United States Patent
`
`[191
`
`Hopper
`
`[11] Patent Number:
`
`4,878,107
`
`[45] Date of Patent:
`
`Oct. 31, 1989
`
`[54] TOUCH SENSITIVE INDICATING LIGHT
`
`[76]
`'
`
`Inventor: William R. Hopper, 119 Errol Street,
`North Melbourne, Victoria 3051,
`Australia
`
`[21] Appl. No.:
`
`84,795
`
`[22] PCT Filed:
`
`Oct. 29, 1986
`
`[86] PCT No.:
`
`PCT/AU86/00324
`
`§ 371 Date:
`
`Jun. 26, 1987
`
`§ 102(e) Date:
`
`Jun. 26, 1987
`
`[87] PCT Pub. No.: WOW/02846
`
`PCT Pub. Date: May 7, 1987
`
`Foreign Application Priority Data
`[30]
`Oct. 29, 1985 [AU] Australia .......... PH3151
`Sep. 9, 1986 [AU] Australia .......................... PH7909
`
`
`Int. Cl.4
`[51]
`.......... H01L 23/28
`[52] US. Cl. ........................................ 357/72; 357/17;
`307/116; 361/174; 340/706; 250/221; 220/21
`R; 341/31
`[58] Field of Search ....................... 357/17, 25, 26, 72,
`357/74, 16; 307/116, 117; 361/173, 174, 178;
`340/365; 250/221
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`........................ 250/221
`3,372,789 3/1968 Thiele et a1.
`..
`.. 357/72
`3,805,347 4/ 1974 Collins et a1.
`
`.. 357/72
`3,855,606 12/1974 Schoberl ........
`3,922,563 1 1/ 1975 Penman ............... 307/116
`
`307/116
`3/1976 Vaz Martins i.
`3,944,843
`
`6/1977 Thome .......
`.. 357/72
`4,032,963
`
`.. 357/72
`4,047,075 9/ 1977 Schoberl
`5/1979 Raupp ............
`307/1 16
`4,152,629
`
`4,514,276 4/1985 Covington et a1.
`.
`357/72
`
`4,550,310 10/1985 Yamaguchi et a1.
`................ 307/116
`5/1986 Komadina et a1. ............. 250/221
`4,591,710
`
`
`4,629,884 12/1986 Bergstrom ...........
`340/365 P
`........ 307/ 116
`5/1987 Skarman ..
`4,668,876
`
`4,737,626
`4/1988 Hasagawa
`340/365 P
`4,761,637
`8/1988 Lugas et a1. ..................... 340/365 P
`
`FOREIGN PATENT DOCUMENTS
`
`0113223
`
`7/1984 European Pat. Off.
`
`........ 340/365 P
`
`OTHER PUBLICATIONS
`
`Ludeman, “Miniature Optical Keyboard”, IBM Tech-
`nical Disclosure Bulletin, vol. 15, No. 11, Apr. 1973, pp.
`3348—3349.
`
`Primary Examiner—Rolf Hille
`Assistant Examiner—Steven Ho Yin Loke
`Attorney, Agent, or Firm—Larson & Taylor
`
`[57]
`
`ABSTRACT
`
`A touch sensitive light emitting diode comprising a
`diode (10) encapsulated in a plastics dome (11) and
`externally operable touch sensitive switching means
`positioned within the dome, the switching means in-
`cluding an output (18) to external electronics and the
`diode providing visual indication of the state of the
`switching means. The switching means may be a resis-
`tive touch switch, a voltage detection touch switch, a
`capacitance detection touch switch, or a proximity
`detection touch switch. The switching means may also
`be in the form of a stress/strain sensitive element (130)
`or a light sensitive element (71) positioned within the
`dome (11) to detect an object or finger-tip in proximity
`to the dome. The light emitting diode may also include
`an integrated circuit (69) positioned within the dome
`(11).
`
`20 Claims, 6 Drawing Sheets
`
`20
`
`1
`
`APPLE 1009
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`APPLE 1009
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`1
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`

`

`US. Patent
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`Oct. 31, 1989‘
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`Sheet 1 of6
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`4,878,107
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`F I G 1
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`/12
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`. FIG 2 [21
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`2
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`US. Patent
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`Oct. 31, 1989
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`Sheet 2 of6
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`4,878,107
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`3
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`US. Patent
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`Oct. 31, 1989
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`Sheet 4 of 6
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`4,878,107
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`5
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`

`

`US. Patent
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`Oct. 31, 1989
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`Sheet 5 of 6
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`4,878,107
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`US. Patent
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`Oct. 31, 1989
`
`Sheet 6 of 6
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`4,878,107
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`1
`
`4,878,107
`
`TOUCH SENSITIVE INDICATING‘ LIGHT
`
`INTRODUCTION
`
`This invention relates to a touch sensitive indicating
`light and more particularly relates to a light emitting
`diode that incorporates a touch or proximity sensor that
`operates as switching means.
`BACKGROUND OF THE INVENTION
`
`Light emitting diodes known as LEDs have become
`very popular in the electrical/electronic industries. A
`light emitting diode is a solid state electronic compo-
`nent typically comprising a forward biased p-n semi-
`conductor junction diode. Light is emitted very close
`to, or within the junction region as injected holes and
`electrons diffuse across the junction and combine. The
`combining holes and electrons release energy and some
`of this energy is released directly as photons. Light
`emitting diodes can be fabricated by diffusing p and 11
`type impurities into a suitable semi-conductor to form a
`junction. A semi-conductor commonly used to fabricate
`light emitting diodes which emit light in the visible
`spectrum, is gallium arsenide phosphide.
`The diode is conventionally encapsulated in plastics
`and defines a cylindrical construction with a domed top,
`the diameter of the assembly being in the order of 5 mm.
`Light emitting diodes perform a significant role in the
`electrical/electronic industry. They provide a simple
`and effective means of indicating a particular state of a
`variety of components in an electronic circuit. Their
`small size and durability makes them ideal for use on
`instrument panels and on circuit boards. They are fre-
`quently used to indicate the state of switching means
`and are often positioned adjacent to a switch to indicate
`the state of the switch.
`In recent times companies such as Hewlett Packard
`have produced LEDs that incorporate integrated cir-
`cuits within the structure of the LED.
`
`SUMMARY OF THE INVENTION
`
`The present invention provides a light emitting diode
`that has built in switching capacity.
`The present invention also provides a touch sensitive
`light emitting diode comprising a diode encapsulated in
`a plastics dome and externally operable touch sensitive
`switching means positioned within the plastics dome,
`the switching means including an output to external
`electronics and the diode providing visual indication of
`the state of the switching means.
`Preferably the switching means comprises a conduc-
`tive region positioned on or adjacent the top of the
`plastic dome.
`The switching means may be a resistive touch switch,
`a voltage detection touch switch, a capacitance detec-
`tion touch switch, or a proximity detection touch
`switch.
`
`In one embodiment the switching means comprises a
`stress/strain sensitive element positioned within the
`plastics dome whereby external pressure on the top of
`the dome is sensed by the stress/strain sensitive element.
`The stress/strain sensitive element may comprise a
`resistor, a piezoelectric or piezoresistive layer, or a
`semi-conductor piezoresistor or a transistor.
`In one other embodiment the switching means com-
`prises a light sensitive element which detects light from
`the light emitting diode reflected from an object or
`finger tip in proximity to the top of the plastics dome.
`
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`2
`The light sensor may comprise a phototransistor or a
`photodiode.
`The term “dome” as used in this specification relates
`to the top portion of the diode which could prevent a
`variety of profiles to the exterior.
`DESCRIPTION OF THE DRAWINGS
`
`Various embodiments of the present invention will
`now be described by way of example only with refer-
`ence to the accompanying drawings, in which:
`FIGS. 1 to 7 are side elevational views illustrating
`various examples of light emitting diodes that incorpo.
`rate switching means;
`FIG. 8 is a block circuit diagram illustrating one
`example of the use of an LED incorporating switching
`means;
`FIG. 9 is a circuit diagram illustrating the LED of
`FIG. 8 used to control a toggling output;
`FIG. 10 is a circuit diagram illustrating the use of an
`LED with switching means to provide a switch with a
`simple “momentarily on” output;
`FIGS. 11a, b and c are respectively plan, side and
`front elevations views of an LED incorporating a
`stress/strain sensor;
`FIG. 12 is a block circuit diagram of an LED incor-
`porating a piezoresistive stress/strain sensor; and
`FIG. 13 is a circuit diagram of a LED incorporating
`a piezoresistive stress/strain sensor.
`
`DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`The various embodiments illustrated in the accompa-
`nying drawings concern light emitting diodes (LEDs)
`that incorporate switching means. Throughout the vari-
`ous embodiments the same reference numerals are used
`
`for like components. In essence the touch sensitive light
`emitting diode comprises an LED 10 of one of many
`known types encapsulated in a translucent plastics
`dome 11. The dome 11 comprises a domed top 12 and a
`cylindrical portion 13 that is mounted on a base plate 14.
`The base plate is usually moulded integrally with the
`dome and incorporates three spaced apart electrical
`terminals which project out of the base plate, namely an
`output terminal 16, a ground terminal 17 and a switch
`output terminal 18. The plastics dome in which the
`LED is encapsulated is preferably a thermo-setting
`resin or produced from the injection moulding of a
`thermo-plastic. The plastics dome is transparent or
`translucent and is produced to have an external diame-
`ter of approximately 5 mm. This is the standard diame-
`ter for conventional LEDs and allows the touch sensi-
`tive LED to be used on existing circuitry and facia
`panels.
`The touch sensitive LEDs described in the embodi-
`ments illustrated in FIGS. 1 to 7 of the accompanying
`drawings incorporate four categories of switching
`means, namely:
`(a) Resistive touch switches.
`The- resistance between two or more electrodes is
`reduced by the contact of a finger or other conductive
`. material. This change is the switching signal and can be
`measured and used by switching electronics within, or
`external to, the touch sensitive LED.
`(b) Voltage detection touch switch.
`The human body acts as a type of antenna and carries
`high impedance AC voltages coupled to it by electro-
`magnetic fields from sources such as mains wiring.
`
`8
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`

`4,878,107
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`3
`These voltages can be detected by a single conductive
`sensor region on the touch sensitive LED. On contact
`or close proximity of a finger these voltages can be
`detected and used as a switching signal. Contact with
`the conductive sensor region is not a requirement as
`these voltages may be capacitively coupled from a fm-
`ger in close proximity to the conductive sensor.
`(c) Capacitance detection touch switches.
`The human body will act as a small capacitor which
`is earthed at one end. This effect can be used by an
`electronic circuit to detect the contact or close proxim-
`ity of a finger to a conductive sensor. This type of touch
`switch can operate with a single conductive sensor
`region on the touch sensitive LED. The change in ca-
`pacitance between the conductive sensor region and
`electrical ground caused by the close proximity or
`contact of a finger is used as the switching signal. This
`change in capacitance can be detected by such methods
`as detecting the current from a high frequency oscilla—
`tor that flows from the conductive sensor to electrical
`ground or by the degradation or harmonic filtering of
`an oscillating signal present at the conductive sensor.
`(d) Conductor proximity touch switches.
`These switches do not require contact and typically
`use more than one conductive sensor region. In these
`switches the proximity of a conductor such as a finger
`changes the inductive or capacitive coupling between
`two or more conductive regions on or within the plas-
`tics encapsulation of the touch sensitive LED.
`From the above it should be understood that the
`expression “touch sensitive” embraces switching means
`in which either an external element positively touches
`the sensor or the external element is placed in close
`proximity to the sensor. The expression “touch sensi-
`tive” is not restricted to positive contact.
`In FIG. 1 there is illustrated a simple form of touch
`sensitive LED 20. This touch sensitive LED contains
`no internal electronics and merely comprises the single
`light emitting diode 10 encapsulated in a plastics dome
`11. The top 12 of the dome is provided with a conduc-
`tive coating/layer 21 applied over the encapsulating
`plastics. This layer may be a conductive oxide or con-
`ductive plastics. Metallic coatings applied by sputter
`evaporative or electrolytic processes have also been
`used to provide the layer. The coating 21 is partially
`transmissive to light from the light emitting diode 10. A
`metallic lead 22 is clipped to one edge of the conductive
`coating 21 and electrically connects this coating to the
`switch output terminal 18. The touch sensitive LED
`also incorporates the ground terminal 17 and the posi-
`tive input voltage terminal 16. In this embodiment the
`touch sensitive LED would be coupled to external
`electronic circuitry.
`In the embodiments illustrated in FIGS. 2 to 7 the
`touch sensitive LEDs all incorporate integrated circuits
`69 that are bonded and electrically connected to the
`zero voltage or ground terminal 17. The positive input
`terminal 16 is coupled to the integrated circuit 69 via a
`fine gold or metallic wire 68 that is bonded to a metallic
`frame element 67 of the terminal 16 and to a metallized
`pad 66 formed on the integrated circuit. Similarly, the
`output or switch terminal 18 is coupled to the integrated
`circuit 69 via a fine wire lead 64. A fine wire 63 couples
`the integrated circuit 69 to the touch sensitive layer 21
`via a lead 62.
`
`A metallic strip 61 is coupled to the LED and the
`integrated circuit for internal construction only. The
`
`4
`lead 61 is clipped off close to the base 14 of the encapsu-
`lation of the LED.
`
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`The integrated circuit and its operation with the
`touch sensitive LED is described later in this specifica-
`tion.
`In the embodiment illustrated in FIG. 3, the dome 12
`of the touch sensitive LED 30 is provided with a con-
`ductive cap 31 made of conductive plastics or other
`conductive material that is wholly or partially transmis-
`sive to light at the frequencies emitted by the light emit-
`ting diode 10. The conductive cap 31 is coupled to the
`integrated circuit 69 via a lead 32.
`In the embodiment illustrated in FIG. 4, the touch
`sensitive LED 40, the conductive cap or conductive
`layer at the top 12 of the dome 11, is replaced by an
`electrode or lead 41 that extends out through the exte-
`rior of the dome to constitute a conductive pin or tab
`that can be directly touched to activate the LED.
`Conductive coatings or layers that can be used in-
`clude a thin coating of a metal such as gold, silver or
`nickel or chromium. This layer is made thin enough to
`be partially transmissive to light from the LED. These
`coatings can be applied using existing evaporative, sput-
`ter or electroplating, or chemical deposition techniques.
`Metal oxide films such as films of indium oxide, tin
`oxide and zinc oxide which are transparent to light and
`are electrically conductive. These oxides are currently
`used for this reason in some optoelectronic compo-
`nents. Oxide films may be used to form the conductive
`touch “pick-up” electrodes. Oxide films are applied
`using existing evaporative processes. Sometimes these
`films are “doped” with impurities to modify or adjust
`their characteristics.
`
`There are also two types of conductive polymers
`namely:
`(i) Normally non-conductive plastics mixed with a
`filler that produces a conductive result. For exam-
`ple, metal or graphite loaded thermopolymers or
`resins. These materials are typically opaque but
`will produce partially transmissive electrodes if
`used as very thin coatings, and
`(ii) true conductive polymers such as polythiphene,
`trans-polyacetylene and polyaniline are electrically
`conductive. These materials will produce partially
`transmissive electrodes when used as thin coatings.
`In the embodiment illustrated in FIG. 5, the touch
`sensitive LED 50 has incorporated in the top 12 of the
`dome 11 two conductive electrodes 51 and 52 in the
`form of conductive layers on the exterior of the dome.
`The conductive electrodes 51 and 52 are positioned
`slightly spaced apart as illustrated in FIG. 5 and are
`coupled to the integrated circuit 69 via leads 53 and 54.
`The contact of a finger will change the resistance be-
`tween the two electrodes, which can be detected by the
`electronics on the integrated circuit. Also the close
`proximity of a finger increases the capacitive coupling
`between the electrodes 51 and 52 and this also can be
`detected by the electronics on the integrated circuit. If
`the sensor is capacitive and does not require direct
`contact, the electrodes may be protected or concealed
`below the surface of the encapsulating plastic dome as
`illustrated in the embodiment of FIG. 6 in which the
`electrode 61 and 62 are positioned a small distance be-
`neath the top 12 of the dome 11.
`FIG. 7 illustrates an embodiment where a pair of light
`emitting diodes 70 and 71 are positioned adjacent one
`another coupled to the integrated circuit 69 so that, in
`use, operation of the switching means, which may be in
`
`9
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`4,878,107
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`5
`any of the forms described above, causes alternate oper-
`ation of the light emitting diodes 70 and 71 which may
`be coloured red or green whereby green “on” indicates
`that the output lead is zero volts and red “on” indicates
`that the output lead is at a positive supply voltage. It is
`understood that a number of light emitting diodes may
`be incorporated within the one touch sensitive LED to
`provide a variety of colours that indicate a range of
`states.
`
`FIG. 8 illustrates a block circuit diagram of the touch
`sensitive LEDs illustrated in FIGS. 2 and 3 and the
`associated electronic circuitry that
`is incorporated
`within the integrated circuit 69 to provide the desired
`operation. The LED is illustrated by reference numeral
`10 and the touch sensitive pick-up by reference numer-
`als 21 and 31. This circuit includes a constant current
`LED driver, a touch detection and debouncing circuit,
`a flipflop to give toggling operation and an output
`driver stroke solid state switch stage. Examples of elec-
`tronics that can be used to implement the switch output
`stage are a transistor, a triac, a diac, an analogue switch
`or a logic level output such as a CMOS inverter. The
`components shown in the block diagram terminate at
`the “voltage in terminal”, the “switching output termi-
`nal” and the “ground terminal”. FIG. 9 is a circuit
`diagram of a single sensor electrode 21, 31, 41 voltage
`detecting touch switch. It is shown, built up from exist-
`ing components and in this form would be suitable as
`external electronics to run a touch sensitive LED with
`no internal electronics as shown in FIG. 1. When the
`electrode 21, is touched the AC electrical signals pres-
`ent in the human body are fed to the input of the
`Schmitt trigger 81. The output of the Schmitt trigger 81
`is then a square wave which is rectified to DC by the
`diode 82, the resistors 83 and 84 and the capacitor 85.
`This DC, present while the electrode is touched is de-
`glitched by the second Schmitt trigger 86. The output
`of the Schmitt trigger 86 drives the clock input to the
`flip-flop 87. The output 18 of the flip-flop toggles be-
`tween zero volts and positive supply voltage with each
`successive touch. When the output 18 is “on”, at posi-
`tive supply voltage this voltage drives the LED 10
`through the resistor 90 which limits the current to the
`LED.
`
`An equivalent to this circuit may be fabricated on an
`integrated circuit and used in embodiments such as
`FIG. 2 and FIG. 3.
`FIG. 10 is a circuit diagram of a two electrode resis-
`tance sensing touch sensitive LED with a simple “on
`while touched” output 18. This circuit is suitable for
`fabrication on an integrated circuit, and would be used
`as shown in FIG. 5.
`
`The circuit consists of complimentary p and n type
`MOS field-effect transistors 98, 99 arranged as an in-
`verter. The resistor 93 normally holds the input to the
`inverter high which sets the output 18, low. Finger
`resistance across the electrodes 51 and 52 pulls the in-
`verter input low which causes the output 18 to lift to
`close to the positive supply voltage. This voltage will
`also drive the LED 10 but the current is limited by the
`resistor 90.
`
`In the embodiments described above the switching
`means of the touch sensitive LEDs all operate by resis-
`tive, voltage detecting inductive or capacitive pick-up.
`In another embodiment the switching means may be
`a light sensor which detects light from the light emitting
`diode reflected back into the plastic dome by an object
`or finger in close proximity. Typically the light sensor
`
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`would be a phototransistor or a photodiode and the
`device may contain more than one light emitting diodes
`of different colours. In this way different switch states
`of the device can be indicated whilst light is always
`emitted to operate the light sensor. One of the light
`emitting diodes may emit infra-red light which is not
`visible and the device appearing to be off.
`The physical layout shown in FIG. 7 may be used to
`implement an optical detection touch sensitive LED.
`One diode, 71, is a light sensitive photodiode and the
`second diode, 72, is a light emitting diode.
`Light from the light emitting diode 72 is reflected
`back onto the photodiode 71 by the proximity of a fin-
`ger tip. The reverse biased current through the photodi-
`ode can be used as a switching signal. In this case with
`only one light emitting diode the different states of the
`switch are indicated by different intensities or flashing
`rates of the light emitting diode, but in each case light
`output is still provided to enable switching, by reflec-
`tion onto the photodiode from a finger tip.
`Typically the photodetector is designed to be sensi-
`tive specifically to light from the light emitting diode
`and is not effected by changes in ambient lighting. The
`photodetector is not exposed to direct light output from
`the light emitting diode and so is sensitive to changes in
`reflected light.
`In the embodiment illustrated in FIGS. 11a, b and c,
`the touch sensitive LED 100 in a manner similar tothe
`
`earlier embodiments incorporates at least one light emit-
`ting diode 10 incapsulated in a dome 11 of a suitable
`plastics material. The LED incorporates a power input
`terminal 16, a ground terminal 17 and a switch output
`terminal 18 as in the earlier embodiments. It is also
`understood that the touch sensitive light emitting diode
`incorporates an integrated circuit 169 coupled in a simi-
`lar manner through gold wires or whiskers to the termi-
`nals in the same manner as described in the earlier em-
`bodiments.
`The major difference between the embodiment illus-
`trated in FIGS. 11a, b and c and the earlier embodi-
`ments is that the switching means comprises a stress/-
`strain sensor 130 that is incorporated within the inte-
`grated circuit to be sensitive to changes in stress/strain
`caused by physical contact at the top 12 of the dome 11.
`To localize the stress/strain when the dome is contacted
`an annular groove 132 is positioned towards the base of
`the dome 111 and a banana shaped cut-out 131 provided
`within the interior of the dome to define a cavity which
`may be evacuated. The combination of the cavity 131
`and the groove 132 causes stress concentrations at the
`stress/strain sensor to ensure that once the dome is
`touched the switch is activated.
`In another alternative the cavity can be replaced by a
`compressive insert that again has the effect of causing
`stress/strain concentration.
`As shown in FIGS. 11 the integrated circuit 169 is
`coupled to the light emitting diode 10 via a fine wire
`lead 114. The integrated circuit is also coupled to the
`metallic output terminal 18 via a fine metallic wire 125
`and to the positive power input terminal 16 via a similar
`fine wire lead 126. The stress/strain sensor 130 may be
`one of three basic types, namely a resistive element, a
`piezoelectric element or a capacitive element.
`In the case of a capacitive element a diaphragm or
`cantilever is etched into or mounted on to the surface of
`the integrated circuit. Strain in the material surrounding
`the integrated circuit will change the capacitance be-
`tween metalized conductors on the diaphragm or coun-
`
`10
`
`10
`
`

`

`7
`terlever and nearby conductors on the integrated cir-
`cuit. This change in capacitance can also be used as a
`switching signal to other electronics on the integrated
`circuit. Integrated circuit accelerometers based on this
`capacitive principle are available commercially.
`In the case of a resistive element the resistive stress
`sensor is incorporated in the integrated circuit via a set
`of resistors which are fabricated on the surface of the
`integrated circuit. These resistors are exposed to strain
`through the encapsulating plastics when a force is ap‘
`plied to the top of the dome. The resistors change in
`resistance with strain and this change is detected and
`interpreted by the other electronic elements that are
`fabricated on the surface of the integrated circuit. Typi-
`cally these resistors change resistance due to piezore-
`sistance and are referred to as piezoresistors.
`The stress/strain sensor illustrated in FIGS. 11 is
`fabricated on an integrated circuit. This sensor may be
`a piezoresistor, or a piezoelectric sensor may be fabri-
`f20
`cated on the silicon chip by coating the sensor area 0
`the substrate with a thin layer of piezoelectric material
`such as crystalline aluminium nitrite. In both of the
`sensor designs described above the sensitivity of the
`devices can be increased by setting the sensor element
`areas on a thin diaphragm or a cantilevered area of the
`silicon chip. To achieve this result the thickness of the
`silicon wafer is reduced in the sensor region using aniso-
`tropic etching micro-machining techniques. By this
`process the underside of the silicon wafer can be etched
`away to produce a diaphragm or counterlevered region.
`The piezoelectric stress sensor device results in a
`charge separation when subjected to strain which
`causes a potential difference across the surfaces of the
`material. The associated change in electrical resistance
`of a conductor or semi-conductor is called the piezore-
`sistive effect. Semi-conductor resistors are fabricated on
`or below the surface of an integrated circuit and exhibit
`the piezoresistive effect and can be incorporated into a
`circuit sensitive to small resistor changes to form a
`strain sensor. These resistors can be fabricated using
`established diffusion, doping techniques, or by deposit-
`ing small areas of the integrated circuit surface with a
`piezoresistive layer such as micro-crystalline silicon. A
`specific example of a piezoresistive stress sensor is a
`metal oxide semi-conductor (MOS) field effect transis-
`tor. When a stress is applied to a semiconductor there is
`in effect a change in charge carrier mobility. The drain
`current of a MOS field effect transistor is proportional
`to the carrier mobility. In this way a MOS field effect
`transistor may be constructed on an integrated circuit
`specifically to act as a piezoresistive based stress/strain
`sensor.
`
`10
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`15
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`25
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`30
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`35
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`40
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`45
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`50
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`4,878,107
`
`8
`smooth noise free transition between voltage levels
`suitable for use with digital electronics.
`The light emitting diode 10 is controlled by the inte-
`grated circuit and is supplied with power when the
`output lead is at the higher voltage state. The light
`emitting diode is supplied with power through a con-
`stant current circuit so that it will operate over a range
`of supply voltages. The supply voltage may vary from
`three to eighteen volts and the output lead can supply
`100 rnilliamps.
`FIG. 13 is a circuit diagram of a touch switch LED
`using a stress/strain sensor and with a simple momen-
`tarily on output. The part of this circuit within the
`dotted line boundary can be directly fabricated on an
`integrated circuit using MOS field-effect transistors 153,
`154, and a piezoresistor 150. The circuit consists of
`complimentary p and 11 type MOS field-effect transis-
`tors arranged as a simple inverter. The piezoresistor 150
`and the fixed resistor 151 form a potential divider. The
`output of this divider 152 is the input to the inverter.
`Stress on the piezoresistor changes the voltage at the
`inverter input which can cause the output voltage to
`change greatly. The output lead also drives the LED
`10, via the resistor 90. The resistor 90 limits the maxi-
`mum current to the LED 10.
`
`Stress/strain sensors fabricated on an integrated cir-
`cuit are used by companies such as Motorola Inc. and
`Honeywell to build miniature gas pressure sensors and
`can be purchased from these companies.
`In all of the embodiments described above the touch
`sensitive light emitting diode incorporates the visual
`light emitting diode as well as suitable switching means.
`This combination of components, together with the
`choice of integrated circuit, allows the device to be used
`in a variety of operations. Typical functions that may be
`built into the device by using an appropriated integrated
`circuit design are:
`1. The output lead toggles between a low and a
`higher (“on”) voltage with successive external
`touches,
`2. A single light emitting diode is controlled to indi-
`cate the state of the device with the light emitting
`diode “on” when the output lead is at positive
`supply voltage and “off” when the output lead is at
`zero volts state.
`
`3. Two different coloured light emitting diodes are
`included in the device with one, for example green,
`indicating that the output lead is at zero volts and
`the second, for example red, indicating that the
`output lead is “on”.
`4. The integrated circuit may include timing func-
`tions such as the output lead being on for ten sec-
`onds and then switching itself “off” after each
`push.
`5. The device is in a specific known state when it is
`first powered up.
`’6. A power down memory capability so that the de-
`vice will remember its function state when its sup-
`ply voltage is removed and will resume this condi-
`tion when next powered up.
`7. An analogue voltage output with a magnitude
`related to the duration or time between the number
`of touched detections. The intensity of, or the
`flashing rate of the light emitting device may also
`be used to indicate the output voltage level.
`It is understood that the choice and design of the
`integrated circuit that detects the function of the touch
`sensitive LED that is the subject of this invention are
`
`11
`
`It is understood that the stress/strain sensor may also
`incorporate other types such as piezoresistive bridges,
`buried piezoresistors or a piezoelectric-film field-effect-
`transistor (PI-FET).
`In the block diagram of FIG. 12 a touch sensitive
`LED incorporating a piezoresistive stress/sensor ele-
`ment is illustrated together with the other components
`of the integrated circuit. The area of the block diagram
`shown in FIG. 12 is within the dotted line of FIG. 11a.
`The device has three electrical terminals. Two of the
`leads 16, 17 are connected across the voltage difference
`of an external power supply, the third or output lead 18
`will toggle between a low and higher voltage level with
`each successive finger touch. The switching action of
`the output lead is “deglitched” in that it provides a
`
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`65
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`11
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`

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`9
`well known to those skilled in the art and are encom-
`passed by this invention in its broadest form.
`The attached sensitive light emitting diode of the
`subject application provides the opportunity for the
`purchase of an off-the-shelf item that incorporates the
`well known function of an LED whilst at the same time
`
`embraces touch control switching with built in elec-
`tronic integrated circuitry. The touch sensitive LED
`combines the advantages already known with regard to
`conventional LEDs as well as the many advantages that
`acrue to touch sensitive switches. The touch sensitive
`
`LED reduces the number of component parts that are
`required in any electronic circuitry and provides an
`integral unit encapsulated in plastics to protect against
`hostile environments such as water and mechanical
`wear and tear. The touch sensitive LED provides in one
`unit both the switching and indicating means at one
`point and thus a number of touch sensitive LEDs may
`be mounted in an array on a panel or directly on circuit
`boards for use in many fields such as the automotive
`industry, laboratory research, hi-fi and consumer equip-
`ment,
`industrial and military applications as well as
`sophisticated test and measurement equipment.
`In some embodiments the function of the touch sensi-
`
`tive LED is increased by the use of transparent or par-
`tially light transmissive electrically conductive materi-
`als or coatings to make up the conductive touch sensi-
`tive region or electrodes. In this way the touch sensitive
`conductive region may be large for a given plastics
`encapsulation size and may be on or close to the front
`surface of the component without obscuring or shadow-
`ing the light output from the LED w