Pin 1. – Ground, The ground pin connects the 555 timer to the negative (0v) supply rail. • Pin 2. – Trigger, The negative input to comparator No 1. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a “HIGH” state. • Pin 3. – Output, The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc – 1.5V so small speakers, LEDs or motors can be connected directly to the output. • Pin 4. – Reset, This pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic “1” level when not used to prevent any unwanted resetting of the output. • Pin 5. – Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise. • Pin 6. – Threshold, The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit. • Pin 7. – Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”. • Pin 8. – Supply +Vcc, This is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V.

1. IC 555 TIMER
By – Anindya Pal, Sandeep Kr Bhanja , Department of Physics
Subject – Electronic Lab (PH2151)
Year - 2024

2. CONTENTS
Introduction
Diagram
Working Principal
Different Modes
Applications
IC 555
IC 55 PIN Diagram

3. INTRODUCTION
• IC 555 was designed in 1971 by Hans R.
𝑪𝒂𝒎𝒆𝒏𝒛𝒊𝒏𝒅𝟏
.
• The 555 timer is one of the most remarkable
integrated circuits ever developed.
• It’s used for different varieties of timer , Pulse
generator , Oscillator.
• Common part numbers are LM555, NE556, LM556,
NE556.
• The 555 timer consists of two voltage comparators,
a bi-stable flip-flop, a discharge transistor & a
resistor divider network
• NE555 and SE555 are used at a commercial range
of 𝟎𝒐
𝑪 𝒕𝒐 𝟕𝟎𝒐
C & −𝟓𝟓𝒐
𝑪 𝒕𝒐 𝟏𝟐𝟓𝒐
C

4. PIN & Circuit Diagram
• PIN1-GROUND
• PIN2-TRIGGER
• PIN3-OUTPUT
• PIN4-RESET
• PIN5-CONTROL
• PIN6-THRESHOLD
• PIN7-DISCHARGE
• PIN8-SUPPLY 𝑽𝑪𝑪
IC555 circuit diagram
IC555 PIN diagram

5. • Pin 1. – Ground, The ground pin connects the 555 timer to the negative (0v) supply rail.
• Pin 2. – Trigger, The negative input to comparator No 1. A negative pulse on this pin “sets” the
internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a
“HIGH” state.
• Pin 3. – Output, The output pin can drive any TTL circuit and is capable of sourcing or sinking up to
200mA of current at an output voltage equal to approximately Vcc – 1.5V so small speakers, LEDs or
motors can be connected directly to the output.
• Pin 4. – Reset, This pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3.
This is an active-low input and is generally connected to a logic “1” level when not used to prevent any
unwanted resetting of the output.
• Pin 5. – Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the
voltage divider network. By applying a voltage to this pin the width of the output signal can be varied
independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor
to eliminate any noise.
• Pin 6. – Threshold, The positive input to comparator No 2. This pin is used to reset the Flip-flop when
the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This
pin connects directly to the RC timing circuit.
• Pin 7. – Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor
which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”.
• Pin 8. – Supply +Vcc, This is the power supply pin and for general purpose TTL 555 timers is between
4.5V and 15V.

7. Different Modes
1. Monostable Multivibrator
2. Astable Multivibrator
3. Bistable Multivibrator
IC555 Has three operating Modes :

8. MONOSTABLE MULTIVIBRATOR
A monostable multivibrator using a 555
timer is a circuit that generates a single
output pulse of a defined duration in
response to an input trigger. This
configuration of the 555 timer is also
known as a one-shot multivibrator
because it returns to its stable state after a
single pulse. The duration of the output
pulse is determined by an RC (resistor-
capacitor) network connected to the timer.
Pulse width

10. ASTABLE MULTIVIBRATOR
Astable multivibrator is also called as Free
Running Multivibrator. It has no stable
states and continuously switches between
the two states without application of any
external trigger. The IC 555 can be made
to work as an astable multivibrator with
the addition of three external
components: two resistors (R1 and R2) and
a capacitor (C). The schematic of the IC
555 as an astable multivibrator along with
the three external components is shown
below.

12. BISTABLE MULTIVIBRATOR
When an astable multivibrator has no stable
states and a monostable multivibrator has a
single stable state, a device with two absolute
stable states is possible. A Bistable multivibrator
is a type of circuit which has two stable states
(high and low). It stays in the same state until
and unless an external trigger input is applied.
Generally, a bistable multivibrator stays low until
a trigger signal is applied and it stays high until a
reset signal is applied. Bistable multi vibrators
are also called as flip-flops or latches. The term
flip-flop is used because it ‘flips’ to one state and
stays there until a trigger is applied and once the
trigger is applied it ‘flops’ back to the original
state.

14. APPLICATIONS
• Monostable
Multivibrator
• FREQUENCY DIVIDER
• PULSE STRETCHER
• PULSE GENERATOR
Astable
Multivibrator
 SQUARE WAVE
OSCILLATOR
 FREE RUNNING RAM
GENERATOR
Bistable
Multivibrator
• FLIP-FLOP
• MEMORY CELL

15. REFERENCES (Hyperlinked) :
[1] Hans Camenzind
[2] Monostable Multivibrator
[3] Astable Multivibrator
[4] Bistable multivibrator