555 TIMER

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 555 Timer
Introduction:
 The 555 Timer is one of the most popular and versatile
integrated circuits ever produced!
 “Signetics” Corporation first introduced this device as the
SE/NE 555 in early 1970.
 It is a combination of digital and analog circuits.
 It is known as the “time machine” as it performs a wide
variety of timing tasks.
 Applications for the 555 Timer include:
• Ramp and Square wave generator
• Frequency dividers
• Voltage-controlled oscillators
• Pulse generators and LED flashers 2
 555 timer- Pin Diagram
The 555 timer is an 8-Pin D.I.L. Integrated Circuit or ‘chip’

Notch

Pin 1

3
 555 timer- Pin Description
Pin Name Purpose
1 GND Ground, low level (0 V)

2 TRIG OUT rises, and interval starts, when this input falls below 1/3 VCC.

3 OUT This output is driven to approximately 1.7V below +VCC or GND.

A timing interval may be reset by driving this input to GND, but the
4 RESET timing does not begin again until RESET rises above approximately
0.7 volts. Overrides TRIG which overrides THR.

5 CTRL "Control" access to the internal voltage divider (by default, 2/3 VCC).

6 THR The interval ends when the voltage at THR is greater than at CTRL.

Open collector output; may discharge a capacitor between intervals.

7 DIS
In phase with output.
8 V+, VCC Positive supply voltage is usually between 3 and 15 V. 4
 555 Timer
Description:
•Contains 25 transistors, 2 diodes and 16 resistors
• Maximum operating voltage 16V
• Maximum output current 200mA
• Best treated as a single component with required
input and output

INPUT PROCESS OUTPUT

If you input certain signals they will be processed / controlled in a

certain manner and will produce a known output.
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 Inside the 555 Timer
+ Truth Table
S R Q Q
Vref
0 0 No Change
0 1 0 1
Threshold
Control Voltage 1 0 1 0
R Q 1 1 X X

S
-
Q

Trigger
Discharge

Fig: Functional Diagram of 555 Timer

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 Inside the 555 Timer
Operation:
• The voltage divider has three equal 5K resistors. It
divides the input voltage (Vcc) into three equal
parts.
• The two comparators are op-amps that compare
the voltages at their inputs and saturate depending
upon which is greater.
• The Threshold Comparator saturates when the voltage
at the Threshold pin (pin 6) is greater than (2/3)Vcc.
• The Trigger Comparator saturates when the voltage at
the Trigger pin (pin 2) is less than (1/3)Vcc
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 Inside the 555 Timer
• The flip-flop is a bi-stable device. It generates two
values, a “high” value equal to Vcc and a “low” value
equal to 0V.
• When the Threshold comparator saturates, the flip flop is
Reset (R) and it outputs a low signal at pin 3.
• When the Trigger comparator saturates, the flip flop is Set
(S) and it outputs a high signal at pin 3.
• The transistor is being used as a switch, it connects
pin 7 (discharge) to ground when it is closed.
• When Q is low, Q bar is high. This closes the transistor
switch and attaches pin 7 to ground.
• When Q is high, Q bar is low. This open the switch and
pin 7 is no longer grounded
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 Uses of 555 timer

What the 555 timer is used for:

•To switch on or off an output after a certain time delay i.e.

Games timer, Childs mobile, Exercise timer.

•To continually switch on and off an output i.e.

warning lights, Bicycle indicators.

•As a pulse generator i.e.

To provide a series of clock pulses for a counter.
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Schematic Diagram of 555 Timer

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 555 Timer operating modes
• The 555 has three operating modes:
1. Monostable Multivibrator
2.Astable Multivibrator
3. Bistable Multivibratior

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555 Timer as Monostable Multivibrator
Description:

 In the standby state, FF holds transistor

Q1 ON, thus clamping the external
timing capacitor C to ground. The
output remains at ground potential. i.e.
Low.

 As the trigger passes through VCC/3, the

FF is set, i.e. Q bar=0, then the transistor
Q1 OFF and the short circuit across the
timing capacitor C is released. As Q bar is
low , output goes HIGH.

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555 Timer as Monostable Multivibrator

Fig (a): Timer in Monostable Operation with Functional Diagram

Fig (b): Output wave Form of Monostable 13
Monostable Multivibrator- Description
• Voltage across it rises exponentially through R towards
Vcc with a time constant RC.
• After Time Period T, the capacitor voltage is just greater
than 2Vcc/3 and the upper comparator resets the FF, i.e.
R=1, S=0. This makes Q bar =1, C rapidly to ground
potential.
• The voltage across the capacitor as given by,
t
v c  V cc (1  e  RC )  If –ve going reset pulse terminal (pin
2
at t  T , v c  V cc
3
4) is applied, then transistor Q2-> OFF,
2 T
V cc  V cc (1  e  RC )
Q1-> ON & the external timing
3
1 capacitor C is immediately discharged.
T  RC ln( )  T  1.1RC sec
3
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 Behavior of the Monostable
Multivibrator
 The monostable multivibrator is constructed by
adding an external capacitor and resistor to a 555
timer.
 The circuit generates a single pulse of desired
duration when it receives a trigger signal, hence it
is also called a one-shot.
 The time constant of the resistor-capacitor
combination determines the length of the pulse.

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Uses of the Monostable Multivibrator

• Used to generate a clean pulse of the correct

height and duration for a digital system
• Used to turn circuits or external components on or
off for a specific length of time.
• Used to generate delays.
• Can be cascaded to create a variety of sequential
timing pulses. These pulses can allow you to time
and sequence a number of related operations.

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 Monostable Multivibrator
Problem:
In the monostable multivibrator of fig, R=100kΩ and the time delay T=100ms.
Calculate the value of C ?
Solution:
T=1.1RC

T 100 x103
C   3
 0.9F
1.1R 1.1x100 x10

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 Applications in Monostable Mode

1. Missing Pulse Detector.

2. Linear Ramp Generator.
3. Frequency Divider.
4. Pulse Width Modulation.

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 1.Missing Pulse Detector

Fig (a) : A missing Pulse Detector Monostable Circuit

Fig (b) : Output of Missing Pulse Detector
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 Missing Pulse Detector- Description
• When input trigger is Low, emitter-base diode of Q is
forwarded biased capacitor is clamped to 0.7v(of
diode), output of timer is HIGH width of T o/p of
timer > trigger pulse width.
• T=1.1RC select R & C such that T > trigger pulse.
• Output will be high during successive coming of
input trigger pulse. If one of the input trigger pulse
missing trigger i/p is HIGH, Q is cut off, timer acts as
normal monostable state.
• It can be used for speed control and measurement.

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2.Linear Ramp Generator

at pin 2 > Vcc/3

Capacitor voltage
at pin 6

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Linear Ramp Generator- Description
Analysis:
Applying KVL around base-emitter loop of Q3
R1
V  V  I R  ( I  I ) R  ( I   I ) R  (1   ) I R   I R  I R  i R
R1  R2
CC BE E E C B E B B E B E B E C E E

( I C  i )

Q3  (  )  (  )
i R E  R1V CC V BE R1 R 2  i  R1V CC V BE R1 R 2
i Ic R1  R2 R1  R2)
R E ((R

Voltage Capacitor,
1t 1 t R1V CC  V BE ( R1  R2 ) 1  (  )
vc  C  idt  C  { }dt  { R1V CC V BE R1 R2 }t
0 0 RE ( R1  R2) C R E ( R1  R2)
When v becomes 23 V at T,
c CC

2
2 R V  V (R  R ) T 3
V CC CR E ( R1  R2)
V CC  1 CC BE 1 2
 T 
3 CR ( R  R )
E 1 2 R1V CC  V BE ( R1  R2)
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 3.Frequency Divider
Description:
A continuously triggered
monostable circuit when triggered by a
square wave generator can be used as a
frequency divider, if the timing interval is
adjusted to be longer than the period of the
triggering square wave input signal.
The monostable multivibrator will
be triggered by the first negative going edge
of the square wave input but the output will
Fig: Diagram of Frequency Divider remain HIGH(because of greater timing

interval) for next negative going edge of the

input square wave as shown fig.
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 4.Pulse Width Modulation

Fig a: Pulse Width Modulation Fig b: PWM Wave Forms

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 Pulse Width Modulation- Description
The charging time of capacitor is entirely depend upon 2Vcc/3.
When capacitor voltage just reaches about 2Vcc/3 output of the timer
is coming from HIGH to Low level.
We can control this charging time of the capacitor by adding
continuously varying signal at the pin-5 of the 555 timer which is
denoted as control voltage point. Now each time the capacitor voltage
is compared control voltage according to the o/p pulse width change.
So o/p pulse width is changing according to the signal applied to
control voltage point. So the output is pulse width modulated form.

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 Pulse Width Modulation
Practical Representation

Fig: PWM & Wave forms

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 Astable Multivibrator

Fig (a): Diagram of Astable Multvibrator

1 – Ground 5 – FM Input (Tie to gnd via bypass cap)

2 – Trigger 6 – Threshold
3 – Output 7 – Discharge
4 – Reset (Set HIGH for normal operation) 8 – Voltage Supply (+5 to +15 V)
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 Astable Multivibrator

R1

VA
A1 A 1
V1 Vo
R2

V2 A2 A 2
VC VT
R3
Q1

Fig (b): Functional Diagram of Astable Multivibrator using 555 Timer

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 Astable Multivibrator- Description
 Connect external timing capacitor between trigger point
(pin 2) and Ground.
 Split external timing resistor R into RA & RB, and connect
their junction to discharge terminal (pin 7).
 Remove trigger input, monostable is converted to Astable
multivibrator.
 This circuit has no stable state. The circuits changes its
state alternately. Hence the operation is also called free
running oscillator.
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Astable 555 Timer Block Diagram Contents
• Resistive voltage divider (equal resistors) sets threshold
voltages for comparators
V1 = VTH = 2/3 VCC V2 = VTL = 1/3 VCC
• Two Voltage Comparators
- For A1, if V+ > VTH then R =HIGH
- For A2, if V- < VTL then S = HIGH
• RS FF
- If S = HIGH, then FF is SET, Q= LOW, Q1 OFF, VOUT = HIGH
- If R = HIGH, then FF is RESET, Q= HIGH, Q1 ON, VOUT = LOW

• Transistor Q1 is used as a Switch

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 Operation of a 555 Astable
1) Assume initially that the capacitor is discharged.
a) For A1, V+ = VC = 0V and for A2, V- = VC = 0V, so R=LOW,
S=HIGH, Q = LOW , Q1 OFF, VOUT = VCC
b) Now as the capacitor charges through RA & RB,
eventually VC > VTL so R=LOW & S=LOW.
FF does not change state.

RA RB
VC(t)
VCC

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 Operation of a 555 Astable
Continued……
2) Once VC  VTH
a) R=HIGH, S=LOW, Q = HIGH ,Q1 ON, VOUT = 0
b) Capacitor is now discharging through RB and Q1 to
ground.
c) Meanwhile at FF, R=LOW & S=LOW since
VC < VTH.
RB
VC(t)

Q1

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 Operation of a 555 Astable
Continued…..
3) Once VC < VTL
a) R=LOW, S=HIGH, Q= LOW , Q1 OFF, VOUT = VCC
b) Capacitor is now charging through RA & RB again.

RA RB
VC(t)
VCC

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Timing Diagram of a 555 Astable
VC(t) 1 2 3
VTH

VTL
t

VOUT(t) TL TH
t
t=0 t = 0'

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 Astable Multivibrator- Analysis
The capacitor voltage for a low pass RC circuit subjected to a step input of Vcc volts is
given by, t
v  V (1  e )
c CC
RC

2
The time t1 taken by the circuit to change from 0 to 2Vcc/3 is, 
V C 3 V CC
2 V CC  t1
 V CC (1  e RC )  t1  1.09 RC
3 1
The time t2 to charge from 0 to vcc/3 is V  3 V
C CC

V  t 2
V
CC
(1 
CC e )  t  0.405 RC
RC
2
3
So the time to change from Vcc/3 to 2Vcc/3 is, t HIGH  t1  t2  1.09RC  0.405RC  0.69RC
So, for the given circuit, t HIGH  0.69( R A  R B )C …… Charging time

The output is low while the capacitor discharges from 2Vcc/3 to Vcc/3 and the
voltage across the capacitor is given by,
V CC  2 t

3 3
V CC e
RC

Contd…. 35
 Astable Multivibrator- Analysis
After solving, we get, t=0.69RC
For the given circuit,  0.69 R B C …… Discharging time
t LOW

Both RA and RB are in the charge path, but only RB is in the discharge path.
 The total time period,
T  t HIGH  t LOW  0.69 ( R A  R B )C  0.69 R B C

 T  0.69 [( R A R B )C  R B C ]  0.69 ( R A  R B  R B )C  0.69 ( R A 2 R B )C

1 1 1.45
Frequency, f    …….1.45 is Error Constant
T 0.69 ( R A  2 R B )C ( R A  2 R B )C

Duty Cycle,
0.69 ( R A  R B )C (  RB)
% D  t HIGH X 100  X 100  R A X 100
T 0.69 ( R A  2 R B )C ( R A 2 R B )
0.69 R B C
% D  t LOW X 100  X 100  RB X 100
T 0.69 ( R A  2 R B )C ( R A 2RB)
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 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 RA
and RB with a time constant (RA + RB)C.
 The capacitor discharges through RB with a time
constant of RBC
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Uses of the Astable Multivibrator
• Flashing LED’s
• Pulse Width Modulation
• Pulse Position Modulation
• Periodic Timers
• Uses include LEDs, pulse generation, logic
clocks, security alarms and so on.

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 Applications in Astable Mode

1.Square Generator
2.FSK Generator
3.Pulse Position Modulator

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 1.Square Generator

( R1 R )
3
10µF DutyCycle  2
X 100  50%
( R1 2 R ) 2

Here R1  0

C1
Fig: Square Wave Generator

 To avoid excessive discharge current through Q1 when R1=0

connect a diode across R2, place a variable R in place of R1.
 Charging path R1 & D; Discharging path R2 & pin 7.

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 2. FSK Generator

Description:
 In digital data communication,
binary code is transmitted by
shifting a carrier frequency
between two preset
frequencies. This type of
Fig: FSK Generator
transmission is called Frequency
Shift Keying (FSK) technique.

Contd….. 41
 FSK Generator
 A 555 timer is astable mode can be used to generate FSK signal.
 When input digital data is HIGH, T1 is OFF & 555 timer works as
normal astable multivibrator.

The frequency of the output wave form given by,

1.45
fO
( R1 2 R )C
2

When input digital is LOW, Q1 is ON then R3 parallel R1

1.45
f O
( R3||R1 2 R )C
2

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 2. Pulse Position Modulator
Description:
 The pulse position modulator can be
constructed by applying a modulating
signal to pin 5 of a 555 timer connected
for astable operation.
 The output pulse position varies with
Fig (a): Pulse position Modulator
the modulating signal, since the
threshold voltage and hence the time
delay is varied.
 The output waveform that the
frequency is varying leading to pulse
Fig (b): Output Wave Form of PPM 43
position modulation.
 Astable Multivibrator
Problem:
In the astable multivibrator of fig, RA=2.2KΩ, RB=3.9K Ω and C=0.1µF. Determine
the positive pulse width tH, negative pulse width tLow, and free-running frequency fo.
Solution:

t HIGH
 0.69( R A  R B )C  0.69( 2.2 K  3.9 K )(0.1X 106 )  0.421ms

t LOW
 0.69 R B C  0.69(3.9 K)(0.1X 106)  0.269ms
1 1.45
fo  ?
T ( R A 2 R B )C Duty Cycle,
(  RB) 2.2 K  3.9
% D  t HIGH X 100  R A X 100  X 100  ?
T ( R A 2RB ) 2.2 K  2 X 3.9 K

3.9
% D  t LOW X 100  RB X 100  X 100  ?
T ( R A 2RB) 2.2 K  2 X 3.9 K

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 Example: Design a 555 Oscillator to produce an approximate
square-wave at 40 KHz. Let C > 470 pF.

One Possible F=40KHz; T=25µs; t1=t2=12.5µs

Solution: For a square-wave RA<<RB; Let RA=1K and RB=10K
t1=0.693(RB)(C); 12.5µs=0.693(10K)(C); C=1800pF
T=0.693(RA+2RB)C: T=0.693(1K+20K)1800pF
T=26.2µs; F=1/T; F=38KHz (almost square-wave).

Example: A 555 oscillator can be combined with a J-K FF to

produce a 50% duty-cycle signal. Modify the above
circuit to achieve a 50% duty-cycle, 40 KHz signal.
One Possible Reduce by half the 1800pF. This will create a T=13.1µs or F=76.35 KHz
Solution: (almost square-wave). Now, take the output of the 555 Timer and connect
it to the CLK input of a J-K FF wired in the toggle mode (J and K inputs
connected to +5V). The result at the Q output of the J-K FF is a perfect
38.17 KHz square-wave.
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 Comparison of Multivibrator Circuits
Monostable Multivibrator Astable Multivibrator
1. It has only one stable state 1. There is no stable state.
2. Trigger is required for the operation 2. Trigger is not required to change the
to change the state. state hence called free running.
3. Two comparators R and C are 3. Three components RA, RB and C are
necessary with IC 555 to obtain the necessary with IC 555 to obtain the
circuit. circuit.
4. The pulse width is given by T=1.1RC 4. The frequency is given by,
1 1.45
Seconds f 
o

T ( R A  2 R B )C

5. The frequency of operation is 5. The frequency of operation is

controlled by frequency of trigger controlled by RA, RB & C.
pulses applied.
6. The applications are timer, frequency 6. The applications are square wave
divider, pulse width modulation etc… generator, flasher, voltage controlled
oscillator, FSK Generator etc..
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 Schmitt Trigger

Fig (b): Output Wave Form

Fig (a): Circuit Diagram of Schmitt Trigger

The use of 555 timer as a Schmitt trigger is shown in fig.

Here the two internal comparators are tied together and externally
biased at Vcc/2 through R1 and R2. Since the upper comparator will
trip at 2Vcc/3 and lower comparator at Vcc/3, the bias provided by R1
and R2 is centered within these two thresholds.
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 Features of IC 555 Timer
The Features of IC 555 Timer are:
1. The 555 is a monolithic timer device which can be used
to produce accurate and highly stable time delays or oscillation. It
can be used to produce time delays ranging from few
microseconds to several hours.
2. It has two basic operating modes: monostable and
astable.
3. It is available in three packages: 8-pin metal can, 8-pin
mini DIP or a 14-pin. A 14-pin package is IC 556 which consists of
two 555 times.

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 Features of IC 555 Timer
4. The NE 555( signetics ) can operate with a supply
voltage in the range of 4.5v to 18v and output currents of
200mA.
5. It has a very high temperature stability, as it is
designed to operate in the temperature range of -55⁰c to
125oc.
6. Its output is compatible with TTL, CMOS and Op-
Amp circuits.

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