555 TIMER IC

TECHNICAL SEMINAR ON

By

K . Vishnu11B61A0459

INTRODUCTION

• The 555 IC was designed in 1971 by Hans

Camenzind under contract to SigNetics

Corporation.

• Basically, 555 timer is a highly stable circuit

used to generate time delays, or Oscillations.

• A single 555 timer can provide time delay

ranging from microseconds to hours.

• It operates from a wide range of power

supplies ranging from + 5 Volts to + 18 Volts

supply voltage.

Pin Configuration

The function of each pin of the IC is given below –

•Pin–1: Ground

•Pin–2: Trigger

•Pin–3: Output

•Pin–4: Reset

•Pin–5: Control Voltage

•Pin–6: Threshold

•Pin–7: Discharge

•Pin–8: Vcc

Functional diagram of 555 timer

+

-

Threshold

(2/3)Vcc

R

Threshold > (2/3)Vcc =>R=logic ‘1’Threshold < (2/3)Vcc =>R=logic ‘0’

+

-

(1/3)Vcc

Trigger

S

Trigger<(1/3)Vcc =>S=logic ‘1’Trigger>(1/3)Vcc =>S=logic ‘0’

R S Q- Q

0 0 Previous state

0 1 0 1

1 0 1 0

1 1 X X

Modes of Operation

555 IC Timer applications can be classified into two main categories:

1. Monostable Multivibrators : Producing a single pulse whentriggered.

2. Astable Multivibrators : Producing a square wave.

1. Monostable Multivibrator

Monostable multivibrator is often called as one-shot multivibrator.

1

3

+VCC

7

6

2

8One-shot operation

R

C

1/3 VCC

The input trigger resets the flip-flop and C then charges

until the upper comparator trips and sets the flip-flop.

R

S

input triggeroutput pulse

Discharge

transistor

~Q

Q

Inverter

+ VCC

555

48

7

6

1

3

2

R

C

t = 1.1RCOutput pulse

Trigger1/3 VCC

The external components determine the output pulse width.

2. Astable Multivibrator

• Astable multivibrators are also known as Free-running Multivibrator.

• Astable do not need trigger pulse for external to change the output.

• The period for LOW and HIGH can be calculated based on resistor and capacitor value that connected at outside of timer.

C charges through RA+ RB

and discharges through RB.

1

3

+VCC

7

6

2

8

Free-running or

astable operationRB

C

RA

R

S

C charges through RA+ RB

and discharges through RB.

Inverter~Q output

Continue…

TH = 0.693 (RA+RB) C TL = 0.693 (RB) C

Period,T = TH + TL = 0.693 (RA + 2RB)C

Frequency, f = 1

T

= 1

(TH+TL)

= 1.44

(RA + 2RB) C

% Duty Cycle = [TH

TH +TL]x 100

=(RA+RB)

(RA+2RB)X100

2/3 VCC

1/3 VCC

output

0

VCC

Timing sequence of astable multivibrator

Capacitor voltage

Applications

Schmitt trigger

PPM

PWM

Linear Ramp generator

Conclusion

Hence 555 IC timer can produce very accurate and stable time

delays, from microseconds to hours. It can be used with supply voltage varying from 5 to 18 V. Timer can be used in monostable mode of operation or astable mode of operation. Its various applications include waveform generator, missing pulse detector, frequency divider, pulse width modulator, burglar alarm, FSK generator, ramp generator, pulse position modulator etc.

References

LINEAR INTEGRATED CIRCUITS ( fourth edition) by D. Roy Choudhury-311 page number.

OP-AMPS AND LINEAR INTEGRATED CIRCUITS (forth edition) by Ramakanth A.Gayakwad-416 page number.

file:///E:/Personal%20data%20of%20me/s/555%20timer/documentations/555%20Timer%20Tutorial%20-%20The%20Monostable%20Multivibrator.html

http://en.wikipedia.org/wiki/555_timer_IC#Design