GB2072915A

GB2072915A - Liquid crystal measuring display

Info

Publication number: GB2072915A
Application number: GB8108745A
Authority: GB
Original assignee: DIGITAL CAR INSTR
Current assignee: DIGITAL CAR INSTR
Priority date: 1980-03-25
Filing date: 1981-03-20
Publication date: 1981-10-07
Also published as: JPS56153213A

Abstract

A liquid crystal display system comprises front and rear plates 1 and 2 carrying aligned arrays of electrodes 5 and 6, each connected in a series string to act as a potential divider. In use, one end 8, 10 of each string is earthed and a variable potential difference is applied across the other ends 7 and 9 of the two strings. The number of cells in each string which are turned ON depends on the magnitude of the applied potential difference, thus providing a bar-graph-type display in which the magnitude of the applied potential voltage is indicated by the position of the boundary between those of the cells which are in the ON state and those which are in the OFF state. The supply is preferably alternating. <IMAGE>

Description

SPECIFICATION Liquid crystal displays This invention relates to liquid crystal display systems.

A conventional liquid crystal display comprises a pair of plates each carrying an electrode on the facing surfaces of the plates and with a liquid crystal held between the two electrodes. When there is a sufficiently large potential difference between the two electrodes the liquid takes up a quasi-crystalline structure which is readily visible.

Liquid crystals can be set in an array with input leads to the electrodes of each liquid crystal so that any combination of crystals can be displayed.

It is an object of this invention to provide a liquid crystal display whereby the total value of a parameter being monitored is indicated by energising all of an array of liquid crystals up to and including the crystal which is related to the instantaneous value of the parameter being monitored. With a conventional display, however, if there were a hundred individual liquid crystals this would require independent input leads to each of the electrodes associates with each crystal which would involve considerable assembly difficulties.

Accordingly this invention provides a liquid crystal display system comprising front and rear plates each carrying a respective array of electrodes, each array being electrically connected in a continuous series string, each electrode on the front plate being aligned with a respective electrode on the rear plate with a liquid crystal held between each such aligned pair of electrodes, and separate inputs to each string whereby a variable potential difference may be applied across the two strings through the liquid crystals.

Each liquid crystal element requires a threshold voltage for energisation and, once energised, a slightly lower hold on voltage is required to maintain the liquid crystal in the on state. Thus the number of liquid crystal elements which will be biased in the on state would depend upon the potential difference applied across the two strings.

As this potential difference varies, the number of liquid crystal elements in the on state will either increase or decrease. Thus this liquid crystal display system can be utilised to indicate an instantaneous value of a varying parameter in a barograph-type display provided by the column of lights formed by the liquid crystal elements.

Whilst the system could be operated by a D.C.

voltage it is much preferred to supply each input by a respective A.C. voltage source as this significantly increases the working life of the liquid crystal elements. The A.C. voltage sources could be in the form of sine waves or biphase square waves. Since the liquid crystal elements are temperature sensitive, if there is likely to be variation in temperature of the system during operation, it is desirable to incorporate a device for determining the threshold potential difference for operating the liquid crystal elements. This threshold potential difference will then be applied between the strings of electrodes as a permanent bias.The threshold determining device can advantageously comprise a further liquid crystal element comprising a liquid crystal held between electrodes on the two plates and a photocell for monitoring the light output from this additional liquid crystal element as a variable test voltage is applied to it.

Ideally the arrays of electrodes are set in linear strings so as to provide a straightforward barographic display of the value of the parameter being monitored.

The electrodes themselves will usually be conductive areas etched on to the plates, such as an indium-tin oxide applied to a glass plate and then etched in the desired pattern. Because these electrodes have a measurable resistance the strings of electrodes in effect form a double potentiometer. For most purposes it will be desirable to ensure that all the electrodes have the same resistivity but for certain applications it is useful to provide that the electrodes have, for example, a continuously varying resistance as one progresses along the string so as to provide a logarithmic type operation or a geometric progression.

The invention may be performed in various ways and one preferred embodiment will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates the main features of the liquid crystal display system of this invention; and Figure 2 is an exploded view of the two plates forming the system shown in Figure 1.

The liquid crystal display system shown in the drawings comprises a pair of glass plates 1, 2 laid on top of each other to form a sandwich 3. Within this sandwich is an array of 100 liquid crystal elements 4. Each liquid crystal element comprises a first electrode 5 etched onto the lower plate 1 and an aligned second electrode 6 etched onto the upper plate 2, with a liquid crystal held between each aligned pair of electrndes. The string of electrodes 5 on the plate 1 are series linked between an input lead 7 and an output lead 8 whilst the string of electrodes 6 on the plate 2 are series linked between an input lead 9 and an output lead 10. The output leads 8 and 10 are connected together to earth whilst the input leads 7 and 9 are connected to independent A.C.

voltage sources.

The A.C. voltage sources for the two strings of electrodes 5 and 6 operate at the same frequency and can be amplitude modulated in synchronisation but are applied 1 80C out of phase to the respective strings. The degree of amplitude modulation is determined by the value of a parameter being monitored so that, as the value of that parameter increases, the amplitude of the potential difference between the two strings of electrodes 5 and 6 will similarly increase. As this potential difference increases the number of liquid crystal elements switched into the on condition will similarly increase thus providing a barographtype display of the value of the parameter being measured. At a normal temperature of 250C, the threshold value for operation of each liquid crystal element is 1.9 v whilst the hold-on voltage is 1.7 v (i.e. 0.2 v less).Thus, for example, if a potential difference between the two strings of electrodes 5 and 6 of 3.9 v is applied, the first eleven of the liquid crystal elements will be held in the on condition.

Changes in the ambient temperature cause variation in the threshold operation voltage of the liquid crystal element. To compensate for such variation the device includes an additional liquid crystal element 11 associated with a photocell 12.

This additional element 11 comprises an electrode 13 on the lower plate 1 and an electrode 14 on the other plate 2 with liquid crystal held therebetween and respective input leads 15, 16.

In use, a variable potential difference is applied to the element 11 until the element switches to the on state which is recognised by the photocell 12.

The photocell 12 may be connected to circuitry for applying the threshold voltage required to energise the photocell 12 as a permanent bias to the signals applied across the input leads 7 and 9 to the string of liquid crystal display elements 4.

The additional liquid crystal element 11 is preferably masked from view.

The system illustrated in the drawings may be manufactured by conventional construction technic,ues. For example, the electrodes 5 and 6 and the connections therebetween may be formed by etching an indium-tin oxide coating on the glass sheets 1 and 2 to the desired pattern. The drive circuits for the string of liquid crystal display elements 4 and the monitoring element 11 will be readily apparent to those skilled in the art.

Alternating voltage supplies to the input leads 7 and 9 are greatly preferred to minimise the reduction in operating life of the liquid crystal elements and can be provided by sine wave inputs or ideally by biphase square wave inputs. It will also be appreciated that any of the available liquid crystal displays may be utilised since the device merely uses their conventional operating characteristics.

The display may be used to illustrate any of a number of parameters whose value tends to increase and decrease with time and operating conditions. For example, it may readily be used to indicate the speed of a vehicle or the temperature of an industrial process.

Claims

1. A liquid crystal display system comprising front and rear plates each carrying a respective array of electrodes, each array being electrically connected in a continuous series string, each electrode on the front plate being aligned with a respective electrode on the rear plate with a liquid crystal held between each such aligned pair of electrodes, and separate inputs to each string whereby a variable potential difference may be applied across the two strings through the liquid crystals.

2. A system according to claim 1 ,further comprising an additional liquid crystal held between electrodes on the two plates and a photocell for monitoring the light output from said additional liquid crystal element.

3. A system according to claim 1 or 2, wherein the resistance of each electrode in each of said series strings is the same as that of the others.

4. A system according to claim 1 or 2, wherein the resistance of the successive electrodes along the length of at least one of the strings varies continuously in a predetermined manner.

5. A liquid crystal display system substantially as hereinbefore described with reference to the accompanying drawing.