Examples
of monostable application |
Pseudo-monostable |
Pseudo-monostable
realized with an Integrated circuit of type 555 |
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Created it, 06/09/09
Update it, 06/09/18
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This new theory will enable you first of all to examine the monostable circuits, then the rockers of Schmitt and the oscillators.
These new circuits call upon concepts of analogical electronics (or fundamental), because of resistances and condensers are generally necessary to their operation.
1. - THE MONOSTABLE ONES
1. 1. - DEFINITION AND FUNCTION
In the preceding theory, various logical circuits were presented to you. It was primarily of the rocker D main slave and rocker J.K. However, you noted that these rockers were characterized by two stable states, these rockers placing from one state to another under the effect of an order external with the circuit. These rockers remain permanently in the state where they are until the control signal makes them rock in the stable state complementary to the preceding state.
This is the definition of the “bistable” circuits.
In this theory, you will see “monostable” circuits having one state of stability. Under the effect of an external order, these monostable can pass in a state complementary to the stable state, but invariably finds their state of stability after one duration determined by the type of circuit. In fact, these circuits have two complementary logical states well, but one is stable, the other is not it.
The stable state is the at-rest state of the circuit. Figure 1 shows you a simple representation of monostable.
The logical signals present at the entry and in exit are presented to you at figure 2.
The state of stability of monostable is characterized by two logical levels L, with the entry and the exit of the circuit.
On the other hand, if one applies a level of input voltage of the circuit to moment t1, the exit passes on a level H transitory which lasts T seconds and falls down on the level L at the moment t2. It is the fixed duration T which is the fundamental characteristic of the monostable one. This duration T is determined by the user according to the application in which the monostable one is inserted.
1. 2. - EXAMPLES
OF APPLICATION
Here two examples which highlight the utility of the monostable ones.
1. 2. 1. - CIRCUIT OF VISUALIZATION OF A SHORT IMPULSE
In an electronic unit, it can be necessary to visualize an impulse of short duration in a given point of this unit.
However, for durations lower than 1 / 10 of second approximately, it is impossible to perceive the illumination of a LED. It will be necessary to use monostable which will play a part of temporization by creating an impulse at its sufficiently long exit to light a pilot LED.
The standard assembly is that indicated on figure 3.
An impulse of exit of 1 second approximately is in this case sufficient for the illumination of the LED.
1. 2. 2. - MEASUREMENT OF FREQUENCY
This application of a monostable circuit makes it possible to measure the frequency of a signal. The general diagram is indicated on figure 4.
With the entry of monostable the signal is applied which one wants to measure the frequency.
At the exit of monostable a network made up of a diode is located, of a condenser and two resistances. The two cases A and B indicated on figure 5 make it possible to include/understand the operation of the circuit.
In case A, the frequency of the signal is relatively weak. With each impulse at the entry, an impulse corresponds to item 1 at exit of the monostable one.
This impulse allows the load of the condenser C through R1 resistance, because the diode is then polarized in the direct direction or passer by. The condenser thus takes care during the period of impulse T, then the tension as in point 1 falls down on the bottom grade. The diode is then polarized in reverse and the condenser tends to discharge through R2 resistance. The tension as in point 2 is thus a continuous positive tension which has a weak undulation. It is possible to compare it to a continuous tension.
In the case B, the frequency is much higher. The operation of the circuit is the same one as in case A, but if the time of load of the condenser is identical, the time of discharge is much shorter.
The condenser tends to much less discharging that in case A and the tension as in point 2 will be higher than in case A. There is thus proportionality between the frequency of the signal which arrives at the entry of the circuit and the tension continues that one collects at exit of the circuit. This assembly is thus a frequency meter or a converter frequency-tension.
These two examples show the realizable applications using monostable circuits.
1. 3. - MONOSTABLE
CIRCUITS
These circuits are subdivided in two categories, first of all that of pseudo-monostable, then that of monostable truths.
1. 3. 1. - PSEUDO-MONOSTABLES
These various assemblies require certain conditions to function into monostable, whereas monostable truths can not observe these conditions.
a) Basic circuit.
It is the circuit represented on figure 6.
The symbol 
indicates only one amplifier, i.e. the output signal varies in the same
direction as that present at point VR.
When the entry passes from the level L on the level H, the condenser behaves like a short-circuit and point VR is carried on the level H. Ensuite, the condenser C takes care through resistance R as indicated in figure 7.
When the tension at point VR crosses the threshold of swing of amplifier A, the exit falls down on the level L. the duration T is thus only given by the values of R and of C. It is worth 0,7 R.C roughly. This and valid in technology CMOS.
It should be noted that the entry must remain on the level H at least as a long time as the duration of the impulse of exit of the monostable one. Indeed, if the signal present at the entry passes by again on the level L before the end of this period, the tension at point VR would pass by again with a value close to the logical level L and the exit would thus pass by again on the level L. the impulse of exit would thus be curtailed. It is because of this particular condition of operation that this type of circuit is classified in the category of themonostable ones.
By replacing amplifier A by a reverser, it is possible to obtain a negative impulse, as indicated on figure 8.
This same type of assembly can also be started by a transition from a level H towards a level L, as indicated in the figures 9-a and 9-b.
Another disadvantage of this basic assembly exists.
Indeed, if fresh impulse of order arrives at the entry of monostable immediately after that which precedes it, it can occur the problem according to illustrated on figure 10.
When the entry passes by again at moment t3 on the level L, the tension at point VR becomes negative because the condenser is then charged. From t3, the condenser starts its discharge. If fresh impulse of order occurs before its complete discharge, the tension at point VR does not go as high as back to moment t1. The tension at point VR will thus cross the logical threshold L before the duration T was not passed. The duration T' of the impulse of exit will be lower than T. This imposes an additional condition for a correct operation of the monostable one.
It is possible to obviate this disadvantage by adding a diode as the assembly of figure 11 shows it.
Indeed, at the moment t3, when the entry passes by again on the level L, the condenser C discharges almost instantaneously through the diode D polarized in the direct direction.
The various tensions are indicated on figure 12. The potential at point VR will not go down in lower parts from - 0,6 volt, threshold of tension of an ordinary diode to silicon.
Thus, this assembly will make it possible to take into account a train of very brought closer impulses.
b) Pseudo-monostable without network RC
For memory, this circuit is indicated on the figure 13 bus it makes it possible to include/understand the function of the monostable one, but is not used in general in an assembly.
Operation rests on the fact that there is a certain travel time through a logical door whatever it is (NAND, NOR…). This is illustrated on the figure 14. Times td and t'd can be equal or not.
In the example quoted on figure 13, the voltage waveform at points A and B and the exit is illustrated on figure 15.
The signal at point A is reversed compared to that at the point B with each transition with a delay TD or T'D due to the sum from the travel times from the three reversers.
In the same way, td and t'd are the travel times of the signal through logical door NAND.
It is visible on the diagram of figure 15 that the time-constant of this monostable is T appreciably with TD. The impulse of order must always be of a greater duration than the duration T of the impulse of exit.
c) Pseudo-monostable
realized with an integrated circuit of type 555.
This integrated circuit is of an employment running in the electronic assemblies. In figure 16, it is assembled into pseudo-monostable (the figures indicate the pins of the circuit).
This integrated circuit includes/understands
primarily a tension divider bridge with three resistances, two comparators
symbolized 
,
a rocker RS whose truth table is deferred on
figure 17, and an electronic switch I.
This switch obeys the following rule of operation: it is opened with powering or when the exit Q is on the level H. When it is on the level L, it is closed.
A comparator has primarily two entries and an exit, and functions as indicated in figure 18.
Tension at the point “a” higher than that at the point “b” ̃ left S on the level H.
Tension at the point “a” lower than that of the point “b” ̃ left S on the level L.
The operation of this pseudo-monostable is illustrated on figure 19.
With the powering, I being opened, C take care through R. When the potential into 6 exceeds 2 / 3 Vcc, the entry R of the rocker passes on the level H.
The exit Q thus passes on the level L, and I is closed. The condenser discharges almost instantaneously and the entry R passes by again on the level L. Since the entry of order is on the level H, the entry S of the rocker is thus on the level L. We thus have R = 0 and S = 0, rocker RS is in position memory. It is the at-rest state of monostable former to moment t1. At this moment, the entry of order places on the level L, S passes on the level H and the exit Q on the level H. Since I is now opened, the condenser starts to take care. When at moment t3, entry 6 reached potential 2 / 3 Vcc, the entry R passes by again on the level H. Ainsi, the exit Q passes on the level L because the entry S inactive, since is carried on the level L.
Indeed, the entry of order passed by again on the level H at the moment t2 former to moment t3. This is an obligatory condition for the correct operation of the assembly, if not, at moment t3, the two entries of the rocker would be on the level H, therefore the exit Q would be forced on the level H.
The duration of the impulse T of this monostable is given by the formula : T = 1,1 RC.
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