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