DE2707671A1 - LIQUID CRYSTAL DISPLAY DEVICE FOR THE TYPE GUEST-HOST EFFECT AND USE OF THIS LIQUID CRYSTAL DISPLAY DEVICE - Google Patents

Abstract

An electronic clock with a guest-host liquid-crystal display is described. The specified drive electronics subject the guest-host liquid-crystal display to frequencies in excess of 32 Hz, preferably about 1 Hz. As a result, the power consumption of the guest-host liquid-crystal display is reduced to such an extent that there is no need for the previously used complex twisted nematic cell. Apart from the price advantage, the elimination of the previously necessary polarisers allows the proposed guest-host liquid-crystal cell to have a greater viewing angle for the display. <IMAGE>

Description

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BBC Aktiengesellschaft Brown, Boveri & Cie., Baden (Switzerland)

Guest-host type liquid crystal display device and use of this liquid crystal display device .

The present invention relates to a liquid crystal display device, consisting of a liquid crystal display with one between two cell plates and At least one guest-host liquid crystal layer each located in the front and one rear electrode and one in the liquid crystal cell integrated reflector as well as from a control circuit, which the liquid crystal display with supplying changing control signals for information display.

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In modern electronic clocks, digital, twisted, nematic rotary cells are predominantly used to display the time used. The commercial

one

usual liquid crystal mixture requires your voltage of about 3 volts. In the case of an electronic wristwatch, for example, this is the case with a rectangular control voltage of 32 Hz an average drive current of 0.3 uA a. This results in an average power of 0.9 uW, which has to be delivered by the battery.

In principle, this power could be reduced somewhat, as the capacitive current consumption of a nematic rotary cell is directly proportional to the control frequency at low frequencies.

In practice, however, the control frequency cannot be reduced below a value of 32 Hz, since there is already a flickering here the display becomes visible to the eye of the beholder. This flickering is the result of the polarity reversal of the control voltage resulting loss of contrast which the viewer perceives and a decrease in the Field strength in the range of half the layer thickness of the liquid, crystal layer arises.

Using the example of an electronic wristwatch, the power consumption of display devices can be put into perspective. A modern electronic wristwatch with quartz

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oscillator and mos integrated circuit takes a power of about 4.5 uW. The previous calculated mean The current consumption of the nematic rotary cell is therefore in the order of magnitude of approx. 20 / S of the total current consumption of the Wrist watch. Experience has shown that such a clock can be operated for at least one year without changing the battery will. The expected further advances in integrated circuit technology will be a reduction allow the power consumption of electronic wristwatches, so that the same with smaller batteries

one
or also / slightly longer operating time without changing the battery

will be possible.

The disadvantage of the current state of the art is the high price of the nematic rotary cell, which means that it is more numerous for the production of necessary process steps.

Liquid crystal displays, which show the guest-host effect (see Physik in our Time, 1975, No. 3, page 71) use, are considerably cheaper to manufacture. and in the case of cholesteric guest-host liquid crystal displays also has the advantage of a wider viewing angle than rotary cells.

The present invention therefore has for its object to provide a liquid crystal display device which

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allows, without accepting any disadvantages, a liquid crystal display with a guest-host liquid crystal layer to use.

This object is achieved in that the following feature is provided according to the invention in an object of the type specified at the beginning: that the control signals alternate voltages or pulsating direct voltages with frequencies f of 32 Hz - ^ - frequency f = r. 0.125 Hz.

With the quantitative specification of the frequency of the control signals mentioned in the patent claim, a technical prejudice is created overcome:

Liquid crystal display devices are being used today with

en

higher frequency voltages »controlled, as it is operated with DC voltage be able to use irreversible electro-chemical processes (see Physics in our time, 6th year, 1975, No. 3).

Accordingly, the known integrated circuits for electronic clocks are for a 32 Hz AC voltage operation the liquid crystal displays, e.g. CAL-TEX MOS integrated circuits, CT 6002 (Contiflex AG, CH-87OO Küsnacht-Zurich).

In the following, on the basis of the drawing, execution

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Examples of the invention explained. It shows:

Fig. 1 The power consumption of a controlled according to the invention Guest-Host Liquid Crystal Display (G-H)

the
as well as ^ comparatively depicted operating

hold a conventional nematic rotating cell

(TN),

Fig. 2 is a block diagram of an electronic wristwatch with guest-host liquid crystal display;

3 shows the representation of the seconds with the control signals particularly suitable for this,

Fig. ¹ J control signals for operating a point flashing every second in a guest-host liquid crystal display.

The basic advantage of the liquid crystal display device according to the invention can be seen from the diagram in FIG. The curve labeled G-H shows the power consumption as a function of the frequency f it is a cholesteric guest-host liquid crystal display of the dimensions standardized for wristwatches, which is operated with a voltage of 5 »5 volts.

In contrast, the curve labeled TN shows the behavior of a nematic rotary cell, which with a voltage is operated by 3 volts. Here it is shown that "that

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the so-called flicker effect when operated below 22 Hz labeled F-E occurs. For this reason and for reasons of service life, the known Liquid crystal displays with drive signals of 32 Hz operated.

This comparison already shows that the usual power consumption of 0.9 μW at 32 Hz for the nematic rotary cell is only reached when the guest-host liquid crystal display is operated above 4 Hz. Accordingly, there is the advantage that, even at frequencies lower than k Hz, lower powers are sufficient for proper operation of the guest-host liquid crystal display. If, furthermore, it is taken into account that the production costs of the guest-host liquid crystal display operating without polarizers are significantly lower, the scope of the invention can be easily estimated accordingly.

From the practical embodiment Fig. 2, an electronic wristwatch, it can be shown that the

\ a guest-host liquid crystal display /

application "V no additional expenditure on control electronics is required. As usual with the previous electronic clocks, the structure of this wristwatch also consists of the basic components: clock circuit 20, display driver logic 30, guest-host liquid crystal display ¹ IO and

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from a battery 10 feeding the overall arrangement with an additional voltage converter V-C.

The clock circuit 20 comprises a quartz oscillator 1, which oscillates at a frequency of 32 * 768 Hz, a Frequency divider chain 2, a setting logic S-L, a second pulse generator 7> a minute pulse generator 8, an hour pulse generator 9 and a 7 segment decoder 7-Sg. The display driver logic 30 consists of a first inverting driver 3, a second (non-inverting) Driver 4, and first and second exclusive-OR drivers 5 and 6, respectively.

The guest-host liquid crystal display 40 has a common rear electrode R and front electrodes F to F _e .

The mode of operation of an arrangement according to FIG. 2 takes place in a manner known per se, such that the quartz oscillator 1 generated oscillation of 32 * 768 Hz in the downstream frequency divider chain 2 of the usual in flip-flops Art is divided into a frequency of 2 Hz or 1 Hz. The 2 Hz signal is routed via the inverting driver 3 and appears at its output as a control signal A- ,, which on the front electrode F ,, which the shape

is one point has supplied]. The 1 Hz signal of the

Frequency divider chain 2 is routed via driver 4

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and appears at its output as a control signal A ″, which is fed to the back electrode R. The other front electrodes F ₀ , F, which correspond to segments 2 and e, are supplied by the outputs of the exclusive OR drivers 5 and 6. The 1 Hz signal is also fed to the first inputs of these exclusive-OR drivers 5, 6, while a selection signal SS , or SS ₁ , is supplied.

This 7-segment decoder 7-Sg is controlled by the seconds pulse generator 7, minute pulse generator 8, and hour pulse generator 9> which deliver the actual time signals. A setting logic serving the basic setting of the time In addition to the frequency divider chain 2, S-L is connected upstream of these pulse generators 7-9 and allows them to be switched on in 1 Hz rhythm.

The entire arrangement according to FIG. 2 is supplied by a battery 10, which has a terminal voltage of 1.5 volts. The electronic voltage converter V-C transforms this 1.5 volt voltage to activate the liquid crystal layer necessary value of 5 »5 volts.

Depending on the type of liquid crystal substance or the desired degree of activation of the liquid crystal layer

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voltages from 2 to 15 volts are applicable, so that for example By connecting two 1.5 volt batteries in series, the voltage converter V-C can be eliminated can.

The arrangement according to FIG. 2 has proven itself in continuous operation; there was no decomposition of the cholesteric Liquid crystal substance is detected; the display worked absolutely during the entire test period flicker free.

in Fig. 3, the individual segments a-g are the seconds denotes a visual numerical display; the selectable selection states .0 .-. I. up to .9 are in the displayed next to each other in the first line.

Using the example of the individual segments e, the Fk layer behind it only when displaying .0., .2., .6. and .8th. is activated, the time course of the control signals normalized between + 1 and - 1 can be clearly seen regard. Where: A_ is the 1 Hz control signal applied to the rear electrode R; that on the front electrode F (segment e) applied control signal is denoted by A ^, while the voltage present between the front electrode F and the rear electrode R is V _ ".

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The time course of the 7-segment decoder 7-Sg output selection - signal S-S ^, which is the actual Contains time information, can be seen in FIG. 3 as the uppermost, non-normalized curve profile.

In FIG. 4, the control conditions at the front electrode F, which is a point flashing every second, are shown. Here, A, is the 2 Hz control signal at the front electrode F,; A »again the 1 Hz control signal applied to the rear electrode R, while the voltage established between the front electrode F and the rear electrode R _{is denoted by Vt 3 d} , where T im

^r l " ^R

present case is 1 second.

FIGS. 3 and 4 clearly show that in the display device according to the invention, pure DC voltages never arise which could destroy the choiesteric liquid crystal; the pulse duty factor of the pulsating DC voltage is 50 % in both cases.

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Claims (1)

BBC Baden 6/77 Claims lj liquid crystal display device, consisting of a liquid crystal display with a guest-host liquid crystal layer located between two cell plates and at least one front and back electrode and a reflector integrated in the liquid crystal cell as well as a control circuit which supplies the liquid crystal display with changing control signals for information display, characterized in that the control signals (A, -A _1. ) are alternating voltages or pulsating direct voltages with frequencies (f) of 32 Hz> »frequency (f) ^ * 0.125 Hz. 2. Liquid crystal display device according to claim 1, with numerical display, in an electronic timepiece with quartz oscillator and frequency divider chain, characterized in that the control signals (A ₂ - A ₁ J) are pulsating DC voltage with a duty cycle of 50i, which have a frequency (f) of 1 Hz and are generated by a frequency divider chain (2). 3. Liquid crystal display device according to claim 1 or 2, characterized in that the control signals 809831/0526 ORIGINAL INSPECTED BBC Baden 6/77 (A, - AJ have such amplitudes that at the guest-host liquid crystal layer, between opposite front and back electrodes in the selected state voltages from 2 volts to 15 volts to adjust. ¹ I. Liquid crystal display device according to claim 1 or 2, characterized in that control signals (A., A2) used to display seconds are pulsating DC voltages with a duty cycle of 50% , the control signal (A,) applied to the back electrode (R). a frequency (f) of 1 Hz and the drive signal (A _{2 ) applied to the front electrode (P 1} ) has a frequency (f) of 2 Hz. 5. Use of a liquid crystal display device according to one of claims 1, 2, 3 or 4 as a display element in portable chronometers, in particular in wristwatches. BBC Public Company Brown, Boveri & Cie. 809831/0526