2.2.3 Astable Circuits

8/12/2019 2.2.3 Astable Circuits

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Topic 2.2.3 Astable Circuits.

Learning Objectives:

At the end of this topic you will be able to;

recall that an astable circuit has two unstable states;

explain the operation of a circuit based on a Schmitt inverter, and

estimate the operating frequency usingRC

f 1 ;

draw the circuit diagram for an astable using a 555 timer i.c.;

select and use formulae for

o the time the output is high: CRRt BAH )(. += 70

o the time the output is low: CRt BL 70.=

o the frequency: CRRf

BA )(. 270

1

+

=

calculate the mar space ratio;

1


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Module ET2

Electronic Circuits and Components.

Astable Circuit.

!n our previous section we looed at the behaviour of a monostable circuit,i.e. a circuit with "ust one stable state. !n this section we will be looing at

the astable circuit. #he astable circuit has no stable state and is constantly

switching between logic $ and logic %. Alternative names for the astable are a

pulse generator or cloc.

#he astable is a very useful circuit in electronics as it the circuit responsible

for causing flashing lights, pulsing bu&&ers in alarm circuits and eeping

counters running as we found out in module '#$, even though we didn(t loo at

how the cloc was produced at that time.

)nce again we will be considering two different methods of producing an

astable circuit. #he first method involves the use of a special type of *)#

gate, called a Schmitt *)# gate, or Schmitt inverter. #he symbol for the

Schmitt inverter is as follows:

#he Schmitt *)# gate has a unique switching characteristic which is very

different to the standard *)# gate. #he following diagrams illustrate the

difference between to two.

2

0 1 2 3 4 5 VIN

VOUT

5

4

3

2

1

00 1 2 3 4 5 V

IN

VOUT

5

4

3

2

1

0

Switching Characteristic

for a stanar NOT !ate

Switching Characteristic

for a Sch"itt NOT !ate

A Q


3/18


+ooing at the two characteristics you should notice that for a standard *)#

gate operating on a 5 supply that the switching point is at the midpoint of

the supply voltage for an increasing or decreasing input voltage.

-omparing this to the Schmitt characteristic reveals a different situation

altogether. As !*increases the voltage has to increase above before the

output voltage changes. )nce the output has changed however, if the input is

then decreased bac to , the output does not change bac, as it would in

the normal case, but now the voltage has to fall to below / before the

output will go high again.

0e have therefore created some hysteresis in the *)# gate with two

distinct switching thresholds. 0e can use this to our advantage to mae an

astable timer with the addition of "ust a resistor and a capacitor. #he circuit

required is shown below.

#his is a very simple circuit, but very reliable, requiring the minimum number

of components, and taing up very little space on a circuit board.

#he frequency of the output is given by the approximationRC

f 1

3


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Module ET2


1ow does it wor2

Assume that initially there is no charge on the capacitor, so the input

to the *)# gate will be +ogic %, so the output is at +ogic $.

#he capacitor begins to charge through the resistor R1and so the

voltage at the input of the *)# gate starts to rise.

0hen the voltage at the input reaches the upper switching threshold,

the output of the logic gate changes to +ogic %.

#he capacitor now starts to discharge through the resistor R1, and

the voltage across the capacitor begins to fall.

0hen the voltage at the input reaches the lower switching threshold,

the output of the logic gate changes to +ogic $ again,

#he capacitor starts to charge through R1again and the wholeprocess repeats as long as the power is switched on.

ote : !"en selecting t"e resistor #or t"e $c"mitt astable circuit %ou

s"ould ensure t"at t"e minimum value o# resistance c"osen is

&'(. T"is )ill limit t"e current #lo)ing to an acceptabl% lo)

value* and prevent over"eating o# t"e $c"mitt i.c. device.

4


5/18


#he addition of an oscilloscope to the circuit, shows this happening, as shown

below:

#he red trace, shows the voltage at the input to the Schmitt *)# gate, the

blue trace shows the voltage at the output of the Schmitt *)# gate.

*otes:

$. #he first cycle lasts longer than subsequent pulses as the

capacitor has to charge up from %, to the upper switching

threshold.

/. After the first cycle the capacitor charges and discharges

between the upper and lower switching threshold of the Schmitt

*)# gate.. #he 3)n( time, and 3)ff( time are of the same duration.

5


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Module ET2


#he Schmitt *)# gate solution is a very simple, neat and reliable solution if a

simple cloc, or pulse generator is required. 1owever if you want to have a

different 3on( and 3off( time then this simple circuit cannot perform thisaction, and we need to consider a more complex solution.

#he second method of maing an astable timer is to use a familiar device in

the 555 timer we used for the monostable timer. #his versatile device can

also be configured to run as an astable timer, by maing the connections

shown below:

#he circuit has many similarities with the monostable option, and it is

important to ensure that you do not confuse the two as %ou are e+pected to

be able to dra) t"is circuit in t"e e+amination.

#here are three formulae that apply to this circuit, allof which areprovided

on the -andidate !nformation page at the front of ever%examination paperso you do nothave to remember them.

o the time the output is high: CRRt BAH )(. += 70

o the time the output is low: CRt BL 70.=

o the frequency: CRRf

BA )(. 270

1

+

=

4rom these formulae we can see that the 3on( time is always greater than the

3off( time, but will be approximately the same if RBRA.

#

$%

$&

C


7/18


0e can confirm this by adding an oscilloscope to the output of the 555

astable circuit to see what the output loos lie.

1ere we can see that with the two resistors equal in value, the 3on( time is

approximately twice as long as the 3off( time. !fR1

is changed to $6 andR2

changed to $%%6 then output becomes as shown below, where the 3on( time is

virtually equal to the 3off( time.

'


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Module ET2


ote : !"en selecting resistors #or t"e ,,, timer circuit %ou s"ould

ensure t"at t"e minimum value o# resistance c"osen is &'(. T"is

)ill limit t"e current #lo)ing to an acceptabl% lo) value* andprevent over"eating o# t"e ,,, i.c. device.

T"e Mar'-$pace atio.

A common way of specifying the parameters of an astable circuit are in terms

of the 7ar8Space ratio. #his sounds complicated but actually is quite

straightforward. #he 37ar( refers to the 3)n( time, the 3Space( is simply the

3)ff( time. So if an astable is specified as having a 7ar8Space ratio of :$,

then the 3on( time must be three times as long as the 3off( time. 9raphically

this would be shown as follows:

ar* S+ace

ti"e


9/18


$tudent E+ercise &:

$. #he following circuit diagram shows a 555 timer configured as anastable timer.

i -alculate the duration of the 3on( pulse for the above circuit.

......................................................................................................................................

......................................................................................................................................

......................................................................................................................................

ii -alculate the duration of the 3off( pulse for the above circuit.

......................................................................................................................................

......................................................................................................................................

......................................................................................................................................

iii 1ence, or otherwise calculate the frequency of the output.

......................................................................................................................................

......................................................................................................................................

......................................................................................................................................

......................................................................................................................................

,


10/18

Module ET2


/. a


11/18


The fo--owing /estions hae een ta*en fro" recent ea"ination +a+ers coering the /se of asta-e

circ/its. So"e reference is a-so "ae to "onosta-e circ/its as the two to+ics are often -in*e in

ea"ination /estions.

1. The fo--owing iagra" shows an asta-e /i-t with a 555 ti"er /se to c-oc* an -.e..

% ata sheet for the 555 asta-e gies the fo--owing infor"ation

(a) Ca-c/-ate the a-/es of T1an T2for the asta-e.

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

2

() +-ain how the circ/it co/- e "oifie to +roie a aria-e "ar*6s+ace ratio.

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

2

11


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Module ET2


2. The fo--owing iagra" shows a 555 ti"er eing /se as an asta-e.

% ata sheet for the 555 asta-e gies the fo--owing infor"ation

(a) Ca-c/-ate the a-/es of T1an T2for the asta-e.

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

2

() Ca-c/-ate the fre/enc7 of the asta-e.

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

2

12


13/18


3. % Sch"itt inerter can e /se as an asta-e circ/it.

(a) Co"+-ete the circ/it iagra" for the asta-e circ/it.

2

() 8raw a s*etch to i--/strate the o/t+/t waefor" +ro/ce 7 the asta-e. 9o/r s*etch

sho/- show c-ear-7 how an asta-e waefor" iffers fro" a "onosta-e waefor".

2

13


14/18

Module ET2


3. The fo--owing a-ar" s7ste" sets off a /::er when the "onosta-e is triggere.

The "onosta-e o/t+/t re"ains high for secons after it is triggere.

The asta-e has an e/a- "ar*6s+ace ratio an a +erio of 2 secons.

The o/t+/t of the %N8 gate is high on-7 when oth its in+/t are high.

(a) 8escrie what the a-ar" oes oer the secon +erio after the "onosta-e is triggere.

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

.............................................................................................................................................................

1

() 8raw a s*etch of the waefor" +ro/ce at the o/t+/t of the %N8 gate oer the 10

secon +erio after the "onosta-e is triggere.

2

14


15/18


4. The fo--owing iagra" shows a 555 ti"er eing /se as an asta-e.

(a) The o/t+/t signa- has a "ar*;s+ace ratio of 4;1. S*etch two c7c-es of the o/t+/t signa-.


16/18

Module ET2


$olutions to $tudent E+ercises

$tudent E+ercise &:

$. i the time the output is high: CRRt BAH )(. += 70

st

st

t

t

CRRt

H

H

H

H

BAH

161

158081

10471023570

10471033102270

70

63

633

.

.

).(.

).(.

)(.

=

=

+=

+=

ii the time the output is low: CRt BL 70.=

st

st

t

CRt

L

L

L

BL

091

08571

1047103370

70

63

.

.

.

.

=

=

=

iii the frequency: CRRf BA )(. 2701

+

=

Hzf

f

f

CRRf

BA

4450

10471026870

1

104710332102270

1

270

1

63

633

.

).(.

).(.

)(.

=

=

+

=

+

=

)r

Hzf

ttf

LH

4440252

1

091161

1

1

....

==

+

=

+

=

1#


17/18


/. a

b

. 0hen faced with a design problem of this type we have a dilemma of

whether to choose a capacitor or resistor value to start with. As the

minimum value of resistor we can use is $6 then this might be a good

value to start with, as the capacitor can be of any value.

FFC

C

RCf

5001050010510002

1

1000

12

1

64===

=

=

=

#his is only one solution, the following

combinations are also acceptable; 56 and

$%%=4, /%6 and /5%=4, 5%%6 and $=4,

/%%6 and /5=4, Substitute them into the formula to chec them out.

>emember you may have come up with another perfectly acceptable solution.

1'

ti"e

ti"e


18/18

Module ET2


$el# Evaluation evie)

Learning Objectives

7y personal review of these ob"ectives:

recall that an astable circuit has two

unstable states;

explain the operation of a circuit

based on a Schmitt inverter, andestimate the operating frequency

usingRC

f 1 ;

draw the circuit diagram for an

astable using a 555 timer i.c.;

select and use formulae for

#he time the output is high:CRRt BAH )(. += 70

#he time the output is low:

CRt BL 70.=

#he frequency: CRRf

BA )(. 270

1

+

=

calculate the mar space ratio;

#argets: $. ??????????????????????????????????????????

??????????????????????????????????????????

/. ??????????????????????????????????????????

??????????????????????????????????????????

1

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2.2.3 Astable Circuits

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