The 555 timer IC can be used to generate precise time delays from microseconds to hours. It operates from 5-18V and has 8 pins including power, ground, trigger, output, reset and control functions. It can be used in monostable or astable modes. In monostable mode, a single output pulse is produced in response to a trigger. In astable mode, it produces a continuous square wave without a trigger. The 555 timer has applications including timers, pulse generators, oscillators and more.

1. 555 TIMER IC
TECHNICAL SEMINAR ON
By
K . Vishnu
11B61A0459

2. 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.

3. 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

4. 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

5. Modes of Operation
555 IC Timer applications can be classified into two main categories:
1. Monostable Multivibrators : Producing a single pulse when
triggered.
2. Astable Multivibrators : Producing a square wave.

6. 1. Monostable Multivibrator
Monostable multivibrator is often called as one-shot
multivibrator.

7. 1
3
+VCC
7
6
2
8
One-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 trigger
output pulse
Discharge
transistor
~Q
Q
Inverter

8. + VCC
555
48
7
6
1
3
2
R
C
t = 1.1RC
Output pulse
Trigger1/3 VCC
The external components determine the output pulse width.

9. 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.

10. 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

11. 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

12. Applications
 Schmitt trigger
 PPM
 PWM
 Linear Ramp generator

13. 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.

14. 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/documentat
ions/555%20Timer%20Tutorial%20-
%20The%20Monostable%20Multivibrator.html
 http://en.wikipedia.org/wiki/555_timer_IC#Design