Abstract:
A capacitive, operator-sensing circuit for mobile power equipment includes a charge-transfer touch sensor that is electrically coupled to at least one sensing electrode mounted on a gripping surface of the mobile power equipment. The charge-transfer touch sensor is preferably a digital integrated circuit that operates to change an output voltage between two levels or states (logic high and logic low) depending upon a quantity of charge sensed on the at least one sensing electrode, the sensed charge on the at least one sensing electrode changing with the presence or absence of an operator&#39;s hand on the at least one sensing electrode. The charge-transfer touch sensor outputs a DC voltage signal, maintaining a given output voltage level until the sensor senses a predetermined change in sensed charge on the sensing electrode. The capacitive operator-sensing circuit additionally includes coupling circuitry coupled between the charge-transfer touch sensor and a component of the equipment. A change in sensor output voltage from a first level to a second level causes a cessation of operation of the component. For example, the component may be an internal combustion engine of the equipment and a change in sensor output voltage from the first level to the second level causes the coupling circuitry to electrically couple a magneto of the engine to be coupled to ground thereby shutting off the engine.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to a capacitive, operator-sensing circuit for disabling a component, such as an internal combustion engine, of mobile power equipment and, more particularly, a capacitive, operator-sensing circuit including a charge-transfer sensor coupled to a capacitive sensing electrode affixed to a gripping surface of the mobile power equipment, the circuit functioning to disable the component upon sensing a change in the charge of the sensing electrode as a result of an operator removing his or her hand from the gripping surface. 
     BACKGROUND ART 
     Various types of mobile power equipment driven by internal combustion engines are in widespread use today such as lawn and garden maintenance equipment, construction equipment, and agricultural implements. Examples of such internal combustion mobile power equipment include power lawnmowers (push and self propelled), rototillers, cultivators, snowblowers, power lawn edgers, riding lawnmowers, garden tractors, etc. Such equipment includes a handle or gripping surfaces which is/are used by the operator to push the equipment (if not self-propelled) and/or direct the equipment movement along a desired path of travel. Gripping surface might include a steering wheel and steering wheel spokes of a riding lawnmower or the steering levers of a skid steer vehicle. A constant concern with such mobile power equipment is insuring that the engine is disabled (turned off) in the event that the operator losses control of the equipment, i.e., the operator losses his or her grip on the handle or gripping surfaces. In some types of mobile power equipment a working implement, such as a cutting blade or an auger, is driven by the motor and there is an electrical or mechanical clutch disposed between the engine, a transmission and the working implement. In such equipment instead of turning off the engine, it may be sufficient to disengage the clutch when the operator losses his or her grip on the handle. Disengaging the clutch insures the working implement is no longer driven by the engine even though the engine continues to run. Such a clutch arrangement is used in equipment having a working implement driven by a power take-off drive system. A power take-off drive system typically includes a drive shaft to which the working implement (cutting blade, auger, etc.) is attached. When the power take-off drive is engaged, the engine rotates the power take; off drive shaft. 
     Various types of mechanical assemblies have been used on such mobile power equipment to shut off the engine and/or disengage the power take-off drive system in the event that the operator losses his or her grip on the handle or gripping surfaces. Generally such mechanical assemblies included a lever or rod that is required to be pivoted to a position adjacent, for example, the handle for the engine to operate. The operator thus grips both the lever and the handle simultaneously to maintain the lever adjacent the handle. The lever is typically biased by a spring to swing away from the handle if the operator releases his or her grip on the handle thereby shutting off the engine and/or drive wheels on self-propelled mobile power equipment and/or disengage a power take-off drive shaft. 
     Such mechanical assemblies have several disadvantages including being costly to install and maintain, adding additional weight to the equipment and continuously exerting pressure against the operator&#39;s hand causing operator fatigue. Additionally, such mechanical assemblies are capable of being defeating by simply tying the lever to the handle with a piece of rope or twine. 
     An electrical operator-sensing circuit would overcome many of the disadvantages of a mechanical assembly including fatigue and defeatability. One such electrical circuit proposal is disclosed in U.S. Pat. No. 4,145,864 to Brewster, Jr. The &#39;864 patent discloses an operator sensing circuit that requires an operator to electrically bridge first and second spaced apart operator contacts mounted on the lawnmower handle of an electric lawnmower for actuation of the electric motor that drives the cutting blade. However, the &#39;864 patent requires that both hands of the operator be on the handle. It is often desirable and necessary to permit continued operation of the equipment if the operator maintains one hand on the handle, e.g., the operator removes one hand from the handle to wipe his or her brow or push low hanging branches out of the way when mowing under a tree. Additionally, the &#39;864 patent teaches the use of a phase lock loop circuit requiring a signal generator and signal receiver which is relatively complex and expensive. 
     What is needed is a simple-to-fabricate and install, low-cost, rugged, durable and difficult to defeat operator-sensing circuit which permits continued operation of the power equipment when at least one hand of the operator is maintained on the equipment handle. 
     SUMMARY OF THE INVENTION 
     A capacitive, operator-sensing circuit for mobile power equipment includes a charge-transfer touch sensor that is electrically coupled to at least one capacitive sensing electrode affixed to a gripping surface of the mobile power equipment. The charge-transfer sensor periodically generates charge bursts that are coupled to the sensing electrode and a floating transfer capacitor. The charge-transfer sensor senses the charge buildup on the sensing electrode resulting from the charge bursts. The sensor changes an output signal between a first and a second states if the sensed charge changes by a predetermined threshold magnitude. 
     The charge-transfer sensor is preferably a digital integrated circuit that operates to change an output voltage signal between two levels or states depending upon sensed charge of the sensing electrode. The gain and sensitivity of the sensor are configured such that the sensed charge of the sensing electrode changes by the predetermined threshold magnitude (or more) when an operator&#39;s hand is moved from a position of gripping or touching the gripping surface to a position of not gripping or touching the surface. The charge-transfer sensor outputs a DC voltage signal, maintaining a given output voltage level until the sensor senses a change in the sensed charged of the predetermined threshold value. 
     The operator-sensing circuit additionally includes coupling circuitry coupled between the charge transfer touch sensor and a component or system of the mobile power equipment. For example, the component may be an internal combustion engine of the equipment and the coupling circuitry may be coupled to a magneto of the internal combustion engine. A change in sensor output voltage signal from a first level to a second level causes the engine magneto to be electrically coupled to ground thereby shutting off the internal combustion engine. 
     The coupling circuit includes a first and second field effect transistors (FETs) coupled to a gate of a triac bilateral switch. The triac is coupled between the magneto and ground and, when the break over threshold voltage of the triac is exceed by application of a sufficiently high positive or negative voltage to the triac gate terminal, the triac conducts in both forward and reverse direction as necessary to ground the magneto and shut off the engine. 
     In one preferred embodiment of the operator-sensing circuit of the present invention, the charge-transfer sensor is coupled to two sensing electrodes which are disposed at spaced apart positions on the equipment handle or gripping surfaces. While either or both of the sensing electrodes is contacted by an operator&#39;s hand, the sensor output voltage signal remains at the first level and the engine magneto is not grounded out. 
     In another preferred embodiment of the operator-sensing circuit of the present invention, the coupling circuit includes a relay which switches between two open and closed conditions as the sensor output voltage changes levels (logic high and logic low). The relay may advantageously be electrically coupled to a controller which disengages a component of the mobile power equipment such as a power take off drive when the operator&#39;s hands are removed from the sensing electrodes. Alternately, in mobile power equipment driven by an electric motor, such a coupling circuit utilizing a relay and controller may be advantageously used disable or turn off power to the electric motor, when the operator removes his or her hands from the sensing electrodes. 
     These and other objects, advantages, and features of an exemplary embodiment of the present invention are described in detail in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a lawnmower including the capacitive operator-sensing circuit the present invention; 
     FIG. 2 is a view partially in elevation and partially in section of a portion of a lawnmower handle showing a first embodiment of a sensing electrode of the capacitive operator-sensing circuitt; 
     FIG. 3 is a view partially in elevation and partially in section of a portion of a lawnmower handle showing a second embodiment of a sensing electrode of the capacitive operator-sensing circuit; 
     FIG. 4 is a schematic circuit diagram showing the capacitive operator-sensing circuit; and 
     FIG. 5 is a schematic circuit diagram showing a second embodiment of a coupling circuit of the capacitive operator-sensing circuit. 
    
    
     DETAILED DESCRIPTION 
     Turning to FIG. 1, a conventional power lawnmower is shown generally at  10 . The lawnmower includes a capacitive operator-sensing circuit  100  of the present invention, shown in detail in FIG.  4  and described below. The lawnmower  10  includes a steel or aluminum housing  12  having a flat upper deck  14  that supports an internal combustion engine  16 . When the engine  16  is operating it rotatably drives a vertical drive shaft  18  and a grass cutting blade  20  affixed thereto. The grass cutting blade  20  is positioned below the deck  14  and is enclosed by the housing  12 . The rotational speed of engine  16  and the cutting blade  20  is controlled by an adjustable throttle  22 . The throttle  22  is mounted on an relatively horizontal upper portion  24  of a handle  26  and is mechanically coupled via a throttle cable  27  to an engine throttle  28  to control the speed of the engine  16 . The handle  26  includes two parallel angled portions  29 . Respective ends  30  of the angled portions  29  are affixed to flanges  32  (only one of which can be seen in FIG. 1) extending upwardly from the housing  12 . 
     Four wheels  34  are attached to the housing  12  so that the lawnmower  10  can be pushed along a desired path of travel so as to mow or cut grass which contacts the rotating cutting blade  20 . An operator controls the movement of the lawnmower  10  by grasping and exerting force against the handle  26  along a gripping portion  36  of the handle upper portion  24  which is closest to the operator. By appropriately grasping and exerting force against the handle gripping portion  36 , the operator is able maneuver the lawnmower  10 , i.e., push and pull the lawnmower  10  as appropriate as the operator walks behind and, turn the lawnmower  10  to the left or right, as necessary, with respect to a straight line of travel. 
     Typically, the operator positions his or her hands at spaced apart gripping positions along the handle gripping portion  36 . Since the handle  26  is typically steel or aluminum which is slippery to an operator&#39;s wet or sweaty hands, for comfort, soft plastic, rubber or neoprene grips  40 ,  42  are advantageously disposed on the handle length  36  to define gripping surfaces and facilitate gripping of the handle  26  by the operator. Alternatively, a single piece of plastic, rubber or neoprene may be disposed to extend the entire length of the handle gripping portion  36  and define a single gripping surface. The grass cut by the lawnmower blade  20  is discharged through an opening in the housing  12  into a grass catcher bag  38  supported by the housing, the angled portions  29  of the handle  26  and a crossmember  30  of the handle  26 . 
     A capacitive, operator-sensing circuit  100  of the present invention is installed on the lawnmower  10  to shut off or stop the engine  16  in the event that the operator&#39;s body no longer is in contact with either of the grips  40 ,  42 , that is, the operator does not have either of his or her hands on one of the two grips  40 ,  42 . Essentially, the operator-sensing circuit  100  (shown schematically in FIG. 4) includes a capacitive charge transfer sensor  130  which periodically transfers or pumps small quantity of electric charge to a pair of sensing electrodes  120 a,  120 b, each of which is affixed to a respective one of the grips  40 ,  42 . If the operator is in contact with either of the two grips  40 ,  42  a portion of the charge transferred to and stored by the sensing electrodes is transferred to ground (or virtual ground if the operator is not in contact with the earth but rather riding on the mobile equipment, i.e., riding on a lawn tractor) through a capacitive conductive path established by the operator&#39;s body. 
     By monitoring the amount of charge present on the sensing electrodes  120   a,    120   b,  the sensor  130  determines if an operator&#39;s hand (or other body part) is overlying or contacting either of the two grips  40 ,  42 . When the sensor  130  determines that there is no operator contact with either of the sensing electrodes  120   a,    120   b,  an output signal of the sensor changes state resulting in a magneto (shown schematically at  50  in FIGS. 1 and 4) of the engine  16  to be grounded thereby shutting off the engine. 
     While the specific lawnmower  10  shown is a push-type power lawnmower, the capacitive operator-sensing circuit  100  of the present invention is equally adapted to be used in connection with other types of mobile power equipment having an internal combustion engine whether the equipment is push-type or self-propelled. Typical examples of such equipment would include push-type and self-propelled lawnmowers, rototillers, cultivators, snowblowers, power grass edgers, riding lawnmowers, garden tractors, skid steer vehicles and other such equipment where the operator holds onto one or more gripping surfaces such as a handle, a lever or levers, or a steering wheel of the equipment to push, guide, or steer the equipment along a desired path of travel. 
     It should also be appreciated that while the specific embodiment of the operator-sensing circuit  100  described herein includes two sensing electrodes  120   a,    120   b,  the circuit  100  and specifically the preferred capacitive, charge transfer sensor  130  is equally capable of functioning properly with a single sensing electrode, i.e. a single electrode spanning the entire gripping surface of a handle or steering wheel or two, three or more sensing electrodes. Only minor adjustments, well known to those skilled in the art, to the sensor  130  are required to support different numbers, size and configurations of sensing electrodes and provide acceptable threshold level and sensitivity values for the circuit  100 . Sensitivity refers to the magnitude of the gain of the sensor  130  while threshold level refers to the percent of absolute signal level at which the sensor changes output state. Sensitivity is related to sensing electrode surface area, orientation with respect to the object being sensed, sensed object composition, and the ground coupling quality of both the sensor circuit and the sensed object. 
     First Preferred Embodiment of Sensing Electrodes 
     The capacitive, operator-sensing circuit  100  comprises a pair of capacitive sensing electrodes  120   a,    120   b  which are permanently affixed to the handle grips  40 ,  42  and electrically coupled to a capacitive, charge-transfer sensor  130 . In a first preferred embodiment of the circuit  100 , the sensing electrodes  120   a,    120   b  comprise sections or lengths of  22  gauge uninsulated conductive copper wire. As can best be seen in FIG. 2, the uninsulated wire section comprising the sensing electrode  120   a  is wrapped in a longitudinal spiral fashion around the outer surface of the handle grip  40  spanning the grip from one end  40   a  to an opposite end  40   b.  The same is true for the grip  42 , an uninsulated wire section comprising the sensing electrode  120   b  is wrapped in a longitudinal spiral fashion around the outer surface of the handle grip  42  spanning the grip from one end  42   a  to an opposite end  42   b.    
     The uninsulated wire sections  120   a,    120   b  are then affixed to the handle grips  40 ,  42  by wrapping respective lengths of electrical tape  121  (or other similar adhesive, dielectric material) around the grips  40 ,  42  in a similar spiral turn fashion between the opposite ends  40   a,    40   b,    42   a,    42   b  of the respective grips  40 ,  42 . 
     While FIG. 2 shows approximately  14  turns of uninsulated wire spanning the grip  40 , the longitudinal length of the grip, the gauge of the wire used and the dielectric characteristics of the overlying adhesive material (electrical tape, duct tape, friction tape, etc.) will determine how many turns will be required. For example, 9 turns of 22 gauge copper “bus” wire overlaid by standard black electrical tape was found suitable for a grip having a longitudinal length of 4 inches and an outer diameter of 1½ inches. 
     Insulated wire sections  122   a,    122   b  are electrically coupled to one end of respective uninsulated spiral wrapped wire sections  120   a,    120   b.  The insulated wire sections  122   a,    122   b  extend from the grips  40 ,  42  to a housing  125  mounted on a crossmember  30  of the handle  26 . The insulated wire sections  122   a,    122   b  are necessary to prevent shorting of the sensing electrodes  120   a,    120   b  by the metal handle  26 . 
     The remaining circuitry of the operator-sensing circuit  100  is mounted on a printed circuit board (shown schematically at  126  in FIGS. 1 and 4) disposed within the housing  125 . The remaining circuitry of the circuit includes the charge-transfer sensor  130 , a DC power supply (battery) (shown schematically as Vcc in FIG. 4) and a coupling circuit  140  for coupling an output signal (at pin  2 ) of the sensor  130  to the engine magneto  50 . Preferably, the charge-transfer sensor  130  is a QT 110 charge transfer integrated circuit manufactured by Quantum Research Group Ltd., 651 Holiday Drive, Bldg. 5/300, Pittsburgh, Pa. 15220. A twelve page specification sheet published in 1999 by Quantum Research Group Ltd. describing the characteristics of the QT110 integrated circuit (IC) entitled “QProx™ QT110 Charge-Transfer Touch Sensor” is incorporated herein in its entirety by reference. 
     Ends of the insulated wire sections  122   a,    122   b  extend through openings (not shown) in the housing  125  and are electrically coupled to a single insulated conductor  128 . The insulated conductor  128 , in turn, is electrically coupled to an input terminal of the charge-transfer sensor  130  (pin  7 —sensel). 
     The charge-transfer sensor  130  output is active low. When the operator of the mower  10  is gripping either or both of the two grips  40 ,  42 , the charge magnitude sensed by the sensor is low and the output signal, Vo, of the sensor  130  (pin  2 ) is a low DC voltage of essentially zero volts, Vo=0 volts. On the other hand, when the operator of the mower  10  is not gripping or touching either of the two grips  40 ,  42 , the charge sensed by the sensor is high and the output signal of the sensor  130  (pin  2 ) is a high DC voltage, having a typical value of Vo=Vcc−0.7 volts, e.g., if Vcc=5 volts, Vo=5 volts−0.7 volts=4.3 volts. 
     An advantage to the Quantum QT110 is that the QT110 IC is highly tolerant to changes in the capacitance of the 0.01 μF reference capacitor  131  because the QT110 IC computes the threshold level ratiometrically with respect to absolute load and does so dynamically at all times. 
     Periodically, the sensor  130  transfers or pumps out a small magnitude of charge to the sensing electrodes  120   a,    120   b  and the floating transfer capacitor  131 . The charge is transferred approximately 10-15 times per second. The sensor  130  monitors the charge buildup on the sensing electrodes  120   a,    120   b  and the floating transfer capacitor  131 . The reference 0.01 microfarad (μF) capacitor  131  functions as a floating transfer capacitor and is coupled between pins  6  and  7  of the sensor  130 . The sensor  130  may be set for one of three internal gain levels (high, medium and low) by using pin  5 . The gain of the sensor  130 , in part determines the sensitivity of the circuit  100 . If pin  5  is left open, the gain of the sensor  130  is high. If pin  5  is tied or electrically coupled to pin  6 , the gain of the sensor is medium. If pin  5  is tied to pin  7 , the gain of the sensor is low. From empirical testing for the particular sensing electrodes tested, the circuit  100  was found to function properly with a medium gain, thus, pin  5  is tied to pin  6  via a jumper  132 . A DC supply voltage Vcc of positive 3-5 volts is supplied to pins  1  and  3  of the sensor  130  via leads  135 ,  136 . The DC supply voltage Vcc may preferably be obtained from a long life battery (nickel-cadmium (Ni—Cd), nickel-metal hydroxide (Ni—MH) or lithium-ion (Li—Ion)), or the like, since the power consumption requirements of the sensor  130  and coupling circuit  140  are low, e.g., under 20 μA for the sensor  130 . 
     If, during a predetermined consecutive number of sensing periods, the sensed charge of the sensing electrodes  120   a,    120   b  decreases by a predetermined threshold amount, the output pin  2  of the sensor  130  will change from a high voltage output to a low voltage output (logic high to logic low). The reduction in sensed charge by an amount greater than the predetermined threshold amount indicates that an operator of the equipment  10  is gripping one or both of the sensing electrodes  120   a,    120   b  thereby bleeding off or discharging the charge of the sensing electrodes  120   a,    120   b  to ground or a virtual ground, i.e., the equipment itself in the event the operator is riding on the equipment and not in contact with true earth ground. As long as the sensed charge remains low, that is, does not increase by a predetermined threshold amount, the output signal of the sensor  130  will remain a low DC output voltage (logic low). 
     If, on the other hand, during a predetermined consecutive number of sensing periods, the sensed charge of the sensing electrodes  120   a,    120   b  increases by the predetermined threshold amount, the output pin  2  of the sensor  130  will change from a low voltage output to a high voltage output (logic low to logic high). The increase in sensed charge by an amount greater than the predetermined threshold amount indicates that an operator of the equipment  10  is no longer gripping either of the sensing electrodes  120   a,    120   b  thus sensed charge increases because the operator is no longer bleeding off or discharging the charge of the sensing electrodes  120   a,    120   b  to ground. 
     The sensor  130  employs a hysteresis effect with regard to the predetermined threshold change magnitude required to switch between logic high and logic low output states to avoid “bouncing” between output states in the event that sensed charge has just crossed a threshold value (resulting in a change of output state) but remains near the threshold value. The charge buildup on the sensing electrodes  120   a,    120   b  is proportional to the effective capacitance of the electrodes. Absent an operator, the electrodes  120   a,    120   b  and the handle gripping portion  36  underlying the electrodes  120   a,    120   b  function as capacitor plates and result in some magnitude of effective capacitance. The charge buildup on the electrodes  120   a,    120   b  is proportional to this effective capacitance in accordance with the general capacitance equation Q=C×V, where, Q is charge on a capacitor, C is capacitance of a capacitor and V is voltage applied across the capacitor plates. The hands of the operator can be thought of as capacitor plates and, upon gripping the grips  40 ,  42 , the proximity of the operator&#39;s hands to the electrodes  120   a,    120   b  changes the magnitude of the effective capacitance of the electrodes  120   a,    120   b.  Essentially, the operator gripping the grips  40 ,  42  establishes a capacitive path to ground and bleeds charge built up on the electrodes  120   a,    120   b.    
     A conductor  130  electrically couples the output signal at pin  2  of the sensor  130  to the coupling circuit  140  which, in turn, is coupled to the magneto  50 . The coupling circuit  140  includes a pair of complementary junction field effect transistors (JFETs)  150 ,  160  and the triac  180 . A gate terminal  152  of the JFET  150  is coupled to the sensor output via the conductor  138 . A drain terminal  156  of the JFET  150  is coupled to Vcc through a 1 MΩ pull-up resistor  170  and a source terminal  154  of the JFET  150  is coupled to circuit common or ground C. The drain terminal  156  of the JFET  150  is coupled to a gate terminal  162  of the JFET  160 , while a drain terminal  166  of the JFET  160  is coupled to a gate terminal  182  of the triac  180  through a 1 KΩ resistor  175 . The JFET  160  is in an open drain configuration. The triac  180  has its terminals  184 ,  186  electrically coupled between the engine magneto  50  (via conductor  190 ) and ground C such that when the voltage applied to the gate  182  of the triac  180  exceeds the gate threshold voltage and current, the triac  180  turns on and grounds the magneto  50 . The triac  180  is advantageously used in the coupling circuit  140  because, when turned on, it is capable of grounding both positive and negative magneto pulses. 
     Other components of the coupling circuit  140  include a 0.0022 μF capacitor  172  coupled in parallel with a 10 KΩ resistor  174  between the drain terminal  166  of the JFET  160  and the magneto  50 . A 1 KΩ resistor  175  is coupled between the drain terminal  166  of the JFET  160  and the gate terminal  182  of the triac  180 . A 1 KΩ resistor  176  and a 0.0022 μF capacitator  178  are coupled in parallel between the gate terminal  182  of the triac  180  and circuit common C. 
     When the output signal, Vo, of the sensor  130  is low (i.e., the sensor output signal is low DC voltage indicative that an operator is gripping one or both of the sensing electrodes  120   a,    120   b ), the JFET  150  is off (in a nonconducting state) since its source is coupled to ground. When the JFET  150  is turned off, the gate of the JFET  160  is high (at Vcc voltage) and since the source  164  is at ground, the JFET  160  is on (conducting state). When the JFET  160  is on, the voltage at the drain terminal of the JFET  160  and at the gate terminal  182  of the triac  180  are essentially at zero volts. A triac gate voltage of zero volts maintains the triac  180  in a nonconducting state and the magneto is not grounded out. Thus, the engine  16  is not turned off. 
     When the output signal, Vo, of the sensor  130  is high (i.e., the sensor output signal is high DC voltage (Vcc—0.7 volts) indicative that an operator is not gripping either of the sensing electrodes  120   a,    120   b ), the JFET  150  is on (in a conducting state). When the JFET  150  is on, the gate of the JFET  160  is low (essentially at zero volts) and, thus, the JFET  160  is off. When the JFET  160  is off, the magnitude of the voltage at the gate terminal  182  of the triac  180  is essentially {fraction (1/12)} of the magnitude of the voltage of the magneto  50  because of the voltage divider formed by the three resistors  174 ,  175 ,  176  coupled in series between the magneto  50  and circuit ground C. Since the magneto primary voltage is high, approximately 300-400 V, a gate voltage of {fraction (1/12)} of the magneto voltage is sufficient to turn the triac  180  on and thereby ground out the magneto  50 . Grounding out the magneto  50  turns the engine  16  off. 
     Second Preferred Embodiment of Sensing Electrodes 
     A second preferred embodiment of a sensing electrode  220  suitable for use with the capacitive operator-sensing circuit  100  is shown in FIG.  3 . While only one sensing electrode is shown in FIG. 3, it should be understood that two (or more) sensing electrodes similar to electrode  220  may be utilized with the capacitive operator-sensing circuit  100  just as was the case with sensing electrodes  120   a,    120   b.  In this second embodiment, the sensing electrode  220  comprises a cylindrical shaped conductive mesh or grid  221  that is fabricated into the grip  240 . The grip  240  is preferably made of a durable, easy to grip material such as rubber, plastic, neoprene, Santoprene® material, etc. 
     The conductive grid preferably is comprised of copper alloy. The conductive grid  221  is disposed at a depth d radially inwardly from an outer peripheral surface S of the grip  240 . A typical depth d for the conductive grid would be 0.005-0.020 inch. 
     The advantage of the conductive grid  221  is that the sensing electrode  220  is integral with the grip  240 , that is, the grip  240  and sensor  220  are of unitary, one-piece construction and, therefore, are more durable than the sensor  120   a,    120   b,  electrical tape and grip  40 ,  42  combinations of the first embodiment. As with the first embodiment, an insulated wire  222  electrically couples the sensor  220  to the conductive lead  128  which, in turn, is coupled to input pin  7  of the sensor  130 . 
     Second Preferred Embodiment of Coupling Circuit 
     Many types of self-propelled power equipment have one or more power take-off drives driven by the engine of the equipment. Typically, when a power take-off drive is engaged engine power is used to rotate a power take-off shaft. Different equipment may be coupled to a power take-off shaft, i.e., a cutting blade, a snowblower auger, etc. In some instances it may be desirable to disengage a power take-off instead of shutting off an engine in the event the operator&#39;s hands are no longer in contact with either of the sensing electrodes  120   a,    120   b.  Other types of mobile power equipment utilize electric motors. In such equipment, stopping the motor involves interrupting the supply of power to the motor as opposed to grounding a magneto in an internal combustion engine. 
     A second preferred embodiment of a coupling circuit  340  is shown in FIG.  5 . The coupling circuit  340  may be advantageously used with the capacitive, operator-sensing circuit  100  to disengage a power take-off drive or stop an electric motor when the operator is not gripping either of the sensing electrodes  120   a,    120   b.  Those components of the operator-sensing circuit  100  that are the same as those described in the first preferred embodiment are labeled with the same reference numbers as in FIG.  4  and operate as described with respect to that embodiment. 
     The coupling circuit  340  of this embodiment includes the pair of JFETs  150 ,  160  as previously described. A drain terminal  166  of the JFET  160  is coupled, in an open drain configuration, to one terminal of a normally closed relay  342  (1 Form C relay—normally closed contacts). The other end of the normally closed relay  342  is coupled to Vcc (+3-5 V DC). Internally, as can be seen in FIG. 5, the normally closed relay  342  comprises a flyback diode  346  coupled in parallel with a coil  344  which, in turn, is inductively coupled to the normally closed switch  348 . 
     One terminal of the normally closed switch  348  is coupled to circuit common C while the other end of the switch  348  is electrically coupled to an input terminal  352  of a controller  350 . The controller  350  controls one or more power units (as shown by the dashed line in FIG. 5) and allows for level shifting of output voltages. For example, the controller  350  may be electrically coupled to an electric motor  52  of a mobile power equipment unit and function to interrupt the supply of power to the electric motor  52  and thereby stop the motor  52  when an operator removes his or her hands from the sensing electrodes  120   a,    120   b.  Alternately, the controller  350  may be electrically coupled to a power take-off drive  54  of a mobile power equipment unit and functions to disengage the power take-off drive shaft being driven by an engine of the unit when an operator removes his or her hands from the sensing electrodes  120   a,    120   b.  The selection of a specific controller from numerous controller products offered by various manufacturers will be dependent on the nature of the system to be controlled, the desired voltage shifting required, etc. as is known to those skilled in the art. 
     The controller  350  senses the voltage state at the input terminal  352  of the controller  350 . The controller  350  includes a 1 MΩ pull-up resistor  354  coupled between Vcc and the input terminal  352 . So long as the switch  348  remains closed, the voltage at the input terminal  352  will remain logic low. If the switch  348  opens, the voltage at the input terminal  352  will switch to logic high. 
     When the operator&#39;s hands are in contact with at least one of the sensing electrodes  120   a,    120   b,  the voltage output, Vo, of pin  2  of the sensor  130  is logic low, the JFET  150  is off and the JFET  160  is in a conductive state, as described previously. When the JFET  160  is conducting, the relay  342  is energized and the normally closed switch  348  remains open and the voltage sensed by the controller  350  at input terminal  352  is logic high. As long as the voltage state of the input terminal  352  is logic high, the controller  350  permits the electric motor  52  to continue to run or permits the power take-off drive  54  to remain engaged. The current sunk by JFET  160  is typically less than 250 mA. 
     If the operator&#39;s hands lose contact with both of the sensing electrodes  120   a,    120   b,  the voltage output, Vo, of pin  2  of the sensor  130  switches to logic high, the JFET  150  switches to a conductive state and the JFET  160  switches off. When the JFET  160  is in a nonconducting state, the relay  342  is deenergized and the normally closed switch  348  closes. Since there is a conductive path to ground through the switch  348 , the controller input terminal  352  is at logic low. The controller  350  switches it output state to shut off the electric motor  52  or disengage the power take-off drive  54 . 
     It should be recognized by those skilled in the art that there are numerous other possibilities for the design of the coupling circuit of the present invention depending on the nature and number of the systems sought to be controlled (internal combustion engine, electric motor, power take-off unit, etc.). For example, a normally closed relay may be used to ground the magneto of an internal combustion engine without the need for a controller should an operator remove both hands from the sensing electrodes  120   a,    120   b.  A unit of mobile power equipment having an internal combustion engine and one or more power take-off drives and/or electric motors may advantageously have a coupling circuit that employs a triac to ground out the internal combustion engine and a relay and controller combination to stop or disengage the electric motor and/or power take-off drive. 
     While the preferred embodiments of the present invention have been described with degree of particularity, it is the intent that the scope of the present invention be construed to include all modifications from and to the disclosed embodiments falling within the spirit or scope of the appended claims.