Created it, 06/10/19
Update it, 06/10/24
N° Visitors
3. - ASSEMBLY OF THE PUSH-BUTTONS GROUP
First of all, remove all the connections carried out in the first experiment. Then give the integrated circuit MM 74C00 in graphited foam.
Now resume work on the digilab, while carrying out the assembly of the push-buttons group.
a) Position one of the push-buttons on the plate of the printed circuit above the P0 initials (figure 4-a). The flat part of the cylindrical higher part is opposed to the P0 inscription. The indications NC, NO, C, marked on the base plate of the push-button must correspond to those serigraphies on the plate of printed circuit (figure 4-b).
b) You ensure that the push-button is introduced thoroughly on the plate and weld the seven terminals.
c) In the same way, weld the second push-button in P1 (figure 4-a).
d) Check the value with the ohmmeter of resistances R26, R27, R28, R29, (4,7 kW, 1 / 4 W, 5%), “yellow, purple, red, gold” and weld these four resistances to the sites indicated onto the plate (figure 4-a). Take the advice given before for the assembly of resistances.
3. 1. - CONTROL HORS-TENSION
Use the controller on the gauge W x 1 000 and refer you with the indications carried in the table of figure 5.
NOTE : The mass corresponds to the grip of the black cord of food.
| N° | Points of connection of the ohmmeter | Position of the push-buttons | Values obtained |
|
1 |
Between R26 side P0 and mass |
Supported P0
Slackened P0 |
0 W
6,3 to 7,7 kW |
|
2 |
Between R27 side P0 and mass |
Supported P0 Slackened P0 |
6,3 to 7,7 kW 0 W |
|
3 |
Between R28 side P0 and mass |
Supported P1 Slackened P1 |
6,3 to 7,7 kW 0 W |
|
4 |
Between R29 side P0 and mass |
Supported P1 Slackened P1 |
0 W 6,3 to 7,7 kW |
Carry out the first control by connecting the black test probe to the crocodile clip of the black cord of the food and touch with the other test probe the terminal directed towards the group of push-buttons of R26 (figure 6).
While pressing on the P0 button, you must find a value of null resistance, while by slackening this push-button, you must find a value of resistance ranging between 6,3 and 7,7 kW.
Then carry out following controls by touching one after the other the points indicated in table figure 5. If you do not obtain the results envisaged, check the weldings carried out, ensure you that there are not short-circuits between close copper tracks and that the push-buttons were assembled correctly.
4. - AN APPLICATION
OF ROCKER R-S
The push-buttons which you mounted function like reversers, each one have three terminals. The central terminal marked by C is the common terminal in contact with terminal NC when the button is at rest (slackened button). While pressing on the button, the terminal C is in contact with the terminal NO (figure 7-a). This type of button is particularly adapted to generate impulses of order, of more or less long duration.
For that, it would be sufficient, at first sight, to carry out the simple circuit represented figure 7-b. At rest, terminal A is with the potential 0 volt. While pressing on the button, terminal A is with the potential + V.
The tension on terminal A compared to the mass is represented theoretically on the figure 7-c under the two conditions, button at rest and inserted button.
In practice, the situation is quite different. The push-button is not a perfect mechanical device and the metal contacts rebound several times one on the other with closing. Practically, the tension on terminal A varies as indicated in the figure 7-d. The amplitude, the duration and the number of the generated rebounds depend on the characteristics of the push-buttons and the circuit to which they are attached.
These impulses of tension created with the closing of contacts C-NO and C-NC are very generally undesirable for the electronic circuits. To avoid this disadvantage, one will use the circuit rocks examined previously. To test this use, proceed in the following way :
a) Introduce the integrated circuit MM 74C00 into support IC1. With cabled resistances R26, R27, R28 and R29, you obtain thus the two identical debouncing circuits represented figure 8.
The two debouncing exits of the
circuit (figure 8-a) are in connection with the contacts of the group of
connectors indicated by the inscription P0
and P0
. Those of the circuit (figure 8-b) are in connections with the
indicated connectors P1
and P1
. For each push-button, the marks NC
and NO indicate the contacts in connections
with the common terminal C in the two
positions, slackened button, supported button.
b) Carry out now the
connections indicated figure 9. You connect the four exits P0
,
P0 ,
P1 ,
P1
respectively to the four LED L3, L2, L1, L0.
Connect the food (pile). You observe that the LED L0
and L2 are extinct while the LED
L1 and L3 are lit.
Indeed, you find
one of the cases seen figure
3. Entry 9 of the circuit MM 74C00
is with the state L (position rest of the P0
button) and entry 13 is with the state H
thanks to the R26 resistance connected to
the positive tension (figure 8-a). Thus the P0
exit
in the state H and L3
is lit ; the P0 exit
is in the state L and L2
extinct.
Now carry out the following test with P0.
c) Press on the P0 button : L2 ignites and L3 dies out. The P0 button in the position indicated in is dotted to the figure 8-a.
Entry 9 of the rocker is thus carried through R27 strength to the level H, while entry 13 is carried on a level L thanks to the P0 push rod.
There is thus swing of the exits, P0
passes to the state H and P0
to the state L.
d) P0,
you slacken find the former situation, i.e. L3 lit
and L2 extinct. In this way, an impulse is
produced on the exit P0
and its duration is equal to the time during which the P0
button remains inserted. This impulse is negative because the exit with the
state H passes on the level L
to return then to the state H. It is
indicated by the symbol .
At the same time, a positive impulse
occurred on the P0 exit
; this exit on the level L passed on
the level H then returned on the level L
after a certain time. This positive impulse is indicated by the symbol .
The figure 7-c represents these two impulses such as you could see them by means of an oscilloscope. There is no phenomenon of rebound due to the mechanical contacts.
Indeed, when you pressed on the P0 button, as soon as the contact was established for the first time between C and NO, the rocker changed state. The contact broke several times before being final, however these successive rebounds of contact C-NO did not involve change of state of the rocker which memorized the first passage to the state L of its entry 13.
The process is the same one when you slacken P0 and that the contact between NC and C is established.
You thus obtain impulses of tension on the exits very close to the ideal impulses represented figure 7-c, moreover the passage of a logical level at the complementary level is very fast because only related to the commutation rate or of very short transition from the rocker.
e) You can carry out the same tests with the second rocker represented figure 8-b.
If the circuit does not function as envisaged, check the connections carried out as well as the continuity of the copper tracks of the printed circuit.
In the event of need, to finish you can replace the integrated circuit MM 74C00 by the other identical circuit in your possession.
Now, leave the circuit MM 74C00 permanently inserted in support IC1 in order to use the two debouncing circuits.
You will have thus on the digilab of two generators impulses which will be used to you to order the logical circuits that you will realize thereafter. This will enable you to have impulses free from parasites.
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