GB2138571A - Transient Signal Detector - Google Patents

Abstract

A transient signal detector for liquid crystal touch switches 17 and 18 comprises a bridge circuit 26 incorporating the switches 17 and 18, and a phase sensitive detector 28 to detect bridge imbalance. Imbalance signals are fed to a band-pass filter 35, and thence to threshold detector circuit 40 to 58 and subsequent non- retriggerable monostable 60. Pressing of a touch switch 17 or 18 produces transient bridge imbalance which is detected and filtered, and which triggers the monostable 60 if the threshold of circuit 40 to 58 is exceeded. After triggering, the monostable (d) is unresponsive for a period corresponding to its internal time constant. This desensitises the detector to switch release, and the threshold detector 40 to 58 desensitises the detector to other unwanted signals. <IMAGE>

Description

SPECIFICATION Transient Signal Dectector This invention relates to a transient signal detector, and more particularly to a detector for transient signals produced by touch-actuated capacitative switches.

Co-pending UK Application No. 8232409 dated the 12th of November 1 982 relates to a liquid crystal touch switch actuated by slight deformation of a wall of the liquid crystal cell. The deformation changes the cell dimensions and corresponding capacitance, but more importantly produces a transient realignment in the liquid crystal molecules with consequent gross change in dielectric constant. A transient change in capacitance also occurs when the cell returns to its original dimensions. Moreover the cell capacitance is sensitive to temperature and to deformations smaller than that typically produced by touch actuation.

It is an object of the present invention to provide a transient signal detector adapted to discriminate between positive actuation of a liquid crystal touch switch and other capacitance changes not caused by positive actuation or initial pressing.

The present invention provides a transient signal detector for a capacitative touchswitch, the detector including an AC signal source for energising the switch, detecting means responsive to changes in signal across the switch, a band-pass filter responsive to typical switchpress pulse frequencies, and means for desensitising the detector both to signals below a given threshold and to signals occurring within a given period of touch switch actuation. The invention provides a detector responsive to initial actuation of a capacitative touch switch but unresponsive to stray pick-up signals, temperature drift, and signals corresponding either to accidental actuation or to switch release.

The detector may alternatively be sensitised to switch release.

Conveniently, the detector may include a bridge circuit incorporating one or more capacitative touch switches as bridge arms. The detector may also include a square wave signal source to energise the bridge and touch switches.

The detecting means may be a phase sensitive detector driven by the signal source.

The band-pass filter is preferably arranged to transmit the frequency of the rising edge of a typical pulse arising from positive actuation of a touch switch.

The detector may conveniently be desensitised to signals below a given threshold by comparing the signals with a preset level and by amplifying the difference therebetween. The amplifier output circuit is arranged to provide a response only to signals exceeding the preset level. In this way, the detector is made unresponsive to signals corresponding to accidental switch actuation.

Desensitisation of the detector to switch release may be provided by the use of a non retriggerable monostable arranged to receive signals produced in response to pulses exceeding the given threshold. The monostable is responsive to an initial signal, but thereafter remains unresponsive for a period set by its internal time constant. The time constant is arranged so that the unresponsive period is slightly longer than that between typical switch press and release.

The-detector may include means incorporating hysteresis, such as a Schmitt trigger circuit, the means being arranged to desensitise the detector to noise.

In order that the invention might be more fully understood, one embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1 and 2 collectively show a circuit diagram of a transient signal detector of the invention, and Figure 3 is an alternative circuit of the invention.

Referring to Figures 1 and 2, a C-MOS multivibrator 10 arranged as a 1 kHz square wave oscillator provides in-phase and anti-phase signals at Q and Q outputs 11 and 12 respectively. The in-phase signal appears across a 50 kohm potentiometer 13 having a slider 14 connected to an earthed capacitor 15. The slider 14 is-also connected to-the common terminal 16 of a series arrangement of two liquid crystal capacitative touch switches 17 and 1 18. The touch switches 17 and 18 are connected to the inverting and non-inverting inputs 1 9 and 20 respectively of a J-FET operational amplifier 21.

The amplifier 21 has a feedback impedance - consisting of a 1 Mohm resistor 22 in parallel with a 1.5 nF capacitor 23. The non-inverting input 20 is connected to earth via a 1 Mohm resistor 24 in parallel with a 1.5 nF capacitor 25.

The amplifier 21 and surrounding impedances 17, 1 8 and 22 to 25 form a bridge arrangement indicated generally by 26. The resistors 22 and 24 provide DC paths to the amplifier inputs 19 and 20.

The amplifier output 27 is connected to a phase sensitive rectifier circuit indicated generally by 28, and comprising a C-MOS analogue switch 29 connected to an amplifier 30. The switch 29 connects the output 27 alternately to the amplifier's inverting and non-inverting inputs 31 and 32, and is driven by the Q output 12 of the multivibrator 10 via a connection 33. The amplifier 30 has an output 34 which is connected (not shown) to a band-pass filter indicated generally by 35. The filter 35 has a 220 kohm input series resistor 36, a grounded 0.22 MF capacitor 37, and 0.22 MF series capacitor 38 and a grounded 2.2 Mohm resistor 39. The filter 35 provides integrating and differentiating time constants of 0.5 sec and 0.05 sec respectively.

Signals from the filter 35 pass both to the noninverting input 40 of a first amplifier 41 and to the inverting input 42 of a second amplifier 43. Both the amplifiers 41 and 43 are J-FET operational amplifiers arranged for open-loop response.

A series resistor chain is arranged between the positive and negative power lines 44 and 45, the chain comprising four resistors of 470 kohm, 10 kohm, 10 kohm and 470 kohm indicated by 46 and 49 respectively. The 10 kohm resistors 47 and 48 have a central common point 50 connected to earth. Two 20 kohm potentiometers 51 and 52 are connected in parallel with the series arrangement of resistors 47 and 48. The potentiometers 51 and 52 have respective sliders 53 and 54 providing variable offset voltages to the inverting input 55 of the first amplifier 41 and to the non-inverting input 56 of the second amplifier 43. The outputs of the amplifiers 41 and 43 are fed via respective diodes 57 and 58 to an input 59 of a non-retriggerable monostable 60 having a Q output 61.The monostable 60 has a time constant variable in the range 0.4 sec to 1.7 sec which is set by an 0.5 MF capacitor 62 in series with a fixed 330 kohm resistor 63 and a resistor 64 variable up to 1 Mohm. The time constant is equal to 2.5 CR; where C is capacitor 62 and R is the sum of resistors 63 and 64.

The circuit in Figures 1 and 2 operates as follows. The 50 kohm potentiometer is adjusted to provide a convenient fraction of the output of the multivibrator 10 to the bridge circuit 26.

Substantialiy equal impedances form the four bridge arms (two touch switches 1 7 and 18, together with feedback impedance 22/23 and earthed impedance 24/25), and so the bridge output 27 is at or near balance initially. The characteristics of the bridge circuit 26 will be described in more detail hereinafter. The action of touching one of the liquid crystal switches 1 7 or 1 8 results in transient imbalance of the bridge 26 both at initial touch and at subsequent release.

The bridge accordingly provides a transient AC output pulse both at press and release, and the signal is in phase or in antiphase with the bridge input signal according to which of the liquid crystal switches 1 7 or 1 8 is pressed. The AC pulses are rectified by the phase-sensitive rectifier 28, which in effect multiplies successive halfcycles by +1, -1, +1 ...etc. This provides positive or negative DC pulses according to which switch is pressed. The pulses pass to the band-pass filter 35, which has a centre frequency tuned as nearly as possible to that of the rising edge of a typical pulse caused by pressing a touch switch.The filter rejects DC and low frequency signals caused by variation in liquid crystal temperature, amplifier drift etc., and also spurious pick-up signals such as mains frequency. The phase sensitive rectifier 28 also tends to eliminate pick-up signals, but passes signals such as drift in the switches 17 and 1 8 occurring at the modulation frequency of the oscillator or multivibrator 10. In view of the connection of the filter 35 to the non-inverting and inverting inputs 40 and 42 of the amplifiers 41 and 43 respectively, the former amplifier supplies a positive output for positive input pulses and the latter a positive output for negative pulses. The diodes 57 and 58 pass only positive outputs to the monostable.The potentiometer sliders 53 and 54 are set to provide offset or threshold voltages at + 15 mV and -15 mV respectively, so that the amplifier 41 provides a positive output for positive pulses exceeding + 15 mV and the amplifier 43 a positive output for negative pulses exceeding 1 5 mV in negative magnitude. Pulses within the +15 mV "window" are effectively rejected as not corresponding to positive pressing of a touch switch. Since the amplifiers 41 and 43 are arranged to operate open-loop, their output pulses will have approximately the positive supply voltage in view of their high gains.

A positive-going pulse from either amplifier 41 or 43 triggers the non-retriggerable monostable 60 into a high output voltage state. The monostable 60 returns to a low voltage output state in a time set by its time constant, around 1 second adjustable by variable resistor 64. Since the monostable 60 is non-retriggerable, it does not respond to any further pulse after it has been triggered into its high voltage state until it returns to low voltage. This introduces an unresponsive period or dead-time. The time constant of the monostable 60 is adjusted to be sufficiently long so that there is no response to liquid crystal switch release after triggering by an initial switch press.

To summarise, the circuit shown in Figures 1 and 2 provides a response to a positive initial press of either liquid crystal touch switch 17 or 18, while being desensitised to switch release, switch temperature drift, amplifier drift, stray signal pick-up and small switch signals corresponding to accidental switch actuation.

The bridge arrangement 26 is employed because it provides a low output impedance combined with input and output which are both single-ended. It is the subject of a co-pending UK Patent Application No in view of certain other characteristics not directly relevant to the present invention. The bridge arrangement 26 is not a true Wheatstone Bridge, since the resistor 24 is connected to earth and not to the feedback impedance 22/23. However, it can be shown that the balance condition for the bridge arrangement 26 is in fact the same as a Wheatstone Bridge.

There are other ways of providing for singleended input and output to and from one or more touch switches. A simple arrangement would comprise a series combination of one touch switch and an equivalent impedance, or alternatively two touch switches, connected from the AC source to earth. The switch output would be taken from the central common point of the series arrangement. This would have a high output impedance corresponding to that of liquid crystal touch switches. Alternatively, one or more touch switches could be connected as arms of a conventional Wheatstone impedance bridge. The AC input would be applied between two diametrically opposite vertices and the output taken between the two other vertices via a comparator or differential amplifier.

The circuit of Figures 1 and 2 may be arranged to indicate which of the two touch switches 1 7 and 1 8 is pressed. This is achieved by connecting each of the amplifier output diodes 57 and 58 to a respective non-triggerable monostable, instead of to the same monostable 60. Each of the monostable outputs would then indicate actuation of a respective switch 1 7 or 1 8.

Referring now to Figure 3, there is shown an alternative form of circuit of the invention suitable for detection of signals from a single touch switch. The circuit includes a square wave oscillator indicated schematically by 70 and equivalent to multivibrator 10 in Figure 1. The oscillator 70 energises a liquid crystal capacitative touch switch 71 via a line 72, and supplies a reference signal to a phase sensitive rectifier indicated schematically by a switch 73.

The reference signal passes along a connection indicated by a chain line 74. The touch switch 71 is connected to the inverting input 75 of an operational amplifier 76 having a feedback impedance comprising a 30 Mohm resistor 77 and a 680 pF capacitor 78 in parallel.

The output of the amplifier 76 passes through the phase sensitive rectifier 73 to a band pass filter indicated generally by 79 equivalent to the filter 35 of Figure 2. From the filter 79, the signal passes to the inverting input 80 of an inverting Schmitt trigger circuit 81. A triggering threshold adjustment for the Schmitt trigger 81 is provided by a 1 kohm potentiometer 82 connected to an earth line 83 and via a 100 kohm resistor 84 to a negative power point 85. The potentiometer 82 has a slider 86 connected via a 4.7 kohm resistance 87 to the non-inverting input 88 of the Schmitt trigger circuit 81, which as a positive feedback resistance 89 of 4.7 Mohm. The output of the Schmitt trigger 81 drives a non-triggerable monostable circuit 90 equivalent to the circuit 60 in Figure 2.The Schmitt trigger circuit 81 is of the inverting type having a high input impedance, and the phasing of the drive to the phase sensitive rectifier switch 73 is such as to provide a negative-going output signal at the circuit 81.

The circuit of Figure 3 operates as follows.

Since the inverting input 75 of the amplifier 76 is a virtual earth, the signal from the osci!lator 70 appears wholly across the touch switch 71. After rectification by the phase sensitive rectifier 73, the filter 79 transmits only transient changes in the signal across the touch switch 71. The Schmitt trigger circuit 81 provides a very sensitive threshold detector, and switches rapidly when the output signal from the filter 79 crosses the threshold voltage set on the potentiometer slider 83. The Schmitt trigger circuit 81 incorporates a small amount of hysteresis, so that once triggered the threshold for triggering in the opposite direction differs. By virtue of this hysteresis, the Schmitt trigger circuit is not immediately retriggered by noise on the signal.

The hysteresis voltage is arranged to be small compared to the threshold voltage of 20 mV.

Claims (9)

1. A transient signal detector for a capacitative touch switch, the detector including: (1) an AC signal source for energising the switch, (2) detecting means responsive to changes in signal across the switch, (3) a band-pass filter responsive to typical switch-press pulse frequencies, and (4) desensitising means for desensitising the detector both to signals below a given threshold and to signals occurring within a given period of touch switch activation.

2. A transient signal detector according to Claim 1 including a bridge circuit incorporating one or more switches as bridge arms.

3. A transient signal detector according to Claim 1 or 2 wherein the AC signal source is a square wave generator.

4. A transient signal detector according to Claim 1,2 or 3 wherein the detecting means comprises a phase sensitive detector.

5. A transient signal detector according to any preceding claim wherein the band-pass filter is arranged to transmit the frequency of the rising edge of a typical pulse arising from positive activation of a touch switch.

6. A transient signal detector according to any preceding claim wherein the desensitising means includes means for providing an output in response only to signals exceeding a preset level.

7. A transient signal detector according to Claim 6 wherein the densensitising means includes a non-retriggerable monostable circuit arranged for triggering in response to signals exceeding the preset level, the monostable circuit having a time constant exceeding a typical time interval between switch press and release.

8. A transient signal detector according to any preceding claim arranged to incorporate hysteresis for desensitisation to noise.

9. A transient signal detector substantially as herein described with reference to and as illustrated in the accompanying Figures 1 and 2 or Figure 3.