Created it, 06/09/09
Update it, 06/09/19
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1. 3. 2. - MONOSTABLE TRUTHS
They are monostable circuits without particular conditions about the control signal. I.e. this one could indifferently be of duration higher or lower than the impulse of exit of the monostable one.
a) Monostable truths with a rocker D.
The typical assembly using a rocker D is indicated to the figure 20-a, whereas the figure 20-b gives the truth table of the rocker.
Figure 21 illustrates the operation of this assembly.
At rest, monostable A its exit Q on the bottom grade (L).
Indeed, if with the powering the exit Q is on the level H, the condenser C takes care and causes, one moment later, the activation of entry CLEAR of the rocker whose exit passes immediately on the bottom grade. The condenser discharges then through the on line polarized diode D.
If one applies to moment t1 a positive impulse to entry CK, the level applied high to the entry D is transferred to the exit Q. This exit being at the high level, the condenser C takes care and there is the phenomenon described previously. Entry CLEAR becomes active at the moment t2, which makes pass by again Q on the bottom grade.
At moment t3, the condenser C is completely discharged. Fresh impulse can be applied to entry CK.
With this assembly, an impulse, even very short, starts the rocker. This same impulse of order can, in addition, one duration old being higher than that of exit. It is thus here about a monostable truth, no particular condition being imposed as for the duration of the impulse of entry.
b) Monostable realized with a specific integrated circuit.
The monostable circuit presented at figure 22 uses an integrated circuit of specific type CMOS.
This integrated circuit comprises mainly a synchronous rocker of type D and a comparator. A Rext resistance and a Cext condenser are associated this integrated circuit to define the time-constant of this monostable.
In the at-rest state, the Q' exit of the rocker is on the level L, therefore G on the level H and Q, left monostable, on the level L. In this case, the transistor T led and shorts-circuit the Cext condenser, imposing a null tension in V1, (entry - comparator). The exit of the comparator is thus on the level H. Entry CLEAR of the rocker is on the level H, therefore inactive.
Figure 23 which follows makes it possible you to include/understand what occurs when the entry of order B places from the level L on the level H.
Entry A of the circuit which can be regarded as an entry of validation is maintained on the bottom grade. At moment t1, the entry B passes from the level L on the level H and thus entry CK of the rocker also : the Q' exit passes on the level H like Q.
The tension at the point G becomes null and the transistor T is blocked.
The Cext condenser starts to take care through Rext resistance. When the tension at the V1 point reaches the threshold of the comparator, this one rocks : its exit passes on the level L, which activates entry CLEAR of the rocker. Q' passes by again then on the level L as well as the exit Q of the monostable one.
At the same time the transistor T led and the comparator Re-rock on the level H to make entry CLEAR inactive. The monostable one found the former state of stability.
It should be noted that entry CLEAR of the rocker D can be activated constantly thanks to entry CLR of the circuit. There is thus average to stop the impulse of exit in Q. This is illustrated on the figure 24.
It is also possible to start this monostable thanks to a downward face applied to entry A. In this case, the entry B must be on the level H.
1. 3. 3. - MONOSTABLE
REDÉCLENCHABLES
It is a last category about which we did not speak up to now. If figure 23 again is looked at, we see that fresh impulse 3 at the entry of order does not have any effect on the exit Q of monostable when this one has been just started by impulse 2. It will be said that it is monostable not a redéclenchable. By opposition, there are monostable assemblies redéclenchables, i.e. fresh impulse of order occurring while the monostable one is started is taken into account and prolongs the impulse of exit one duration equivalent to that past between the beginning of the first order and the beginning of the second. This is illustrated on figure 25.
The total duration is to T' well the sum of T (time-constant of the circuit) and “t3-t2”. A continuation sufficiently close to impulses at the entry E would maintain permanently the exit S on the level H.
a) Realization with a rocker D.
The diagram comprises only one difference with that of figure 20. Indeed, on figure 26, you note that the diode D is connected to the entry and either to the exit.
This assembly functions with a particular condition for the control signal. It is necessary that this last has one duration higher than the time-constant T of the monostable one. Thus, it is about pseudo-monostable a redéclenchable.
Indeed, if entry CK is brought back prematurely to the level L, the diode D, on line polarized, conduit and entry CLEAR never reach the level H since the condenser C discharges at once. The exit Q would thus remain permanently with the state H.
For the operation of the circuit into monostable redéclenchable, it is enough after a first impulse to order, to bring back the entry to the level L and immediately to bring back it to the level H before the period T was not passed.
To each time the entry is brought back to the level L then on the level H, a new cycle of load of the condenser starts again.
b) Realization with an integrated circuit.
The assembly illustrated on figure 27 is a monostable truth redéclenchable which uses a specific integrated circuit of type 74122 or 74C122.
In the at-rest state, the exit Q is always at the logical level L.
To start this monostable, it is possible to use the four entries A1, A2, B1 and B2.
It is enough to send a positive transition either on B1, or on B2, provided that the other entry (B1 or B2) or on the level H and that one of the two entries A1 or A2 at least or on the level L.
If B1 and B2 are on the level H, it is also possible to start this monostable by a negative transition in A1 or A2.
The two components cabled outside Cext and Rext determining the duration of the impulse of exit T.
Active entry CLEAR on the level L has priority, i.e. it positions the exit Q on the level L as soon as it is activated.
An example of application of monostable the redéclenchables is provided by the systems of detection of breakdown in the circuits of clock.
Indeed, it is enough to choose a time-constant of monostable slightly higher than the period of clock. It will be enough that only one face of clock misses so that the exit of monostable falls down to the at-rest state and allows the description of an incident.
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