Bill-posters
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Bill-posters
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Created it, 06/09/09
Update it, 06/09/25
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2. 3. - MULTIPLEXED BILL-POSTERS
Let us suppose that we want to post a number with 4 digits. By the preceding method, it will be necessary 4 bill-posters, 4 decoders for us, 28 resistances of connection is 29 connections on the whole of the bill-posters.
This assembly is represented figure 40.
To post 12 digits, which is not rare on a computer, one would need 85 files of connection and 12 circuits of binary decoding - 7 segments.
In order to simplify wiring, there are bill-posters with several figures known as multiplexed.
Figure 41 represents the internal diagram of a multiplexed bill-poster.
The principle of these bill-posters is very simple :
Diodes LED have a response time very short
and in any event shorter than that of the human eye.
The retinal persistence of the eye does not make it possible to perceive the
flutter of a lamp of which the period of recurrence is lower than 20
ms (indicative value for a cyclic report/ratio of 1).
It is thus enough to send successively on the bus lines a, b, c, d, e, f, g the codes corresponding to the figures to be posted and this in a cyclic way : unit, ten, hundred, a thousand, unit, ten, hundred, a thousand, unit,…. and so on.
One calls bus lines : a whole of drivers allowing simultaneously to distribute information to several circuits.
It will be obviously necessary to select simultaneously the anode of the bill-poster concerned so that the figure appears only over the decade and only the decade concerned : unit, ten,….
The distribution of signals is carried out by means of an electronic switch that we will call multiplexer and who will be examined later on, this is why it is schematized by a contact in figure 42.
There are decoders specialized for circuits multiplexed bill-posters.
Figure 43 shows the use of such a decoder.
In this figure a block of 4 bill-posters is represented as well as the decoder/multiplexer which is associated for him.
One can also meet blocks of bill-posters much more important containing decimal points and signs (more or less).
In this field each manufacturer presents a different line of goods which can meet all the needs.
2. 4. - BILL-POSTERS BY MATRIX LED
A type of bill-poster very much used employs 37 LED in the form of points laid out according to a matrix with 7 lines and 5 columns plus 2 decimal points (DP1 and DP2), one located on the right and the other on the left (figures 44 and 45).
As for the bill-poster with 7 segments by lighting the adapted LED, it is possible to obtain the 10 decimal digits ; moreover, thanks to the number of more important LED, it is possible to represent other characters among which all letters of the alphabet and various signs like +, -, /, (,) and others still, as represented on figure 46.
These bill-posters require because of the greatest number of LED of the more complex circuits of decoding.
With regard to the order of these circuits, it is easy to understand that considering the principle of the matrix with diode (figure 44) if one wants for example to light the diodes with the intersections line 3 column 3 and line 4 column 4, one will light the diode located inevitably line 4 column 3.
One can thus deduce from it that it is necessary to have a circuit carrying out the multiplexing of the entries line and column so that the lighting of the diodes is not simultaneous but takes place the ones after the others in order to remove this defect.
This type of circuit, also called circuit of sweeping, can be external or internal with the bill-poster.
The multiplexers being still unknown to us, will not say we longer on this type of bill-poster.
There is also another type of bill-poster at points of which the use is simpler and in which the characters available are limited to those of the hexadecimal code in other words to the ten digits from 0 to 9 and to the letters of A to F, as you can see it figure 47.
The LED are fewer, that is to say 20 for the characters and 2 for the decimal point and these circuits thus do not require multiplexing.
In figure 48, you can see how are laid out diodes LED and the circuits of memory (Latch), decoding and piloting, are incorporated in the same case as the LED.
The decoder :
The most important characteristic of this decoder is the fact that this one is included in the case what makes it possible to directly send the binary number provided by the meter on the bill-poster.
Circuit latch with 4 bits :
It is used to store and post at the time desired the provided binary number.
That is very useful bus often the number to be visualized is available only during one very insufficient short moment to be perceived by the eye. By memorizing it in the latch, one can then visualize it during a time independent of logic so that it is seen correctly.
Pilot circuit :
This circuit makes it possible to provide to the LED the current food necessary by means of a generator of constant current. It is equipped with a terminal to which one can apply a signal which orders the simultaneous extinction of all the LED. This is very useful when numbers with several figures must be posted with an independent bill-poster for each figure.
If one wants to post a number much lower than the maximum capacity of the bill-poster, this number will appear with one or more 0 on the left.
For example with 4 bill-posters, the hexadecimal number that one can post will be FFFF. A number much smaller such 23 will appear in form 0023.
The posted number is perfectly comprehensible but one can wish for reasons esthetic to remove these 0. In this case while acting on the entry of extinction, one can erase them.
2. 5. - OTHER TYPES
OF BILL-POSTERS
2. 5. 1. - OTHER BILL-POSTERS WITH LED
There also exists of other types of bill-posters with LED with 9, 14 or 16 segments (figure 49-a).
Each time, it will be enough to have the adequate decoder.
Most advanced (16 segments) allows all the representations alphanumeric.
There are bill-posters known as bill-posters with going beyond (called sometimes half Digit).
These bill-posters comprise only 5 segments, and are used to indicate the sign + or -, as well as figure 1.
In a voltmeter with digital display for example, the first bill-poster (on the left) will be 1 / 2 Digit.
The figure 49-b represents a posting of 3 Digits 1 / 2.
2. 5. 2. - BILL-POSTERS A LIQUID CRYSTALS
The bill-posters with crystals liquid or L.C.D. (Liquid Cristal Display) include/understand segments like the bill-posters with LED.
a - Physics of the liquid crystals
Certain bodies, although liquids, have a structure close to that of the solid crystals. They are characterized by a specific alignment their molecules.
One distinguishes three types of structures :
The smectite state :
The molecules are divided into layers which regularly pile up the ones on the others. It is of currently any industrial interest (figure 50-a).
The state cholesteric :
In each plan, the molecules are parallel. When one passes successively from a plan to another, the axis of the molecules turns of a constant angle.
So this structure is called: helical structure (figure 50-b).
The nematic state :
The large axes of the molecules are parallel to a certain direction but their centers of gravity are distributed randomly as in an ordinary liquid.
The molecules can move in all directions in three dimensions (figure 50-c).
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