The 555 timer is a very
popular IC used in electronic industry which is reliable, easy to use and
cheaper in cost. It was invented in 1971 by American company signets. It is an integrated circuit (chip) used in variety of
timer, pulse generation, and oscillator applications. The 555 can be used to
provide time delays, as an oscillator, as a flip-flop element, temperature
measurement,monostable and astable multivibrator.It operates basically in one
of the two modes either as a monostable multivibrator or as an astable
multivibrator.The name 555 timer has come from the three resistor of 5k each
within the timer circuits.
Figure 1: 555 Timer
Pin
configuration
Pin1: Grounded
terminal
All voltage is measured with respect to
this terminal.
Pin
2:
Trigger terminal
It is an inverting input to a comparator
that is responsible of flip flop from set to reset. The output of the timer
depends on the amplitude of the external trigger pulse applied to this pin.
Pin
3:
output terminal
There are two ways in which load can be
connected to the output terminal either between pin 3 and ground or between Pin
3 and Pin 8 and is usually called a load and normally off load respectively.
Pin
4:
Reset terminal
To disable or reset the timer a negative
pulse is applied to this pin due to which it is referred to as reset terminal.
When this pin is not to be used for reset purpose, it should be connected to
Vcc to avoid any possibility of false triggering.
Pin5: Control
voltage terminal
Its function is to control the threshold
and trigger level. Thus either the external voltage or a pot connected to this
pin determines the pulse width of the output waveform. The external voltage
applied to this pin can also be used to modulate the output waveform. When this
pin is not used it should be connected to ground through a 0.01uF capacitor to
avoid any noise pollution.
Pin
6:
Threshold terminal
This is non inverting input terminal of
comparator , which compares the voltage applied to this terminal with a
reference voltage of +2/3 v. the amplitude of voltage applied to this terminal
is responsible for the set state of flip flop.
Pin
7:
Discharge terminal
This pin is connected internally to the
collector of transistor and mostly a capacitor is connected between this
terminal and ground. It is called discharge terminal because when transistor is
at saturation capacitor discharge through the transistor. When the transistor
is at cut off, the capacitor changes at a rate determined by external resistor
and capacitor.
·
The voltage divider consisting of 5k
resistors provides a trigger level of 1/3 VCC and a threshold level of 2/3 VCC.
·
The control voltage input (pin 5) can be
used to externally adjust the trigger and threshold levels to other values of
necessary.
·
When the trigger input (at +5v)
momentarily goes below 1/3 VCC the output of comparator B switches from 0 to 5v
and sets the S-R latch, causing the output pin3 to go +5v and turning the
discharge transistor Q1 OFF.
·
The output will stay at +5V until the
threshold input (at 0V) goes above 2/3 VCC and causes the output of comparator
A to switch from 0 to +5 V.
·
This resets the latch causing the output
to go back to 0Vand turning the discharge transistor ON the external reset input can be used to reset
the latch independent of the threshold input the trigger threshold inputs (pin
2 and 6) are controlled by external components connected to produce either
monostable or astable action.
Monostable
operation of 555 timer
A monostable vibrator is often called a one shot
multivibrator.In such vibrator one state of output is stable while the other is
unstable.It makes use of external resistor(R) and a capacitor(C).The pulse
width of the output is given by
tp = 1.1 R1
C1
|
Figure 2: Monostable Multivibrator
|
Astable Operation
Figure 3: Astable Multivibrator
It is often called as a free-running multivibrator
which is used to generate rectangular wave. It does not require external
trigger to change the state of the output. It can be produced by adding two
resistors and a capacitor to the basic timer IC.The output is either high or
low and is determined by the two resistors and capacitors.
Pin 1 is grounded, Pins 4 and 8 are
shorted and then tied to supply +vcc, Output is taken from pin3, Pin 2 and 6
are shorted and then connected to ground through a capacitor (C).Pin 7 is
connected to supply +Vcc through a resistor RA and between pins 6 and 7 are
resistor RB is connected.At pin 5 either a bypass capacitor of 0.01 uF s
connected or Modulation input is applied.
Circuit
operation
·
When the power is turned ON, the
capacitor is uncharged and thus the trigger voltage (pin2) is at 0v.
·
The output of comparator b is at +5v and
the output of comparator A to be at 0v forcing the output of the latch and the
base of Q1 to be at 0v.As a result the transistor Q1 is OFF, c1 is charging
through R1 and R2.
·
When the capacitor voltage reaches 1/3
Vcc, comparator B switches to its 0 output state and when the capacitor reaches
2/3Vcc, comparator A switches to its +5V state.
·
This resets the latch causing the base
of Q1 to go +5V. As a result Q1 turns ON. This sequence creates a discharge
path for the capacitor through R2.
·
The capacitor now begins to discharge
causing comparator A to go to 5V at the point where the capacitor discharges
down to 1/3 Vcc, comparator B switches to +0V level.
·
This sets the latch, making the base of
Q1 to go down to 0V.
·
As a result Q1 turns OFF. Another charging
cycle begins and the entire process repeats.
·
This produce a rectangular wave output
whose duty cycle depends on the values of R1 and R2.
·
The frequency of oscillator of a 555
timer in astable mode is given by the expression,
F=1.44/(R1 + 2R2) C1
·
By selecting R1 and R2, the duty cycle
of the output can be adjusted since C1 charges through R1+R2 and discharges
only through R2, a duty cycle approaching 50% can be achieved of R2>>R1
so that charging and discharging times are approximately equal.
