Created it, 06/10/19
Update it, 06/10/27
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9. - EIGHTH EXPERIMENT : USE OF A CIRCUIT COMPARATOR AND MONOSTABLE TO CARRY OUT A CIRCUIT A COMBINATIONS
You now will carry out a circuit which is a new play : to find the combination of four switches in a limited time, fixed in advance.
The gaining combination is programmed by that which carries out the circuit. There are sixteen possible combinations.
The goal of this handling is not the play in itself, but the examination of an integrated circuit which you do not know yet and who is called binary comparator.
The comparator that you will use is the circuit MM 74C85 and its diagram is deferred on figure 15. This integrated circuit can compare four binary digits with four others and recognize if they are equal or not.
On the diagram of the MM 74C85, you notice four noted entries A0, A1, A2, A3 ; to these entries correspond four other respectively noted entries B0, B1, B2, B3.
This circuit compares the eight entries two to two, i.e. it compares the level of tension of the A0 entry with that of the B0 entry. In the same way, it compares between them the entries A1 and B1, A2 and B2, A3 and B3.
If the signals on the entries are equal two to two, i.e. if A0 is on the same logical level as B0, like A1 with B1, A2 with B2, A3 with B3, the exit connected to pin 3 (and who is noted A = B) goes on the level H.
In all the other cases, even if if only one couple of entry is not on the same level, the exit A = B goes on the level L. the integrated circuit MM 74C85 lays out, moreover, other entries (pins 4, 5, 6) and left (pins 12 and 13) noted A > B, A < B, A = B.
These other entries and exits are however not taken into account in this experiment and their function will be developed later in the course when the arithmetic binary one is treated.
9. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food and remove matrix all the connections and the components of the preceding experiment.
b) Insert on the matrix the integrated circuits MM 74C221 and MM 74C85, a resistance of 1 MW and an electrolytique capacitor with the tantalum of 10 µF by respecting the polarities of its terminals and by conforming you on figure 16.
Carry out then the connections illustrated on this same figure : of the three connections traced in dotted line, carry out only that which connects pin 3 of the integrated circuit MM 74C85 with the contact L4 (LED L4).
The electric diagram of the connections carried out on the comparator MM 74C85 is represented on figure 17.
The entries A0, A1, A2, A3 are connected respectively to switches SW0, SW1, SW2, SW3 : the entries B0 and B2 are connected to the positive tension, therefore are on the level H, while the entries B1 and B3 are connected to the mass and are thus on the level L.
Do not omit the integrated circuit MM
74C74 on support IC3.
c) Introduce the integrated circuit MM 74C08 (which contains four doors AND) into support ICX.
9. 2. - OPERATIONAL TESTS
Before supplementing the circuit so as to also use the monostable one and a circuit AND, check before all the operation of the comparator.
a) Put all the switches on position 0 and connect the food.
Observe the LED L4 : it is extinct. It is thus because entries A are not all on the same level as the corresponding entries B. Indeed, the levels of tension present on entries A and B being indicated in the table of figure 18, it appears that two couples of entries only on four are on the same level. Under these conditions, the exit A = B is on the level L and the LED L4 is extinct.
b) Put the switches on various positions, you thus test the various possible combinations on entries A.
You note that the LED L4 ignites only when entries A are on the same level as the corresponding entries B and that occurs only for the following combination of levels :
A0 = H
A1 = L
A2 = H
A3 = L
Naturally, it is possible to change the combination of levels on the entries B by connecting each one of them to the positive tension or the mass, so as to obtain the combination of desired reference.
While laying out suitably switches SW0, SW1, SW2 and SW3, you will manage to light the LED L4.
c) Disconnect the food and remove the connections going of pin 3 of the MM 74C85 to the L4 contact and connect the L0 contact with contact OUT 1 of the group of connectors.
Figure 19 illustrates the circuit carried out. Note that the integrated circuit MM 74C74 is that assembled definitively on support IC3 of the digilab.
Still note that the exit of the binary comparator is connected to the entry of the circuit AND and that the other entry of this same circuit is connected to the exit of the monostable one.
The operation of the circuit is as follows. While supporting and by slackening the P1 push-button, the monostable one and the rocker which orders the lighting of L0 put at zero. Then while acting on the P0 button, the monostable one generates a positive impulse lasting one approximately 10 second, which thus validates the exit of the comparator.
So during this time (10 seconds approximately), you succeed in positioning the switches on a combination such as entries A and B is equal two to two, the exit of the comparator passes on the level H and through the circuit AND, creates a face going up on the entry of clock of the rocker. One obtains the lighting of L0 thus.
To check the operation of the circuit, proceed as follows :
d) Put all the switches on position 0 and connect the food : the LED L0 will probably ignite. Press on the clearing button P1 so that the LED L0 dies out and that the circuit is in starting position.
e) Actuate the P0 button. You have 10 seconds to find the good combination with the switches which determines the lighting of L0.
For you who know the correct combination, that will be easy, but for another person, it will be more difficult and the difficulty could be increased by reducing the serviceable time to find the good combination. This can be obtained by decreasing the value of Cext or Rext.
Each time L0 ignites, it is necessary to give the circuit under the initial conditions while pressing on the P1 button.
Of course, if the gaining combination were not found in assigned time, you can remake an attempt by actuating the P0 button again.
The serviceable time to find the good combination can be visualized by connecting the LED L7 to the exit of monostable (pin 13) as indicated in dotted in figures 16 and 19.
In the next practice, you will examine two other circuits of great utility, the rockers of Schmitt (or trigger of Schmitt) and the oscillators
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