IC 555 TIMER

M.S.P.V.L. Polytechnic College,

Pavoorchatram

What is the 555 timer?

• The 555 timer is one of the most remarkable integrated

circuits ever developed. It comes in a single or dual

package and even low power cmos versions exist -

ICM7555.

• Common part numbers are LM555, NE555, LM556,

NE556. The 555 timer consists of two voltage

comparators, a bi-stable flip flop, a discharge transistor,

and a resistor divider network.

Block Diagram of Timer 555 IC

– The voltage divider (blue) has three equal 5K resistors.

It divides the input voltage (Vcc) into three equal

parts.

– The two comparators (red) 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

Inside the 555 Timer

– The flip-flop (green) 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 (purple) is being used as a switch, it connects pin

7 (discharge) to ground when it is closed.

• When Q is low, Qbar is high. This closes the transistor switch and

attaches pin 7 to ground.

• When Q is high, Qbar is low. This open the switch and pin 7 is no

longer grounded

What are the 555 timer applications?

• Applications include – precision timing,

– pulse generation,

– sequential timing,

– time delay generation and pulse width modulation (PWM).

Pin configurations of the 555 timer

• Pin Functions - 8 pin package

• Ground (Pin 1)

• Not surprising this pin is connected directly to ground.

• Trigger (Pin 2)

• This pin is the input to the lower comparator and is used to set the latch, which in

turn causes the output to go high.

• Output (Pin 3)

• Output high is about 1.7V less than supply. Output high is capable of Isource  up to

200mA while output low is capable of Isink  up to 200mA.

• Reset (Pin 4)

• This is used to reset the latch and return the output to a low state. The reset is an

overriding function. When not used connect to V+.

• Control (Pin 5)

• Allows access to the 2/3V+ voltage divider point when the 555 timer is

used in voltage control mode. When not used connect to ground through a

0.01 uF capacitor.

• Threshold (Pin 6)

• This is an input to the upper comparator.

• Discharge (Pin 7)

• This is the open collector to Q14 in figure 4 below.

• V+ (Pin 8)

• This connects to Vcc and the Philips databook states the ICM7555 cmos

version operates 3V - 16V DC while the NE555 version is 3V - 16V DC.

Note comments about effective supply filtering and bypassing this pin

below under "General considerations with using a 555 timer"

Types of 555-Timer Circuits

• Astable Multivibrator puts out a continuous sequence of pulses

5V

Ra

C

0.01

uF

LED

NE555

2

5

3

7

6

4 81

TR

CV

Q

DIS

THR

R

VC

CG

ND

Rb

5V

12

1K

0.01

uFC

R

LED

NE555

2

5

3

7

6

4 81

TR

CV

Q

DIS

THR

R

VC

CG

ND

Monostable Multivibrator (or one-shot) puts out one pulse each time the switch is connected

• Monostable Multivibrator (One Shot)

+V

-V

-

+

+V

-V

-

+R

S

Q

Q

3

4

1

7

2

6

8

R

R

R

Control Flip-FlopTrigger Comparator

Threshold Comparator

Output

ResetVcc

Trigger

Monstable MultivibratorOne-Shot

C

Racc

2V

3

cc

1V

3

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.

– 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.

Uses of the Monostable Multivibrator

Astable Pulse-Train Generator (Multivibrator)

+V

-V

-

+

+V

-V

-

+R

S

Q

Q

3

4

1

7

2

6

8

R

R

R

Control Flip-FlopTrigger Comparator

Threshold Comparator

Output

Vcc

Astable Pulse-Train Generator

C

R1

R2

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

– Flashing LED’s

– Pulse Width Modulation

– Pulse Position Modulation

– Periodic Timers

Uses of the Astable Multivibrator

Flashing LED’s

• 40 LED bicycle light with 20 LEDs flashing alternately at 4.7Hz

Understanding the Astable Mode Circuit

• 555-Timers, like op-amps can be configured in different ways to create different circuits. We will now look into how this one creates a train of equal pulses, as shown at the output.

First we must examine how capacitors charge

• Capacitor C1 is charged up by current flowing through R1

• As the capacitor charges up, its voltage increases and the current charging it decreases, resulting in the charging rate shown

VV V

R1

1k

U2

TOPEN = 01

2C1

1uF

U1

TCLOSE = 0

1 2

0

V110V

IV V

R

V

kC A P A C IT O R C A P A C IT O R

1

1

1 0

1

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(U2:1) V(R1:2) V(V1:+)

0V

2V

4V

6V

8V

10V

Capacitor Voltage

Capacitor Charging Equations

• Capacitor Current

• Capacitor Voltage

• Where the time constant

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msI(R1) I(C1)

0A

2mA

4mA

6mA

8mA

10mA

Capacitor and Resistor Current

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(U2:1) V(R1:2) V(V1:+)

0V

2V

4V

6V

8V

10V

Capacitor Voltage

I I eo

t

V V eo

t

1

R C R C m s1 1 1

Understanding the equations

• Note that the voltage rises to a little above 6V in 1ms.

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(U2:1) V(R1:2) V(V1:+)

0V

2V

4V

6V

8V

10V

Capacitor Voltage

( ) .1 6 3 21 e

Capacitor Charging and Discharging

• There is a good description of capacitor charging and its use in 555 timer circuits at http://www.uoguelph.ca/~antoon/gadgets/555/555.html

555 Timer • At the beginning of the

cycle, C1 is charged through resistors R1 and R2. The charging time constant is

• The voltage reaches (2/3)Vcc in a time

1)21(693.01arg CRRTt ech

1)21(arg CRRech

555 Timer

• When the voltage on the capacitor reaches (2/3)Vcc, a switch (the transistor) is closed (grounded) at pin 7.

• The capacitor is discharged to (1/3)Vcc through R2 to ground, at which time the switch is opened and the cycle starts over. 1)2(arg CRedisch

1)2(693.02arg CRTt edisch

555 Timer

• The frequency is then given by

fR R C R R C

1

0 6 9 3 1 2 2 1

1 4 4

1 2 2 1. ( )

.

( )

Output voltage high turns off upper LED and turns on lower LED

Capacitor is charging through Ra and Rb

Output is high for 0.693(Ra+Rb)C

555 Animation

http://www.williamson-labs.com/pu-aa-555-timer_slow.htm

Output is low so the upper LED is on and the lower LED is off

Capacitor is discharging through Rb

Output is low for 0.693(Rb)C

555 Animation

PWM: Pulse Width Modulation

• Signal is compared to a sawtooth wave producing a pulse width proportional to amplitude

What Can Be Done With PWM?

• Question: What happens if voltages like the ones above are connected to a light bulb? Answer: The longer the duty cycle, the longer the light bulb is on and the brighter the light.

Low

Duty Cycle

Medium

Duty Cycle

High

Duty Cycle

What Can Be Done With PWM?

• Average power can be controlled• Average flows can also be controlled by fully opening and

closing a valve with some duty cycle

The End

…..Thank you…..