Bill-posters
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Created it, 06/09/09
Update it, 06/09/25
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b - Constitution of bill-posters L.C.D.
The bill-posters with liquid crystals consist of a layer of liquid crystal ranging between two plates glass covered with conducting materials.
The alignment of the molecules of the crystal can be changed by application of a terminal voltage of its two plates.
This change of alignment makes it possible to visualize segments comparable with those of a bill-poster LED with this close which they emit no light.
The electrical contact with the liquid crystal is carried out by means of a transparent driver. As the appearance of the sign depends on alignment on the molecules, the direction with which the light strikes the crystal is critical. For this reason, one uses filters polarizing the light which are fixed at before and at the back of the bill-poster in order to control this one.
c - Principle of operation of bill-posters L.C.D.
It is based on the principle of the “reflection” or the “not reflection” of the light through polarizing filters.
The light is an electromagnetic wave length ranging between 400 and 750 nanometers. In the case more running, the oscillations of the electric fields is based in any direction. One calls “natural” light.
There is coherent” or “polarized” light called a “whose oscillations of the electric field are done in a quite precise direction (case of the laser), (figure 51-a).
(The oscillations of the electric field are done in a precise direction).
The light “polarized” (or coherent) is obtained by diffusing wild light in an opaline medium or “filters polarizing”. These filters are consisted crystals directed in a well defined direction (figure 51-b).
It is important to note that these filters are transparent and nonopaque.
Experiment of the two polarizing filters :
Let us suppose a source of light, resulting from a bulb, or obtained by reflection using a polished surface.
While placing on this source of light two filters directed in the same direction, one notes that the light the cross-piece, without notable absorption, as the figure 51-c represents it.
By crossing one of the two filters gradually, one notes that the light passes more and more badly until total absorption (crossed 90° filters as represented with the figure 51-d).
Now let us introduce a crystal thin, transparent, between the two filters (sheet of Plexi for example), the light crosses the two filters again. This is due to the optical activity of the crystal, which makes set out again the light in a well defined direction and cancels the effect of crossing of the polarized filters.
The liquid crystal, introduced in a thin layer between the two crossed filters, will restore, like the plexi, the passage of the light.
An electric field crossing this unit locally, will disorientate the crystals of the liquid crystal, and again, the light will not pass any more (figure 51-e).
d - Various types of bill-posters L.C.D.
There are two types of bill-posters with liquid crystals :
Systems with dynamic diffusion :
They use a pure nematic crystal to which one applies an electric field accompanied by passage of current.
In the absence of tension applied, the liquid crystal is perfectly transparent. When a tension is applied, the crystal strongly becomes diffusing because the molecules are directed in all directions.
Systems with field effect :
When one subjects a nematic liquid crystal to an electric field, it follows from there a variation in the direction of the alignment of the molecules (helical rotation of the molecules) without involving phenomenon of dynamic diffusion. One adopts an alternating voltage of frequency higher than the frequency of relieving of the liquid crystal but too weak to cause the oscillation of the molecules, is approximately 5 kHz.
In the systems with field effect, one uses the effect of helical nematic which makes it possible to vary the rotation of the polarized light in a continuous way of 0 with 90°.
One thus causes an optical delay which makes it possible to break up the white light. One obtains thus red, yellow, green contrary to the systems with dynamic diffusion which are them achromatic (not clean colors).
The nematic used is a butyl-aniline derivative, the transparent electrodes are out of tin oxide or indium oxide.
These systems are of very low fuel consumption and their high contrast makes it possible to use them in the watches.
e - Control circuits of bill-posters L.C.D.
For the reasons seen previously but also to avoid a deposit by electrolysis which could damage the bill-posters those are ordered by an alternating signal not having a continuous component.
For the bill-posters called to field effect, one uses a tension of ordering from 2 to 10 V and a few kilocycles of frequency.
For the bill-posters with dynamic diffusion, one uses a tension of order between 7 to 30 V and 20 to 400 Hz.
One orders the bill-posters with liquid crystals by means of special decoders providing the suitable square wave voltages.
Figure 52 shows the connection of a decoder / driver with a bill-poster L.C.D. with field effect.
Figure 53 represents the internal structure of a decoder / driver for bill-poster L.C.D.
The decoder is supplied between VSS and VDD is 5 to 15 volts.
The bill-poster as for him is fed between VDD and VEE, which makes it possible thanks to the circuit adapter of level to lay out on the exit of the decoder of a level of amplitude 15 V even if the level available on entries B.C.D. is only of 3 V for example.
Strobe the valid entry to 1 makes it possible to transfer code B.C.D. present at the entry from the integrated circuit into the entry of the adapter of levels.
Entry DF (Frequency Display) is fed by a square signal of 5 V of amplitude and frequency between 30 Hz and 200 Hz.
When a code BCD is present at the entry of the adapter of level for DF = 0, the corresponding exit of the segment is high.
If a square signal is present at entry DF, the corresponding exit of the selected segment is fed by a signal out of phase of 180° compared to DF. The exits of the not selected segments are fed by a square signal in phase with DF, so that only the selected segments see appearing a potential difference on their terminals (tension between exit of the ordering of bill-poster for the selected segment and DF).
The chronogram of figure 54 shows the various signals such as they were described.
f - Favor bill-posters with liquid crystals and disadvantages :
Their consumption is quasi null from where
them generalized use in the watches with quartz (consumption:
approximately 15 nano-amps per square
millimetre).
Their contrast increases with
illumination.
They do not produce a light by themselves
and thus require a good illumination.
Their response time is high : 100 to 300 milliseconds.
Their price is high.
Owing to the fact that they require an alternating signal, the multiplexing is
difficult.
2. 5. 3. - BILL-POSTERS WITH FILAMENTS WITH INCANDESCENCE
In this technology, each segment is in fact a filament identical to that of an incandescent lamp.
The advantage of these bill-posters is their great luminosity and the possibility with a filter of obtaining any color.
Their principal disadvantage is their consumption.
Figure 55 shows the stitching and the internal organization of a bill-poster with filament minitron as well as an example of connection (decoder 7447 being represented only by its synoptic).
Decoder 7447 allows to the direct order bill-posters filaments because its exits can receive up to 40 mA (see figure 37).
The figure 55-c represents the connections to be carried out to order the circuit.
We can see that all the filaments are ordered between 0 and 5 V. All 5 V (pins 2, 5, 10, 12, 13) are connected together. The loose lead of the filament is then ordered by the exit corresponding at the low state of the decoder thus running a current through the filament.
It is recommended for this kind of bill-posters to feed the filaments by a source of food to constant current independent of the load in order to have a uniform luminosity on all the segments of all the bill-posters.
2. 5. 4. - FLUORESCENT
TUBES
Technology employed points out that of the vacuum tubes.
A filament heated just below the incandescence acts as cathode and thus ensures the emission of electrons.
Metal segments laid out as those of the bill-posters 7 segments act as anodes.
When an anode is selected, electrons emitted by cathode - filament come “to bombard” the selected segments. The tube containing a gas derived from phosphorus, it follows an emission of blue light from there in the vicinity of the selected segments.
This technique allows the realization of panels from 8 to 16 digits.
Taking into account its low cost, Japanese uses this technology in the computers of offices.
2. 5. 5. - CATHODE
RAY TUBES
Their principle is that of the vacuum tube.
The electrons emitted by a cathode hot by a filament F are concentrated by a called special grid Wehnelt, are accelerated by an anode A and are concentrated in a fine brush then deviated by deflection plates horizontal and verticals then projected on the screen consisted the bottom of the tube covered with phosphorus. The electronic “bombardment” of the screen then produces a light or spot which one can vary the form.
A cathode ray tube is represented figure 56-a. Its internal constitution is represented figure 56-b. Its principle of operation is illustrated appears 56-c.
Certain cathode ray tubes have three guns making it possible to excite phosphorus giving a red, green or blue light in order to restore the color. One calls these “trichromatic” tubes in opposition to the other tubes known as “monochromic”.
The cathode ray tubes require high tensions (several thousands of volts).
The cathode ray tubes allow a considerable flexibility in use. Indeed, they make it possible to represent alphanumerics or alphabetical, decimal digits, special symbols, graphs…
They equip a multitude with computer terminals.
However, it is necessary to have a generator of characters in order to modulate the brush of electrons emitted by cathode to obtain the desired sign.
Figure 57 gives the synoptic diagram of the unit.
2. 5. 6. - COMPARISONS
BETWEEN THE VARIOUS BILL-POSTERS
You can see on the table of figure the 58 all possible comparisons enter the various types of bill-posters.
It is to be however noticed very low fuel consumption of the bill-posters with liquid crystals.
You can also notice the very long time of “reaction” of these bill-posters.
2. 5. 7. - CONCLUSION
In this theory, we saw a significant number of circuits of posting and circuits decoders.
We saw a number important of codes, other codes exist still the such code U.S.A.S.C.I.I. which will be treated in the continuation when we examine the microprocessors.
In the next theory, you will discover the adders, comparators as well as the multiplexers/demultiplexers
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