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To construct and study the wave forms at the base and collector of the transistors
of a free running multivibtrator. To construc and study of the height , duration and
time period of the output pulses in a monostable and Bistable multivibrators with
reference to input trigger.
We have seen that Multivibrators and CMOS Oscillators can be easily constructed
from discrete components to produce relaxation oscillators for generating basic square
wave output waveforms. But there are also dedicated IC’s especially designed to
accurately produce the required output waveform with the addition of just a few extra
timing components.
One such device that has been around since the early days of IC’s and has itself
become something of an industry “standard” is the 555 Timer Oscillator which is
more commonly called the “555 Timer”.
The basic 555 timer gets its name from the fact that there are three internally
connected 5kΩ resistors which it uses to generate the two comparators reference
voltages. The 555 timer IC is a very cheap, popular and useful precision timing device
which can act as either a simple timer to generate single pulses or long time delays, or
as a relaxation oscillator producing a string of stabilized waveforms of varying duty
cycles from 50 to 100%.
The 555 timer chip is extremely robust and stable 8-pin device that can be operated
either as a very accurate Monostable, Bistable or Astable Multivibrator to produce a
variety of applications such as one-shot or delay timers, pulse generation, LED and
lamp flashers, alarms and tone generation, logic clocks, frequency division, power
supplies and converters etc, in fact any circuit that requires some form of time control
as the list is endless.
The single 555 Timer chip in its basic form is a Bipolar 8-pin mini Dual-in-line Package
(DIP) device consisting of some 25 transistors, 2 diodes and about 16 resistors
arranged to form two comparators, a flip-flop and a high current output stage as
shown below. As well as the 555 Timer there is also available the NE556 Timer
Oscillator which combines TWO individual 555’s within a single 14-pin DIP package
and low power CMOS versions of the single 555 timer such as the 7555 and LMC555
which use MOSFET transistors instead.
A simplified “block diagram” representing the internal circuitry of the 555 timer is given
below with a brief explanation of each of its connecting pins to help provide a clearer
understanding of how it works.
555 Timer Block Diagram
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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
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”.
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Pin 8. Supply +Vcc, This is the power supply pin and for general purpose
TTL 555 timers is between 4.5V and 15V.
The 555 Timers name comes from the fact that there are three 5kΩ resistors
connected together internally producing a voltage divider network between the supply
voltage at pin 8 and ground at pin 1. The voltage across this series resistive network
holds the negative inverting input of comparator two at 2/3Vcc and the positive non-
inverting input to comparator one at 1/3Vcc.
The two comparators produce an output voltage dependent upon the voltage
difference at their inputs which is determined by the charging and discharging action
of the externally connected RC network. The outputs from both comparators are
connected to the two inputs of the flip-flop which in turn produces either a “HIGH” or
“LOW” level output at Q based on the states of its inputs. The output from the flip-flop
is used to control a high current output switching stage to drive the connected load
producing either a “HIGH” or “LOW” voltage level at the output pin.
The most common use of the 555 timer oscillator is as a simple astable oscillator by
connecting two resistors and a capacitor across its terminals to generate a fixed pulse
train with a time period determined by the time constant of the RC network. But the
555 timer oscillator chip can also be connected in a variety of different ways to
produce Monostable or Bistable multivibrators as well as the more common Astable
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Basic Astable 555 Oscillator Circuit
In the 555 Oscillator circuit above, pin 2 and pin 6 are connected together allowing
the circuit to re-trigger itself on each and every cycle allowing it to operate as a free
running oscillator. During each cycle capacitor, C charges up through both timing
resistors, R1 and R2 but discharges itself only through resistor, R2 as the other side
of R2 is connected to the discharge terminal, pin 7.
Then the capacitor charges up to 2/3Vcc (the upper comparator limit) which is
determined by the 0.693(R1+R2)C combination and discharges itself down to 1/3Vcc
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(the lower comparator limit) determined by the 0.693(R2*C) combination. This results
in an output waveform whose voltage level is approximately equal to Vcc 1.5V and
whose output “ON” and “OFF” time periods are determined by the capacitor and
resistors combinations. The individual times required to complete one charge and
discharge cycle of the output is therefore given as:
Astable 555 Oscillator Charge and Discharge Times
Where, R is in Ω and C in Farads.
When connected as an astable multivibrator, the output from the 555 Oscillator will
continue indefinitely charging and discharging between 2/3Vcc and 1/3Vcc until the
power supply is removed. As with the monostable multivibrator these charge and
discharge times and therefore the frequency are independent on the supply voltage.
The duration of one full timing cycle is therefore equal to the sum of the two individual
times that the capacitor charges and discharges added together and is given as:
555 Oscillator Cycle Time
The output frequency of oscillations can be found by inverting the equation above for
the total cycle time giving a final equation for the output frequency of an Astable 555
Oscillator as:
555 Oscillator Frequency Equation
By altering the time constant of just one of the RC combinations, the Duty
Cycle better known as the “Mark-to-Space” ratio of the output waveform can be
accurately set and is given as the ratio of resistor R2 to resistor R1. The Duty Cycle
for the 555 Oscillator, which is the ratio of the “ON” time divided by the “OFF” time is
given by:
555 Oscillator Duty Cycle
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The duty cycle has no units as it is a ratio but can be expressed as a percentage ( % ).
If both timing resistors, R1 and R2 are equal in value, then the output duty cycle will
be 2:1 that is, 66% ON time and 33% OFF time with respect to the period.
555 Oscillator Example No1
An Astable 555 Oscillator is constructed using the following components, R1 =
1kΩ, R2 = 2kΩ and capacitor C = 10uF. Calculate the output frequency from the 555
oscillator and the duty cycle of the output waveform.