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Signet :

Using an eight-way Multiplexer to provide the Function of a Gate Network :


So far we have used the multiplexer to switch logic signals. In the following experiment, however, the multiplexer will be used to replace a logic gate network that would have required the use of several integrated circuits.

As an example of application, we will take into consideration the heating installation whose schematic representation is given in Figure 35.


A boiler is fed by two different tanks, one main and one reserve when one of the two valves that regulate the arrival of the fuel is open. This circuit must also indicate whether the main valve and the reserve valve are closed simultaneously.

To detect if the valves are open or closed, two switches S1 and S2 controlled by the valves are inserted in the installation.

To detect the presence of flame in the boiler, a photodiode S0 is used as the sensor. This, in the presence of light, behaves like a closed switch and in the absence of light, like an open switch.

The circuit to be realized will be connected to the photodiode and switches as shown in Figure 36.


When closing a valve, it closes the corresponding switch and the latter ground the input of the control circuit.

When a valve is open, the corresponding switch is opened and a positive voltage is applied to the input of the control circuit through the resistance connected to the "+".

In the same way, the light of the flame makes the photodiode conductive, bringing the corresponding input of the control circuit to the high level. In the absence of a flame, the photodiode is blocked and this same input goes low.

By associating as usual logical state 1 with a high voltage and the ground voltage with logic state 0, the three sensors provide the following information :

S2 = 0    main valve closed

S2 = 1    main tap open

S1 = 0    closed supply valve

S1 = 1    open reserve valve

S0 = 0    no flame

S0 = 1    presence of flame

There may be three different situations, namely :

  • Everything is normal ;

  • We are using the reserve ;

  • The operation is abnormal.

The first situation is true when :

  • S2 = 0     (main valve closed).         No heating during the summer

  • S1 = 0     (closed supply valve).       No heating during the summer

  • S0 = 0     (no flame).                        No heating during the summer

or when :

  • S2 = 1    (main valve open)          Heating period

  • S1 = 0    (closed supply valve)     Heating period

  • S0 = 1    (there is a flame)            Heating period

When we use the reserve, we are in the second situation and we have :

  • S2 = 0    (main valve closed)        Heating period

  • S1 = 1    (open supply valve)        Heating period

  • S0 = 1     (there is a flame)           Heating period

The third situation (abnormal operation) is true for all other combinations of the three sensors and must correspond to an alarm signal.

The different situations considered are summarized in the table of Figure 37.

Fig. 37. - Operating table of the control circuit.
S2 S1 S0 Output
   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

Let's look again at the eight-way multiplexer truth table in Figure 28 and rewrite it as shown in Figure 38.

Fig. 38. - Truth table indicating the input of the multiplexer connected to the output thereof when S0 is connected to control input A, S1 to input B and S2 to input C.

STROBE S2 S1 S0 Output Y follows the value of the input
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

Connect the Y output to the L0 LED. This will be responsible for indicating the three situations ("normal", "alarm", "reserve") as follows :

Normal : L0 off, Y = 0 

Alarm : L0 flashes at the frequency of 10 Hz, Y = 10 Hz 

Reserve : L0 on, Y = 1.

To achieve these three conditions, it is sufficient to connect the desired signal to the input which is enabled by the combination of the control signals of the multiplexer.

For example, if S2, S1 and S0 have the logic value 0, the L0 LED should indicate the "normal" state and should therefore be off. It is therefore necessary to connect the input D0 to ground.

The different entries will be linked as follows :

  • Input D0 connected to ground 

  • Input D1 connected to 10 Hz

  • Input D2 connected to 10 Hz

  • Input D3 connected to "+"

  • Input D4 connected to 10 Hz

  • Input D5 connected to ground

  • Input D6 connected to 10 Hz

  • Input D7 connected to 10 Hz.


a) Remove from the matrix all the links and integrated circuits used in the previous experiment.

b) Insert the integrated circuit MM 74C151 in the position indicated in Figure 39-a and make connections related to this assembly.


You notice that the control inputs A, B, C are respectively connected to the switches SW0, SW1, SW2. These simulate the switches of the two valves and the photodiode.

SW2 replaces S2, SW1 replaces S1, while SW0 performs the function of the photodiode S0.


a) Place SW0, SW1 and SW2 on the 0 position.

b) Turn on the Digilab and observe L0 : it is off.

c) When operating the switches, try all possible combinations and check if the situations listed in the following table are true.

SW2 SW1 SW0 L0
0 0 0 Off
0 0 1 Blinks to 10 Hz
0 1 0 Blinks to 10 Hz
0 1 1 Lit
1 0 0 Blinks to 10 Hz
1 0 1 Off
1 1 0 Blinks to 10 Hz
1 1 1 Blinks to 10 Hz

d) After the experiment is complete, turn off the Digilab.

You have thus found that using a single multiplexer, one can realize a montage of a certain complexity.

To realize the savings made by the use of a multiplexer instead of a network of doors, both in the number of components used and in the connection, just look at the diagram in Figure 40 which illustrates one of the solutions allowing 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 over, the next one will be devoted to the examination of the memoirs.

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