114  of magneto-sensitive unit  100 , as well as a third terminal  136  connected to power supply terminal  128  of magnetic sensor element  120 . Buffered power supply unit  130  is configured to generate a continuous (buffered) auxiliary voltage Va from signal terminal  114  and reference terminal  116  of magneto-sensitive unit  100  and to provide that continuous auxiliary voltage Va at its third terminal  136  which is connected to terminal  128  of magnetic sensor element  120 , in order to reliably supply magnetic sensor element  120  with a sufficient operating voltage level, even when the impeller is rotating and electronic switching element  124  of magnetic sensor element  120  intermittently short circuits its output terminals  126 ,  127  and the voltage at signal terminal  114  periodically collapses close to zero. 
     In a first preferred embodiment of the invention, as shown in  FIG. 2 , buffered power supply unit  130  comprises a diode  320 , preferably a Schottky diode, as a rectifying element which is connected between second terminal  134  and third terminal  136 , and a capacitor  310  as a buffering element which is connected between first terminal  132  and third terminal  136 . Alternatively, also a supercapacitor or an accumulator or rechargeable battery may be used as a buffering element. Rectifying element or diode  320  restricts the direction of the current flowing between second terminal  134  and third terminal  136  such that capacitor  310  can be charged when the voltage at signal terminal  114  is at HIGH level, i.e. transistor  124  is in its open state or high resistance state, but discharging of capacitor  310  is prevented when voltage at signal terminal  114  is at LOW level, i.e. transistor  124  is in its closed state or low resistance state. Capacitor  310  preferably comprises either a smaller ceramic capacitor having good high frequency characteristics and an electrolytic tantalum capacitor having a larger capacity, for example 6.8 μF to 10 μF or the like, or a MLCC (multi layer chip capacitor) which provides a compromise between high frequency response and large capacitance in a single component, to provide sufficient buffering capacity for the power supply of magnetic sensor element  120 . Commercially available magneto-sensitive chips which can be used as magnetic sensor element  120  may require a supply voltage of about 3 V and a supply current of about 0.2 mA or less. In this way, the buffered power supply of magnetic sensor element  120  can be ensured by bridging usual LOW level signal periods appearing at signal terminal  114  without any significant drop of the auxiliary voltage level Va supplied to power supply terminal  128  of magnetic sensor element  120 . In this manner, a reliable operation of magneto-sensitive unit  100 , and thus, of the flow meter according to the invention can be ensured. 
     According to a second embodiment of the invention, as shown in  FIG. 3 , buffered power supply unit  130  comprises an active rectifier  330  as a rectifying element. This active rectifier  330 , which is per se known in the art, may have, in comparison to the two terminal diode  320 , a further terminal which is connected to first terminal  132 , but not shown in  FIG. 3 . Active rectifier  330  is advantageous over diode  320  because the voltage drop at the diode (which is about 0.2 to 0.3 V in case of a Schottky diode) can be further reduced to a voltage drop at the active rectifier of less than 0.1 V, so that buffered power supply unit  130  according to the second embodiment can provide a higher auxiliary voltage level at voltage supply terminal  128  of magnetic sensor element  120 . In this way, a greater variety of commercially available sensor chips can be used for magnetic sensor element  120 . 
     If the fluid stops and the impeller or vane wheel of the flow meter is stopped in a position in which the movable magnetic element applies a strong magnetic field or flux to magneto-sensitive unit  100  (second condition), then transistor  124  is switched to its conducting (closed) state for a longer period of time. In this case, also the voltage level at signal terminal  114  remains at a very low level for a longer period of time, so that capacitor  310  is gradually discharged by the current drawn by magnetic sensor element  120 , and auxiliary voltage Va gradually decreases. As soon as auxiliary voltage Va falls below the nominal operating voltage of magnetic sensor element  120 , magnetic sensor element  120  may enter an undefined operating state which may provide an undefined resistance appearing between signal terminal  114  and reference terminal  116 . This may either lead to a floating state providing a medium resistance between signal terminal  114  and reference terminal  116 , or to an oscillation of the auxiliary voltage Va and the resistance appearing between signal terminal  114  and reference terminal  116 , and thus to an oscillation of the voltage level at signal terminal  114 . If such oscillations are crossing a LOW voltage threshold value and a HIGH voltage threshold value of control unit  10 , control unit  10  detects a pulse sequence indicating a fluid flow, although the fluid flow has stopped. In order to prevent such malfunction of the flow meter according to the invention, the following provision may be made. 
     In a further embodiment of the invention, magnetic sensor element  120  is configured such that it slides into and maintains a more or less stable third state providing a medium resistance between signal terminal  114  and reference terminal  116  when the movable element is stopped so that the magnetic flux applied by the magnetic element is continuously strong or present (second condition). In this case this medium resistance must be sufficiently low to keep the voltage at signal terminal  114  below a HIGH level threshold (first digital signal level) of digital input terminal  14  of control unit  10 , so that no HIGH level pulses are generated and detected at the digital input terminal  14 . Simultaneously, the voltage level at signal terminal  114  of magneto-sensitive unit  100  is high enough to maintain this third state by holding transistor  124  in a floating state between its open state and its closed state. In this manner, no impulses will be detected at digital input terminal  14  of control unit  10  when the fluid flow and the impeller are stopped in a position in which a strong magnetic field or flux is applied to magneto-sensitive unit  100 . 
     In another embodiment, magneto-sensitive unit  100  further comprises an adapting unit  140  which is connected to magnetic sensor element  120 . Adapting unit  140  may comprise further resistor, inductor or capacitor elements, or may also comprise active electronic elements such as transistors, in order to adapt magnetic sensor element  120  such that magnetic sensor element  120  or magneto-sensitive unit  100 , respectively, slides into and maintains said third state. In this manner, also commercially available sensor chips may be used as magnetic sensor element  120  which would per se not exhibit the desired behavior. 
     In a further embodiment, magnetic sensor element  120  is configured such that, when the movable element is stopped so that the magnetic flux applied by the magnetic element is continuously strong or present (second condition), it enters into a stable oscillation state in which it oscillates between a fourth state providing a relatively low resistance between signal terminal  114  and reference terminal  116 , so that a LOW level signal (second digital signal level) is generated at signal terminal  114 , and a fifth state providing a relatively high resistance between signal terminal  114  and reference terminal  116 , so that a HIGH level signal (first digital signal level) is generated at signal terminal  114 . This stable oscillation preferably exhibits a predetermined characteristic frequency, which may be a resonant frequency of the flow meter sensor arrangement. Alternatively or additionally, this stable oscillation exhibits a predetermined characteristic pulse duty factor, i.e. a characteristic ratio between the durations of the fourth and fifth states. The control unit or evaluation circuit  10  is in this case configured to detect this predetermined characteristic frequency and/or this predetermined characteristic pulse duty factor, which preferably do not appear during normal operation of the flow meter, and thus determines that the fluid flow and the impeller of the flow meter have stopped. 
     In another embodiment, magneto-sensitive unit  100  further comprises an adapting unit  140  which is connected to magnetic sensor element  120 . Adapting unit  140  may comprise further resistor, inductor or capacitor elements, or may also comprise active electronic elements such as transistors, in order to adapt magnetic sensor element  120  such that magnetic sensor element  120  or magneto-sensitive unit  100 , respectively, enters a stable oscillation between the fourth state and the fifth state with the predetermined frequency and/or the predetermined pulse duty factor, as described above, when the magnetic flux applied by the magnetic element is continuously strong or present. In this manner, also commercially available sensor chips may be used as magnetic sensor element  120  which would per se not exhibit the desired behavior. 
     Adapting unit  140  may, for example, comprise a capacitor connected between output terminal  127  and power supply terminal  128  of magnetic sensor element  120  and/or a resistor connected between output terminal  126  and power supply terminal  128  in order to suppress any oscillations, or alternatively, to impose a predetermined characteristic resonant frequency or a predetermined characteristic pulse duty factor. 
     All components of magneto-sensitive unit  100  are preferably mounted on a single circuit board together with the plug which matches with the sensor socket of the flow meter. Thus, magneto-sensitive unit  100  can be easily replaced by another magneto-sensitive unit  100  or by a common reed contact unit having a similar plug and similar dimensions. This provides additional freedom during the production of home appliances, so that a manufacturer can arbitrarily choose between a cheaper reed contact unit  50  and a more reliable magneto-sensitive unit  100  according to the present invention, depending on the requirements to be met. 
     In alternative embodiments of the invention, limiting element  12  and control unit  10  may be arranged within a flow meter module, or may be mounted on another component of the home appliance. For example, the function of control unit  10  may be integrated in a control device of the home appliance. Limiting element  12  of  FIG. 1 , which connects digital input terminal  14  of control device  10  to a first power supply voltage Vs 1  may be a resistor as described above, or may be a constant-current source or any other limiting circuit which, similar to an ohmic resistance, generates at digital input terminal  14  a HIGH level voltage signal when electronic switching element  124  is in its non-conducting (open) state, and a LOW level voltage signal close to zero (ground) when electronic switching element  124  is in its conducting (closed) state. For example, a so called weak transistor may be used as limiting element  12 , such as a FET configured to maintain a conducting state with increased resistance (medium conducting state). 
     In the above description of the preferred embodiments of the invention, it is assumed that the first state of magneto-sensitive unit  100  provides a high resistance between its terminals  114 ,  116 , the second state of the same provides a low resistance or short circuit between its terminals  114 ,  116 , and that the first condition of the magnetic field or flux corresponds to a weak or absent magnetic field or flux, and the second condition of the magnetic field or flux corresponds to a strong or present magnetic field or flux. Moreover, it is assumed that the first digital signal level is HIGH and the second digital signal level is LOW. It is however conceivable that the first state of magneto-sensitive unit  100  provides a low resistance or short circuit between its terminals  114 ,  116  and the second state of the same provides a high resistance between its terminals  114 ,  116 . Alternatively or additionally, it is also possible that the first condition of the magnetic field or flux corresponds to a strong or present magnetic field or flux, and the second condition of the magnetic field or flux corresponds to a weak or absent magnetic field or flux. Alternatively or additionally, it is also possible that that the first digital signal level is LOW and the second digital signal level is HIGH. Such amendments are simple inversions of the disclosed principles and fall within the meaning of the present invention. A skilled person is able to adapt the disclosed circuit diagrams and the construction accordingly, e.g. by swapping the polarity of certain terminals or components (such as diode  320  or capacitor  310 ) or by replacing some components with similar components of a complementary type (e.g. replacing n-type FET  24  shown in  FIG. 1  with a p-type FET). 
     LIST OF REFERENCE SIGNS 
       10  control unit 
       12  limiting element 
       14  digital input terminal 
       16 ,  18  first and second power supply terminals 
       50  reed contact 
       100  magneto-sensitive unit 
       114  signal terminal of magneto-sensitive unit 
       116  reference terminal of magneto-sensitive unit 
       120  magnetic sensor element 
       122  magneto-sensitive component 
       124  electronic switching element 
       126 ,  127  first and second output terminals of magnetic sensor element 
       128  power supply terminal of magnetic sensor element 
       130  buffered power supply unit 
       132 ,  134 ,  136  first, second and third terminals of buffered power supply unit 
       140  adapting unit 
       310  buffering element, capacitor 
       320  diode 
       330  active rectifier