Transcript Uses of the Astable Multivibrator

Astable Multivibrator
Introduction
 An astable multivibrator, often called a free-running multivibrator, is a rectan-
gular-wave generating circuit. Unlike the monostable multivibrator, this circuit does
not require any external trigger to change the state of the output, hence the name freerunning.
 An astable multivibrator can be produced by adding resistors and a capacitor to the
basic timer IC, as illustrated in figure. The timing during which the output is either
high or low is determined by the externally connected two resistors and a capacitor.
The
details
of
the
astable
multivibrator
circuit
are
given
below.
Cont...,
• Pin 1 is grounded;
• pins 4 and 8 are shorted and then tied to supply +Vcc, output (VOUT is
taken form pin 3;
• pin 2 and 6 are shorted and the connected to ground through
capacitor C,
• pin 7 is connected to supply + VCC through a resistor RA;
• and between pin 6 and 7 a resistor RB is connected.
• At pin 5 either a bypass capacitor of 0.01
• F
is
connected
or
modulation
input
is
applied.
Astable Multivibrator & Waveforms
2) Astable Multivibrator(Oscillator)
8 UCC
charge
R1
.
R2
first
uC=0
+
uC
–
.
5KΩ
5
6
2
.
.
V
A
>2/3
<1/3
5K
Ω UUCC
CC
VB
.
7 5K
Ω
T
.
1
discharge (grand)
4
 10
+ +
C1
 10
+ +
C2
01
RD Q
SD Q
RD=1
SD=0
uO
13
+VCC
uC
2/3UCC
R1
.
R
u .
2
C
5 8 4
6
3
2
71
.
C
1/3UCC
t
uO
uO
t
tp1 =(R1+R2)C ln2=0.7(R1+R2)C
tp2 =R2C ln2=0.7R2C
T=tp1+tp2
=0.7(R1+2R2)C
Duty Cycle = tp1/T
tp1 tp2
Astable Multivibrator Operation
 In figure, when Q is low or output VOUT is high, the discharging transistor is cut-off
and the capacitor C begins charging toward VCC through resistances RA and RB.
Because of this, the charging time constant is (RA + RB) C.
 Eventually, the threshold voltage exceeds +2/3 VCC, the comparator 1 has a high
output and triggers the flip-flop so that its Q is high and the timer output is low.
With Q high, the discharge transistor saturates and pin 7 grounds so that the
capacitor C discharges through resistance RB with a discharging time constant RB
C.
 With the discharging of capacitor, trigger voltage at inverting input of comparator
2 decreases.
 When it drops below 1/3VCC, the output of comparator 2 goes high and this reset
the flip-flop so that Q is low and the timer output is high. This proves the autotransition in output from low to high and then to low as. Thus the cycle repeats.
555 Timer Astable Multivibrator Circuit
Cont..,
 This circuit diagram shows how a 555 timer IC is configured to
function as an astable multivibrator.
 An astable multivibrator is a timing circuit whose 'low' and 'high'
states are both unstable.
As such, the output of an astable
multivibrator toggles between 'low' and 'high' continuously, in effect
generating a train of pulses.
 This circuit is therefore also known as a 'pulse generator' circuit.
Cont..,
 In this circuit, capacitor C1 charges through R1 and R2, eventually building up
enough voltage to trigger an internal comparator to toggle the output flip-flop.
Once toggled, the flip-flop discharges C1 through R2 into pin 7, which is the
discharge pin.

When C1's voltage becomes low enough, another internal comparator is triggered
to toggle the output flip-flop. This once again allows C1 to charge up through R1
and R2 and the cycle starts all over again.
 C1's charge-up time t1 is given by: t1 = 0.693(R1+R2)C1. C1's discharge time t2 is
given by: t2 = 0.693(R2)C1. Thus, the total period of one cycle is t1+t2 = 0.693
C1(R1+2R2).
 The frequency f of the output wave is the reciprocal of this period, and is therefore
given by: f = 1.44/(C1(R1+2R2)), wherein f is in Hz if R1 and R2 are in mega ohms
and C1 is in microfarads.
Behavior of the Astable Multivibrator
 The astable multivibrator is simply an oscillator. The astable multivibrator
generates a continuous stream of rectangular off-on pulses that switch
between two voltage levels.
 The frequency of the pulses and their duty cycle are dependent upon the RC
network values.
 The capacitor C charges through the series resistors R1 and R2 with a time
constant
(R1 + R2)C.
 The capacitor discharges
through R2 with a time
constant of R2C
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Uses of the Astable Multivibrator
– Flashing LED’s
– Pulse Width Modulation
– Pulse Position Modulation
– Periodic Timers (see mushroom timer in the
experiment).
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