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.used to generate time delays, or Oscillations.
• Asingle 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–5: Control Voltage
•Pin–6: Threshold
•Pin–7: Discharge
•Pin–8: Vcc
4. Pin Configuration
555TIMER
1. Timer IC 555 is a
monolithic timer IC
The basic building blocks of 555
timer are:-
a)RS Flip Flop,a)RS Flip Flop,
b)two comparators
c)ReferenceVoltage Source
d) Discharge transistor
5. (2/3)Vcc
Functional diagram of 555 timer
Threshold
+
R
-
Threshold > (2/3)Vcc =>R=logic ‘1’
Threshold < (2/3)Vcc =>R=logic ‘0’
1 1
(1/3)Vcc
+
S
-
Trigger
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
X X
6. Modes of Operation
555 IC Timer applications can be classified into two main categories:
1. Monostable Multivibrators : Producing a single pulse when
triggered.triggered.
2. Astable Multivibrators : Producing a square wave.
7. 1. Monostable Multivibrator
Monostable multivibrator is often called as one-shot
multivibrator.
Resistiv
e
divider + is the pos. of the
battery=9VR R R
+3 V
comparato
r
FF=flip-flip, depicted as a DPDT
switch.
+6 V
- is the neg. term. of the
battery=0V
(Assumption: pin #3 is connected to
0V)
8. One-shot operation+VCC
DischargeR
8
C
input trigger
7
R Q
S~
Q
transistor
3
Inverter
6
1/3
The input trigger resets the flip-flop and C then charges
until the upper comparator trips and sets the flip-flop.
input trigger
1/
The i
until
~
output pulse2
VCC
1
9. 2
+ VCC
R 8 4
555C
7
6 3
1
2
1.1RC
Output pulse
The external components determine the output pulse width.
1/3 VCC Trigger
t =
1
1.1
10. 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 atcapacitor value that connected at
outside of timer.
C charges through RA+ RB
and discharges through RB.
11. +VCC A B
7
RB
S ~Q
RA C charges through R + R
8 and discharges through RB.
R
Free-running or
astable operation
3
Inverter output
R
6
C
2
1
12. CC
1
Frequency, f =
=
0
2/3 V 1Capacitor voltage
1. 44
Continue…
TH = 0.693 (RA+RB) C
V output
TL = 0.693 (RB) C
Period,T = TH + TL = 0.693 (RA + 2RB)C
T
CC
(TH+TL) 1. 44
=
CC TH + TL
(RA+2RB)
T
CC
(TH+TL)
(RA + 2RB) C
1/3 V % Duty Cycle = [ TH
]x 100
Timing sequence of astable multivibrator (RA+RB)
14. 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, pulsewidth modulator, burglar alarm, FSK generator, ramp generator, pulse
position modulator etc.
15. 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 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
