Use
of the C.I. LM 555 to produce a Astable Multivibrator
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Created it, 06/10/19
Update it, 06/11/01
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4. - FIRST EXPERIMENT : USE OF THE LM 555 LIKE MONOSTABLE
Associated a network RC, the circuit LM 555 allows to carry out monostable of which the duration of the impulse will be according to the time-constant RC.
4. 1. - REALIZATION OF THE CIRCUIT
a) Insert on the matrix the integrated circuit LM 555, the C1 condenser of 0,01 µF, the electrolytique capacitor with C2 tantalum of 1 µF and R1 resistance of 1 MW like illustrated appears 8-a.
b) Carry out the connections indicated in this same figure.
The electric diagram of the assembly is given figure 8-b.
4. 2. - OPERATIONAL TEST
a) Put Digilab under tension.
b) Support, then slacken P0 immediately. You notice that the LED L0 ignites during approximately 1 second.
c) If you support on P1
before the LED L0 is not extinct, you notice
that it dies out immediately. Indeed, the entry RESET
connected to the P1 contact 
is activated.
d) Put Digilab not under tension.
e) Replace the electrolytique capacitor of 1 µF - 12 V by an electrolytique capacitor of 10 µF - 12 V.
f) Give under tension Digilab.
g) Support on P0. You note that the LED L0 ignites during 10 seconds, that is to say ten times longer than during the preceding test.
h) The test is finished, you can put Digilab not under tension.
To analyze the operation of this monostable, you defer on figure 9.
.gif)
Under the initial conditions of rest,
the exit 
of the rocker is on the level H and led
transistor T1. The
C2 condenser is thus shorted-circuit and the tension on its terminals
is close to zero.
The exit of the buffer (pin 3) is on the level L.
The entry (pin 2) integrated circuit (entered inverseuse of the comparator of release) is on the level H, therefore the exit of this comparator is on the level L since the noninverseuse entry is with potential 1 / 3 of Vcc, that is to say approximately 1,67 volt.
When one supports on P0,
the inverseuse entry of this comparator passes on the level L,
therefore on a level of tension lower than that of the noninverseuse entry. The
exit of the comparator passes on the level H. the
exit
of the rocker thus passes on the level L and
the exit of the buffer on the level H, as
indicated figure 10-a.
Transistor T1 is blocked and the C2 condenser takes care through R1 resistance.
The figure 10-b represents the variation of the terminal voltage of C2.
This tension is applied to the noninverseuse entry of the comparator of threshold. The tension on the inverseuse entry of this comparator has a value of : 2 / 3 x 5 = 3,33 volts.
When the potential on pin 6 exceeds this tension of 3,33 volts, the exit of the comparator passes on the level H.
The exit
of rocker RS thus returns on the level H,
while transistor T1 is saturated and makes
it possible the C2 condenser to very quickly
discharge.
The cycle of monostable is thus finished.
The duration of the impulse of exit is directly proportional to the product R1 x C2. It is equal to 1,1 x R1 x C2.
In this case, we have :
T = 1,1 x R1 x C2 = 1,1 x 106 x 10-6 = 1,1 second
While pressing on the P1 button, the entry RESET is activated and gives to zero rocker RS, which stops the load of the condenser if this last were in phase of load.
If this entry of restoring is not used, it is good to cable it with tension Vcc, to prevent that parasites can modify the duration of the impulse of exit.
The C1 condenser, connected to the inverseuse entry of the comparator of threshold, is used to eliminate from possible parasites being able to disturb the operation of the circuit (condensing of decoupling).
5. - SECOND EXPERIMENT : USE
OF THE INTEGRATED CIRCUIT LM 555 TO PRODUCE A MULTIVIBRATOR ASTABLE
With a restricted number of components and external connections, the integrated circuit LM 555 can function like multivibrator astable.
5. 1. - REALIZATION OF THE CIRCUIT
a) Remove matrix the connections and the components relating to the preceding experiment, while leaving the LM 555 in place.
b) Insert on the matrix the two condensers C1 of 0,01 µF and C2 of 1 µF and two resistances R1 of 47 kW and R2 of 120 kW in the position indicated figure 11-a.
c) Carry out the connections illustrated in this same figure 11-a.
The electric diagram is given figure 11-b.
5. 2. - OPERATIONAL TEST
a) Put Digilab under tension. You observe that the LED L0 flickers at a frequency of 5 Hz.
b) Put Digilab not under tension.
As you noted, the circuit oscillates freely and generates a rectangular signal. In order to analyze the operation of this assembly, observe the electric diagram of the circuit on figure 12.
.gif)
We know that the exit
is on the level L with the powering. The
transistor is blocked and the C2 condenser
takes care through R2 and R1.
When the terminal voltage of C2
reaches 2 / 3 Vcc, the exit of the
comparator of threshold passes on the level H
and
passes on the level H.
The chronogram of figure 13 represents the evolution of the terminal voltages of C2 and at exit of the oscillator.
The transistor is saturated, which allows the discharge of C2 through R2 during time t2. You can note that in the case of the monostable one, the discharge of the condenser was practically immediate since there was no resistance.
In addition, in the oscillating
assembly, the entry of release is connected to the entry of threshold, therefore
when the terminal voltage of C2 goes down in
lower parts from 1 / 3 Vcc, the comparator
of release rocks and the exit
turns over to the level L.
The transistor is blocked again and the C2 condenser can be reloaded. A new cycle starts again. As figure 13 indicates it, the terminal voltage of C2 varies alternatively between 2 / 3 Vcc and 1 / 3 Vcc.
The C2 condenser takes care during time : T1 = 0,693 x (R1 + R2) x C2.
It discharges during time : t2 = 0,693 x R2 x C2.
The period T of the rectangular signal is worth : T = t1 + t2 = 0,693 x (R1 + 2R2) x C2
With the experimental values we obtain :
t1 = 0,693 x (47 x 103
+ 120 x 103)
x 10-6
= 0,12 second
t2 = 0,693 x (120 x 103
x 10-6)
= 0,08 second
T = 0,12 + 0,08 = 0,2 second
The frequency is worth :
F = 1 / T = 1 / 0,2 = 5 Hz.
6. - THIRD EXPERIMENT : RECTANGULAR GENERATOR
OF SIGNAL A VARIABLE FREQUENCY
While preserving the preceding assembly, you in this experiment will produce a multivibrator astable whose frequency of oscillation can vary using a potentiometer.
6. 1. - REALIZATION OF THE CIRCUIT
a) Withdraw assembly only resistances R1 and R2, the connection with the LED L0 and that located between the pin 3 of the LM 555 and one end of R2 resistance of 120 kW. (See figure 14-a).
b) Supplement the circuit while inserting on the matrix the potentiometer of 10 kW, two new R1 resistances and R2 of 1 kW, the electrolytique capacitor to C3 tantalum of 10 µF and the cord of the loudspeaker, as well as the new connections indicated figure 14-a.
c) Introduce the male card jack into catch SPK of Digilab.
The electric diagram of the circuit carried out is given figure 14-b.
You notice that the loudspeaker is connected on the outlet side of the multivibrator astable. The rectangular electric signal is thus translated into acoustic signal by the loudspeaker.
6. 2. - OPERATIONAL TEST
a) Put Digilab under tension. You immediately hear a sound whose frequency is a function of the position of the cursor on the potentiometer.
b) Turn the screw of the potentiometer. You note that the frequency of the acoustic signal varies.
c) Put Digilab not under tension.
The frequency of oscillation lies between two limiting values : the minimal frequency and the maximum frequency.
To calculate these two values, it is enough to use the following formula :
While turning the adjustable tangent towards the line at bottom (P = 10 kW), it is possible to obtain the minimal frequency, that is to say :
Conversely, while turning it at bottom towards the left, one obtains the maximum frequency, that is to say :
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