WO1995019661A1

WO1995019661A1 - Proximity sensing arrangement

Info

Publication number: WO1995019661A1
Application number: PCT/US1995/000398
Authority: WO
Inventors: Robert K. Parrott
Original assignee: Loyer, Daniel, W.
Priority date: 1994-01-12
Filing date: 1995-01-11
Publication date: 1995-07-20

Abstract

This invention includes a proximity sensing arrangement having a square wave oscillator (12); a Schmitt trigger (14-1); a first R-C circuit (16-1) or an integrator coupled between the square wave oscillator (12) and the input of the Schmitt trigger (14-1); and a proximity sensing pad (18) coupled between the resistive and capacitive components of the R-C circuit (16-1).

Description

PROXIMITY SENSING ARRANGEMENT This invention relates to a proximity sensing arrangement.

Such an arrangement can, for example, be used to form a touch switch, ie a device whereby switching can be effected by proximity of the operator's finger to a touch pad. Electrical contact with the touch pad is not required, and the switch has no moving parts. The arrangement of the invention can also, for example, be used to form a thickness detector.

The invention attempts to provide a proximity sensing arrangement which is inexpensive and which is able to operate at relatively low voltages - ie the voltages used in electronic devices such as, for example, pocket calculators.

SUMMARY OF THE INVENTION

According to the invention there is provided a proximity sensing arrangement which comprises a square wave oscillator, a Schmitt trigger, an R-C circuit between the output of the square wave oscillator and the input of the Schmitt trigger, and a proximity sensing pad coupled to a point between the resistive and capacitive components of the R-C circuit.

In one form of the invention the R-C circuit may be configured as an integrator, to serve as a pulse delay for the output of the square wave oscillator, the output of the Schmitt trigger being coupled to the output of a first edge detector, there being a second R-C circuit configured as an integrator to serve as a second pulse delay for the output of the square wave oscillator, the second R-C circuit being connected to a second edge detector, and there further being means for comparing the outputs of the first and second edge detectors, which means may, for example, be in the form of an AND gate. In another form of the invention the R-C circuit may be configured as a differentiator.

IN THE DRAWINGS

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings.

Figure 1 is a block diagram of a touch switch in accordance with a first embodiment of the invention;

Figure 2 is a more detailed circuit diagram of the touch switch of Figure 1;

Figure 3 are some waveforms to illustrate the operation of the touch switch of Figures 1 and 2;

Figure 4 is a block diagram of a touch switch in accordance with a second embodiment of the invention; Figure 5 is a more detailed circuit diagram of the touch switch of Figure 4;

Figure 6 are some waveforms to illustrate the operation of the touch switch of Figures 4 and 5; and

Figure 7 is a more detailed circuit diagram showing another form which the touch switch of Figures 4 and 5 can take.

Appendix A is a detailed description of an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring first to Figures 1 and 2, reference numeral 10 generally indicates a touch switch which comprises a square wave oscillator 12 which will typically have a frequency of between 50 to 150Hz, and, preferably, lOOHz, a first Schmitt trigger 14.1, a first R-C circuit 16.1 connected between the output of the square wave oscillator and the Schmitt trigger, and an insulated touch pad .18 connected between the. resistive and capacitive components of the first R-rC circuit. The output of the first Schmitt trigger 14.1 is connected via a first trailing edge detector 20.1 to the firεt input of an AND gate 22. The touch switch 10 further comprises a second Schmitt trigger 14.2, and a second R-C circuit 16.2 connected between the output of the square wave oscillator 12 and the second Schmitt trigger. The output of the second Schmitt trigger 14.2 is connected via a second trailing edge detector 20.2 to the second input of the AND gate 22. The resistive component of the second R-C circuit 16.2 includes a trimpot 24 to permit adjustment of the time constant of the R-C circuit. Each of the trailing edge detectors 20.1 and 20.2 comprises an R-C circuit 26 which is configured as an integrator, and a Schmitt trigger 28. The output of the AND gate 22, which produces a pulse output, is connected to a retriggerable monostable oscillator 30 to provide a level output on its output terminal 32. Referring now to Figure 3, the waveform of Figure

3a indicates the output of the square wave oscillator 12.

The waveforms of Figures 3b and 3c indicate the outputs of the two Schmitt triggers 14.1 and 14.2, and the waveforms of Figures 3d and 3e indicate the outputs of the corresponding Schmitt triggers 28. The R-C circuits 16.1 and 16.2 serve as pulse delays for the output of the square wave oscillator 12, and the trailing edge detectors 20.1 and 20.2 provide pulse outputs as illustrated in Figures 3d and 3e at the trailing edges of the respective delayed square wave outputs of the Schmitt triggers 14.1 and 14.2.

The pulse delay caused by the R-C circuits 16.1 and 16.2 depends on the time constant of the R-C circuits. The time constant of the R-C circuit 16.2, once it has been adjusted by means of the trimpot 24, remains fixed, whereas the time constant of the R-C circuit 16.1 changes when the touch pad 18 is touched. The values of the components are so selected that, when the touch pad 18 is not touched, the pulse output of one of the Schmitt triggers 28 is out of coincidence with the pulse output of the other Schmitt trigger 28. The output of the AND gate 22 is, in these circumstances, always low and, likewise, the output of the monostable oscillator 30 is always low as indicated by the solid line in Figure 3f. The arrangement is further such that when the touch pad 18 is touched, the pulse output of one of the Schmitt triggers 28 shifts at least partly into coincidence with the pulse output of the other Schmitt trigger 28. This causes a pulse signal to appear on the output of the AND gate 22 as indicated by dotted lines in Figure 3f, at a frequency corresponding to the frequency of the square wave oscillator 12, and having a pulse width which depends on the degree of coincidence. The "on" time of the monostable oscillator 30 is longer than the period of the square wave oscillator 12, so that the monostable oscillator converts the pulse output of the AND gate 22 to a level output. If desired, the output of the monostable oscillator 30 can be connected to a flip-flop circuit to form a latching switch. It will be appreciated that that part of the circuit formed by the first R-C circuit 16.1, the first Schmitt trigger 14.1, the first edge detector 20.1, the AND gate 22, and the retriggerable monostable oscillator 30 may be repeated several times to form as many touch switches, whereas that part of the circuit formed by the second R-C circuit 16.2, the second Schmitt trigger 14.2, and the second edge detector 20.2 may be used to form a common reference for the several touch switches.

Whilst, in the embodiment illustrated, the pulse outputs of the Schmitt triggers 28 shift into coincidence when the touch pad 18 is touched, it will be appreciated that the arrangement may also be such that the pulse outputs of the Schmitt triggers 28 are in coincidence when the touch pad 18 is not touched and shift out of coincidence when the touch pad is touched.

The Schmitt triggers are preferably of the CMOS type (eg SN74HC14, CD40106) .

Referring now to Figures 4 and 5, there is shown a touch switch 10.2 which, as in the Figures 1 and 2 embodiment comprises a square wave oscillator 12, a Schmitt trigger 14, an R-C circuit 16 connected between the output of the square wave oscillator and the Schmitt trigger, and an insulated touch pad 18 connected to a point between the resistive and capacitive components of the R-C circuit. In this instance the R-C circuit 16 is configured as a differentiator. The output of the Schmitt trigger 14 is connected direct to the input of a retriggerable monostable oscillator 30 similar to the one used in the Figures 1 and 2 embodiment.

Referring now to Figure 6, the square wave output of the oscillator 12 is indicated in Figure 6a and the pulse output of the Schmitt trigger 14 is indicated in Figure 6b. The Schmitt trigger 14 produces a positive-going pui a as indicated in Figure 6b, at each trailing edge of the output of the square wave oscillator, when the touch pad 18 is not touched. When the touch pad 18 is touched, there is in effect inserted an additional capacitance between the input of the Schmitt trigger 14 and earth and this causes a decrease in the pulse width of the pulse output of the Schmitt trigger 14. The components of the R-C circuit 16 are selected so that the pulse width of the output of the Schmitt trigger 14 decreases to such an extent that when the touch pad 18 is touched the pulse disappears altogether. Suitable values have been found to be 100KΩ for the resistive component of the R-C circuit, and 10 to 33pF for the capacitive component of the R-C circuit, where the supply voltage is 1.5V. However, these values may vary as will be obvious to those skilled in the art, once working with the circuits. As in the Figures 1 and 2 embodiment, the monostable oscillator 30 converts the pulse output of the Schmitt trigger 14 to a level output as indicated in Figure 6c. The output of the monostable oscillator 30 is normally high as indicated by the solid line in Figure 6c and goes low as indicated by the dotted line if the touch pad is held.

Referring now to Figure 7, there is shown an arrangement similar to the one shown in Figure 5, the same reference numerals being used to indicate the same parts, the arrangement in Figure 7 differs from that in Figure 5 in that the resistive component of the R-C circuit 16 is connected between the Schmitt trigger 14 and earth instead of between the input of the Schmitt trigger and the positive rail as in the Figure 5 embodiment. The effect of this is to provide inverted pulses on the output of the Schmitt trigger 14. This will require an alternative configuration of the monostable oscillator 30, as illustrated in Fiqure 7.

The oscillator frequency is not critical. The lower it is the less is the power consumption of the circuit. The sensitivity (ie the distance between the operator's finger and the touch pad that is required for the switch to operate) of the touch switches of the Figures 4 to 7 embodiments is adjustable by adjusting the pulse width. This can be done by providing the resistive component of the R-C circuit 16 with a trimpot and/or by providing the capacitive component of the R-C circuit with a trimmer capacitor.

The square wave oscillator 12 can drive a number of touch switches in parallel. It will easily be able to drive eleven touch switches, making use of only two SN74HC1415 integrated circuits. The "on" period of the monostable oscillator 30 should be longer than the period of the square wave oscillator, but short enough to give a sufficiently rapid response to the touch pad 18 being touched or released.

Claims

1. A proximity sensing arrangement having: a square wave oscillator; a Schmitt trigger' a first R-C circuit or an integrator coupled between the square wave oscillator and the input of the Schmitt trigger; and a proximity sensing pad coupled between the resistive and capacitive components of the R-C circuit.

2. The device of claim 1 wherein said R-C circuit is configured as an integrator.

3. The device of claim 1 or claim 2 further comprising a second R-C circuit or a second integrator.