The 555 timer IC is a versatile integrated circuit used in timer, pulse generation, and oscillator applications. It contains transistors, resistors, and diodes on a silicon chip. The 555 can be used in monostable, bistable, and astable modes to generate pulses or oscillations. It is commonly used in applications like blinking LEDs, timers, oscillators, and more due to its low cost, ease of use, and stability.

1. 555
Timer IC

2. INTRODUCTION
• Introduced by Signetics in 1972, most popular is NE555 by
STM electronics and Fairchild Semiconductor.
• The 555 timer IC is an integrated circuit (chip) used in a
variety of timer, pulse generation, and oscillator applications.
• The 555 is used to provide time delays, as an oscillator, and
as a flip-flop element.
• It gets its name from the three 5k ohm resistors which give
the two comparators reference voltage.
• Depending on the manufacturer, the standard 555 package
includes 25 transistors, 2 diodes and 15 resistors on a silicon
chip installed in an 8-PIN DIP (Dual in-line) package.
• It is available in low power CMOS type ICM7555 package and
556 Dual Timer (14 pin) with two timer in one IC and 558
which is Quad timer.

3. INTERNAL LAYOUT

4. VOLTAGE DIVIDER
2/3 Vcc
1/3 Vcc

5. VOLTAGE COMPARATOR
Whenever the voltage is higher at the + input, the output value is +.
The output is 0 volts under all other conditions.

6. THE FLIP-FLOP

7. THE DISCHARGE TRANSIST

8. THE OUTPUT BUFFER

9. PIN DESCRIPTION
PIN 1-Ground, The ground pin connects the
555 timer to the negative (0v) supply rail.
PIN 2-Trigger, when < 1/3 Vcc ('active low')
this makes the output high (+Vcc). It
monitors the discharging of the timing
capacitor in an astable circuit.
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, when less than about 0.7V ('active low') this makes the
output low (0V), overriding other inputs. When not required it
should be connected to +Vcc.

10. PIN DESCRIPTION
PIN 5-Control Voltage, this can be used to
adjust the threshold voltage which is set
internally to be 2/3 Vcc. Usually this function is
not required and the control input is
connected to 0V with a 10nF capacitor to
eliminate electrical noise.
PIN 6-Threshold, when > 2/3 Vcc ('active high')
this makes the output low (0V)*. It monitors
the charging of the timing capacitor in astable
and monostable circuits.
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 (18V Absolute Maximum).

11. MODES OF OPERATION
The 555 Timer IC has three modes of operation:
MONOSTABLE: in this mode, the 555 functions as a "one-shot"
pulse generator. Applications include timers, bounce free switches,
touch switches, frequency divider and so on.
BISTABLE: the 555 can operate as a flip-flop, if the DIS pin is not
connected and no capacitor is used. Uses include bounce-free
latched switches.
ASTABLE, the free running mode: the 555 can operate as an
oscillator. Uses include LED and lamp flashers, pulse generation,
logic clocks, tone generation, security alarms, pulse position
modulation and so on. The 555 can be used as a simple ADC,
converting an analog value to a pulse length.

12. MONOSTABLE MODE
In the monostable mode, the 555 timer
acts as a “single" pulse generator.
The pulse begins when the 555 timer receives
a signal at the trigger input that falls below a
third of the voltage supply.
The width of the output pulse is determined by
the time constant of an RC network, which
consists of a capacitor (C) and a resistor (R).
The output pulse ends when the voltage on
the capacitor equals 2/3 of the supply
voltage. The output pulse width can be
lengthened or shortened to the need of the
specific application by adjusting the values of
R and C.

13. MONOSTABLE MODE CIRCUIT DI
The output pulse width of
time t, which is the time it
takes to charge C to 2/3 of
the supply voltage, is given
by
t=RC ln(3)
where t is in seconds, R is
in ohms and C is in farads.
While using the timer IC in
monostable mode, the
main disadvantage is that the time span between any two triggering
pulses must be greater than the RC time constant.

14. BISTABLE MODE
In bistable mode, the 555 timer acts as a
flip-flop. The trigger and reset
Inputs are held high via Pull-up resistors
while the threshold input is simply
grounded. Thus configured, pulling
the trigger momentarily to ground acts as
a 'set' and transitions the output pin to
Vcc (high state). Pulling the reset input to
ground acts as a 'reset‘ and transitions
the output pin to ground (low state). No
capacitors are required in a bistable
configuration. Pin 5 (control) is
connected to ground via a small-value
capacitor (usually 0.01 to 0.1 uF); pin 7
(discharge) is left floating.

15. ASTABLE MODE
In astable mode, the 555 timer puts out a
continuous stream of rectangular pulses
having a specified frequency.
the capacitor is charged through R1 and R2, and
discharged only through R2, since pin 7 has low
Impedance to ground during output low intervals of
the cycle, therefore discharging the capacitor.
In the astable mode, the frequency of the
pulse stream depends on the values
of R1, R2 and C:
High time:
Low time:

16. APPLICATIONS OF 555 TIMER
The 555 Timer IC being easy to use, low price, and good stability is
used widely in many applications.
Some of which we are going to show you now, namely:
1. Blinking LED lights
2. Police Car Siren
3. Light Detector
4. Mosquito Repellant
5. Railway Lights

17. BLINKING LED Lights
The 555 timer is used to blink an array of LEDs’, The timer is used in
ASTABLE mode which gives us pulsed output.
According to the above formula
the frequency of the blink is
approximately 2.2Hz.
The frequency can be changed
by either changing the value of
resistance or the capacitor.

18. POLICE CAR SIREN
Police siren circuit given here is a circuit based on 555 timer IC. This
circuit uses two 555 ICs and which are the core working parts of this
circuit. Here two timer ICs are wired to work as astable
multivibrators.

19. WORKING OF POLICE CAR S
But these two timer ICs are working at different frequencies. First 555
Timer is slow astable multivibrator operating at a frequency of 20Hz
and a duty cycle of 50% whereas second 555 timer works at 600Hz as
fast astable multivibrator. The output of first timer is fed to the
control pin (pin5) of second 555 timer. By this connection, the output
frequency of second 555 timer will be modulated with the output
frequency of first 555 timer.
The circuit can be powered from a DC supply voltage ranging between
6V and 15V. Power supply used here is 12V DC supply. The loudness of
output can be further increased by inserting a power amplifier circuit
at the output stage of this circuit.

20. LIGHT DETECTOR
In this circuit the 555 timer runs in ASTABLE mode, where the
output is controlled by varying the potential of the RESET (pin 4). To
control the voltage an LDR is used. When light is shown in the LDR
its resistance decreases and its potential is almost equal to Vcc.
Hence the internal
flip flop is not
reset, which in turn
produces output
pulses of desired
frequency which
makes the buzzer
beep. The
sensitivity can be
controlled by the
connected
potentiometer.

21. MOSQUITO REPELLANT
This circuit produces a tone above human audible range and this is
supposed to keep the mosquitoes away. You need a piezo diaphragm
that will respond to 15kHz.
The 555 timer works in
ASTABLE mode in this circuit.

22. RAILWAY LIGHTS
In this circuit, the 555 timer works in ASTABLE mode. One LED glows
due to sinking of current and the other due to sourcing of current.
This happens simultaneously because the Duty Cycle of this circuit is
approximately 50%.
Frequency=3Hz