Astable Multivibrator

Introduction

An astable multivibrator, often called a free-running multivibrator, is a

rectan gular-wave generating cir cuit. Unlike the monostable multivibrator, this

circuit does not require any ex ternal trigger to change the state of the output,

hence the name free-running.

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

UCC

+ +C1

+ +C2

QQRD

SD

.

.

.

5KΩ

5KΩ

5KΩ

VA

VB

T

1

32

4

56

7

8

(grand)

uO

2) Astable Multivibrator(Oscillator)

firstfirstuuCC=0=0

0

1

11

0

0

>2/3 UCC

RD=1

SD=0

.

.

uC

R1

R2

.+

–

charge

discharge

1

<1/3 UCC

485627 1

3

+VCC

uO

.

uC..

C

R1

R2

ttp1p1 =(=(RR11++RR22))CC ln2=0.7( ln2=0.7(RR11++RR22))CC

ttp2p2 ==RR22CC ln2=0.7 ln2=0.7RR22CC

TT==ttp1p1++ttp2p2 =0.7(=0.7(RR11+2+2RR22))CC

tp1 tp2

2/3UCC

1/3UCC

uC

t

uO

t

Duty Cycle = ttp1p1/T/T

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 auto-transition 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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