Capacitive sensing, solid state touch button system

Abstract: A solid state touch or control button assembly ( 10 ), with no moving button parts and operated by capacitive sensing by monitoring the phase shift of a signal applied to the face of the button, including in a first embodiment ( Figs. 2A & 2B ) auto balancing operation and in a second embodiment ( Figs. 3A & 3B ) a constant pressure operation. If skin or other material comes into contact with the touch surface (1), the capacitance is changed and sensed, causing the electrical or electronic function controlled by the touch button to be activated (or deactivated, depending on the design, or otherwise altered). LEDs then are activated, providing visual feedback to the button pusher through a light ring ( 2A ) surrounding the button surface ( 1 ). False activation of the button by residual impedance and external influences, such as temperature change, cleaner residue build-up and other deposits, are avoided by compensating for them by slowly auto balancing to all such phase shifts, with the rate of auto balance being set to be faster than the rate of change of the steady state phase shift; and, additionally, false activations by noise transients, including those due to EMI, RFI and other environmental fluctuations, are avoided by delaying the activation of the button by an appropriate amount of time, such as, for example, of the order of about one hundred (100 msec.) milliseconds.

Description: The present invention relates to touch buttons typically used to activate or deactivate or otherwise control some electrical or electronic function, such as signaling, when touched typically by a human operator touching or "pushing" on the button with a finger. The invention more particularly relates to a solid state touch button system with no moving parts operated by capacitive sensing, which can be used in many different applications, including, for example, as a touch or push button for elevator car calling or control. There is a need for a reliable button that does not use or need moving parts or mechanical contacts, to be used in, for example, elevator applications. It is desirable that such a button be aesthetically pleasing in appearance and use, highly reliable, low in cost and not be activated by extreme environmental changes. It is known that the human body has some amount of capacitance to ground. A basic, previously known concept or approach is to monitor a button face for capacitance to ground, and, if a certain amount (or greater) capacitance is present, to activate the button. In the present invention, this monitoring of the button surface is accomplished by monitoring the phase shift of a signal applied to the face of the button. However, due to the residual impedance of the button and the presence of external influences, the steady state phase shift of the button can fluctuate. Such external influences include, for example, temperature changes, cleaner residue build-up and other deposits or extreme environmental changes, etc. This fluctuation has the potential to falsely activate a button, which is based on the monitoring of phase shift, although in fact no human operator is touching the button. US-3761736 discloses a switch comprising an insulated electrode whose capacitance can be changed. The electrode is coupled to a bridge circuit which has a delayed self-balancing action. DE-3117185 discloses a touch switch circuit which is not affected by temperature changes by means of a self-regulating circuit comprising a differential amplifier with negative feedback. A distinguishing characteristic of such above mentioned exemplary fluctuation is that it would occur slowly over a period of time relative to a phase shift induced by the presence of a person. To avoid this problem, according to the preferred "auto-balancing" embodiment of the invention the steady state phase shift fluctuation is compensated for by slowly auto balancing to all phase shifts. The rate of auto balance is set to be faster than the rate of change of the steady state phase shift. The rate of change of phase shifts induced by a person is faster than the auto balance rate, which allows the button to be operated. A second potential source of false activations of such a button are phase shifts induced by transient noise. These potential transients include, for example, electromagnetic interference (EMI) and radio frequency interference (RFI). A distinguishing characteristic used in the invention to help prevent false activations caused by transient noise is that such noise would not last for a long period of time relative to the interaction time with a person. Activations due to transient noise thus are avoided in the invention by delaying the activation of the button for a certain minimum period of time. Thus the present invention is directed to a reliable button that does not need, and preferably does not use, moving parts or mechanical contacts and can be used in, for example, elevator applications. According to the invention, there is provided a capacitive sensing touch button for controlling a function, said button comprising a button surface member for receiving a reference signal, said button surface member outputting a signal shifted in phase, relative to the reference signal, upon contact of said button surface member by a user; an oscillator to provide the reference signal to said button surface member, said oscillator having a predetermined duty cycle; a phase shift to pulse width converter operatively connected to said button surface member and said oscillator to convert the phase-shifted signal into a pulse width based on the amount of the phase shift relative to the reference signal; auto-balancing compensation means, operatively connected to said phase shift to pulse width converter for avoiding false activation of the controlled function due to residual impedance and/or external influences at said button surface member, said auto-balancing compensation means receiving the output of said phase shift to pulse width converter; characterised in that said auto-balancing compensation means includes at least two integrators, each having a different time constant, each of said integrators receiving the pulse width and converting the pulse width into a dc voltage, said auto-balancing compensation means outputting either a SET signal or a RESET signal based on the relative dc voltage values produced by said integrators thereby avoiding false activation of the controlled function due to residual impedance and/or external influences at said button surface member and in that said button further comprises delay means operatively connected to said auto-balancing means to receive an output signal of said auto-balancing compensation means and to control the controlled function when said signal output by said auto-balancing compensation means is present for at least a predetermined time period, thereby avoiding false activation of the controlled function due to transient noise. The present invention may provide a button that is aesthetically pleasing in appearance and use, highly reliable, is low in cost and is not activated by extreme environmental changes. Finally, and most importantly, the present invention is directed to a phase shift monitoring, capacitive sensing button that avoids false activation of the button by residual impedance and external influences, such as temperature change, cleaner residue build-up and other deposits. The preferred "auto-balancing" embodiment of the invention achieves this by compensating for them by slowly auto balancing to all such phase shifts, with the rate of auto balance being set to be faster than the rate of change of the steady state phase shift. Additionally, the present invention avoids false activations by noise transients, including those due to EMI and RFI by delaying the activation of the button, and the associated operative signal that causes it to be activated, by an appropriate amount of time, such as, for example, a time period of the order of about one hundred (100 msec.) milliseconds. According to the preferred "auto balancing" embodiment, three integrators are used to provide the auto balancing feature of the invention. A "medium" speed integrator provides a "trigger" signal, while relatively "slow" and "fast" integrators provide "set" and "reset" thresholds. In accordance with another aspect of the invention, the purpose of the delay on the timer is for erroneous signal rejection, as generally mentioned above. The erroneous signals of primary concern are those which could be caused by extreme environmental changes. Testing has shown that any set pulses caused by exemplary, extreme environmental changes typically do not last longer than one hundred (100 msec.) milliseconds with an input resistance of, for example, one (1 MΩ) Mega Ohms. Further testing with the input resistor set to, for example, two hundred and sixty-one (261 kΩ) kilo-ohms resulted without any set pulses caused by extreme environmental conditions. From these results a delay "on" time of the order of about one hundred (100) milliseconds is deemed to be appropriate. According to the preferred "auto balance" embodiment the delay time is achieved by a set pulse charging a capacitor through resistors and a diode to a threshold bet by other resistors. The voltage across the charging capacitor will reach the threshold, if a set pulse or series of set pulses transpire without the occurrence of any "reset" pulses. Once the threshold is reached, the output of a comparator is released from common. The voltage on the charging capacitor is pulled up and held high by a hysteresis resistor, pending the occurrence of a "reset" pulse. At any time the voltage across the charging capacitor can be quickly discharged by a "reset" pulse. The voltage across the charging capacitor is discharged through a current limiting resistor, which resets the timer. Benefits of the invention include the facts that the button is: heat resistant; EMI resistant; RFI resistant; ESD resistant; and is capable of providing light and/or other feedback. The invention may be practised in a wide variety of applications, including but not restricted to, elevator car call or control buttons, utilizing known technology, in the light of the teachings of the invention, which are discussed in detail hereafter. Other features and advantages will be apparent from the specification and claims and from the accompanying drawings. Figure 1 is an exploded, perspective view of an exemplary solid state push button. Figures 2A & 2B are interconnected schematic diagrams of an exemplary circuit for the button system of the present invention with auto balancing characteristics, with the two schematics being connected in the "Delay On Timer" block at the "SET/RESET" line; As can be seen in Figure 1 , the exemplary "solid state button" (SSB) 10 of the present invention preferably includes a non-moving, capacitive sensing button surface 1 , that can be used, for example, as a call button in the car operating panel (COP) and/or hall fixtures of an elevator system. The SSB is capable of capacitively sensing a human touch, preferably providing both visual feed back (illumination) to the button pusher, as well as communication to the operational control of the elevator system through, for example, a remote station interface that the button has been actuated, so that the system accordingly can react. The exemplary button of Figure 1 includes the non-moving button element 1 fitted within a light ring element 2 , in which ring is carried a circular array of light emitting diodes (LEDs) 3 at its bottom. A printed circuit board 4 , into which the light ring element 2 is pin inserted, is carried on the back side of the button elements 1 , 2 , and is held to the button elements by a bracket 5 and rear bolt 6 . The interconnecting bolt or stem 7 has a front, threaded, male end 7A , which is screwed into the back side of the button surface 1 , and a rear, threaded, male end 7B , which is screwed into the front end 6A of the rear bolt 6 with a lock washer 7C . The interconnecting bolt 7 extends through a center, circular opening (unseen) in the light ring element 2 and through an opening in the PC board 4 , while the head 6B of the rear bolt 6 fits into a notch in a "U" shaped rear strap 8 , which is part of the bracket holder 5 . When assembled, the intermediate elements of the button assembly 10 are held in compression between the button surface 1 and the head 6B of the bolt 6. The bracket 5 includes a series of peripherally spaced, lateral extensions 9 through which screw pins 9B are placed for fastening the button assembly 10 to a face plate or panel. When so fastened, the only elements of the button assembly 10 which are seen by the user is the non-moving, circular button surface 1 surrounded by the translucent ring 2A, which is lit up by the internally contained LEDs when the button is actuated. It is noted that the touch button elements of Figure 1 are basically symmetrical about their longitudinal center-line, except for the PC board and its associated holder. The printed circuit board 4 carries on it the electronic components and circuitry which perform the SSB monitoring functions of the present invention. According to the preferred embodiment of the present invention an "auto balancing" solid state button (ABSSB) is schematically illustrated in detail in Figures 2A & 2B. As generally noted above, the purpose of the auto balancing feature is to automatically accommodate for static changes in components, packaging and the environment, while maintaining a relatively high sensitivity to sensing when the button actually has been actuated. Due to the basic operation of auto balancing, the ABSSB is not a pure, "constant pressure" type button and, in the exemplary application of an elevator system, is not applied to, for example, "door open," "door close" and other applications requiring a pure constant pressure (CP) feature. If a button touch is constantly maintained for, for example, one hundred (100 msec.) milliseconds, the button module 10 will turn "on" its output and illumination long enough to be read and controlled by the operational control system without loss of the call or illumination. The illumination input preferably is controlled by the operational control system. The ABSSB is reset upon the removal of the illumination control input. The basic functions are as follows: Power Oscillator Phase Shift To Pulse Width Converter Integrators Set & Reset Comparators Electrostatic Discharge (ESD) Protection Delay on Timer Dwell on Timer Output Control High Out Illumination Current Regulator Power Up & Control System Reset Functions Figures 2A & 2B are schematics of the preferred "auto balance" embodiment. The power aspects of the preferred embodiment represents a standard approach and its operation (and many alternatives thereto) are known to those of ordinary skill. - OSCILLATOR - The oscillator generates, for example, a square wave of an appropriate cycle. The threshold voltage to the non-inverting input of comparator U1A is set by resistors R2 & R3 , and the state of the output of U1A , which controls the hysteresis resistor R4 . The "on" state threshold is higher than the "off" state threshold. The oscillator is controlled by the charging and discharging of capacitor C1 . When comparator U1A is in the "on" state, capacitor C1 will charge to the "on" state threshold, and, as a result, the comparator will turn "off ". Conversely, when comparator U1A is in the "off" state, capacitor C1 will discharge to the "off" state threshold, and, as a result, the comparator will turn "on". This is standard oscillator circuitry, and its operation and various alternatives are known to those of ordinary skill. - PHASE SHIFT TO PULSE WIDTH CONVERTER - ** Auto Balance SSB ** The phase shift to pulse width converter functions as follows. The oscillator is fed directly into the non-inverting input of U1C . When the oscillator is high, the output of U1C is released, providing the rising edge of a pulse. The inverting input of U1B monitors the phase shift of the oscillator through resistor R7 across the button input impedance. (The factors which contribute to the button input impedance are the capacitor C3 , the ESD protection circuit and what is applied to or acquired by the button face.) When the voltage at the inverting input of U1B reaches the threshold voltage on the non-inverting input, the output of U1B is pulled to common, at the falling edge of the output pulse of the phase shift to pulse width converter. There is always a pulse, even without anything applied to the button face 1 . The pulse is due to the impedance of capacitor C3 , the ESD protection circuit and any residuals in the circuit. Capacitor C3 is used to prevent any DC voltage from being placed on the button face. The value of the capacitor C3 should be significantly larger than the capacitive sensitivity to be obtained. The larger the value of the resistor R7 , the larger the phase shift is for a given input impedance, including the effects of resistance and noise. The value of the resistor R7 preferably is chosen to generate as large a phase shift as possible from the input. The integrators convert the pulse into a DC voltage. The DC voltage is equal to the duty cycle of the pulse multiplied by "Vcc," the regulated supply voltage. The pulse from the "phase shift to pulse width converter" is fed into three different R-C integrators. Each integrator has a different time constant, providing relatively "slow" (R13 x C4), "medium" ((R14 in parallel with R16) x C5) and "fast" (R15 x C6) time constants. The "medium" integrator has a resistor (R16) in parallel with its capacitor to act as a voltage divider. The voltage divider insures that the steady state DC voltage of the "medium" integrator will be less than that of the "slow" and "fast integrators. The three integrators of Figure 2A provide the auto balancing feature of the invention. The "medium" speed integrator provides the "trigger" signal, while the "slow" and "fast" integrators provide the "set" and "reset" thresholds. The selection criteria for the integrator time constants is given below. The purpose of the set comparator (U2A) is to provide a set pulse resulting from an increase in the pulse width from the "phase shift to pulse width converter." A set pulse is defined as a continuous release, from common, of the set comparator's output for any duration of time. The set comparator will release its open collector output, whenever the DC level of the "medium" integrator is greater than the DC level of the "slow" integrator. The purpose of the reset comparator (U2B) is to provide a "reset" pulse resulting from a decrease in the phase shift to pulse width converter's pulse width. A "reset" pulse is defined as a continuous pull to common of the reset comparator's output for any duration. The reset comparator will pull its open collector output to common, whenever the DC level of the "fast" integrator is less than the DC level of the "medium" integrator. The "slow" integrator time preferably is set as slow as required, in order to maintain the required time constant differences between the integrators and minimize the ripple voltage of the "fast" integrator. The "medium" integrator time constant and voltage divider preferably is chosen to obtain a "set" pulse with a duration equal to the delay "on" time. The "fast" integrator's time constant preferably is chosen to obtain a "reset" pulse upon the removal of a minimal input. A minimal input is defined as the smallest amount of impedance which could cause a set pulse. Exemplary values for the "Integrators" block are set out below: Component Value R13 200KΩ R14 100KΩ R15 100KΩ C4 1µF C5 0.68µF C6 0.047µF The primary part of the electrostatic discharge (ESD) protection circuit is the spark gap SG1 . In the event of an ESD the spark gap will activate and provide a low impedance path to earth via connector J4-1 . Connector J4-1 preferably is connected to the face plate of the button with a short wire. In hall fixture applications the face plate should be connected to the masonry box with, for example, an insulated flat braided conductor. The masonry box in turn should be bonded to building steel through wiring conduit or a flat braided conductor to the closest building steel. The spark gap is a relatively slow device, and, therefore, the zener diode CR1 is used to limit any incoming voltage to, for example, thirty volts (30V). The purpose of resistor R1 is to limit the current through the zener diode CR1 and absorb the major portion of the energy. Resistor R1 may be selected to be of carbon composition due to their pulse energy handling capability and size. On the auto balance SSB the printed circuit (PC) board is conformably coated to prevent uncontrolled arcing on the PC board during an ESD. During an ESD the potential of J4 will rise due to the very high frequency components of the ESD and the inductance of the chassis ground lead. Appropriate spacing is required from the button connection to the button face. The electrical connection to the button face can be achieved with, for example, appropriate wire soldered into the PC board and a ring terminal to the intermediate button stem or interconnecting bolt 7 (note Fig. 1 ), which bolt holds and electrically connects the button face and the PC board. In accordance with the invention the purpose of the delay on timer is for erroneous signal rejection. As generally discussed above, the erroneous signals of primary concern are those which could be caused by heat or other like extreme environmental changes. Testing has shown that any set pulses caused by environmental changes typically do not last longer than one hundred (100 msec.) milliseconds with an input resistance ( R7 ) of, for example, one (1 MΩ) megohms. Further testing with the resistor R7 set to, for example, two hundred and sixty-one (261 kΩ) kilo-ohms resulted without any set pulses caused by extreme environmental change. From these results a delay "on" time of, for example, one hundred (100) milliseconds is deemed to be more than adequate for exemplary purposes. The delay time may be achieved by a set pulse charging capacitor C8 through resistors R17 , R18 and diode CR2 to a threshold set by resistors R20 and R21 . The voltage across capacitor C8 will reach the threshold, if a set pulse or series of set pulses transpire without the occurrence of any "reset" pulses. Once the threshold is reached, the output of comparator U2C is released from common. The voltage on capacitor C8 is pulled up and held high by hysteresis resistor R22 , pending the occurrence of a "reset" pulse. At any time the voltage across C8 can be quickly discharged by a "reset" pulse. The voltage across capacitor C8 is discharged through the current limiting resistor R19 , which resets the timer. - AUTO BALANCE SSB DWELL ON TIMER - The purpose of dwell "on" time is to insure that, once an input to the button face has activated the button, a call will be registered regardless of subsequent changes to the input. When the output of U2C is released from common, the capacitor C9 is quickly charged through R23 and CR3 . Once the voltage across the capacitor C9 reaches the "inactive" threshold, the output of comparator U2D is pulled to common. When the output of U2D is pulled to common, the button is activated. The threshold voltage to the non-inverting input of comparator U2D is set by resistors R24 & R25 and the state of the output of U2D , which controls the hysteresis resistor R27 . The dwell "on" time begins when the input to the button face is removed, and, as a result, the output of U2C is pulled to common. The voltage across the capacitor C9 is discharged through resistor R26 , providing the dwell "on" time. Once the voltage across the capacitor C9 reaches the "active" threshold, the output of comparator U2D is released from common, deactivating the output control. Resistor R16 limits the discharge current to protect the output of U1C . Resistors R17 and R16 create a voltage divider to which capacitor C6 will discharge. Resistor R17 was selected to set the voltage of the divider lower than the "active" threshold voltage. Diode CR7 protects Q6 by limiting the emitter to base voltage. When the output of comparator U1C is released and comparator U1D is pulled to common, a voltage divider is created by R17 , CR7 , and R19 . This voltage divider sets the voltage on the capacitor C6 , while the button 1 is being held in the active state. The difference between the voltage divider and the "active" threshold level provides the "dv" or difference in voltage for the dwell time. The output control provides the active pull to common required by the "high out" and "illumination current regulator" functions. Transistor Q1 of the output control provides the signal inversion required between the output of comparator U2D and the output driving transistor Q2 . Transistor Q2 was chosen to have the current rating required to drive the "high out" and "illumination current regulator" functions. The button assembly 10 is designed to interface to a remote station module in an elevator system. The output to the module is required to be an active high level. The high level is provided through resistor R40 , when the transistor Q3 is driven into saturation by transistor Q2 , pulling resistor R39 to common. Resistor R40 is a current limiting resistor, which protects the transistor Q3 in the case of an accidental shorting of the output to ground. The illumination is controlled by the button 1 through transistor Q2 or the module through connector J1-2 . The illumination is comprised, for example, of two external strings of LEDs being fed by dedicated current regulators through connectors J2 and J3 . The current through each string is regulated to, for example, thirty (30mA) milliamps, by controlling the voltage across the resistors R42 and R43 with zener diode CR7 . The value of resistor R41 was selected to provide the proper current dependent voltage across diode CR7 . The signal diode CR8 is used to isolate the high level output from the illumination control of the remote station module. This isolation allows the operational controller to be signaled, when the button is released. Due to the minimum input voltage requirement of, for example, twenty and four-tenths volts DC (20.4 VDC) the nine light ring LEDs were divided into two strings. - POWER UP AND CONTROL SYSTEM RESET FUNCTIONS - The power up reset insures that the button does not activate due to power outages of arbitrary duration. Upon the application of power, capacitor C10 will charge at a rate set by the resistors R31 and R32 to a threshold set by resistors R33 and R34 . The output of comparator U1D is pulled to common, while the capacitor C10 is charging. The output of the comparator U1D being pulled to common prevents the capacitor C8 from charging, while the integrators charge to their steady state levels. The purpose of diode CR4 is to quickly discharge the capacitor C10 in the event of short power outages. A system reset is provided due to the remote possibility that noise could activate and latch the button 1 . Under normal operation the button 1 , when activated, will provide a high output to its remote station. The operational control system will read in and acknowledge the call by turning on the appropriate remote station output. The output will pull the " /ILLUM. " input to common, turning "on" the illumination. Once the elevator arrives at the floor, the control system typically will check the button output to see if a person is trying to hold the doors open with the call button. If the button output is active, the control system will hold the doors open and leave the button's illumination on. In the event the button is latched active or the person "fell asleep," the operational control system could remove its pull to common on the " /ILLUM. " input, turning the illumination "off" and providing a reset. When the " /ILLUM. " input is pulled to common, the bias across capacitor C11 is reversed. The capacitor C11 will discharge through diodes CR5 and CR6 and current limiting resistor R36 . After discharging, the capacitor C11 will be charged to a voltage set by resistors R33 and R34 . When the " /ILLUM. " input pull to common is removed, the charge across the capacitor C11 will temporarily increase the voltage to the inverting input of comparator U1D . While the voltage of the non-inverting input is greater than the threshold set by resistors R31 and R32 , the output of comparator U1D will be pulled to common discharging the capacitor C8 . Diode CR6 is used to prevent the voltage to the non-inverting input of the comparator U1D from going too far below common, when the bias on the capacitor C11 is reversed. Diode CR6 is used to prevent any current from flowing through resistor R35 from the "Illumination Current Regulator" function. Diode CR9 is used to isolate the reset function from the Output Control. - CIRCUITRY DETAILS - Of course the circuits shown and described are exemplary and subject to great variation. The specific values of each of the resistors, capacitors and diodes are not key to the invention and many workable values of them are available and known to those of ordinary skill. The exemplary solid state button assembly described in detail above is designed to be applied in a hall fixture and car operating panel (COP) of an elevator, although, of course, many other uses and applications are possible. The exemplary unit described is a low cost, easily replaceable device, taking, for example, five (5) minutes to replace. Although this invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes in form, detail, methodology and/or approach may be made without departing from the scope of this invention as defined in the Claims.

Claims: A capacitive sensing touch button (10) for controlling a function, said button (10) comprising: a button surface member (1) for receiving a reference signal, said button surface member (1) outputting a signal shifted in phase, relative to the reference signal, upon contact of said button surface member (1) by a user; an oscillator to provide the reference signal to said button surface member (1), said oscillator having a predetermined duty cycle; a phase shift to pulse width converter operatively connected to said button surface member (1) and said oscillator to convert the phase-shifted signal into a pulse width based on the amount of the phase shift relative to the reference signal; auto-balancing compensation means, operatively connected to said phase shift to pulse width converter for avoiding false activation of the controlled function due to residual impedance and/or external influences at said button surface member (1), said auto-balancing compensation means receiving the output of said phase shift to pulse width converter; characterised in that: said auto-balancing compensation means includes at least two integrators, each having a different time constant, each of said integrators receiving the pulse width and converting the pulse width into a dc voltage, said auto-balancing compensation means outputting either a SET signal or a RESET signal based on the relative dc voltage values produced by said integrators thereby avoiding false activation of the controlled function due to residual impedance and/or external influences at said button surface member; and in that said button (10) further comprises: delay means operatively connected to said auto-balancing means to receive an output signal of said auto-balancing compensation means and to control the controlled function when said signal output by said auto-balancing compensation means is present for at least a predetermined time period, thereby avoiding false activation of the controlled function due to transient noise. The capacitive sensing touch button (10) of claim 1, wherein: said delay means generates a signal to control the controlled function when said SET signal is present for at least a predetermined time period. The capacitive sensing touch button of claim 1 or 2, wherein: said auto-balancing compensation means includes three integrators, a first integrator having a relatively slow time constant, a third integrator having a relatively fast time constant, and a second integrator having a time constant between those of the first and third integrators, each of said integrators receiving the pulse width and converting the pulse width into a dc voltage; said auto-balancing compensation means outputting the SET signal when the voltage output from said second integrator is greater than the voltage output from said first integrator; and said auto-balancing compensation means outputting the RESET signal when the voltage output from said third integrator is less than the voltage output from said second integrator. The capacitive sensing touch button (10) of any preceding claim, said touch button (10) further including electrostatic discharge protection circuitry operatively connected between said button surface member (1) and said phase shift to pulse width converter, said protection circuitry comprising: a spark gap (SG1) connected between said button surface (1) and a first grounding path; a resistor (R1) having a first terminal connected to said button surface member (1) and a second terminal connected to said phase shift to pulse width converter; and a Zener diode (CR1) connected between said second terminal of said resistor (R1) and a second grounding path, wherein said first and second grounding paths are separate.

Inventors: LEACH ROBERT B, SLABINSKI CHESTER J, YOULA JEAN, LEACH, ROBERT B., SLABINSKI, CHESTER J., YOULA, JEAN
Applicants: OTIS ELEVATOR CO, OTIS ELEVATOR COMPANY
Application year: 1990
Main classification: H03K17
IPC codes: B66B1, H03K17