The Astable Multivibrator

Lesson 1456 Experiment 3

Astable Operation

A timer IC produces an astable (or

free running) square-wave output

whose frequency can be varied by

changing the external RC time

constant.

Astable Multivibrator

OAstable Multivibrators generate a

rectangular waveform.

OThis rectangular-wave oscillator

is useful in generating signals for

digital circuitry, lamp flashers,

sound generators and various

other applications. .

Basic operation

OThe astable multivibrator works

with both comparators tied

together .

OThis means that pin 6 of the

upper comparator is tied to pin 2

of the lower comparator.

O When power is applied, C1 is

discharged and the flip-flop is

reset.

O C1 begins to charge through RA

and RB.

O When the voltage across C1

reaches 2/3 VCC, the upper

O comparator is activated and the

flip-flop is set.

O This turns on the DISCHARGE

transistor, which discharges C1

through RB.

O The lower comparator is reset

when the voltage across C1 drops

to 1/3 VCC.

Things to note before constructing Figure 11

OYou will not be asked to measure

the period or frequency

OThis is due to the wide tolerances

of the Electrolytic Capacitors.

OElectrolytic Capacitors have a

tolerance on their marked values

which can range from -20%,

+80%.

OThis means the capacitor you use

can have a value which is quite

different than the one we used

preparing the experiment.

Astable Multivibrator, Fig 11

Discussion on Experiment 3

OYou will be changing the

values of RA and RB to help

you understand how these

values affects the duty cycle

of the astable multivibrator.

OWhat is the Duty Cycle?

OThe time the output of the

astable multivibrator is high

(t1).divided by the period of

the waveform (T). D = t1 /T

OThe duty cycle is usually

expressed as a percentage of

the period of the waveform

OThe astable’s frequency is

determined by two time periods.

O (t1 ) is the time it takes for C1 to

charge up through RA and RB.

t1 = 0.693(RA+RB)C1

O (t2) is the time it takes for C1 to

discharge through RB.

t2 = 0.693(RB)C1

What is the Period of the waveform?

OT = t1 + t2 = 0.693(RA + 2RB)C1

Frequency is the reciprocal of time

or the reciprocal of the period T.

O f = 1/T = 1.44/(RA + 2RB)C1

OWhen using an oscilloscope, you

may often be converting the

period of the signal to frequency.

Building your circuit

ORemember the basics

ORemember where Pin 1 and

8 are located on the IC

OCenter the IC on the divider

strip on the Breadboard

OMake sure you do not short out

the component leads

OSpread your circuit out to make it

easier to follow the connections.

OUse different color jumpers for

the VCC and GND connections

OMake sure you pay attention to component polarities

OElectrolytic capacitors have a polarity which you need to follow.

ODiodes have a polarity as well

OThe band is the cathode on a diode

OThe short lead or notch side on an LED is the cathode

Astable Multivibrator Built

Astable Multivibrator, Fig 11

Close-up of Astable Multivibrator

Modify Fig 11 as indicated

OChange RB to the 100KΩ potentiometer (on the trainer) in series with a 10kΩ Resistor.

OChange C1 from 100µF to 10µF.

OThis will give you the Variable-frequency oscillator of Fig 12.

Variable Frequency Oscillator

Fig 12 Variable Freq. Osc.

Close-up of Variable Freq. Osc.

Brief Review of Experiment 3

OWe have seen changing the

components in the external RC

circuit changes the frequency and

duty cycle of the oscillator .

OPin 3 output was high when the

capacitor was charging through

RA and RB

OPin 3 output was low when the

capacitor was discharging

through RB.

Experiment 4 Overview

OYou are going to vary the frequency

of the astable multivibrator using

two methods;

1. Vary the frequency using a

variable RB

2. Vary the frequency by using a

control voltage on Pin 5.

3. It is important to keep your

working circuit from experiment 3

intact.

a) Modify the previous circuits as

needed to build Figs. 13, 14 &15.

b) You will be changing the output

device and the timing circuit

OYou are again going to vary the

frequency of the astable

multivibrator using two methods;

1. Vary the frequency using a

variable RB.

2. Vary the frequency by using a

control voltage on Pin 5.

O Remember: Pin 5 should be

connected to GND through a

capacitor when not in use.

Questions?

The End

Developed and Produced by the Instructors in the CIE Instruction Department.

© Copyright 01/2012

All Rights Reserved / Jan. 2012