Created it, 06/10/19
Update it, 06/10/30
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11. - TENTH EXPERIMENT: USE Of a MULTIPLEXER WITH EIGHT WAYS TO PROVIDE the FUNCTION Of a NETWORK OF DOORS
Until now, we used the multiplexer to commutate logical signals. In the experiment which will follow, on the other hand, the multiplexer will be used to replace a network of logical doors which would have claimed the use of several integrated circuits.
Like example of application, we will take into account the heating installation of which the diagrammatic representation is given on figure 35.
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A boiler is supplied by two different tanks, one principal and the other of reserve when one of the two taps which regulate the arrival of fuel is on. This circuit must moreover indicate if the principal tap and the tap of reserve are turned off simultaneously.
To detect if the taps are opened or turned off, one inserts in the installation two switches S1 and S2 ordered by the taps.
To detect the presence of flame in the boiler, one uses a S0 photodiode like sensor. This one, in the presence of light, behaves like a closed switch and in absence of light like an open switch.
The circuit which one must carry out will be connected to the photodiode and the switches as one sees it on figure 36.
When a tap is turned off, the corresponding switch is closed and this one puts at the mass the entry of the checking circuit.
When a tap is on, the corresponding switch is opened and a positive tension is applied to the entry of the checking circuit through the resistance connected to “+”.
In the same way, the light of the flame makes the photodiode conducting, which carries the entry corresponding of the checking circuit at the high level. In the absence of flame, the photodiode is blocked and this same entry passes on the low level.
By associating as usual the logical state 1 A a high tension and the tension of mass at the logical state 0, the three sensors provide following information:
S2 = 0 turned off principal tap
S2 = 1 open principal tap
S1 = 0 tap of reserve closed
S1 = 1 open tap of reserve
S0 = 0 absence of flame
S0 = 1 presence of flame
It can present three different situations, namely :
All is normal ;
One is using the reserve ;
Operation is abnormal.
The first situation is checked when :
S2 = 0 (turned off principal tap). No heating during the summer
S1 = 0 (tap of reserve closed) No heating during the summer
S0 = 0 (there is no more flame) No heating during the summer
or when :
S2 = 1 (open principal tap) Period of heating
S1 = 0 (tap of reserve closed) Period of heating
S0 = 1 (there is a flame) Period of heating
When the reserve is used, one is in the second situation and one a :
S2 = 0 (turned off principal tap) Period of heating
S1 = 1 (open tap of reserve) Period of heating
S0 = 1 (there is a flame) Period of heating
The third situation (abnormal operation) is checked for all the other combinations of the three sensors and it must correspond to an alarm.
The various situations considered are summarized in the table of figure 37.
| S2 | S1 | S0 | Exit |
|
0 |
0 | 0 | Normal |
|
0 |
0 | 1 | Alarm |
| 0 | 1 | 0 | Alarm |
| 0 | 1 | 1 | Reserve |
| 1 | 0 | 0 | Alarm |
| 1 | 0 | 1 | Normal |
| 1 | 1 | 0 | Alarm |
| 1 | 1 | 1 | Alarm |
Again let us examine the truth table of the multiplexer with eight ways of figure 28 and rewrite it in the way indicated on figure 38.
| STROBE | S2 | S1 | S0 | The Y exit follows the value of the entry |
| 0 | 0 | 0 | 0 | D0 |
| 0 | 0 | 0 | 1 | D1 |
| 0 | 0 | 1 | 0 | D2 |
| 0 | 0 | 1 | 1 | D3 |
| 0 | 1 | 0 | 0 | D4 |
| 0 | 1 | 0 | 1 | D5 |
| 0 | 1 | 1 | 0 | D6 |
| 0 | 1 | 1 | 1 | D7 |
| 1 | X | X | X | Y = 0 |
Let us connect the exit Y to the LED L0. This one will be charged to indicate the three situations (“normal”, “alarm”, “reserve”) in the following way :
Normal : Extinct L0, Y = 0
Alarm : L0 flickers at the frequency of 10 Hz, Y = 10 Hz
Reserve : Lit L0, Y = 1.
To carry out these three conditions, it is enough to connect the signal wished to the entry which is validated by the combination of the control signals of the multiplexer.
For example, if S2, S1 and S0 have the logical value 0, the LED L0 must announce the “normal” state and must thus be extinct. It is thus necessary to connect the D0 entry to the mass.
The various entries will be connected as follows :
D0 entry connected to the mass
D1 entry connected to the 10 Hz
D2 entry connected to the 10 Hz
D3 entry connected to “+”
D4 entry connected to the 10 Hz
D5 entry connected to the mass
D6 entry connected to the 10 Hz
D7 entry connected to the 10 Hz.
11. 1. - REALIZATION OF THE CIRCUIT
a) Remove matrix all the connections and the integrated circuits used in the preceding experiment.
b) Insert the integrated circuit MM 74C151 in the position indicated to the figure 39-a and carry out the connections relating to this assembly.
You notice that the entries of order A, B, C are respectively connected to switches SW0, SW1, SW2. Those simulate the switches of the two taps and the photodiode.
SW2 replaces S2, SW1 replaces S1, while SW0 provides the function of the S0 photodiode.
11. 2. - OPERATIONAL TESTS
a) Place SW0, SW1 and SW2 on position 0.
b) Energize Digilab and observe L0 : it is extinct.
c) By actuating the switches, test all the possible combinations and control if the situations enumerated in the following table are checked.
| SW2 | SW1 | SW0 | L0 |
| 0 | 0 | 0 | Extinct |
| 0 | 0 | 1 | Flicker to 10 Hz |
| 0 | 1 | 0 | Flicker to 10 Hz |
| 0 | 1 | 1 | Lit |
| 1 | 0 | 0 | Flicker to 10 Hz |
| 1 | 0 | 1 | Extinct |
| 1 | 1 | 0 | Flicker to 10 Hz |
| 1 | 1 | 1 | Flicker to 10 Hz |
d) The experiment being finished, put not under tension Digilab.
You thus noted that by using only one multiplexer, one can carry out an assembly of a certain complexity.
To realize of the saving made by the use of a multiplexer instead of a network of doors, as well in the component count used as in that of the connections, it is enough to observe the diagram of figure 40 which illustrates one of the solutions making it possible to obtain the same result but with logical doors.
The circuit of figure 40 would require three standard integrated circuits for its realization.
This practice is finished, the following one will be devoted to the examination of the memories.
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