Examination
of a Rocker of Schmitt |
Use
of a rocker of Schmitt to produce an Oscillator |
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Created it, 06/10/19
Update it, 06/10/27
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With this new practice, we will examine the rockers of Schmitt and the oscillators.
The rockers of Schmitt are circuits very much used to give in the shape of the signals of bad quality, either because they are parasitized, or because they have rising faces and too slow descendants, therefore not very effective to order digital circuits.
The oscillators are circuits which generate cyclic signals. They are essential in the synchronized digital circuits to which they give the rate of operation so that certain components, as the rockers work in a synchronous way.
We saw indeed that the majority of the rockers have an entry called CLOCK. By sending on this entry the signal of an oscillator which is precisely destined for this occasion SIGNAL CLOCK, one authorizes the rocker to change state only in the presence of one impulse of this clock.
If one sends the same clock signal to the rockers and the circuit components having an entry CLOCK, their operation will be synchronized, i.e. all will change state at the same time.
1. - PREPARATION OF MATERIALS
In this practice, you will use following materials :
1 resistance of 12 kW 1 / 4 W tolerance 5 % (chestnut - orange red - but)
1 resistance of 22 kW 1 / 4 W tolerance 5 % (red - orange red - but)
1 resistance of 15 kW 1 / 4 W tolerance 5 % (chestnut - green - orange - but)
1 resistance of 100 kW 1 / 4 W tolerance 5 % (chestnut - black - yellow - but)
1 resistance of 470 kW 1 / 4 W tolerance 5 % (yellow - purple - yellow - but)
1 resistance of 2,2 MW 1 / 4 W tolerance 5 % (red - red - green - but)
2 resistances of 1,5 MW 1 / 4 W tolerance 5 % (chestnut - green - green - but)
3 resistances of 150 kW 1 / 4 W tolerance 5 % (chestnut - green - yellow - but)
1 electrolytique capacitor with the tantalum of 0,33 µF - 10 V
1 electrolytique capacitor with the tantalum of 1 µF - 10 V
1 condenser polyester of 0,1 µF
1 condenser polyester of 0,01 µF
1 ceramic condenser disc of 330 pF
1 linear multi-turn potentiometer of 10 kW
2 diodes 1N 4148
2 integrated circuits MM 74C14
1 integrated circuit MM 74C175
1 integrated circuit MM 74C02
1 integrated circuit MM 74C04
1 integrated circuit MM 74C74
1 braid of insulated rigid wire (red and black)
2. - FIRST EXPERIMENT : EXAMINATION OF THE OPERATION OF A ROCKER
OF SCHMITT
In this handling, you initially will compare the operation of a circuit carried out with two reversers and the operation of a rocker of Schmitt available in the form of integrated circuit.
2. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food, remove the connections and the components left on the matrix at the end of the preceding experiments and remove the integrated circuit of support ICX.
b) Insert in the matrix the integrated circuit MM 74C04 containing six reversers, the multi-turn potentiometer of 10 kW and the resistance of 22 kW 1 / 4 W 5 % (red - red - orange - but) in the position indicated to the figure 1-a ; carry out then the connections illustrated by this same figure.
Do not put for the moment the resistance of 100 kW drawn in dotted line.
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The diagram of the circuit thus carried out is deferred in the figure 1-b.
The entry of the first REVERSER (pin 1) is connected to the cursor of the potentiometer through the resistance of 22 kW (R1).
This cursor, by means of the adjustable tangent, can be moved towards an end or the other of the potentiometer and thus towards the mass or the positive tension of food.
Thus, it is possible to apply to the entry of the circuit a variable tension continuously between zero volt and the supply voltage.
The exit of the circuit (pin 4) is connected to the LED L0.
2. 2. - OPERATIONAL TEST
a) Connect the food.
b) Connect the controller positioned on continuous tension gauges 10 V, between the mass (black test probe) consisted the negative pole of the pile and the terminal of R1 which is connected to the cursor of the potentiometer (red test probe) as illustrated on the figure 2-a.
On the figure 2-b, an enlarging shows you how this last connection must be carried out, by using a piece of wire rigid isolated from suitable length whose stripped end will be rolled up on the end of the red test probe and the other inserted in the point of measurement.
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c) firmly Maintain in place with a finger the potentiometer and with a screwdriver, turn the adjustable tangent in the direction of the needles of a watch until the voltmeter indicates zero volt (figure 2-a). Under these conditions, the cursor is in race end, towards the end connected to the mass, and you note that the LED L0 is extinct.
d) Now, slowly turn the screw of the potentiometer in the contrary direction of the needles of a watch until the moment when the LED L0 ignites and note on a paper sheet the value of tension indicated by the voltmeter.
e) Continue to turn the screw in the same direction until the tension read on the apparatus reaches the value of the supply voltage (+ 4,5 V approximately), which indicates that the cursor reached the other end connected to the positive tension. You note that the LED L0 remains lit.
f) Again turn the screw in the direction of the needles of a watch to lower the tension of entry of the circuit by stopping you as soon as you see the LED dying out and note the value of tension read on the voltmeter.
You notice that this value coincides with that measured in the phase “d” when you turned the screw in the contrary direction of the needles of a watch and that the LED had ignited.
The tension thus measured is called TENSION OF THRESHOLD.
With this test, you noted that the tension of threshold for which the LED ignites or extinguishes according to whether one makes grow or to decrease the tension of entry is approximately half of the supply voltage.
g) Disconnect the food now and insert on the matrix R2 resistance of 100 kW 1 / 4 W 5 % (chestnut - black - yellow - but) represented in dotted on the figure 1-a.
h) Connect the food again. While proceeding as described previously, measure the values of the tensions corresponding to the lighting and the extinction of the LED.
You note that by increasing the tension of entry of the circuit starting from 0 V, the LED ignites with a value of tension higher than that measured previously. This new value is called HIGHER THRESHOLD. By reducing the tension of entry then, you measure another value of tension of threshold called LOWER THRESHOLD, corresponding to the extinction of the LED. By comparing the value of the higher threshold with that of the lower threshold, you can note that they are different.
With the values of resistances used and for a value of supply voltage of 4,5 V, the higher threshold is indeed of 2,8 V approximately while the lower threshold is approximately 1,8 V.
This property of the circuit is indicated by the term OF HYSTERESIS; the variation of hysteresis, i.e. the difference between the two values of the tensions of higher and lower threshold is given by the product of the supply voltage and the report/ratio between resistances R1 and R2 (figure 1-b).
In our case, it is of :
In conclusion, the tested circuit is characterized by two thresholds of commutation, one valid when the tension of entry grows and different the tension of entry decrease.
This property is very useful for the signals disturbed by the parasites.
Indeed, as soon as the input signal crossed the higher threshold, the circuit rocks and remains in this state as long as the input signal does not go down again in lower part of the level of the lower threshold as we saw previously.
Thus, even if the input signal should undergo important fluctuations, the circuit would be unaware of them as much as the amplitude of these fluctuations is lower well off hysteresis, i.e. the difference between the values of the two thresholds of the rocker.
In figure 3, the behavior of this circuit, known under the name of rocker of Schmitt or trigger of Schmitt, is compared with that of a normal circuit CMOS.
In the case of the ordinary circuit, since the tension of entry crosses several times the threshold of commutation, the output voltage undergoes as many changes of states.
On the contrary, the rocker of Schmitt gives at exit a signal which is not prone to the undesirable fluctuations which appear in the vicinity of the thresholds of commutation.
2. 3. - EXAMINATION OF THE ROCKER OF INTEGRATED SCHMITT MM 74C14
To have a rocker of Schmitt, it is not necessary to resort to the circuit which you have just carried out. Indeed, this rocker is available in the form of integrated circuit.
A typical specimen is the integrated circuit MM 74C14 which you surely have in your possession and whose electric diagram is represented figure 4.
As you can see it, it includes/understands six reversers, each one of them having the characteristic of a rocker of Schmitt.
You point out that this characteristic is specific to the entries of the circuits and that the behavior of the reversers is identical to that of a normal reverser from the point of view of the table of operation and truth table.
To check the operation of the MM 74C14, proceed as follows :
a) Disconnect the food, remove matrix the integrated circuit MM 74C04 and replace it by the MM 74C14. Also remove R2 resistance of 100 kW. Leave in place, on the other hand, the other components and connections.
You thus simply replaced the normal reversers by reversers with rockers of Schmitt.
b) Connect the food and vary the tension of entry of the first reverser while operating the potentiometer.
You note that the behavior of this new circuit is the same one as that raised for the preceding circuit when, with the insertion of the resistance of 100 kW, you obtained two thresholds different of commutation.
The two threshold values which you measured are those of the characteristics of the integrated circuit in question.
Besides that used, there exists about it the different one in the trade, as certain doors NAND whose entries have the particular characteristic of the rockers of Schmitt.
3. - SECOND EXPERIMENT : USE OF A ROCKER OF SCHMITT
TO PRODUCE AN OSCILLATOR
An oscillator is a circuit which does not have a stable state but which, as its name indicates it, oscillates continuously between two states known as unstable.
The number of oscillations which the circuit achieves in one second is the frequency, measured in Hertz (Hz). A Hertz is equivalent to an oscillation a second.
The oscillators are essential in all the synchronized digital circuits in which the various components work in a synchronous way (i.e. state or swing at one precise time ordered by a clock signal changes generated by a single oscillator).
During this handling, you will examine a very simple oscillator, but however effective, which will be then assembled definitively on the printed circuit of the digilab and will constitute a very useful generator of signal.
3. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food and remove all the components and connections of the circuit tested previously by leaving however the integrated circuit MM 74C14 in place.
b) Introduce on the matrix resistance R of 1,5 MW - 1 / 4 W - 5 % (chestnut - green - green - but) and the electrolytique capacitor with the tantalum (C) of 0,33 µF - 10 V (by respecting the polarities of its terminals) with the sites indicated in the figure 5-a which also illustrates connections to be carried out. The electric diagram of the circuit thus carried out is represented figure 5-b.
3. 2. - OPERATIONAL TEST
a) Connect the food and observe the LED L0 : you note that it ignites and dies out periodically approximately once a second. That means that the output voltage passes alternatively from a level L to a level H and vice versa, which determines the lighting and the extinction of the LED.
In this case also, as for the monostable one, the operation of the circuit is based on the load and the discharge of the condenser through resistance.
With the powering, the condenser C is discharged, therefore the entry of the reverser is on the level L. the exit passes consequently on the level H. Then the condenser starts to take care through resistance R. After a time t depend on the time-constant RC according to the relation t = RC, the terminal voltage of the condenser reached a value such as it exceeds the higher threshold of commutation of the rocker and thus the exit passes to the state L. Consequently, the condenser starts to discharge through resistance.
When the terminal voltage of C reaches the lower threshold, the rocker commutates again, going back under the initial conditions since the exit is now on the level H and thus C starts to take care.
The load and the discharge of the condenser are repeated periodically in the described way, therefore the rocker continuously commutates from one state to another, generating a output voltage which has the pace of a rectangular signal, as represented on figure 6. In our case, with R = 1,5 MW and C = 0,33 µF, the time of load and discharge of the condenser is of :
t = R x C = 1,5 x 106 x 0,33 x 10-6 = 0,495 s is approximately one 1 / 2 second.
Since the circuit commutates each 1 / 2 second, it is necessary one second whole so that the output voltage achieves a complete cycle.
Time necessary so that the tension achieves a complete cycle is called PERIOD and is indicated by the letter T.
In our case, we have T = 2t = 2 R x C
The shorter the period is, the more the number of complete cycles that the output voltage can achieve in one second will be large.
The number of cycles at the second, i.e. the frequency, is given by the report/ratio :
who is written in manner symbolic system :
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with f expressed in Hertz and T in second.
In our case, by replacing T by its value of 1 s, one obtains :
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b) Disconnect the food, replace the resistance of 1,5 MW by another of 470 kW 1 / 4 W 5 % (yellow - purple - yellow - but) and replug the food.
You notice that the LED ignites and at a frequency three times die out higher than that of the preceding case.
Indeed, the period of the oscillation produced by the circuit is equal to :
T = 2 RC = 2 x 470 x 103 x 0,33 x 10-6 = 0,31 s
One deduces the frequency from it from the generated signal :
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c) By disconnecting each time the food, replace resistance or the condenser by other components having increasingly low values.
You observe that the frequency of the oscillation increases more and more, until the LED remains lit permanently.
That does not mean that the oscillator does not function any more, but simply that the phases of illumination and extinction of the LED follow one another so much quickly that the eye is not able any more to appreciate them.
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