CA1099041A - Liquid crystal display with heat responsive switch - Google Patents
Liquid crystal display with heat responsive switchInfo
- Publication number
- CA1099041A CA1099041A CA308,930A CA308930A CA1099041A CA 1099041 A CA1099041 A CA 1099041A CA 308930 A CA308930 A CA 308930A CA 1099041 A CA1099041 A CA 1099041A
- Authority
- CA
- Canada
- Prior art keywords
- liquid crystal
- display
- thick film
- potential difference
- electrodes
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Liquid Crystal (AREA)
- Liquid Crystal Display Device Control (AREA)
Abstract
Abstract of the Disclosure Each element of a display operated in a scan mode is series connected to a thick film switch which sets a voltage threshold to isolate the element from periodic scan pulses unless coincident with an element selecting data pulse in which case the thick film switches and the element, for example, a nemaric liquid crystal cell, operate.
- i -
- i -
Description
11~9904~
This invention relates to a threshold circuit for a voltage sensitive element and extends to a system of such elements and circuits in matrix form and to operation of the system. The invention finds particular application in liquid crystal displays.
In a typical liquid crystal display a number of electric field sensitive cells are located at the interstices of a matrix of row and column electrodes. A data signal applied to the display has a signal component consisting of scan pulses which are applied to successive ones of one set, say the rows, of electrodes and data pulses which are applied to appropriate ones of the columns of electrodes. The data pulses may be positive +VD or negative -VD whereas the scan pulses are arranged to be single valued, say vS. Thus when the potential difference across a cell is a maximum VD + vS~ the liquid crystal molecules are orientated so that the cell twists the plane of polarization of polarized light and when the potential difference is a minimum, v5 -VD the liquid crystal molecules adopt random orientation and the plane is not twisted.
In fact, because of the rate at which the scan pulses are applied, the cells respond to the r.m.s. of the applied voltage, so cells although not selected for data pulses, receive a voltage higher than vs-VD and consequently exhibit a partial "on" state. This wastes power and can reduce contrast between "on" and "off" parts of the display. The invention envisages a threshold circuit which is operable to enhance the display contrast.
According to the invention there is provided a threshold circuit for an element controlled by potential difference applied across a pair of control electrodes, said circuit comprising a thick film switch connected in series with the element between the control electrodes. In one embodiment of the invention the element '~
~99~41 comprises a liquid crystal cell in parallel with a resistor. The liquid crystal cell can be a pair of crossed polarizing plates with a layer of nematic liquid crystal material sealed therebetween.
In an alternative embodiment of the invention the element comprises resistive material coated with a layer of cholesteric liquid crystal material on a darkened substrate.
In the first embodiment a sufficiently high potential difference existing across the cell so orientates the nematic liquid crystal molecules that the plane of polarization of light incident on one polarizing plate is not extinguished by the other polarizing plate after passing through the cell.
In the second embodiment the thick film switch permits current to flow through and thereby heat the resistive material which, in turn, produces a colour change in the cholesteric film.
Preferably the threshold circuit has two such series connected thick film switches, one each side of said element and connecting the element to the respective control electrodes.
In another aspect of the invention, a panel comprises a first plurality of scan electrodes, a second plurality of data electrodes, a plurality of elements, each element connected in series with a thick film switch across a respective combination of one scan electrode and one data electrode, said element being controlled by a potential difference supplied across the scan and data electrodes.
The panel elements may be liquid crystal cells, based for example on nematic twisted crystal structure, or may be resistive material coated with cholesteric liquid crystal as previously proposed.
A method of operating the panel comprises applying scan pulses to successive scan electrodes and applying data pulses ~W91~41 to specific ones of said data electrodes according to the data content of a data input, the voltage pulse levels being so chosen that a predetermined cummulative voltage level of a data pulse and a scan pulse activates the thick film switch.
Two embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:-Figure 1 shows a circuit arrangement of part of aliquid crystal display indicating electrical connections between elements of the display and scan and data electrodes;
Figure 2 is a graphical representation of the percentage light transmission through a liquid crystal cell as a function of the potential difference applied across the cell, Figure 3 shows the components of a signal for operating the display of Figure l;
Figure 4 shows the circuit arrangement of Figure 2 enhanced by the addition of threshold circuit; and Figure 5 shows a circuit arrangement similar to that shown in Figure 3 but where elements of the display comprise heat responsive layers of cholesteric liquid crystal material.
Referring to the drawings in detail, Figure 1 shows in circuit schematic form a number of liquid crystal cells 10 connected into a cartesian matrix of electrodes 11 and 12. The cells 10 comprise two crossed polarizing plates between which is a layer of nematic liquid crystal material. The effect of applying electric field across the cell is to align molecules of material in such a way that the plane of polarization of light entering the cell through one polarizing plate is twisted and so a component of that light escapes through the other polarizing plate. A large number of the cells connected into the matrix is used as a display panel. In operation, B
lW9~9Ll one component of a display signal is applied successively to row electrodes 11 while another component of the display signal is applied to appropriate ones of the column electrodes 12. According to the total potential difference, and therefore electric field, produced across the liquid crystal cell, the cell transmits a percentage of incident light as indicated graphically by plot I in Figure 2.
A scanning pulse is applied cyclically to the electrodes 11 which are termed the scan electrodes, the applied scanning pulses being of fixed amplitude and sense. On the other hand, the electrodes 12, termed the data electrodes, receive, as appropriate, a data pulse, the timing of the pulse and the selection of electrode corresponding to a character to be displayed and being derived through pulse generation circuitry (not shown). As shown in Figure 3 the data pulse may be positive or negative according to whether the particular element is required to transmit light or not. If the scan pulse is at +v5 and the data pulse is VD then the total potential difference across the cell may be v5-VD and the cell characteristics are so chosen that in the resulting electric field the cell does not transmit light. Similarly if the potential difference across the cell is v5+VD, then the corresponding electric field will be such that the cell transmits.
Figure 2 shows graphically the relationship between electric field and light transmission for a liquid crystal cell.
It is clearly shown that the transmission characteristic of a liquid crystal cell changes over a relatively extensive range of electric field strengths. Consequently when the scanning pulse is applied to a scan electrode without a data pulse being applied to a data electrode, there is still some light transmission. Because the scan pulses are applied at a very high rate, the liquid crystal responds to the r.m.s.
of the scan pulse voltage the effect being that the liquid crystal ~99~41 cell is, throughout use, in a partially "on" state. This is undesirable for two reasons: firstly, power is dissipated and secondly contrast suffers.
Referring now to Figure 4 there is shown part of a matrix of liquid crystal cells with connections to scan electrodes and data electrodes, respectively 11 and 12. Between every scan electrode and data electrode are connected in parallel a liquid crystal cell 10 and a resistance 15. This parallel combination is series connected via thick film switches 16 which are chosen to operate when a voltage of vs+VD is applied between a pair of electrodes 11 and 12. As soon as the thick film switches 16 are brought into a low resistance state, terminals of the liquid crystal cell 10 are connected to a high potential difference, and the liquid crystal immediately changes its light transmission as shown at II
in Figure 3. For an unselected cell, the thick film switch is "off"
; and has a resistance approximately 104 X its resistance when conducting.
Consequently very little current flows through the resistance 15, potential across the liquid crystal cell 10 is negligible, and the cell is accordingly maintained in a completely "off" condition.
In another embodiment of the invention shown in Figure 5 a similar matrix of electrodes are used. Between each scan electrode 11 and data electrode 12 is connected in series a thick film switch 18 and a resistance 19 of the order of 50K ohms. The resistance 19 is in the form of a resistive strip which is coated with cholesteric liquid crystal material. The nature of cholesteric material is that across a certain temperature range it will change colour and at a threshold temperature an anisotropic to isotropic transition occurs accompanied by a change from opaqueness to transparency. Cooling the cholesteric material down results in the reverse sequence.
,,~,s~
9~4~
In a preferred embodiment the resistive material, by means of which the cholesteric liquid crystal material is heated to change its light transmission characteristics, is formed integrally with a thick film switch. A cartesian matrix of row and column electrodes has thick film switches deposited at the interstices of the electrodes. The substrate in the neighbourhood of the thick film switches is coated with a dark ink and the cholesteric liquid crystal is sprayed onto the ink before being encapsulated by a transparent protective cover of plastic or glass.
While the thick film switch is in its non-conducting state, the cholesteric liquid crystal material will be at a low temperature and will exhibit a bright colour. When an information signal is applied to the matrix of electrodes as described with reference to the previous embodiment, the thick film switches become conducting, and are thereby resistively heated to a threshold temperature at which the cholesteric liquid crystal material changes state. In the embodiment described, the presence of a data pulse on a data electrode 12 will be such as to produce a change from colour to darkness in its associated display panel element.
Other cholesteric liquid crystals are known which are colourless below a threshold temperature and exhibit colour above this threshold. In a corresponding display panel, the appearance will be of coloured elements on a black background.
A typical thick film switch for use in operating both the nematic liquid crystal display and the cholesteric liquid crystal display is vanadium oxide (V02) which switches at approximately 70C. Appropriate cholesteric material is obtainable from Ashley Butler Inc. under specification CR (colourless to coloured;
operating at a temperature within the range of 0C to 150C), or specification CC (coloured to colourless; operating at a temperature range 20C to 70~C).
This invention relates to a threshold circuit for a voltage sensitive element and extends to a system of such elements and circuits in matrix form and to operation of the system. The invention finds particular application in liquid crystal displays.
In a typical liquid crystal display a number of electric field sensitive cells are located at the interstices of a matrix of row and column electrodes. A data signal applied to the display has a signal component consisting of scan pulses which are applied to successive ones of one set, say the rows, of electrodes and data pulses which are applied to appropriate ones of the columns of electrodes. The data pulses may be positive +VD or negative -VD whereas the scan pulses are arranged to be single valued, say vS. Thus when the potential difference across a cell is a maximum VD + vS~ the liquid crystal molecules are orientated so that the cell twists the plane of polarization of polarized light and when the potential difference is a minimum, v5 -VD the liquid crystal molecules adopt random orientation and the plane is not twisted.
In fact, because of the rate at which the scan pulses are applied, the cells respond to the r.m.s. of the applied voltage, so cells although not selected for data pulses, receive a voltage higher than vs-VD and consequently exhibit a partial "on" state. This wastes power and can reduce contrast between "on" and "off" parts of the display. The invention envisages a threshold circuit which is operable to enhance the display contrast.
According to the invention there is provided a threshold circuit for an element controlled by potential difference applied across a pair of control electrodes, said circuit comprising a thick film switch connected in series with the element between the control electrodes. In one embodiment of the invention the element '~
~99~41 comprises a liquid crystal cell in parallel with a resistor. The liquid crystal cell can be a pair of crossed polarizing plates with a layer of nematic liquid crystal material sealed therebetween.
In an alternative embodiment of the invention the element comprises resistive material coated with a layer of cholesteric liquid crystal material on a darkened substrate.
In the first embodiment a sufficiently high potential difference existing across the cell so orientates the nematic liquid crystal molecules that the plane of polarization of light incident on one polarizing plate is not extinguished by the other polarizing plate after passing through the cell.
In the second embodiment the thick film switch permits current to flow through and thereby heat the resistive material which, in turn, produces a colour change in the cholesteric film.
Preferably the threshold circuit has two such series connected thick film switches, one each side of said element and connecting the element to the respective control electrodes.
In another aspect of the invention, a panel comprises a first plurality of scan electrodes, a second plurality of data electrodes, a plurality of elements, each element connected in series with a thick film switch across a respective combination of one scan electrode and one data electrode, said element being controlled by a potential difference supplied across the scan and data electrodes.
The panel elements may be liquid crystal cells, based for example on nematic twisted crystal structure, or may be resistive material coated with cholesteric liquid crystal as previously proposed.
A method of operating the panel comprises applying scan pulses to successive scan electrodes and applying data pulses ~W91~41 to specific ones of said data electrodes according to the data content of a data input, the voltage pulse levels being so chosen that a predetermined cummulative voltage level of a data pulse and a scan pulse activates the thick film switch.
Two embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:-Figure 1 shows a circuit arrangement of part of aliquid crystal display indicating electrical connections between elements of the display and scan and data electrodes;
Figure 2 is a graphical representation of the percentage light transmission through a liquid crystal cell as a function of the potential difference applied across the cell, Figure 3 shows the components of a signal for operating the display of Figure l;
Figure 4 shows the circuit arrangement of Figure 2 enhanced by the addition of threshold circuit; and Figure 5 shows a circuit arrangement similar to that shown in Figure 3 but where elements of the display comprise heat responsive layers of cholesteric liquid crystal material.
Referring to the drawings in detail, Figure 1 shows in circuit schematic form a number of liquid crystal cells 10 connected into a cartesian matrix of electrodes 11 and 12. The cells 10 comprise two crossed polarizing plates between which is a layer of nematic liquid crystal material. The effect of applying electric field across the cell is to align molecules of material in such a way that the plane of polarization of light entering the cell through one polarizing plate is twisted and so a component of that light escapes through the other polarizing plate. A large number of the cells connected into the matrix is used as a display panel. In operation, B
lW9~9Ll one component of a display signal is applied successively to row electrodes 11 while another component of the display signal is applied to appropriate ones of the column electrodes 12. According to the total potential difference, and therefore electric field, produced across the liquid crystal cell, the cell transmits a percentage of incident light as indicated graphically by plot I in Figure 2.
A scanning pulse is applied cyclically to the electrodes 11 which are termed the scan electrodes, the applied scanning pulses being of fixed amplitude and sense. On the other hand, the electrodes 12, termed the data electrodes, receive, as appropriate, a data pulse, the timing of the pulse and the selection of electrode corresponding to a character to be displayed and being derived through pulse generation circuitry (not shown). As shown in Figure 3 the data pulse may be positive or negative according to whether the particular element is required to transmit light or not. If the scan pulse is at +v5 and the data pulse is VD then the total potential difference across the cell may be v5-VD and the cell characteristics are so chosen that in the resulting electric field the cell does not transmit light. Similarly if the potential difference across the cell is v5+VD, then the corresponding electric field will be such that the cell transmits.
Figure 2 shows graphically the relationship between electric field and light transmission for a liquid crystal cell.
It is clearly shown that the transmission characteristic of a liquid crystal cell changes over a relatively extensive range of electric field strengths. Consequently when the scanning pulse is applied to a scan electrode without a data pulse being applied to a data electrode, there is still some light transmission. Because the scan pulses are applied at a very high rate, the liquid crystal responds to the r.m.s.
of the scan pulse voltage the effect being that the liquid crystal ~99~41 cell is, throughout use, in a partially "on" state. This is undesirable for two reasons: firstly, power is dissipated and secondly contrast suffers.
Referring now to Figure 4 there is shown part of a matrix of liquid crystal cells with connections to scan electrodes and data electrodes, respectively 11 and 12. Between every scan electrode and data electrode are connected in parallel a liquid crystal cell 10 and a resistance 15. This parallel combination is series connected via thick film switches 16 which are chosen to operate when a voltage of vs+VD is applied between a pair of electrodes 11 and 12. As soon as the thick film switches 16 are brought into a low resistance state, terminals of the liquid crystal cell 10 are connected to a high potential difference, and the liquid crystal immediately changes its light transmission as shown at II
in Figure 3. For an unselected cell, the thick film switch is "off"
; and has a resistance approximately 104 X its resistance when conducting.
Consequently very little current flows through the resistance 15, potential across the liquid crystal cell 10 is negligible, and the cell is accordingly maintained in a completely "off" condition.
In another embodiment of the invention shown in Figure 5 a similar matrix of electrodes are used. Between each scan electrode 11 and data electrode 12 is connected in series a thick film switch 18 and a resistance 19 of the order of 50K ohms. The resistance 19 is in the form of a resistive strip which is coated with cholesteric liquid crystal material. The nature of cholesteric material is that across a certain temperature range it will change colour and at a threshold temperature an anisotropic to isotropic transition occurs accompanied by a change from opaqueness to transparency. Cooling the cholesteric material down results in the reverse sequence.
,,~,s~
9~4~
In a preferred embodiment the resistive material, by means of which the cholesteric liquid crystal material is heated to change its light transmission characteristics, is formed integrally with a thick film switch. A cartesian matrix of row and column electrodes has thick film switches deposited at the interstices of the electrodes. The substrate in the neighbourhood of the thick film switches is coated with a dark ink and the cholesteric liquid crystal is sprayed onto the ink before being encapsulated by a transparent protective cover of plastic or glass.
While the thick film switch is in its non-conducting state, the cholesteric liquid crystal material will be at a low temperature and will exhibit a bright colour. When an information signal is applied to the matrix of electrodes as described with reference to the previous embodiment, the thick film switches become conducting, and are thereby resistively heated to a threshold temperature at which the cholesteric liquid crystal material changes state. In the embodiment described, the presence of a data pulse on a data electrode 12 will be such as to produce a change from colour to darkness in its associated display panel element.
Other cholesteric liquid crystals are known which are colourless below a threshold temperature and exhibit colour above this threshold. In a corresponding display panel, the appearance will be of coloured elements on a black background.
A typical thick film switch for use in operating both the nematic liquid crystal display and the cholesteric liquid crystal display is vanadium oxide (V02) which switches at approximately 70C. Appropriate cholesteric material is obtainable from Ashley Butler Inc. under specification CR (colourless to coloured;
operating at a temperature within the range of 0C to 150C), or specification CC (coloured to colourless; operating at a temperature range 20C to 70~C).
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-
1. A display cell comprising a plurality of display elements, each display element defined by a pair of spaced electrodes flanking an electro-optic material whose optical transmissivity is dependent on potential difference applied thereacross, means for applying a potential difference between the electrodes of each element, a plurality of switch elements series-connected to respective display elements, each switch element comprising a thick film material having a large negative temperature coefficient of resistance in a predetermined switching temperature zone, wherein, in use, switch elements subjected to the predetermined threshold potential difference experience joule heating to said switching temperature zone and the proportion of the applied potential difference appearing across the display element is thereby markedly and suddenly increased.
2. A display cell as claimed in claim 1 in which said electro-optic material is a liquid crystal.
3. A display cell as claimed in claim 2 in which the spaced electrodes and said thick film switch elements are formed on spaced transparent plates flanking a nematic liquid crystal.
4. A display cell as claimed in claim 2 in which said electro-optic material is a cholesteric liquid crystal overlying a resistive material on a reflective substrate, the resistive material being adapted to undergo joule heating when said thick film switch switches whereby to heat the liquid crystal and so induce an optical transmissivity change therein.
5. A display cell as claimed in claim 1, in which said material comprises vanadium dioxide (VO2).
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA308,930A CA1099041A (en) | 1978-08-08 | 1978-08-08 | Liquid crystal display with heat responsive switch |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA308,930A CA1099041A (en) | 1978-08-08 | 1978-08-08 | Liquid crystal display with heat responsive switch |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1099041A true CA1099041A (en) | 1981-04-07 |
Family
ID=4112078
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA308,930A Expired CA1099041A (en) | 1978-08-08 | 1978-08-08 | Liquid crystal display with heat responsive switch |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1099041A (en) |
-
1978
- 1978-08-08 CA CA308,930A patent/CA1099041A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |