Project Work

00 Table of contents

Chapter 1

Introduction

1.1 Problem statement

1.2 Objectives

1.3 Significance of project

Chapter 2

Literature Review

2.1The 555 Timer IC

*2.1.1 555 Timers in Astable Multivibrator Mode

2.2 NAND Gate Astable Multivibrator 2.3 CMOS Astable Multivibrator

2.4 OP- Amp Astable multivibrator

Chapter 3

Methodology

3.1 Design Methods

3.2 Modules design

3.3 Design structure 3.4 Electronic Component

*3.4.1 Transistor (2SC945)

*3.4.2 Electrolytic capacitor (22uf)

*3.4.3 LEDs

3.5 Hardware resources and procurement

3.6 Construction

List of figures

Fig 1.1 Example of astable multivibrator

Fig 2.1 Capacitance-Frequency graph

Fig 2.2 555 connected as an astable multivibrator

Fig 2.3 NAND gates in Astable Mode

Fig 2.4 output of the NAND gate.

Fig 2.5 CMOS in Astable mode

Fig-2.6 is the circuit of the 741 in astable mode

Fig2.7 Square wave output of the op-amp

CHAPTER1

1.0 Introduction:

Background

The first electronic flip-flop was invented in 1918 by William Eccles and Jordan. It was

initially called the Eccles–Jordan trigger circuit and consisted of two active elements

(vacuum tubes). Such circuits and their transistorized versions were common in

computers even after the introduction of integrated circuits, though flip-flops made from

logic gates are also common now.

Early flip-flops were known variously as trigger circuits or multivibrators. A

multivibrator is a two-state circuit; they come in several varieties, based on whether each

state is stable or not: an astable multivibrator is not stable in either state, so it acts as a

relaxation oscillator; a monostable multivibrator makes a pulse while in the unstable

state, then returns to the stable state, and is known as a one-shot; a bistable multivibrator

has two stable states, and this is the one usually known as a flip-flop. However, this

terminology has been somewhat variable, historically.

An astable multivibrator is also known as a free-running multivibrator. It is called free

running because it alternates between two different output voltage levels during the time

it is on. The output remains at each voltage level for a definite period of time. If you

looked at this output on an oscilloscope, you would see continuous square or rectangular

waveforms. The astable multivibrator has two outputs, but NO inputs.

A multivibrator is an electronic circuit used to implement a variety of simple two-state

systems such as oscillators, timers and flip-flops. An astable multivibrator has two states,

neither one stable. The circuit therefore behaves as an oscillator with the time spent in

each state controlled by the charging or discharging of a capacitor through a resistor. The

astable multivibrator may be created directly with transistors or with use of integrated

circuits such as operational amplifiers (op amps) or the 555 timers. A positive and

negative rail voltage, the output never able to exceed these rail voltages, powers most

operational amplifiers. Depending upon initial conditions, the op amp’s output will drive

to either positive or negative rail. Upon this occurrence, the capacitor will either charge

or discharge through the resistor R2, its voltage slowly rising or falling. As soon as the

voltage at the op amp’s inverting terminal reaches that at the non-inverting terminal (the

op amp’s output voltage divided by R1 and R2), the output will drive to the opposing rail

and this process will repeat with the capacitor discharging if it had previously charged

and vice versa.

Once the inverting terminal reaches the voltage of the non-inverting terminal the output

again drives to the opposing rail voltage and the cycle begins again. Thus, the astable

multivibrator creates a square wave with no inputs. Period of astable multivibrator

displayed. An astable multivibrator generates a string of pulses.

1.1 Problem Statement:

Most astable multivibrator fabricated on chips require more power to drive loads. This

could affect system performance since the generated heat has adverse effects on the ICs

and other surrounding components. Moreover these astable ICs have poor voltage

Figure 1.1: Astable Multivibrator

regulation and require input clock pulse to activate them. However, astable multivibrators

fabricated with discrete components have excellent voltage regulations and require no

clock activation.

1.2 Objective:

To design and construct a BJT astable multivibrator as a flasher.

1.3 Significance of project:

This project aims at developing an astable multivibrator that will:

* Provide less power to drive a load.

*Produce an accurate voltage regulation and no input clock pulse for its activation

CHAPTER 2

2.0 Literature Review.

2.1The 555 Timer IC

One of the most common linear integrated circuits is the 555 timers. SE 555/NE 555 IC

was first introduced in early 1970 by Signetics Corporation and was called "The IC Time

Machine" and was also the very first and only commercial timer IC available. It provided

circuit designers and hobby tinkerers with a relatively cheap, stable, and user-friendly

integrated circuit for both monostable and astable applications. Since this device was first

made commercially available, a myrad of novel and unique circuits have been developed

and presented in several trade, professional, and hobby publications.

The past ten years some manufacturers stopped making these timers because of

competition or other reasons. Yet other companies, like NTE (a subdivision of Philips)

picked up where some left off. Some typical application of the 555 timer is monostable

and astable multivibrator, DC-DC converter, digital logic probes, waveform generators,

analog frequency meter and tachometers, temperature measurement and control, infrared

transmitters, burglar toxic gas alarms, voltage regulators, etc. The 555 timers is a

monolithic timing circuit that is showing accurate and highly stable time delays and

oscillations.

The 555 timers are reliable, easy to use and economical. The 555 timer is available as 8-

pin metal can, 8-pin mini DIP or 14-pin DIP. The SE 555 is having large operating

temperature range (-55 oC to 125 oC) whereas other version of timer IC, NE 555 is having

small operating temperature (0 oC to 70 oC).

When the low signal input is applied to the reset terminal, the timer output remains low

regardless of the threshold voltage or the trigger voltage. Only when the high signal is

applied to the reset terminal, the timer's output changes according to threshold voltage

and trigger voltage. When the threshold voltage exceeds 2/3 of the supply voltage while

the timer output is high, the timer's internal discharge Tr. turns on, lowering the threshold

voltage to below 1/3 of the supply voltage. During this time, the timer output is

maintained low. Later, if a low signal is applied to the trigger voltage so that it becomes

1/3 of the supply voltage, the timer's internal discharge Tr. turns off, increasing the

threshold voltage and driving the timer output again at high.

2.1.1 555 Timers in Astable Multivibrator Mode

The 555 timers can generate a very wide frequency range, depending on the values of

R1, R2 and C. The following figure shows how to choose the timing resistors. The

designing equation is given as, charge time (output high): 0.693*(R1+R2)*CDischarge

time (output low): 0.693*(R2)*C, Period: 0.693*(R1+2*R2), Frequency: 1.44 /

((R1+2*R2)*C). Duty cycle: Time High / Time Low: (R1+R2) / R2With a 5-volt supply,

the resistors can range from 1KΩ (minimum value of R1 or R2) through 3.3MΩ

(maximum value of R1 and R2 in series)

Fig 2.1 Capacitance-Frequency graph

Credit: www.tele.pitt.edu/resources/lab_manuals/555Timer.pdf

FIG 2.2 555 connected as an astable multivibrator

Credit: www.tele.pitt.edu/resources/lab_manuals/555Timer.pdf

Best results are obtained with capacitors of 1000pF or larger, but smaller values can be

used with lower values of R1 and R2. The maximum operating frequency is around 1

MHz, but best operation is obtained below 300 kHz. The minimum operating frequency

is limited only by the size and leakage of the capacitor you use. For instance, a 10μF

capacitor and a 3.3 Ω resistor will give a time interval of 23.1 seconds if the leakage of

the capacitor is low enough. By making R2 large with respect to R1, we can get an

essentially symmetrical square-wave output.

For instance, if R1 is 1KΩ and R2 is 1MΩ, the difference in charging and discharging

resistance is only 0.1%, and good symmetry results. Any symmetry you want from 50%

through 99.9% can be obtained by a selection of the ratio of R1 and R2. Only a small

frequency variation occurs due to power supply variation but variation due to temperature

changes is large, so any precise instrumentation projects require more stable crystal

clock.

An astable timer operation is achieved by adding resistor RB to and configuring as

shown. In the astable operation, the trigger terminal and the threshold terminal are

connected so that a self-trigger is formed, operating as a multi vibrator. When the timer

output is high, its internal discharging Tr turns off and the VC1 increases by exponential

function with the time constant (RA+RB)*C. When the VC1, or the threshold voltage,

reaches 2Vcc/3, the comparator output on the trigger terminal becomes high, resetting the

F/F and causing the timer output to become low.

This in turn turns on the discharging Tr. and the C1 discharges through the discharging

channel formed by RB and the discharging Tr. When the VC1 falls below Vcc/3, the

comparator output on the trigger terminal becomes high and the timer output becomes

high again. The discharging Tr. turns off and the VC1 rises again. In the above process,

the section where the timer output is high is the time it takes for the VC1 to rise from

Vcc/3 to 2Vcc/3, and the section where the timer output is low is the time it takes for the

VC1 to drop from 2Vcc/3 to Vcc/3.

Important Features

The 555 timers basically operate in one of the two modes either as a monostable (one

shot) multivibrator or as an astable (free running) multivibrator. In the one-shot mode, the

555 acts like a monostable multivibrator. A monostable is said to have a single stable

state that is the off state. Whenever an input pulse triggers it, the monostable switches to

its temporary state. It remains in that state for a period of time determined by an RC

network.

It then returns to its stable state. In other words, the monostable circuit generates a single

pulse of fixed time duration each time it receives and input trigger pulse. Thus the name

one-shot, One-shot multivibrators are used for turning some circuit or external

component on or off for a specific length of time. It is also used to generate delays. When

multiple one-shots are cascaded, a variety of sequential timing pulses can be generated.

Those pulses will allow you to time and sequence a number of related operations.

The other basic operational mode of the 555 is as and astable multivibrator.

An astable multivibrator is simply and oscillator. The astable multivibrator

generates a continuous stream of rectangular off-on pulses that switch between two

voltage levels. The frequency of the pulses and their duty cycle are dependent upon the

RC network values.

The important features of the 555 timer are as follows:

(a) Can operate on +5V to +18V supply voltage.

(b) Having adjustable duty cycle.

(c) Timing from microseconds to hours.

(d) Producing high current output.

(e) Having capacity to source or sink current of 200 mA.

(f) Output can drive TTL.

(g) Having temperature stability of 50 ppm per oC change in temperature

or 0.005% per C.

(h) Is reliable, easy to use, and low cost.

The NE 555 timer is the bipolar version of timer. This primer is about this fantastic timer,

which is after 30 years still very popular and used in many schematics. Although these

days the CMOS version of this IC, like the Motorola MC1455, is mostly used, the regular

type is still available; however there have been many improvements and variations in the

circuitry. But all types are pin-for-pin plug compatible. This can operate over a supply

voltage range of +2V to +18V and has output current sinking and sourcing capabilities of

100 mA and 10 mA. Advantages of CMOS version timer are low power requirement and

very high input impedance.

2.2 NAND Gate Astable Multivibrators

Fig 2.3 NAND gates in Astable Mode

Credit: www.electronics-tutorials.ws/waveforms/bistable.html

The astable multivibrator circuit uses two CMOS NOT gates such as the CD4069 or the

74HC04 hex inverter ICs, or as in our simple circuit below a pair of CMOS NAND such

as the CD4011 or the 74LS132 and an RC timing network. The two NAND gates are

connected as inverting NOT gates.

Suppose that initially the output from the NAND gate U2 is HIGH at logic level "1", then

the input must therefore be LOW at logic level "0" (NAND gate principles) as will be the

output from the first NAND gate U1. Capacitor, C is connected between the output of the

second NAND gate U2 and its input via the timing resistor, R2. The capacitor now

charges up at a rate determined by the time constant of R2 and C.

As the capacitor, C charges up, the junction between the resistor R2 and the capacitor, C,

which is also connected to the input of the NAND gate U1 via the stabilizing resistor, R2

decreases until the lower threshold value of U1 is reached at which point U1 changes

state and the output of U1 now becomes HIGH. This causes NAND gate U2 to also

change state as its input has now changed from logic "0" to logic "1" resulting in the

output of NAND gate U2 becoming LOW, logic level "0".

Capacitor C is now reverse biased and discharges itself through the input of NAND gate

U1. Capacitor, C charges up again in the opposite direction determined by the time

constant of both R2 and C as before until it reaches the upper threshold value of NAND

gate U1. This causes U1 to change state and the cycle repeats itself over again.

Fig 2.4 Output of the NAND gate.

Credit: www.electronics-tutorials.ws/waveforms/bistable.html

2.5 CMOS Astable Multivibrator

Fig 2.5 CMOS in Astable mode

Credit: www.googleimages.com

The LM556 Dual timing circuit is a highly stable controller capable of producing

accurate time delays or oscillation. The 556 is a dual 555. An external resistor and

capacitor for each timing function provide timing. The two timers operate independently

of each other sharing only VCC and ground. The circuits may be triggered and reset on

falling waveforms. The output structures may sink or source 200mA.It results in

providing effective solutions for timing and pulse circuit applications.

Applications such as:

Pulse generation

Sequential timing

Time delay generation

Pulse width modulation

Pulse position modulation 0

Linear ramp generator

Operation

With the output high (+Vs.) the capacitor C1 is charged by current flowing through R1

and R2. The threshold and trigger inputs monitor the capacitor voltage and when it

reaches 2/3Vs (threshold voltage) the output becomes low and the discharge pin is

connected to 0V.

The capacitor now discharges with current flowing through R2 into the discharge pin.

When the voltage falls to 1/3Vs (trigger voltage) the output becomes high again and the

discharge pin is disconnected, allowing the capacitor to start charging again.

This cycle repeats continuously unless the reset input is connected to 0V which forces the

output low while reset is 0V.An astable can be used to provide the clock signal for

circuits such as counters.

A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher

frequency flashes are too fast to be seen clearly. Driving a loudspeaker or piezo

transducer with a low frequency of less than 20Hz will produce a series of 'clicks' (one

for each low/high transition) and this can be used to make a simple metronome.

An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from a

loudspeaker or piezo transducer. The sound is suitable for buzzes and beeps. The natural

(resonant) frequency of most piezo transducers is about 3kHz and this will make them

produce a particularly loud sound.

Duty cycle

The duty cycle of an astable circuit is the proportion of the complete cycle for which the

output is high (the mark time). It is usually given as a percentage.

For a standard 555/556-astable circuit the mark time (Tm) must be greater than the space

time (Ts), so the duty cycle must be at least 50%:

2.6 OP- Amp Astable multivibrator

The 741 chip: An important and very useful group of integrated circuits is the

“operational amplifier” or “op-amp” group. These devices have a very high gain, an

inverting input and a non-inverting input. There are many op-amps but we look at the

“741” which has an open-loop gain of 1000,000 times. All operational amplifiers work in

the same way in theory. The way they operate in a circuit is controlled by the external

components attached to them.

They can operate as inverting amplifier, a non-inverting amplifier (buffer), a comparator,

an Astable multivibrator and many more.

.

Credit: circuittoday.com

Fig-2.6 is the circuit of the 741 in astable mode

A capacitor C is connected to the inverting terminal (2) of the operational amplifier from

the ground. Similarly a resistance R1 is connected to the non-inverting terminal (3) of the

operational amplifier from the ground. The output terminal (6) of the amplifier is fed

back to inverting and non-inverting terminals of operational amplifier through resistors R

and R2 respectively. Here R2 is fixed resistor and R is variable resistor. To observe the

output waveform, the output terminal (6) is connected to CRO Y- Plates phase terminal

and the other terminal of CRO is grounded. The terminals (7) and (4) of the op. amp are

connected to +12 V and -12 V of the D.C. power supplies separately. The output terminal

(6) is also grounded through a series combination of two zener diodes connected in

reverse order as shown in the fig2.6.

Theory: - First the inverting terminal (2) is at zero potential (V2 = 0, the inverting

terminal 2 is virtually grounded) and the input at the non-inverting terminal (3) has some

potential V1 i.e. the voltage across R1. This occurs due to the power supply of the

operational amplifier.

This ‘+ ve’ voltage drives the output of operational amplifier into ‘+ ve’ saturation

voltage (+Vsat). This large saturation voltage is due to the high gain of the operational

amplifier i.e. the comparator character of the amplifier. When the + Vsat is fed back to

the inverting terminal (2) through the resistor R, the capacitor C gets charged and the

potential of the right side plate of the capacitor gradually rises (or) the V2 value rises

(Even though the inverting terminal 2 is virtually grounded but it is not mechanically

grounded). When V2 becomes slightly more than V1, the input (Vi = V1 – V2) becomes

‘–ve' and immediately this ‘–ve’ voltage drives the output of the operational amplifier in

to ‘–ve’ saturation voltage (- Vsat).

Now the capacitor discharges gradually. When V2 becomes less than V1 and (V1 – V2)

becomes ‘+ve’ and the output drives to +Vsat. The same process is repeated and the

output of the operational amplifier swings between two saturation voltages i.e. between +

Vsat and - Vsat.

The output eo of the operational amplifier is square wave. So, operational amplifier can

function as a square wave generator. The wave shape is as shown in Fig2.7.

Fig2.7 Square wave output of the op-amp

Credit: cicuirttoday.com

CHAPTER 3 METHODOLOGY

Overview

The purpose of this section is to outline and examine the design requirement and process

of Implementation based on the requirement analysis. This section will also give explicit

information about the system under development. The design stage produces a prototype

that includes performance, reliability, constraint and all relevant information about the

system.

A systematic examination and evaluation of data or information, by breaking it into its

component parts to uncover their interrelationships and also the breakdown of the topic

into simpler units in order to achieve a better understanding.

3.1 Design Methods

Approaches that will enhance the design of a two state device.

*Getting a suitable circuit diagram that will function as an astable multivibrator.

* Simulating to get the right components that will meet our design or requirement.

*Implementing our design

*Observing the output waveforms from generated by every module.

*Observing the final output waveforms from the structure.

3.2 Modules design

During this stage of the module the charging of the LED1 depends on the

transistor Q1 being connected to the negative plate of the C1. The period of the square

wave at the Q1 Outputs in this mode of operation is a function of the external

components employed.

3.2.1

During this stage of the module the charging of the LED2 depends on the

transistor Q2 being connected to the negative plate of the C2. The period of the square

wave at the Q1 Outputs in this mode of operation is a function of the external

Components employed

3.3 Design structure

R210k

R310k

R1390

R4390

C1

22µF

C2

22µFV112 V

LED1LED2

Q2

2SC945Q1

2SC945

36 re

d LE

Ds

40G

reen

LE

Ds

A high level on the ASTABLE input enables Astable operation. The

Period of the square wave at the Q1 and Q2 Outputs in this mode of operation

is a function of the external components employed. "True" input pulses on

the ASTABLE input or "Complement" pulses on the ASTABLE input allow

the circuit to be used as a getable multivibrator. The OSCILLATOR output

Period will be half of the Q terminal output in the astable mode. However, a

50% duty cycle is guaranteed at this output.

The characteristics of the two transistors are not exactly the same. When the circuit is

first switched on, the current through one transistor, say Q1, will increase faster than the

current through Q2.

Due to the rise of current through R1, the voltage across it will increase, causing the

collector voltage of Q1 to fall.  This fall in voltage is coupled to the base of Q2. This

causes the collector current of Q2 to fall, and its collector voltage to rise, due to less

voltage being dropped across R4.

This rise in collector voltage is cross-coupled to the base of Q1, increasing the forward

bias of Q1 and increasing its collector current. Since the collector current was already

rising, its rise is aided by this rising forward bias. The effect is cumulative and Q1

becomes rapidly fully on and Q2 completely off.

The collector voltage of Q1 is now low, and that of Q2 is high. C1 now begins to charge

from the supply rail, via R2. As the voltage on the right hand side of C1 starts to rise, Q2

starts to conduct. Again we have the cumulative effect and Q2 rapidly comes on and Q1

goes off. The collector voltage of Tr1 is now high and that of Q2 low. It is now the turn

of C2 to charge from the supply via R3.

As the voltage on the left hand side of C2 begins to rise, the base voltage of Q1 increases,

turning it on and turning Q2 off.

3.4 Electronic Component

NAME COMPONENTUSED DESCRIPTION NO OF

COMPONENTS

REQUIRED

2SC945 2SC945 TRANSISTOR 2

RES 1,4

RES 2,3

RC390

RC 10K RESISTOR

2

2

CAP Electrolyte (22uf) CAPACITORS 2

VDC VDC DC VOLTAGE 1

LED (2mA) GREEN, RED LED 2

3.4.1 Transistor (2SC945)

A BJT transistor is an electronic device made by doped semiconductor material and can

be made use of in amplifying or switching functions. It is a three-terminal electronic

device. 2SC945 is an NPN bi-polar junction transistor. A transistor, stands for transfer of

resistance, is commonly used to amplify current. A small current at its base controls a

larger current at collector & emitter terminals. 2SC945 is mainly used for amplification

and switching purposes

The transistor terminals require a fixed DC voltage to operate in the desired region of its

characteristic curves. This is known as the biasing. For amplification applications, the

transistor is biased such that it is partly on for all input conditions. The input signal at

base is amplified and taken at the emitter

3.4.2 Electrolytic capacitor (22uf)

Electrolytic capacitors are polarized and they must be connected the correct way round, at

least one of their leads will be marked + or -. They are not damaged by heat when

soldering.

An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic

conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume

than other types.

They are often referred to in electronics usage simply as "electrolytic". They are used in

relatively high current and low frequency electrical circuits, particularly in power supply

filters, where they store charge needed to moderate output voltage and current

fluctuations in rectifier output. . Electrolytic capacitors also have relatively low

breakdown voltage, higher leakage current and inductance, poorer tolerances and

temperature range, and shorter lifetimes compared to other types of capacitors.

3.4.3 LEDs

An LED is often small in area (less than 1 mm2) and is easily populated onto printed

circuit boards, and integrated optical components may be used to shape its radiation

pattern. LEDs present many advantages over incandescent light sources including lower

energy consumption, longer lifetime, improved robustness, smaller size, faster switching,

and greater durability and reliability.

LEDs powerful enough for room lighting are relatively expensive and require more

precise current and heat management than compact fluorescent lamp sources of

comparable output. The low energy consumption, low maintenance and small size of

modern LEDs has led to uses as status indicators and displays on a variety of equipment

and installations. Their efficiency is not affected by shape and size, unlike fluorescent

light bulbs or tubes

3.5 Hardware resources and procurement

ITEM QTY UNIT PRICE TOTAL

AMOUNT

TRANSISTOR (2SC945) 2 50p Gh1

RESISTOR (390 ohms)

(10K ohms)

2

2

50p

50p

Gh1

Gh1

ELECTROLYTIC

CAPACITORS (22UF)2 50p Gh1

DC VOLTAGE 1 GH5 Gh5

LED 80 10 Gh10

BREAD BOARD 1 Gh2 Gh2

Total Ghc 21

3.6 Construction

After we have designed your circuit, perhaps even bread boarded a working

prototype, and now it's time to turn it into a nice Bread Circuit Board

design.

* The four resistors fit flat against the board. To make them sit neatly, bend the leads to

90° with a sharp bend and push them up to the board before soldering. 

* The two 100u electrolytic are next. The positive hole is marked on the board for each

electro. This is the longer lead. The negative lead is marked on the component with a

black stripe.

*Fit the two NPN transistors. We have used 2SC 945 but any general-purpose

NPN low-power transistor will be suitable. They are pushed to the board.

*The red and green LEDs can be fitted to either position on the board. The short lead is

Cathode and this is the bar on the symbol. 

* The project is now ready to turn on

390R

390R

10k

10k

22uf 22uf 40 green

CHAPTER 4 Result And Analysis

4.1 Simulation results on Q1

The entire normal range of silicon transistor operation involves a change in base-emitter

voltage of only about two-tenths of a volt. This is because the base-emitter diode is

forward biased. One of the constraints on transistor action is that this voltage remains at

about 0.6-0.7 volts.

Fig 4.1 Multimeter showing Emitter-Base Voltage (VEBO )

Multimeter showing a voltage reading of 0.73volts which means that the transistor is

operating at normal range.

4.1.2 Waveform signal of Q1

The graph below depicts the the 0.73v waveform signal on the oscilloscope.

Fig 4.2 Oscilloscope showing the signal of Q1

4.2 Simulation results on Q2

Multimeter showing a voltage reading of 0.73volts which means that the transistor is

operating between the normal range of 0.6-0.7 volts on Q2.

Fig 4.3 Multimeter showing Emitter-Base Voltage (VEBO )

4.2.1 Waveform signal of Q1

The graph below depicts the the 0.73v waveform signal on the oscilloscope.

Fig 4.4 Oscilloscope showing the signal of Q1