NZ503599A - Touch sensor with strobe electrode and circuitry - Google Patents

Abstract

A touch proximity sensor includes first and second electrodes 16, 18 and a strobe electrode 22 adjacent the other electrodes and energized with a square wave signal of 5 volts at 30 kHz. The electric field associated with the electrodes 16, 18 is generated in response to the signals provided to the strobe electrode and is affected by the presence of a stimulus in proximity to the electrodes 16, 18. The differential circuit 32 with a plurality of transistors Q1 and Q2 is referenced to the strobe signal and produces an output signal on terminal 24 indicative of the presence of a stimulus in proximity to the electrodes 16, 18.

Description

Patents Form No. 5 Our Ref: JT213459 This is a divisional out of application number 331543 dated 8 December 1997 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION DIFFERENTIAL TOUCH SENSORS AND CONTROL CIRCUIT THEREFOR We, TOUCHSENSOR TECHNOLOGIES, L.L.C., a Delaware limited liability company, USA of 201 North Gables Road, Wheaton, Illinois 60187, United States Of America, United States Of America hereby declare the invention, for which We pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: PT0578010 (followed by page 1 a) -1 a- DIFFEREHTIAL TOUCH SENSORS AND CONTROL CIRCUIT THEREFOR of -the Invention The present invention relates to a touch panel 5 system, and more particularly, to touch sensors attached to one side of a substrate for detecting user contact of the opposite side of the substrate.

Background of the Invention Touch panels are used in various applications 10 to replace conventional mechanical switches: e.g., kitchen stove, microwave ovens, and the like. Unlike mechanical switches, touch panels contain no moving parts to break or wear out. Mechanical switches used with a substrate require some type of opening through the 15 substrate for mounting the switch. These openings, as well as openings in the switch itself, allow dirt, water and other contaminants to pass through the substrate or become trapped within the switch. Certain environments contain a large number of contaminants which can pass 20 through substrate openings, causing electrical shorting or damage to the components behind the substrate. However, touch panels can be formed on a continuous substrate sheet without any openings in the substrate. Also, touch panels are easily cleaned due to the lack of 25 openings and cavities which collect dirt and other contaminants. ^ Existing touch panel designs provide touch pad electrodes attached to both sides of the substrate; i.e., on both the "front" surface of the substrate and the 30 "back" surface of the substrate. Typically, a tin antimony oxide (TAO) electrode is attached to the front surface of the substrate and additional electrodes are SUBSTITUTE SHEET (RULE 26) (followed by page 2) attached to the back surface. The touch pad is activated when a user contacts the TAO electrode. Such a design exposes the TAO electrode to damage by scratching, cleaning solvents, and abrasive cleaning pads. Further-5 more, the TAO electrode adds cost and complexity to the touch panel.

Known touch panels often use a high impedance design which may cause the touch panel to malfunction when contaminants such as water or other liquids are present 10 on the substrate. This presents a problem in areas where liquids are commonly found, such as a kitchen. Since the pads have a higher impedance than the water, the water acts as a conductor for the electric fields created by the touch pads. Thus, the electric fields follow the path of 15 least resistance; i.e., the water. Also, due to the high impedance design, static electricity can cause the touch panel to malfunction. The static electricity is prevented from quickly dissipating because of the high touch pad impedance.

Existing touch panel designs also suffer from problems associated with crosstalk between adjacent touch pads. The crosstalk occurs when the electric field created by one touch pad interferes with the field created by an adjacent touch pad, resulting in an erroneous 25 activation such as activating the wrong touch pad or activating two pads simultaneously.

Known touch panel designs provide individual pads which are passive. No active components are located in close proximity to the touch pads. Instead, lead lines 30 connect each passive touch pad to the active detection circuitry. The touch pad lead lines have different lengths depending on the location of the touch pad with respect to the detection circuitry. Also, the lead lines have different shapes depending on the routing path of the 35 line. The differences in lead line length and shape cause the signal level on each line to be attenuated to a SUBSTITUTE SHEET (RULE 26) different level. For example, a long lead line with many corners may attenuate the detection signal significantly more than a short lead line with few corners. Therefore, the signal received by the detection circuitry varies 5 considerably from one pad to the next. Consequently, the detection circuitry must be designed to compensate for large differences in signal level.

Many existing touch panels use a grounding mechanism, such as a grounding ring, in close proximity 10 to each touch pad. These grounding mechanisms represent additional elements which must be positioned and attached near each touch pad, thereby adding complexity to the touch panel. Furthermore, certain grounding mechanisms require a different configuration for each individual 15 touch pad to minimize the difference in signal levels presented to the detection circuitry. Therefore, additional design time is required to design the various grounding mechanisms.

The use of conventional touch panels or touch 20 sensors in stoves, microwave ovens, and the like, places such touch sensors in an environment where they can potentially come into frequent contact with conductive liquids or contaminants. The presence of a conductive liquid on any touch sensor could create a false output 25 thereby causing the control circuit to initiate an output action where none was intended. Such liquids, when in the form of a large puddle or drops, can actually span two or more individual touch sensors. This again leads to the potential for false input signals. 30 Recent improvements in touch panel design include techniques which lower the input and output impedance of the touch sensor itself, thereby making the sensors highly immune to contaminants and false activations due to external noise sources. U.S. Patent No. 35 5,594,222 describes such a technique. Even though this approach has several advantages over the prior art, there SUBSTITUTE SHEET (RULE 26) are some attributes that may limit its application. For instance, the resulting sensor may be inherently sensitive to temperature variations. As long as the temperature variations at the output are small relative to legitimate 5 signal changes and are small relative to signal variations due to transistor variations, then a single transistor or other amplifying device will be quite satisfactory. However, in applications where there is little dynamic range to allow for compensation by software and where 10 temperature changes are significant relative to legitimate ^ signal changes, another approach would be useful to eliminate or greatly reduce the effects of temperature. Also, even though the low impedance approach of this technique can differentiate between contaminants with some 15 finite amount of impedance and a human touch with some finite amount of impedance, this technique may not be enough to inherently differentiate extremely low levels of impedance. Such examples of this situation would exist when a sensor (i.e., both the inner and outer electrode) 20 is covered with a large amount of contaminants, greatly reducing the impedance of the inner pad. Another example would be where a conductive material such as a metal pan covers an entire singular sensor. ) Thus, it would be desirable to provide a touch panel which prevents false signal generation in the presence of highly conductive materials, relatively substantial temperature changes, and other effects common to both the inner and outer electrode and associated circuitry, or at least provide the public with a useful alternative. intellectual property office of nz. 1 3 SEP 2001 RECEIVED 4a Summary of Invention A first aspect of the invention provides a sensor apparatus for detecting the presence of a stimulus to be detected, said apparatus comprising: at least one first electrode; a strobe electrode positioned in proximity to said at least one first electrode; a signal source for providing electrical signals to said strobe electrode; wherein an electric field associated with said at least one first electrode is generated in response to the signals provided to said strobe electrode by said signal source; wherein the electric field is affected by the presence of the stimulus in proximity to said at least one first electrode; a circuit having a first input node connected to said at least one first electrode, a second input node connected to a reference electrical potential, and an output node; wherein said circuit generates a signal at its output node relating to the difference in voltage at the first and second input nodes, whereby the signal at the output node relates to the presence of the stimulus in proximity to said at least one first electrode; and wherein said circuit comprises a plurality of active components A second aspect of the invention provides a sensor apparatus for detecting the presence of a stimulus to be detected, said apparatus comprising: at least one first electrode; a strobe electrode positioned in proximity to said at least one first electrode; a signal source for providing electrical signals to said strobe electrode; 50359 intellectual property office of nz 1 3 SEP 2001 RECEIVED 503599 wherein an electric field associated with said at least one first electrode is generated in response to the signals provided to said strobe electrode by said signal source; wherein the electric field is affected by the presence of the stimulus in proximity to said at least one first electrode; a low-impedance circuit having a first input node connected to said at least one first electrode, a second input node connected to a reference electrical potential, and an output node; and wherein said circuit generates a signal at its output node relating to the difference in voltage at the first and second input nodes, whereby the signal at the output node relates to the presence of the stimulus in proximity to said at least one first electrode; and wherein said circuit comprises a plurality of active components A third aspect of the invention provides a sensor apparatus for detecting the presence of a stimulus to be detected, said apparatus comprising: at least one first electrode; a signal source for providing electrical signals to said at least one first electrode, for providing electrical signal to said at least one first electrode; wherein an electric field associated with said at least one first electrode is generated in response to the signals provided to said at least one first electrode by said signal source; wherein the electric field is affected by the presence of the stimulus in proximity to said at least one first electrode; a circuit having a first input node connected to said at least one first electrode, a second input node connected to a reference electrical potential, and an output node; and wherein said circuit generates a signal at its output node INTELLECTUAL PROPERTY OFFICE OF NZ. 1 3 SEP 2001 RECEIVED 5035 relating to the difference in voltage at the first and second input nodes, whereby the signal at the output node relates to the presence of the stimulus in proximity to said at least one first electrode; and wherein said circuit comprises a plurality of active components In its preferred form, the present invention greatly improves, if not completely solves, the above mentioned problems by providing for a comparison between two electrodes that make up a touch sensor. The inventive touch sensor has one or more first electrodes and one or more second intellectual property office of n z. 1 3 SEP 2001 RECEIVED electrodes coupled to a circuit means for measuring the difference in electrical potential between the first and second electrodes. The first and second electrodes typically would be placed on the same surface of the 5 substrate, opposite the side of the substrate that would be used as the touch surface. The first electrode is spaced proximate to the second electrode such that a comparison can be made between the voltage on the first electrode and the second electrode when affected by a 10 touch input. The differential measuring circuit will provide for the rejection of common mode signals such as temperature, electrical noise, power supply variations, and other inputs that would tend to affect both electrodes equally.

The inventive touch pad can be used in place of existing touch pads or to replace conventional switches. The touch pad is activated when a user contacts or approaches near to the substrate with a human appendage, such as a fingertip. The touch pad can be used, for 20 example, to turn a device on or off, adjust temperature, set a clock or timer, or any other function performed by a conventional switch. In addition to improving and solving problems associated with existing touch pad designs, the present invention also is useful in applica-25 tions which presently use membrane switches. The touch pad of the present invention is well suited for use in environments where temperature variations are extreme, where substantial amounts of contaminants are present or where metal objects may be placed on or over the touch 30 pad.

In the preferred form, a strobe electrode is connected through a first resistor to a first electrode and through a second resistor to a second electrode. An electric field is generated at each electrode in response 35 to a strobe signal being applied to the strobe electrode.

An electric potential is developed at each electrode. Two SUBSTITUTE SHEET (RULE 26) transistors are arranged in a differential measuring circuit which is connected to the first and second electrodes for measuring the difference in voltage between the first and second electrodes. A sense line is attached 5 to the output of the differential measuring circuit, which in the preferred embodiment carries a detection signal to a peak detector circuit. The output of the differential measuring circuit is altered when the substrate is touched by a user.

In the preferred form, two matched transistors are configured as a differential pair, each located in close proximity to the touch pad. The transistors work together to amplify the differential input signal, to buffer the touch pad from the effects of strobe and sense 15 traces, and to reduce the output impedance of the touch pad. Also, by using matched transistors, the output of the differential circuit will change little with temperature variations.

The inner and outer electrodes are connected to 20 separate inputs to the differential circuit such that when a first electrode is affected more by the induced electric field than the second electrode, the differential circuit will provide a higher output voltage level. Also, in the preferred embodiment the circuit will generate a lower 25 output when the second electrode is affected more than the first electrode by the electric field. When both electrodes generate equal or similar responses, the output of the differential circuit will change little. These conditions will be created, for example, when a fingertip 30 substantially covers the first electrode but not the second electrode. This will generate a higher output signal. Another condition is created when contaminates substantially cover the second (outer) electrode but not the first (inner) electrode. This will generate a lower 35 level output signal. Another condition would be when a metal pan covers both of the first and second electrodes.

SUBSTITUTE SHEET (RULE 26) Given this condition, in the preferred embodiment, the response of the two electrodes will be substantially equal and, therefore, the output of the differential measuring circuit will change little from the previous no-touch 5 condition.

TVHp.-F Description of The Drawing The various features, advantages and other uses of the present invention will become more apparent by referring to the following detailed description and 10 drawings in which: FIG. 1 illustrates an inventive touch pad as viewed from the back surface of the substrate; FIG. 2 is a cross-sectional view generally taken along line 2-2 in FIGURE. 1; FIG. 3 is a cross-sectional view, similar to FIGURE 2, but showing an alternate mounting of the active components to the substrate; FIG. 4 is an electrical schematic representation of the touch pad shown in FIGURES 1 and 2; 20 FIG. 5A, 5B, 5C and 5D are waveforms of the sense output under various input stimuli; FIG. 6 illustrates the strobe signal waveform; and FIG. 7 is a view, similar to FIGURE 1, but of an alternate embodiment of the inventive touch pad. 25 Similar indicia numbers in FIGS. 1, 2, 3, 4, and 7 indicate similar elements.

Desc-ription of The Preferred Embodiments Referring to FIGS, l and 2, a single touch pad 13 is shown attached to a dielectric substrate 10. It 30 should be understood that many, if not most, applications will include multiple touch pads and related circuitry on the substrate.

Substrate 10 can be manufactured from any' type of dielectric material, such as glass, ceramic, plastic SUBSTITUTE SHEET (RULE 26) or* similar materials. In the preferred embodiment, substrate 10 is manufactured from glass and has a uniform thickness of approximately 3 mm. The thickness of substrate 10 varies with the particular application such 5 that a thicker substrate is used where additional strength is required. If substrate 10 is manufactured from glass, typical substrates can be as thin as approximately 1.1 mm and as thick as approximately 5 mm. If substrate 10 is manufactured from plastic, the substrate can be less than 10 l mm thick, similar to the material used in plastic membrane switches.

Substrate 10 has a front surface 12 and an opposite back surface 14. A user activates the touch pad 13 by touching front surface 12 of substrate 10, providing 15 the necessary stimuli.

The touch pad 13 includes a first, conductive or inner electrode pad 16 and a second, conductive or outer electrode 18 which substantially surrounds the first electrode. A space is located between first electrode 16 20 and second electrode 18. Preferably, first electrode 16 has dimensions such that the electrode may be covered by a user's fingertip or other human appendage when the front surface is touched.

In the preferred embodiment, first electrode 16 25 is square and second electrode 18 has a square shape which conforms to the shape of the first electrode 16. However, it will be understood that various geometric shapes may also be used for the first electrode 16 including, but not limited to, rectangles, trapezoids, circles, ellipses, 30 triangles, hexagons, and octagons. Regardless of the shape of first electrode 16, second electrode 18 at least partially surrounds the first electrode 16 in a spaced apart relationship.

It may be recognized that even though the pad 35 geometry in FIG. 1 is one way to arrange the electrode structure, there are many other shapes and sizes that SUBSTITUTE SHEET (RULE 26) would work also, depending on the application and size of the appendage. One example could be an arrangement where a hand might be the appendage of interest instead of a finger. In this case, the spacing between the two electrodes could be spaced farther apart and the two electrodes would be much larger.

Similarly, it may be recognized that even though the pad geometry in FIGS. 1 and 7 each show specific ways to arrange the electrode structure, there are many other shapes and sizes that would work here also, depending on the application and size of the appendage. One example would be where the two electrodes are spaced farther apart and the two electrodes are larger.

Preferably, first electrode 16 is a solid conductor. However, first electrode 16 may also have a plurality of apertures or may have a mesh or grid pattern.

In the preferred embodiment, a third electrode, strobe electrode 22 is provided, as Shown in FIG. 1. The strobe electrode 22 is a thin conductor formed on the substrate 10. The strobe electrode 22 is spaced across from the second electrode 18. Preferably, the strobe electrode 22 is spaced from both sides of the second electrode 18 as shown in FIG. 1. The strobe electrode 22 is also adjacent the first electrode 16. In this manner, one portion of the strobe electrode 22 is spaced between the second electrode 18 and the first electrode 16 such that the single strobe electrode 22 acts as a strobe line for both the first electrode 16 and the second electrode 18, as seen in FIG. 1.

As shown in FIG. 1 the strobe electrode or line 22 is connected to a voltage source 60.

Strobe line 22 carries a strobe signal such as, for example, a square wave in the preferred embodiment, (shown in FIG. 6) from a source 60. In the square preferred embodiment, the wave oscillates between 0 and +5 volts at a frequency between 25 khz and 50 khz.

SUBSTITUTE SHEET (RULE 26) Alternatively, the strobe signal may have a frequency less than 25 khz or greater than 50 khz, depending on the detection circuitry used. Furthermore, the strobe signal may oscillate between 0 and +3 volts, 0 and +12 volts, o and +24 volts, -5 volts and +5 volts, or any other voltage range, depending on the voltage readily available from the device being controlled.

Preferably, the strobe signal has a rise time of approximately 7 nsec. However, rise times up to 110 nsec or even larger may also be used. Faster rise times, such as 7 nsec, provide lower input impedances and may be preferred. The strobe signal creates an electric field at the touch pad, as described hereinafter.

The strobe signal has a sharp rising edge 15 (shown in FIG. 6) which creates a difference in the electrical potential between the strobe line 22 and each of second electrode 18 and first electrode 16. This difference in potential between electrodes 15, 18 and 22 creates an arc-shaped electric field between the elec-20 trodes, as shown by the dashed lines in FIG. 2. The electric field extends past front surface 12 and through substrate 10. Although not shown in FIG. 2, the electric field between electrodes 16, 18 and 22 follows a similar arc-shaped path away from the back surface 14 of the 25 substrate 10. This path is almost a mirror image of the dashed lines shown in FIG. 2, extending downwardly rather than upwardly.

As shown in FIG. 2, the electric fields created are in opposition to one another. For example, the field 30 paths shown in FIG. 2 originate from strobe electrode 22, at opposite sides of the first electrode 16, and from strobe electrode 22 to second electrode 18.

Referring again to FIG. 1, a sense or output line 24 is attached to substrate 10 connected to the 35 output of differential circuit 32, which is described hereinafter. Sense line 24 carries a detection or operate SUBSTITUTE SHEET (RULE 26) PCT/U S97/22738 signal from the touch pad 13 to activate suitable detection or control circuitry as described in detail in my United States Letters Patent 5,594,222 which issued January 14, 1997, the contents of which is incorporated herein.

As shown in FIGS. 1,2 and 4, surface mount components are electrically connected to the touch pad 13. The surface mount components include resistor 28 connected between the strobe electrode 22 and the second electrode 18, and resistor 30 connected between the first electrode 16 and the strobe electrode 22. The resistors 28 and 30 may have a value of 2.2 K ohms, as shown in the preferred embodiment, thereby providing a relatively low discharge input impedance for the touch pad 13.

The differential circuit denoted generally by reference number 32 is also connected to the electrodes 16, 18 and 22. The differential circuit 32 includes two transistors Q1 and Q2 arranged in a differential pair with the emitters of both transistors Q1 and Q2 connected to strobe electrode 22 through resistor 34.

The base of transistor Q1 is connected at second electrode 18 to resistor 28, with its collector connected to ground. The base of transistor Q2 is connected to resistor 30 via first electrode 18. The collector of transistor Q2 is connected to the sense line 24 and to ground through resistor 48.

Preferably, each transistor Q1 and Q2 is a PNP transistor, such as transistor model number MPS3906. Alternately, a NPN transistor, MOSFET, or any other active, triggerable electrical component may be used in •s place of a PNP transistor.

FIG. 4 also schematically illustrates stray, parasitic and other capacitance coupling between the various electrodes 16, 18 and 22. Capacitor 37 represents capacitive coupling between the strobe electrode 22 and the second electrode 18. Capacitor 33 represents capaci- SUBST1TUTE SHEET (RULE 26) tive coupling between the strobe electrode 22 and the first electrode 16. Capacitor 35 represents first electrode field disturbance (i.e., modeled as capacitive coupling between the first electrode 16 and the ground).

Capacitor 36 represents stray sense line capacitance.

Capacitor 38 represents second electrode 18 field disturbance. Capacitor 40 represents stray strobe line capacitance. Resistor 29 represents the resistance of strobe electrode 22. Resistor 30 in the present embodiment 10 serves to bias transistor Q2 on during the leading edge of the strobe pulses and forms a discharge path for capacitors 33 and 35. Similarly resistor 28 forms a discharge path for capacitor 37 and 38 and biases transistor Q1 on during the leading edge of the strobe pulses. 15 The differential circuit 32 operates in such a way that transistors Q1 and Q2 act as a differential pair. Common emitter resistor 34 serves to generate negative feedback which will generate the differential action of the sensor circuit. If the base of transistor Q2 is 20 biased higher than the base of Ql, more current will flow through the collector of Q2 thereby generating an increase of voltage across resistor 48. If the base of transistor Ql is biased higher than the base of transistor Q2, then the majority of the emitter current will flow through the 25 collector of transistor Ql thereby leaving less current to flow through the collector of transistor Q2 generating a decrease of voltage across resistor 48. If the bias applied to the base of transistor Ql is increased and the bias applied to the base of transistor Q2 is also in-30 creased to a voltage equal to the bias on the base on the base of transistor Ql, then the differential circuit is balanced, and there is no appreciable increase in the collector current of Q2 and the voltage change across resistor 48 will be small, if any. 35 The differential circuit 32 provides several advantages with respect to the operation of the touch SUBSTITUTE SHEET rRULE 26> -13 sensor 13. This operation can be seen in FIGS. 5A-5D, which depict output voltage on sense line 24 in response to various stimuli or lack of stimuli applied to the first and second electrodes 16 and 18. As shown in FIG. 5A where there is no first or second electrode stimuli, the signal 220 on strobes line 22 will rise from 0 volts to a maximum of approximately 5.0 volts. Although there is a relatively small output voltage 240 on the sense line 24, essentially due to the slight difference in the biasing of the transistors Ql and Q2, the output voltage on sense line 24 is at a minimal steady state amount.

As shown in FIG. 5B, with a stimulus applied to the first electrode 16 (i.e., a finger tip placed on front surface 12 in the area of first electrode 16) , and no stimulus to the second electrode 18, the output voltage on sense line 24 rises to a maximum of over 3.0 V, which is appreciably greater than the steady state amount, and then falls off exponentially. In FIG. 5C, a stimulus applied only to the second electrode 18 results in a voltage on sense line 24 which is less than "the steady state voltage. Finally, as shown in FIG. 5D, when stimuli are applied to both first and second electrodes 16 and 18, the output voltage is close to the steady state voltage.

The differential circuit 32 acts to generate output proportional to a difference between the stimuli applied to first and second electrodes 16 and 18. Thus, the output 24 is substantially more sensitive to a difference in stimuli applied to first and second electrodes 16 and 18 then to the magnitude of the stimuli. If substantial amounts of contaminants or conductive materials are placed over both the first and second electrodes 16 and 18, there will be various responses from the touch sensor 10 depending on the nature of the contaminates, with higher conductivity contaminants tending to generate lowered responses. Such a substantial amount of contaminate need only be as large as the SUBSTITUTE SHEET (RULE 26) enclosed area of the second electrode 18. This arrangement makes the touch sensor 10 highly immune to false triggering due to substantial contamination or conductive material at a localized area while allowing responses to 5 small differences between the first and second electrodes.

Further, differential circuit 32 minimizes drift due to temperature changes in the active components since the bias of both transistors Ql and Q2 will change together such that the current through resistor 48 will 10 not change substantially. Finally, changes relating to power supply, input signals, component drift electrical noise, etc., common to both of electrodes 16 and 18 and transistors Ql and Q2 will tend not to affect the output of differential circuit 32. 15 In addition to differential circuit 32, other methods may be used to process the differential signal associated with the first and second electrodes 16 and 18. Current differencing techniques and mirrors typically used in Norton amplifiers, MOS type transistors, and voltage 20 input operational amplifiers are examples of the types of circuits that could be used.

With reference to an alternative embodiment shown in FIG. 3, electrodes 16, 18, and 22, and sense line 24 are attached to a flexible carrier 25 manufactured from 25 a polyester material such as Consolidated Graphics No. HS- 500, Type 561, Level 2, 0.005 inches thick. Electrodes 16, 18, and 22, and sense line 24 are formed using a conductive silver ink, such as Acheson No. 427 SS, 0.5 mills thick. The active components Ql and Q2 are then 30 attached to the electrodes and lines. A dielectric layer 27 is placed over the electrodes and lines to protect the conducting surfaces. Preferably the dielectric 27 is Acheson No. ML25089, 1.5 mills thick. The flexible carrier 25 is then bonded to substrate 10 using an 35 adhesive 29 such as 3M No. 457. The flexible carrier 25 SUBSTITUTE SHEET (RULE 26) can be curved and twisted to conform to the shape of substrate 10.

Alternatively, with reference to FIG. 2, electrodes 16, 18, and 22, and sense line 24 can be attached directly to substrate 10. The active components are then attached to electrodes 16, 18 and 22, and to sense line 24.

In operation, the touch pad 13 is activated when a user applies stimuli by contacting or approaching substrate 10. The touch pad 13 will sense contact by a fingertip or other appendage which causes a sufficient disruption of the electric field potential between electrodes 16 and 18.

The base current of transistors Ql and Q2 is determined by the equation Ib=C(dV/dT) where Ib is the base current, C is the capacitance of the touch pad field, and dV/dT is the change in voltage with respect to time. The change in voltage with respect to time is created by the change in voltage level of the oscillating strobe signal. When a user contacts the touch pad 13 formed by electrodes 16, 18 and 22, the field capacitance of capacitor 33 is reduced while the field capacitance of capacitor 3 5 is increased. Due to the relative close proximity of electrode 18 on back surface 14 to the user contact on front surface 12 in the preferred embodiment, there will be an increase of field capacitance on capacitor 38 also, though not as great as the field capacitance of capacitor 35.

In the preferred embodiment, transistor Q2 amplifies and buffers the detection signal in close •> proximity to the touch pad 13. This reduces the difference in signal level between touch pads caused by different lead lengths and lead routing paths. By providing a more uniform detection signal level, greater amplification is possible while maintaining the signal level between, for example, 0 and +5 volts.

SUBSTITUTE SHEET (RULE 26) In the embodiment shown in FIG. 7, the strobe electrode 22 is eliminated. The bases of transistors Ql and Q2 are still connected to the second and first electrodes 18 and 16, respectively. The strobe signal is 5 applied directly to the bases of Ql and Q2 through resistors 50 and 52. Ql is biased on by the resistor 50 and the field capacitance developed by electrode 18. In a similar manner, Q2 is biased on by the resistor 52 and the field capacitance developed by the electrode 16. A 10 field potential difference generated by the transient voltages applied to electrodes 16 and 18 is developed. The potential difference will cause the bias on Ql and the bias on Q2 to differ proportionally with the field potential difference associated with electrodes 18 and 16. 15 This embodiment provides less isolation between first and second electrodes 16 and 18 as compared to the circuit of FIG. 1. Even with less isolation, there are many applications where the level of performance provided by this embodiment is adequate. The benefits derived from the 20 insensitivity of the differential circuit arrangement to common mode influences, such as the effects associated with the application environment, etc., mentioned above, is preserved in this alternative embodiment of FIG. 7.

While only two embodiments of the present 25 invention have been shown, it will be obvious to those skilled in the art that numerous modifications may be made without departing from the spirit of the claims appended hereto.

SUBSTITUTE SHEET (RULE 26) 17

Claims (5)

WHAT WE CLAIM IS:

1. A sensor apparatus for detecting the presence of a stimulus to be detected, said apparatus comprising: at least one first electrode; a strobe electrode positioned in proximity to said at least one first electrode; a signal source for providing electrical signals to said strobe electrode; wherein an electric field associated with said at least one first electrode is generated in response to the signals provided to said strobe electrode by said signal source; wherein the electric field is affected by the presence of the stimulus in proximity to said at least one first electrode; a circuit having a first input node connected to said at least one first electrode, a second input node connected to a reference electrical potential, and an output node; wherein said circuit generates a signal at its output node relating to the difference in voltage at the first and second input nodes, whereby the signal at the output node relates to the presence of the stimulus in proximity to said at least one first electrode; and wherein said circuit comprises a plurality of active components.

2. The apparatus of claim 1 wherein said active components are in close proximity to said first electrode and said strobe electrode.

3. The apparatus of claim 1 or 2 wherein said plurality of active components comprises at least a first transistor and a second transistor.

4. A sensor apparatus for detecting the presence of a intellectual property office of nz. 1 3 SEP 2001 RECEIVED 5035 50359? stimulus to be detected, said apparatus comprising: at least one first electrode; a strobe electrode positioned in proximity to said at least one first electrode; a signal source for providing electrical signals to said strobe electrode; wherein an electric field associated with said at least one first electrode is generated in response to the signals provided to said strobe electrode by said signal source; wherein the electric field is affected by the presence of the stimulus in proximity to said at least one first electrode; a low-impedance circuit having a first input node connected to said at least one first electrode, a second input node connected to a reference electrical potential, and an output node; and wherein said circuit generates a signal at its output node relating to the difference in voltage at the first and second input nodes, whereby the signal at the output node relates to the presence of the stimulus in proximity to said at least one first electrode; and wherein said circuit comprises a plurality of active components.

5. A sensor apparatus for detecting the presence of a stimulus to be detected, said apparatus comprising: at least one first electrode; a signal source for providing electrical signals to said at least one first electrode, for providing electrical signal to said at least one first electrode; wherein an electric field associated with said at least one first electrode is generated in response to the signals provided to said at least one first electrode by said signal source; INTELLECTUAL PROPERTY OFFICE OF N Z. 1 3 SEP 2001 RECEIVED 503599 wherein the electric field is affected by the presence of the stimulus in proximity to said at least one first electrode; a circuit having a first input node connected to said at least one first electrode, a second input node connected to a reference electrical potential, and an output node; and wherein said circuit generates a signal at its output node relating to the difference in voltage at the first and second input nodes, whereby the signal at the output node relates to the presence of the stimulus in proximity to said at least one first electrode; and wherein said circuit comprises a plurality of active components. END OF CLAIMS intellectual property office of n z. 1 3 SEP 2001 E