Use
of the integrated circuit MM 74C221 in a circuit Controller of
Reflexes |
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Created it, 06/10/19
Update it, 06/10/26
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5. - FOURTH EXPERIMENT : EXAMINATION OF THE INTEGRATED CIRCUIT MM 74C221
The monostable truth studied in the preceding experiment was carried out with a rocker D. It exists however integrated circuits designed especially to function like monostable under optimal conditions. You use the MM 74C221 here. This integrated circuit contains two monostable independent. The representation is given to the figure 6-a.
It is necessary to add an external resistance and a condenser with the integrated circuit to make function the monostable one.
The pins to which these two components are connected are located by R / C ext. and C ext. indicated on figure 6.
The monostable ones being two, the pins of the first carry index 1, those of the second index 2.
The terminals C ext. and R / C ext. relating to the first monostable one are located by 1 C ext. and 1 R / C ext. ; those relating to the second by 2 C ext. and 2 R / C ext.
The duration T of the impulse of exit of monostable, measured in seconds is equal to the product of Rext, expressed in megohm and Cext expressed in microfarad.
By connecting a resistance of 1
MW and a condenser of 1 µF, the
impulse will thus last 1 second. The
monostable one has two principal entries A
and B, an entry CLR
(abbreviation of CLEAR) ; and two
exits Q and 
.
5. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food and remove all the components and the connections carried out during the preceding experiment.
b) Carry out the assembly indicated to the figure 7-a by respecting the orientation of the integrated circuit and the polarities of the condenser into tantalum.
c) Preserve the integrated circuit MM 74C00 in position IC1 (debouncing).
The electric diagram of the circuit is represented with the figure 7-b. You notice that the second monostable one is not used.
The entry 1A
is connected to the contact P0
, the entry 1B with switch SW0
and entry CLR with switch SW1.
The exit 1Q order the LED L0.
5. 2. - OPERATIONAL TEST
a) Connect the food, the LED L0 is extinct. It can ignite an initial short moment. Indeed, the monostable one is in a stable state which corresponds to a level L for the exit Q.
b) Support and slacken the P0 button. The LED L0 ignites during 1 second.
c) Now, support on P0 and maintain it in this position : L0 remains extinct.
d) Slacken P0 : L0 ignites during 1 second. You thus note that it is by slackening the P0 button that the monostable one starts and delivers an impulse at exit. This monostable is started by a downward face applied to entry A.
e) Now put SW0 on position 0. The entry B is on a level L.
f) Support and slacken P0. The LED L0 does not ignite. The entry B is thus an entry of validation. The monostable one can be started only if this entry B is on a level H.
g) Now, interchange the
connections relating to two entries A and B.
entry A is connected to SW0,
the entry B in P0
.
h) With SW0 on position 0, you impose the state L on entry A.
i) Support and slacken the P0 button : The LED L0 ignites 1 second.
j) Put SW0 on position 1, entry A is with the state H. Actionnez P0. The LED L0 remains extinct.
The functions of entries A and B are now reversed : Entry A became the entry of validation and the entry B orders the monostable one.
You can now check if one needs a rising face or a downward face applied to the entry B to order the monostable one.
k) For that, support on P0 and maintain to it a certain time in this position, then slacken it. You observe if the LED L0 ignites while supporting or by slackening P0. You defer to the table of operation of figure 8. It is well a face going up applied out of B which starts the monostable one.
It remains to examine the operation of entry CLR.
l) Support on P0, the LED L0 ignites, immediately put switch SW1 on 0 and bring back it on 1.
You thus apply during a short moment a level L to entry CLR.
You note that the LED L0 dies out before the duration of impulse of 1 second was not passed.
You can also increase the value of R and of C. entry CLR is the entry of restoring priority. It can be activated constantly by a level L.
m) Check the variations of
by connecting pin 4 of the integrated
circuit into the LED L1 and by repeating the
preceding tests.
All these results are consigned in the table of operation.
In this table, the arrow 
indicates a positive transition from the low state towards the high state ;
the arrow 
indicates a negative transition. The symbol “X”
in this table indicates that the logical state can be indifferently 0
or 1.
6. - FIFTH EXPERIMENT : USE OF THE
MONOSTABLE
DOUBLE
MM 74C221 IN
A CIRCUIT CONTROLLER OF REFLEXES
Like example of application of the integrated circuit MM 74C221, you will now carry out a circuit with which it is possible to check the speed of the reflexes and to have the notion of the time which passes.
6. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food and remove only the connections relating to the last circuit carried out and leave the components in place.
b) Insert an integrated circuit MM 74C74 in support ICX.
c) Carry out the connections indicated in the figure 9-a also illustrating the site where two other components must be inserted : a resistance of 1 MW and a condenser of 1 µF forming cell RC for the second monostable one of the MM 74C221.
While inserting the condenser, respect the polarities of the terminals while referring to you with the figure 9-a.
d) Place switch SW0 on position 1.
You thus carry out the circuit represented by the diagram of the figure 9-b. As you can observe it, the two monostable ones are connected one following the other ; the first monostable one is ordered by the P1 button.
The rocker D has its entry DATED connected to the exit Q from the second monostable one and receives on its entry CLOCK the control signal of the P0 button ; switch SW0, connected to entry CLEAR, makes it possible to give the rocker to zero if one positions it on 0.
The signals present at the exits Q of the two monostable ones and the rocker D are visualized by means of the LED L4, L1 and L0.
6. 2. - OPERATIONAL TESTS
a) Connect the food and observe the LED L1 and L4.
You note that they are extinct or remain lit during a short moment, while the LED L0 probably ignites.
In this case, give the rocker to zero while temporarily putting SW0 on position 0 and bring back it on position 1. Now, the circuit is ready to function.
b) Support and slacken the P1 push-button.
The LED L4 ignites as soon as button is slackened and remains lit approximately 1 second. Immediately after, as soon as L4 dies out, L1 also ignites during 1 second.
If the circuit does not function as envisaged, carefully check the connections as well as the insertion of the integrated circuits and the components.
c) Actuate P1 again and while L1 is lit, support on P0. The LED L0 ignites and remains lit even after the extinction of L1.
The goal of this assembly is to check the speed of the reflexes: it is a question of supporting on P0 during the short moment when L1 is lit. If you support during this lapse of time, L0 ignites and attests good test result. In the contrary case, it remains extinct.
The first monostable one is used to make run out a sufficient time between the pressure of P1 and the moment when it is necessary to support on P0. Naturally, with the LED which remain lit during 1 second, the play is rather easy. To make it more difficult, it is enough to reduce the time of illumination of L1.
d) For that, disconnect the food and remove R2 resistance of value 1 MW connected between pin 7 of the MM 74C221 and the positive tension and replace it by a resistance of 470 kW.
e) Connect the food, put at zero the rocker by actuating SW0 and remade the experiment.
You observe that L1 remains lit during a shorter time : 1 / 2 second approximately instead of 1 second what makes more difficult to light L0 of the first blow.
While decreasing the value of R2 further, you make the play even more difficult ; for example by taking R2 = 330 kW, the effective time to support on P0 become equal to 1 / 3 s, the value close to the limit of the minimum time of human reaction.
Another means of making the play more difficult consists in disconnecting the LED L4 so as to more not have the indication which precedes lighting by L1. One can also change the R1 value to vary the moment when L1 ignites. By using resistances and the condensers at your disposal, you can obtain various combinations of the delay with which L1 ignites and of time during which it remains lit. You make thus the control of the reflexes very easy or very difficult.
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