Non-contact capacitive sensing system for use in toys

Abstract

For toys that respond to touch to trigger a particular response, an activation system utilizes a non-contact capacitive proximity sensing system that permits activation when a finger, lips or other body part is close to a sensing area in the form of a hidden flat conductor under the surface of a toy so that actual touching of the sensor is not required to activate any of the functions of the toy. Low capacitance coaxial cable buried in the toy is used to connect the sensing area to the capacitance detection circuit so that only the capacitance of the sensing area is measured. Proximity sensing activation occurs when there is an increase in capacitance at the sensing area due to the proximity of a body part, with the change in capacitance being detected through the use of an RC circuit in the feedback loop of an oscillator whose frequency decreases when sensed capacitance increases. Self-calibrating techniques involving adaptive threshold adjustment provide for fail safe sensing in all environments and across unit-to-unit component variations, with the thresholds being set each time the toy is turned on, then adjusted over time as necessary. In one embodiment, multiple sensing areas are sequentially addressed through a multiplexing circuit and all audio circuitry is turned off during sensing to prevent capacitance sensing errors.

Description

FIELD OF INVENTION

[0001] This invention relates to non-contact capacitative sensing and more particularly to its use in toys.

BACKGROUND OF THE INVENTION

[0002] In most toys, a child's interface with an electronic toy is through the pressing of switches. This results in a rather unnatural interaction with said toy in order to elicit the desired electronic response, such as having to squeeze a doll's hand instead of holding it, or having to press down on a stuffed animal instead of stroking its fur.

[0003] One way to overcome this drawback is through the use of capacitive sensing to sense human touch. Capacitive sensing has been used in many toys in the past to sense touch using conductive areas on the surface of the toy. The basic premise of this technology is that when the conductive area is touched, the capacitance from the person increases the capacitance of the conductive area, which can be sensed in many different ways. The problem with this technology is that a conductive area on a toy is not desirable visually or tactilely. For example, it is difficult to put external conductive sensing areas on a stuffed animal with soft fur, or on the face of a doll, without it being visually and tactilely unappealing. Another disadvantage with this type of touch sensing is that having the conductive areas external to the toy makes it much harder to pass CE ESD immunity standards. Because the user has direct access to the internal circuitry through these conductive areas, it is quite easy to disrupt and/or damage the circuit with static discharge. Another disadvantage with having conductive sensors on surfaces of dolls is that when children dress the doll in clothes, it hides the conductive areas, rendering them useless.

[0004] Another method of proximity sensing which has been used in toys is the sensing of a special stylus which is wired to the toy itself. One drawback to this method is that if the wire to the stylus ever breaks or becomes intermittent through metal fatigue, the toy is rendered useless. Also, for younger children who haven't learned to write with a pencil, the use of a stylus as a pointing instrument is awkward.

[0005] Another method used by toys to sense the user's “touch” without the use of switches is through the use of light-sensing elements embedded within the doll. When the person's hand covers the light-sensing element, the decrease in light level is interpreted as a “touch”. The disadvantage of this technology is that any object that happens to block light to the area is falsely interpreted as a “touch”.

SUMMARY OF THE INVENTION

[0006] This invention also uses capacitive sensing, but differs from the prior art in the fact that the conductive surface is buried inside a toy, and the surface is standard molded plastic, rubber, simulated fur, cloth, or paper, commonly used in toys. The sense areas are undetectable from the outside of the toy, so the visual aesthetics of the toy and tactile textures are preserved. Note that the human body has thin skin and large amounts of fluid inside. Thus, a human body part such as a finger is a relatively good conductor inside the skin, which acts like a bag of water with a thin dielectric covering. The body part thus makes a relatively good capacitor to earth ground. Even though the sensing area is referenced to its own ground, which is the negative terminal of one of the batteries, it has some coupling with earth ground as well. So when a human body part that acts like a capacitor to ground comes close to the sensing area, which is one plate of a capacitor, it changes the frequency of the an RC-controlled oscillator inside the toy enough to be able to detect it.

[0007] Thus, the capacitive sensing used is proximity sensing of human skin rather than touch sensing. This proximity sensing must sense much smaller changes in capacitance than standard touch sensing because of the finite distance between the conductive sensing area and the human skin. Also, this method must be very cost-effective in order to be practical for low-cost toys. This type of sensing allows very natural, intuitive interaction with a toy to reliably trigger an electronic response. For example, a child kissing a doll's cheek or a young toddler pointing at a letter can be detected by the electronic toy and can elicit the desired response.

[0008] In one embodiment, in a self-calibrating sequence when the toy is turned on, the entire capacitive threshold for each sensing area is first set to zero. A capacitive reading is then taken for each sensing area. This reading is the capacitance in terms of the number of oscillator pulses in a predetermined time period.

[0009] If there is no change in sensed capacitance over the appropriate number of tries, a threshold corresponding to the number of oscillator counts is stored. It is against this stored value that subsequent samples of the sensing area are tested. A body part near the sensing area will cause the capacitance to rise, and the oscillator frequency to fall, which results in a decreased number of oscillator pulses and thus a decreased count. When this count is beneath the previously set threshold by a predetermined amount or delta, then a ‘touched’ condition is triggered, and the toy responds appropriately.

[0010] For self-calibration, the capacitance threshold is adaptive in the sense that if the count representing the capacitance sensed is above the previously established threshold for a long enough period, then the threshold is reset to this value.

[0011] Thus for instance, if the sensing area is in the hand of a doll and the child is grasping the hand of the doll when the system is turned on, then the initial threshold will reflect a capacitance of the sensing area plus body part. Later, when the sensing area is sampled and the child is no longer clutching the hand of the toy, the capacitance will go down and the count of oscillator pulses will go up. If this occurs for a number of cycles, then the threshold is incremented to this new higher number.

[0012] Note that a finger, lips or other body part can be sensed when the body part is non-contacting, for instance, ¼-inch away from the sensing area. This means that the sensing area can be buried beneath the skin of the toy and even underneath synthetic fur in the case of a teddy bear, with the toy's actions being triggered by proximity of the body part. Thus, unsightly sensing areas are eliminated, making the toy much more appealing.

[0013] In summary, for toys that are to respond to touch to trigger a particular response, an activation system utilizes a non-contact capacitive proximity sensing system that permits activation when a finger, lips or other body part is close to a sensing area in the form of a hidden flat conductor so that actual touching of the toy is not required to activate any of the functions of the toy. Low capacitance coaxial cable buried in the toy is used to connect the sensing area to the capacitance detection circuit so that the system is shielded from capacitance other than at the sensing area. Proximity sensing activation occurs when there is an increase in capacitance at the sensing area due to the proximity of a body part, with the change in capacitance being detected through the use of an RC circuit in the feedback loop of an oscillator whose frequency goes down when sensed capacitance goes up. Self-calibrating techniques involving adaptive threshold adjustment provide for fail safe sensing in all environments and across unit-to-unit component variations, with the thresholds being set each time the toy is turned on, then adjusted over time as necessary. In one embodiment, multiple sensing areas are sequentially addressed through a multiplexing circuit and all audio circuitry is turned off during sensing to prevent capacitance sensing errors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] These and other features of the subject invention will be better understood in connection with the Detailed Description in conjunction with the Drawings of which:

[0015] FIG. 1 is a diagrammatic illustration of one embodiment of the subject invention in which a child is kissing the cheek of a doll having a sensing area located beneath the skin of the check, also showing buried coaxial cable coupling a sensing area to detection circuitry within the doll;

[0016] FIG. 2 is a top view of a sensing area which is a small piece of copper tape in this embodiment, to which the center conductor of a coaxial cable is soldered;

[0017] FIG. 3 is a block diagram of the subject system indicating low capacitance coaxial cable connecting sensing areas through a multiplexing circuit to an oscillator, the output of which is coupled to an external event counter within a micro-controller that counts the number of positive-going pulses from the oscillator within a certain predetermined period of time, which is stored as the sensed capacitance;

[0018] FIG. 4 is a simplified schematic representation of the Schmitt-trigger RC oscillator and a single sensing area when it is selected by the multiplexer of FIG. 3, the sensing area acting as a variable capacitance to ground in parallel with the fixed capacitance of the coaxial cable, PCB, and semiconductor components, there being a fixed resistor which forms an RC circuit in the feedback path of the Schmitt-trigger inverter which causes the circuit to oscillate at a frequency inversely proportional to the total capacitance; and,

[0019] FIG. 5 is a simplified flow chart showing the programming of the microprocessor of FIG. 3.

DETAILED DESCRIPTION

[0020] This proximity-sensing scheme involves both the capacitance sensing areas themselves, the circuitry used to detect a small change in capacitance, as well as the wiring used to connect the two.

[0021] Referring now to FIG. 1, what is depicted is a child's toy in the form of a doll, here illustrated at 10, in which the doll has a number of sensing areas 12 underneath the skin of the doll. In this embodiment, the sensing areas are in the feet, hands, stomach, eyes, mouth, and cheeks of the doll. Each of these sensing areas is fitted with buried coaxial cables shown in dotted outline at 14 to connect the sensing areas to the control circuitry of the doll.

[0022] Also shown is a child generally indicated at 16 using her lips 18, to activate the doll to perform one of a number of functions by kissing the doll on the cheek. Other types of functions may be activated when the child's body part is adjacent to the other sensing areas so that the doll can be made to respond in different manners to increased capacitance sensed at different sensing areas.

[0023] It will be noted that the sensing areas are buried within the skin of the doll, as are the lengths of coaxial cable to couple the various sensing areas to control circuitry carried within the doll.

[0024] Referring to FIG. 2, one such sensing area is illustrated as including a strip of copper tape 20 to which the center conductor 22 of a coaxial cable 24 is soldered as illustrated at 26. This inexpensive and simple sensing area comprises a sensor which can be used in the proximity sensing described above.

[0025] Referring to FIG. 3, a number of sensing areas 30, 32, 34 and 36 are coupled via low capacitance coaxial cable 40 to a multiplexing circuit 42 shown symbolically as having a number of taps 44 contacted through a wiper arm 46 to a common tap 48. Each of these contact points is connected to the center conductor of the associated coaxial cable, with a resistor R connected between the center conductor of this coaxial cable and the output of an oscillator 50. In one embodiment, this oscillator is a Schmitt-trigger inverter acting as an RC-controlled oscillator. It will be appreciated that the frequency of the output pulses generated by this Schmitt-trigger inverter are inversely proportional to the capacitance at point 44 of the selected tap, with the output of the oscillator coupled to an external event counter 56 within a micro-controller 58, with the micro-controller controlling the addressing of the sensing areas as illustrated by signals 60. In this embodiment, the micro-controller has an associated 6-megahertz crystal 68 and a speaker 70 for an audio output, should such be desired.

[0026] The microprocessor continually scans each of the sensor areas using the multiplexer to select each one sequentially. When a body part comes into proximity of a sensing area, micro-controller 58 can sense the increase in capacitance of the particular sensing area and cause the toy to respond in a preprogrammed way, depending on the sensing area activated. For instance, if the user touches the doll's lips, the doll may be made to make a kissing sound, emanating from speaker 70. Additionally there are many types of movements or sounds that the doll can make, depending on what is preprogrammed into micro-controller 58.

[0027] Referring to FIG. 4, oscillator 50 has as its output a frequency which is inversely proportional to the total capacitance. The total capacitance is the sum of the variable capacitance Cv, and the fixed capacitance Cf. Fixed capacitance Cf is due to the capacitance of the low capacitance coaxial cable, circuit board, and other electrical components in the circuit, whereas Cv is variable and depends upon the proximity of a body part near a sensing area. The RC circuit is formed by a fixed resistor R in the feedback path of the Schmitt-trigger inverter oscillator such that the frequency of the output of the oscillator is changed by variable capacitor Cv due to the proximity of a human body part to the sensing area.

[0028] In operation, the capacitance sensing areas are small conductive surface areas made of conductive tape, copper-clad-PCB, flat copper braid, or any of the many available low-cost conductive materials used in electronics manufacturing. The material can be chosen based on cost, manufacturability, and the tactile quality desired. If the sensing area is to be used in a plush stuffed animal, for example, the sensing area should be soft and malleable, such as copper tape or flat copper braid, as to be undetectable from the outside when the stuffed animal is squeezed. The surface area used in the preferred embodiment is approximately one square inch.

[0029] Another feature of the subject invention is the method by which the above sensing areas are wired to the detection circuitry, which in one embodiment is on a central circuit board in the toy. The sensing areas are likely to be spread out all over the toy, and may be over twelve inches from the circuit board. Low-capacitance coaxial cable, such as low-cost standard 75-ohm video coax cable, can be used. The center conductor of the coax cable is used to connect the capacitive sensing area to the detection circuitry. The outside shield of the coax cable is connected to ground, and this prevents the detection circuitry from detecting false capacitance changes due to human skin near the cabling itself. This method keeps the capacitance sensing localized to the sensing area only.

[0030] The detection circuitry must be able to reliably sense very small changes in capacitance at the remote sensing area, usually a few picofarads. This small increase in capacitance is a small percentage of the total capacitance of the coax cable, input capacitance of the detection circuitry, and stray capacitances on the circuit board. It would be possible for a very fast micro-controller to time how long it takes to charge this capacitance through a resistor with fine enough resolution to detect this small change in capacitance.

[0031] However, the preferred embodiment of the invention uses an RC oscillator scheme to allow a low-speed, low-power micro-controller to detect this small change. In the illustrated embodiment, micro-controller 58 allows oscillator 50 to run for a pre-determined amount of time, and counts how many low-to-high transitions occurred. This allows the minute difference in oscillator frequency to add up over many cycles, making it easy for a slower micro-controller to accurately detect the percentage of change in the capacitance.

[0032] The detection circuitry consists of a single Schmitt-trigger inverter acting as the RC oscillator 50, which oscillates at a frequency inversely proportional to the capacitance that it is connected to. There is an analog multiplexer 42 which selects which sensing area is connected to the oscillator. A transistor circuit in an emitter-follower configuration on each sense area may be used which acts as an analog buffer in order to greatly reduce the impedance so that the large capacitance of the analog multiplexer does not affect the oscillation frequency. The output of oscillator 50 is connected to the external event counter 56 input on the micro-controller. Note that a low-dropout regulator may be used which regulates the voltage to the analog oscillator/multiplexer circuit and the micro-controller. This helps to keep the oscillator frequency from drifting over time, and reacting to noise on the battery supply. All of the components used in the circuit are commonly available, mature, low-cost components.

[0033] The software algorithm used in the micro-controller in the preferred embodiment is described in the flowchart of FIG. 5.

[0034] Referring now to FIG. 5, this flow chart represents the algorithmic operation of the subject system. In the flowchart and elsewhere, the term ‘PAD’ is used to refer to a sensing area, and the two terms are interchangeable. Also, the term ‘touched’ is used to indicate when a person's skin is near enough to the sensing area to trigger the capacitive sensing mechanism. The person doesn't necessarily have to be ‘touching’ the sensing area for this to occur, since it is proximity sensing. However, the word ‘touched’ is used throughout this patent in order to make it easier to understand.

[0035] The capacitance reading that will be referred to in FIG. 5 reflects the actual capacitance at the sensing area, and is therefore inversely proportional to the actual number of oscillations counted by the microprocessor. The microprocessor sets the multiplexer to select the sensing area in question, then counts the number of oscillations based on its capacitance in a predetermined period of time, then takes the inverse of that count to arrive at the capacitance reading. For example, assume that the counter is an 8-bit counter, the predetermined period is 1 ms (one millisecond), and that a particular PAD has a quiescent ‘untouched’ capacitance such that it oscillates at 200 kHz when the PAD is connected to the RC oscillator. When the microprocessor selects this PAD using the multiplexer, it will read a count of 200 when it counts for 1 ms. In one embodiment, the microprocessor could subtract this count from the 8-bit maximum, 255, in order to arrive at a ‘capacitance’ reading, in this case a value of 55. When this PAD is touched, the capacitance will rise, and cause the oscillator frequency to fall, let's say to 180 kHz. Now, when the microprocessor takes a reading of this PAD, it will get a count of 180 in 1 ms. Subtracting this from 255, it would arrive at a ‘capacitance’ reading of 75. As shown in this example, the capacitance reading as referred to in FIG. 5 reflects the actual capacitance of the sensing area, not the RC oscillation frequency or the raw oscillation count.

[0036] More particularly, the micro-controller starts up in a power-up block 80, then advances to a threshold initialization block 82, where thresholds of all of the sensing areas are initialized to the maximum value. Next, the microprocessor advances to a multiplexer initialization block 84, where it selects the first of the sensing areas. In this case, the first pad is selected. As can be seen by block 86, the capacitive sensing algorithm is inhibited if audio is currently being played through a speaker. If audio is not currently playing, the counter is allowed to count the external pulses from the oscillator during a given time interval, as illustrated at 90. The algorithm prevents readings from being taken while a sound is being played through the speaker so that any electrical noise created on the board due to high current spikes from the audio playback do not give false readings.

[0037] As illustrated in block 90, the current reading is saved, and at a decision block 94, it is ascertained if the capacitance reading for the current pad is less than the pad's calibrated threshold. It should be noted that the thresholds for all sensing areas are initially set to the maximum value, so that block 94 is always true when the toy has just been turned on.

[0038] If, as illustrated at block 94, the current capacitance reading for the given pad is smaller than its threshold, this indicates that the threshold may need to be reset. It is first determined if there were enough readings below the threshold to warrant a new threshold value, as seen in block 122. This debounce feature of requiring X number of consecutive readings below the threshold is utilized to prevent a single noisy reading from erroneously adjusting the threshold. If there hasn't been X number of consecutive readings below the threshold, then this pad is marked as ‘untouched’, and the threshold value is unchanged, as illustrated in block 112. If there has been X number of consecutive readings below the threshold, then this pad is marked as ‘untouched’, and the current capacitance reading is established as the new threshold as seen in block 124, and the process iterates back.

[0039] In order to ascertain if the current pad is being ‘touched’, a determination is made at block 98 whether the current reading is above this pad's threshold by a given amount. If so, the pad is marked as being touched.

[0040] As illustrated in block 98, what constitutes a sensing area being touched is that the current reading minus the current threshold is larger than a predetermined delta value. If so, then the PAD is considered ‘touched’, as illustrated at 100. The delta value being set to a large enough constant so that the system is not triggered by noise, which causes minor changes in the sensed capacitance readings. It should be noted that this delta value may be set to a different value for each sensing areas. This is useful for deliberately setting the touch sensitivity differently on the various sensing areas. In block 98, if the current reading is not larger than the threshold, or the difference is less than the predetermined delta, then the pad is marked as ‘untouched’ as indicated at 112 and at the micro-controller proceeds to block 102.

[0041] Below is an example of how the threshold initially adjusts to the quiescent capacitance of each sensing area, and why the continual adaptive threshold algorithm is important. When the unit is first turned on, all thresholds are set to maximum value. At this time, each PAD is always going to have a reading that is less than the threshold in block 94. This is how the system automatically stores the initial quiescent settings for each pad. For instance, let's assume that the current capacitance reading is 100 on the left hand of the doll, the threshold starts at 255. The microprocessor will continually read 100 for the left hand sensing area until block 122 is true, and 100 is now set as the new threshold value to which all subsequent readings of the left hand sensing area is compared.

[0042] The adaptive threshold algorithms is also important in the case when, for instance, the left hand of the doll is touched when the unit is first turned on. The quiescent ‘untouched’ reading should be 100, but the sensing area repeatedly returns a reading of 120 because the left hand is being held by the child, so the threshold gets set to 120. When the child lets go of the hand, the reading will jump down to 100 and stay there. When that happens, the algorithm notices that the reading is less than the stored threshold, and the ‘untouched’ reading of 100 will now correctly be stored as the new threshold.

[0043] In one embodiment, even if a PAD is determined to be ‘touched’, it is not acted upon immediately. All of the PADS are read in a single scanning cycle before the appropriate response is determined. This scheme allows for multiple-PAD detection. For example, if only one hand is touched, the doll may say “I love to hold hands with Mommy”, but if both hands are held, the doll may react differently, for example, by singing “Ring around the Rosie”. Block 102 checks to see if all of the PADS have been read, and if not, the next PAD is selected in block 108 and the reading process is repeated. If at block 102, it is determined that all PADS have been read, then block 104 resets the multiplexer to the first PAD. Block 106 takes into consideration which PAD or combination of PADS were marked as ‘touched’, and triggers the appropriate response.

[0044] In summary, the subject invention has sensing areas that can detect when human skin is in close proximity. All sensing areas are inside of a toy, where it is visually and tactilely undetectable by the user. Moreover, the software algorithm self-calibrates all of the sensing areas each time it is turned on, so the absolute capacitance of a given sensing area, its cabling, and detection circuitry are irrelevant. Moreover, The sensitivity of each area can be set separately.

[0045] While the subject system has been described in connection with its use within a doll, it will be appreciated that the subject system is useful anywhere that proximity sensing is required. It will be noted that the system may be activated by a person's finger or other body part which is spaced from the actual sensor itself. This makes burying of the sensors for whatever reason practical so that a covering or other layer of material may be interposed between the sensor and the body part doing the activation of the system.

[0046] Having now described a few embodiments of the invention, and some modifications and variations thereto, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by the way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention as limited only by the appended claims and equivalents thereto.

Claims

1. In a battery-operated toy, a system for sensing the proximity of a human body part to a sensing area at said toy for the activation of a selected toy response, comprising:

A non-contact capacitance sensing unit within said toy including an electrically conductive sensing pad, a proximity sensing circuit for the activation of said toy response and a means for connecting said pad to said proximity sensing circuit, whereby said pad may be buried beneath the outer covering of said toy to avoid unsightly visible activation apparatus.

2. The system of claim 1, wherein said means for connecting said pad to said proximity sensing circuit includes a shielded conductor, whereby capacitance sensing is localized to changes in capacitance at said pad.

3. The system of claim 1, wherein said toy includes multiple pads distributed about said toy and wherein said proximity sensing circuit includes a multiplexer for sequentially accessing said pads.

4. The system of claim 1, and further including an automatic calibrating unit for sensing capacitances absent the proximity of a body part and for setting a corresponding capacitance threshold level, the calibrating unit being self-adaptive.

5. The system of claim 4, wherein said proximity sensing circuit includes an RC-controlled oscillator and a counter coupled thereto, the frequency of said oscillator being inversely proportional to the capacitance associated with said pad, and wherein said automatic calibrating unit has said corresponding capacitance threshold level established by the count in said counter.

6. The system of claim 1, wherein said selected toy response is a predetermined sound, and wherein said toy has a sound generator for generating said predetermined sound when activated by said toy, and further including means for inhibiting said proximity sensing circuit when said sound generator is activated, thereby to eliminate any false readings of capacitance due to the electrical noise of said sound generator during capacitance sensing.

7. The system of claim 4, wherein said calibrating unit is activated when said toy is turned on to establish said capacitance threshold level.

8. The system of claim 5, wherein said calibrating unit includes means for adaptively setting said capacitance threshold level.

9. The system of claim 8, wherein said means for adaptively setting said capacitance threshold level includes a unit for setting said capacitance threshold level to a maximum value, means for taking a current capacitance reading, means for ascertaining if the current capacitance reading is less than said threshold and means responsive thereto for saving the current capacitance as the new threshold level.

10. The system of claim 9 and further including means for establishing if the current capacitance reading is larger than said threshold by a predetermined margin and for activating said toy response responsive thereto.

11. A non-contact capacitance sensing system, comprising:

an electrically conductive patch;

a length of coaxial cable having a center conductor coupled to said conductive patch;

an RC-controlled oscillator coupled to said coaxial cable and having an output frequency inversely proportional to the capacitance associated with said patch;

a counter coupled to the output of said oscillator; and

a threshold circuit coupled to the output of said counter for indicating the presence of a body part adjacent to said patch when the count from said counter varies from said threshold, whereby the adjacency of said body part to said patch is sensed.

12. Apparatus for triggering a predetermined response in a toy, comprising:

a non-contact capacitive proximity system having a conductive pad for sensing the proximity of a body part adjacent said pad; and,

a threshold circuit for generating a trigger when the proximity of a body part is sensed.

13. The apparatus of claim 12, wherein the threshold set by said thresholding circuit is adaptively set.

14. The apparatus of claim 13, wherein said adaptively set threshold is set when said toy is turned on.

15. The apparatus of claim 14, wherein said adaptively set threshold is further adjusted after the original setting when said toy is turned on.

16. The apparatus of claim 15, wherein capacitances associated with said pad are periodically monitored, with said threshold being adaptively set when sensed capacitance associated with said pad is sufficiently different from a previously established threshold.

17. The apparatus of claim 16, wherein said threshold is reset if the current sensed capacitance is lower than that associated with said threshold, whereby a capacitance threshold that is set during the proximity of a body part is adjusted upon removal from proximity of said body part from said pad.