A brief description on integrated circuit 555 timer with its applications and states in which it works !

1. 555 TIMER IC
MAYANK RAJ SINGH

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’

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

12. 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.