CHAPTER ONE

1.1 INTRODUCTION

In this imperfect world, it is impossible to avoid faults because as the reliability of

devices depreciates with time, a time will come when the device or equipment will fail and

when this happens, if the equipment is in a delicate position, the loss of lives and properties is

inevitable. To reduce this loss, alarms are made to inform the people that a fault has been

detected.

A tone generator is any device that generates a steady sound signal at a particular

frequency from the oscillation of the voltage from a given voltage source. In this case, the

circuit generates a sound signal with two tones, hence the name.

This alarm is one that can be applied to any purpose as it can be on depending on which

fault you want to detect, this is because the circuit can be closed in many cases. Examples of

these cases are the detection of fire in a building, short-circuiting of a circuit and security

trespass in a secured area.

1.2 STATEMENT OF THE RESEARCH PROBLEM

This project brings a solution to the high loss of equipments and valuable materials due to

faults of different types depending on the sensor. This circuit can also be of help in the

alerting of people to a certain emergency, for example, the passing of a train, the passing of a

police van or the passage an ambulance.

1.3 OBJECTIVES OF THE RESEARCH

The objective of this research is to design and construct a tone generator that produces

sound of only two tones that can be constantly connected to a power source and will be able

to create or generate this sound when the sensor it is connected to senses a fault in the

environment it is connected to. An example is the fire sensor, when the fire sensor detects a

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rapid change in temperature; it closes the switch of this tone generator therefore causing the

generator to generate its sound as an alarm to alert the people of this development.

Also, an objective is to design a circuit that would create a tone when a person wants to

alert group of people of a certain condition. This can be used as a police siren or ambulance

siren that would alert people in case of any emergency.

Finally, we hope to design a tone generator that would cost less and would use

components that are readily available so that other students can also use the project to

understand the working principles of the alarm and modify it.

1.4 SCOPE OF THE RESEARCH

There were times when people used only shouts to alarm people of a certain danger; that

era has been wiped from existence since the creation of electrical and mechanical alarms. The

electrical alarms were made by varying the output frequency that a device called an oscillator

produces. An oscillator is an electronic circuit that produces a repetitive, oscillating signal,

often a sine wave or a triangular wave. The varying of the output frequency from oscillators

give off sounds of different pitch which are usually alarming. In this modern era, the study of

electronics has shot technology to a very high place where these oscillators and and all the

components of a tone generator are all compacted into one integrated circuit.

For the reason of our limited knowledge and practical know how, this research is done

within the scope of our knowledge as students of this department in 500 level. To this regard,

this project revolves in the knowledge of digital oscillators called 555 timer, resistors and

sound amplifier to amplify the signal generated from the 555 timer.

1.5 SIGNIFICANCE OF RESEARCH

In our everyday life, we see faults emerge in our different places of work, school and

recreation; this motivated us to learn the working principle of this device. The applicability of

this device is priceless as a little cost in the production of this device can help in saving

properties worth ten times its cost. In this project, we also see the possibility of the project to

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influence organizations in the state and country at large to make use of this type of devices as

it has many advantages and is cheap.

1.6 LITERATURE REVIEW

Tone generators are electronic devices that convert electric signals into sound waves.

These devices apply in many areas of use in our everyday life, it can be found nearly

everywhere, in public locations and private residences; examples of its use is as an alarm in

our place of work, banks, and other government parastatals with high security alert. Also, for

private use, Tone generators are used for door bells, music, and children’s play toys. It can

also be used as a way of informing people of the state of emergency of a certain item e.g.

police siren, ambulance siren, train horn, fire alarms and others.

In this project, we concentrated on the design and construction of an alarm inform people

of a certain emergency when the switch is closed. This is done using a 555 timer which was

created for many uses ranging from oscillators, time delays, pulse generation and pulse width

modulation but in this project, we made use of it as oscillator and a pulse width modulator.

This tone generation can also be designed using a transistor, Bipolar Junction Transistor

(BJT) or a Field Effect transistor (FET). These weren’t used because the 555 timer is an

integrated circuit that is basically made of transistors, resistors, capacitors and logic gates.

Therefore, the 555 timer is an upgrade of the transistors.

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CHAPTER TWO

2.0 THEORETICAL BACKGROUND

A tone generator is a device that oscillates an input DC voltage to give an oscillated

output of a particular frequency. This project makes use of a digital integrated circuit called

555 timer and these timers are generally referred to as oscillators. The general block diagram

of this tone generator circuit can be shown as

Fig. 2.1: Block Diagram of a Tone Generator

We shall divide this theoretical background into three subtopics with respect to the

number of block diagrams, these subtopics are as follows;

i. Power supply unit

ii. Oscillating unit and

iii. Transducer unit.

2.1 Power Supply Unit

4

Every electrical circuit needs a source of power, we could have used a battery as our

source of power but in its stead, we used a circuit that converts AC voltage input of our

laboratory to a regulated DC voltage needed by the circuit to improve its reliability and

longitivity as battery is mainly temporary and doesn’t last long.

The power supply unit can be divided into these stages;

1. Transformer

2. Rectifier

3. Filter and

4. Voltage regulator.

They can be represented in block as

Fig 2.2: Block diagram of the power supply stage

2.1.1 Transformer

A transformer makes use of Faraday's law and the ferromagnetic properties of an iron

core to efficiently raise or lower AC voltages. Since a transformer cannot increase the input

power, when the voltage is raised, the current is proportionally lowered and vice versa.

This device transfers energy by inductive coupling between its winding circuits. A

varying current in the primary winding creates a varying magnetic flux in the transformer’s

core and thus, a varying magnetic flux in the secondary winding. This varying magnetic flux

induces a varying electromotive force (EMF) or voltage in the secondary winding.

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The figure below gives a pictorial diagram of a transformer representing its flow of

current and magnetic flux.

Fig. 2.3: A diagram of a transformer showing its current and magnetic flux paths

The voltage across the secondary coil can be expressed mathematically from Faraday’s

law of induction as

V s=Es=N sd∅dt

Where Vs = ES = Voltage across secondary coil

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Ns = Number of coils

d∅dt = Derivative of the magnetic flux through one coil

Transformers are made of input (primary) coils and output (secondary) coils that define

the ratio at which the voltage is stepped up or down. The formula for this ratio is defined

below;

E s

Ep=

N s

N p=

I p

I s

Where E s = transformer output EMF

Ep = transformer output EMF

N s = number of output coils

N p = number of input coils

I p = transformer input current

I s = transformer output current

2.1.2 Rectifier

A rectifier is an electrical device that converts alternating current (AC), which

periodically reverses direction, to direct current (DC), which flows in only one direction. he

rectifier is classified into;

Half wave rectifier and,

Full wave rectifier

Half wave rectifier

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These rectifiers convert AC voltage by conducting only the positive signal parts. They are

made of a diode and sometimes a resistor, the diode conducts positive signals as forward bias

but blocks the negative part as it assumes it to be in a reverse bias configuration. Its diagram

is shown below;

Fig 2.4: A half wave rectifier

Full wave rectifier

This rectifier operates by conducting both negative parts of the wave but it inverts the

negative part to make it all a pulsating DC voltage. The diagram is shown below

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Fig 2.5: A full wave rectifier

In this project, the full wave rectifier was used as it has fewer ripples compared to the

half wave rectifier.

2.1.3 Filter

When rectifiers convert this AC signal to a DC signal, the DC signal possesses many

ripples causing us to by all means reduce this ripple by any means necessary to the barest

minimum. The circuit or component used to remove these ripples off a DC voltage signal is

called a filter.

A ripple is a small unwanted residual periodic variation of the direct current (DC) output

of power supply which has been derived from an alternating current (AC) source.

In this case, the filter used is a capacitor and is a basic type of power supply filter, the

capacitance of the capacitor to be used is calculated from the equation below;

C=0.7 × If Rv

Where C = capacitor value

I = current input

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R v = peak to peak ripple voltage

f = frequency

after the signal goes through the filter, the ripple of the signal is reduced, the effect of the

transformer, rectifier and filter on the signal is shown in the figure below;

Fig. The effects of transformer, rectifier and filter on the AC signal

2.1.4 Voltage Regulator

Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable

output voltages. They are also rated by the maximum current they can pass. Negative voltage

regulators are available, mainly for use in dual supplies. Most regulators include some

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automatic protection from excessive current ('overload protection') and overheating ('thermal

protection'). Many of the fixed voltage regulator ICs has 3 leads and look like power

transistors. They include a hole for attaching a heat sink if necessary.

2.2 OSCILLATORS

It consists of capacitors, resistors and 555 timers. The components of the oscillators are

hereby discussed below;

2.2.1 A Resistor

A resistor is a two terminal electronic component that is designed to oppose the flow of

electric currents by producing a voltage drop between its terminals hat is proportional to the

electric current that is passing through it.

In accordance to Ohm’s law, we have that,

V=IR

Where V = voltage across the resistor,

I = current passing through resistor and

R = the resistance.

Resistors are used as part of an electrical network and electronic circuits as well. They are

also used in voltage division for input into some part of a circuit where much voltage is not

needed. There are two types of resistors which are;

i. Fixed resistors

ii. Variable resistors

The figure below shows the variable and the fixed resistors,

R1

100k

11

Fixed resistor variable resistor

Fig. 2.6: Circuit representations of fixed and variable resistors.

A capacitor is a passive electronic component consisting of a pair of conductors separated

by a dielectric. When voltage potential difference exists between the conductors, an electric

field is present in the dielectric. The field stores energy and produces a mechanical force

between the plates. The effect is greatest between wide, flat, parallel, narrowly separated

conductors. An ideal capacitor is characterized by a single constant value, capacitance, which

is measured in farads. This is the ratio of electric charge on each conductor to the potential

difference between them. In practice, the dielectric between the plates passes a small amount

of leakage current.

C=QV

The conductors and leads introduce an equivalent series resistance and the dielectric has

an electric field strength limit resulting in breakdown voltage. The properties of capacitors in

a circuit may determine the resonant frequency and quality factor of a resonant circuit, power

dissipation and operating frequency in a digital logic circuit, energy capacity in a high-power

system, and many other important aspects. Capacitors are used in a bypass condition in an

A.C filter network. There are of three types: the variable, polarized and non-polarized

capacitor. They are shown in the figure below. The capacitors in this circuit are used to set the

timing interval of the output signal and for noise reduction.

C1

1.0pF

C1

1.0pF

C2

0.1uF-POL

Non polarized capacitor variable capacitor polarized capacitor

Fig. 2.7: Types of capacitors.

2.2.3 The 555 Timer

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The 555 timer is an integrated circuit used in a multitude of precise timing and waveform

generation applications. In this discussion, we will consider the 555 configured as an Astable

multi-vibrator and as a monostable multi-vibrator or one shot. Essentially, the Astable

Multivibrator is a circuit that outputs a quasi-rectangular waveform whose frequency and duty

cycle are established by the choice of external resistors and capacitor. In comparison, the one

shot receives an appropriate trigger signal and outputs a single pulse whose duration is set by

the selection of an external resistor and capacitor. The 555 timer requires a power supply and

external circuitry to achieve the desired operating characteristics. As illustrated in the table

below, the chip has eight pins and their functions are stated with them.

Pin Designation Description

1 GND Ground, low level (0 V)

2 TRIG OUT rises, and interval starts, when this input falls below 1/3

VCC.

3 OUT This output is driven to +VCC or GND.

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4 RESET A timing interval may be interrupted by driving this input to GND.

5 CTRL Voltage control that defines the mark-space ratio of a timer

6 THR The interval ends when the voltage at THR is greater than at

CTRL.

7 DIS Open collector output; may discharge a capacitor between

intervals.

8 V+, VCC Positive supply voltage is usually between 3 and 15 V.

2.2.3.1 Functional Block Diagram Of A 555-Timer

The main functional components of a 555-timer are two voltage comparators, a voltage

divider network with three equal resistors of 5k each, one RS flip flop and a discharge

transistor.

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Fig. 2.8: Simplified Functional Block Diagram of a 555-Timer

As seen in the figure above, since the voltage divider has equal resistors, the top

comparator has trip point called;

UTP, Upper trip point of 23

V cc

The lower comparator has trip point called;

LTP, Lower trip point of 13

V cc

Pin 6 is connected to the upper comparator and the voltage on it is called the threshold.

The voltage comes from external components connected to the timer and when it is greater

than the UTP, the upper comparator has a high output.

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Pin 2 is connected to the lower comparator and the voltage on it is called the trigger.

When the voltage is lower or equal to the LTP, the lower comparator produces a high output.

The RS flip flop is a single memory cell with complementary outputs Q and . If the

voltage at the S input goes high, the flip flop is set and the output goes low while the Q

output goes high. If the voltage at the R input goes high, the flip flop is reset and then output

goes high at the same time as the Q output goes low.

The discharge transistor acts as a switch that is open when the flip flop is reset ( low)

and closed when the flip flop is set (Q high).

The three identical resistors are used to obtain the reference voltages Vcc/3 and 2Vcc/3 if

the control input is left open. The flip flop is then reset if the voltage at the trigger goes below

Vcc/3. To set the flip flop, the voltage at the threshold input has to go above 2Vcc/3.

2.2.3.1 Astable Configuration of the 555-Timer

As mentioned earlier, in an astable configuration, the 555-timer has no stable state which

means that it cannot remain indefinitely in either state. It produces rectangular waves with

variable duty cycle as shown in fig. 5 below.

Fig. 2.9: 555-Timer Used in Astable Mode

2.2.3.3 Functional Block Diagram Of A 555-Timer In Astble Mode

In the astable operation of the 555-timer, three external components (two resistors and

one capacitor) are required to set the frequency of oscillations. In building this circuit, is

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recommended to use a 10nF capacitor from pin 5 (CTRL) to ground (GND). The capacitor’s

function is to suppress noise and deliver energy during the transition time of the output. The

functional block diagram of the 555-timer in astable mode is shown in the figure shown

below.

Fig. 2.10: 555-Timer Connected For Astable Operation

When Q is high, the transistor is cut off and the capacitor is charging through a total

resistance of R = R1 + R2. Because of this, the charging time constant is given as

T H=0.693(R1+R2)C

And as the capacitor charges, the threshold voltage (pin 6) increases. Eventually, it

exceeds 2Vcc/3 and then the upper comparator resets the flip flop. With Q low, the transistor

saturates and grounds (pin 7), the capacitor then discharges through R2. Therefore, the

discharging time constant is given as

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T L=0.693 R2C

When the capacitor voltage drops to slightly less than Vcc/3, the lower comparator sets the

flip flop. The figure below shows the wave forms.

Fig. 2.11: Capacitor and output waveforms for Astable operation

The timing capacitor has exponentially rising and falling voltages between UTP and LTP.

The output is a rectangular wave that swings between 0 and +Vcc. Since the charging time

constant is longer than the discharging time constant, the output is non-symmetrical.

Depending on the resistances R1 and R2, the duty cycle is between 50 and 100 percent.

By analyzing the equations for charging and discharging, the formulas can be derived;

The period of the output is given by;

T=0.693(R1+2 R2)C

The reciprocal of the period is the frequency given by;

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f = 1T

= 10.693(R1+2 R2)C

Therefore, the frequency equation can be simplified as

f = 1T

= 1.44(R1+2 R2)C

The duty cycle is obtained by dividing the pulse width by the period i.e.

duty cycle=T H

T=

0.693(R1+R2)C0.693 (R1+2R2)C

Therefore, the duty cycle can be reduced to

D=R1+R2

R1+2 R2

When R1 is much smaller than R2, the duty cycle approaches 50%, also, when R1 is much

greater than R2, the duty cycle approaches 100%.

2.3 TRANSDUCER UNIT

Transducers are devices in which energy can be converted from one form to another, the

energy transmitted could be of any form. The two main types of transducers are

Active transducer and

Passive transducer

Passive transducers

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These are transducers that do not require an external supply to effect the conversion of

one form of signal to another e.g. resistance change transducers and capacitor change

transducers, loudspeakers etc.

Active transducers

These are transducers that are connected to an auxiliary source of power which supplies a

major part of the output power while the signal supplies only insignificant portion e.g.

thermoelectric transducers and electromagnetic transducers.

2.3.1 Sound Output Transducer (speaker)

The electrical signal from the second 555-timer is fed to the speaker which is an output

transducer; output transducers are transducers that converts signal from an electrical signal to

an audio signal. It is also basically made up of a diaphragm that varies as the electrical signal

is fed into the speaker.

CHAPTER THREE

DESIGN AND ANALYSIS

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3.0 DESIGN CONSIDERATION

In this project, we are expected to design a two tone generator that gives off two tones of

different frequencies; these frequencies are expected to vary periodically as time goes. The

design for this two tone generator is shown below;

U1

LM555CMGND

1

DI S7OUT 3RST4

VCC8

THR6

CON5TRI2

C1100nF

U2

LM555CMGND

1

DI S7OUT 3RST4

VCC8

THR6

CON5TRI2

R33.0k

C210uF-POL

R236k

R13.0k

R41.0k

R54.3k

C30.1uF-POL

VCC

12V

Fig. 3.1: Circuit diagram of the two tone generator

We shall consider each stage of this project and show how this project was designed; the

analysis of the stage shall also be considered. The stages that would be considered are;

i. Power supply stage.

ii. Modulating Astable multivibrator stage.

iii. Pulse width modulation stage and

iv. Output transducer stage.

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3.1 The Power Supply Unit

The circuit diagram of the 12V DC power supply for this project is shown below;

U1

2

V1

240 V 50 Hz 0Deg

5 6

7

1

C12200uF-POL

U2LM7812CT

LI NE VREG

COMMONVOLTAGE

4

2 12V

0

3

Figure 3.2 Diagram of 9V Regulated Power Supply

The transformer used in this circuit is the 15 Volts step down transformer and the four

diodes serves as a rectifier to convert the input AC signal to a DC signal.

The Capacitor C1 is called a filter capacitor and the purpose of the filter capacitor is to

remove or smooth out the ripple in the rectified ac voltage. The residual amount of ripple is

determined by the value of the filter capacitor; the larger the value of the filter capacitor the

smaller the ripple.

The equation below is used to calculate the value of the filter capacitor:

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C1=0.7 × IRv × F

Where

F=Ripple frequency=2× frequency of the ac supply

R v=Peak¿ Peak ripplevoltage

I=Current

The multiplication of the line frequency by two to get the ripple frequency is because the

rectifier is a full wave rectifier. if the rectifier had been a half wave rectifier, the ripple

frequency would have been equal to the line frequency.

Therefore, for this power supply,

f =50 Hz ,

I=500 m A=0.5 A

From the data sheet of the LM7812, it was confirmed that the dropout voltage of the

LM7812 is 2.5 V. Therefore, the valley of the peak-to-peak ripple should be

12 V+2.5V =1 4 .5V

Since the rectified DC voltage is 15 V, the minimum negative peak of the ripple voltage

is

15 V – 14.5V=0.5V

The peak-to-peak ripple voltage is double the peak voltage, so the peak to peak voltage of

the capacitor is 1.0V.

Plugging these values into the formula;

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C=0.7 × 0.51 ×100

C=0.0035 F=3500 μF

Since 3500 μF is not a standard value, we scale the capacitor to the nearest standard value

of capacitor which is 3900 μF

The output voltage of the capacitor is

√2× rectifier output

Where, Rectifier output=2V m

π

Also, V m=√2 × RMS input AC voltage

V m=√2 ×15

V m=21.21V

Therefore, Rectifier output=2× 21.21π

Rectifier output=13.50 V

Therefore, the output voltage=√2× 13.5

the output voltage=27V

Hence, we chose a 50 V capacitor.

3.2 ASTABLE MULTIVIBRATOR STAGE

This stage creates a low frequency signal that modulates the frequency signal of the

second stage; it consists of a 555 timer, three resistors and a discharging capacitor. The circuit

below shows the placement of the components in this stage.

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U1

LM555CMGND

1

DI S7OUT 3RST4

VCC8

THR6

CON5TRI2

C1100nF

R33.0k

C210uF-POL

R236k

VCC

12V

R13.0k

Fig.3.3: Astable Multivibrator Circuit Diagram

This first stage is used to modulate the output of the second stage, this modulation varies

the frequency of the second stage. Therefore, the low pulse time (TL) and the high pulse time

(TL) of this stage determines time the second stage remains in a particular frequency.

In this project, we want the low pulse time (TL) to be 25 seconds and so we choose a

discharge capacitor value to be 10µF. therefore, if

T L=0.693 R2C

Therefore,

R2=T L

0.693 ×C

25

R2=0.25

0.693 ×10 ×10−6

R2=36,075 Ω=36 KΩ

If we choose the high pulse time to be 0.27 seconds, the value of the R1 resistor is gotten

by using the equation below;

T H=0.693(R1+R2)C

Therefore, R1=T H

0.693×C−R2

R1=0.27

0.693 ×10 ×10−6 −36000

R1=2,961 Ω=3K Ω

Also, duty cycle is

D=R1+R2

R1+2 R2

Therefore,

D= 3+363+2×36

=0.52

The duty cycle is therefore 52% meaning the ration of the high pulse time to the total

period of the wave is 0.52.

The resistor R3 is used to create a little voltage drop so that the maximum control

voltage of the second stage 555-timer would be a little lower than the input voltage. We chose

the value as

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R3=3 K Ω

3.4 PULSE WIDTH MODULATION STAGE

The first stage is wired as a slow astable multivibrator operating at around 2Hz at 52%

duty cycle and this stage is wired as a fast astable multivibrator operating at around 1500Hz.

The output of first astable mutivibrator is connected to the control voltage input (pin5) of this

second stage 555-timer. This makes the output of the second stage to be modulated by the

output frequency of the first stage, giving a siren effect. Therefore, it is correct to say that the

output frequency of the second stage is controlled by the output of the first stage.

The circuit below shows the circuit diagram of the second stage of this two tone

generator.

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U2

LM555CMGND

1

DI S7OUT 3RST4

VCC8

THR6

CON5TRI2

VCC

12V

R41.0k

R54.3k

C30.1uF-POL

FROM STAGE 1

Fig.3.4: Circuit diagram of the carrier astable multivibrator

If we choose our output frequency to be 1500 Hz, then the period of the output signal

would be gotten as

T=1f= 1

1500

Therefore, T=0.00067 seconds

Let, T L=0.0003=0.693× R5 ×C3

If C3=0.1 μF

Then R5=0.0003

0.693 ×0.1 ×10−6

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R5=4330 Ω=4.3 K Ω

Also, if T H=0.00037=0.693 ×(R4+R ¿¿5)×C3 ¿

Then R4=0.00037

0.693× 0.1 ×10−6 −R5

R4=1000 Ω=1 K Ω

From the values of resistors and capacitors calculated, the duty cycle can be calculated as

D=R1+R2

R1+2 R2

Therefore,

D= 1+4.31+2 ×4.3

=0.55

The duty cycle is therefore 55% meaning the ration of the high pulse time to the total

period of the wave is 0.55.

Therefore, the above found values of resistors and capacitor is used to give the output carrier frequency of 1500 Hz.

After simulating this circuit, the simulation gave a representation of the output waveform on an oscilloscope as shown below;

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fig.3.5: Representation of output on the oscilloscope

3.4 THE OUTPUT TRANSDUCER STAGE

This stage is made up of the speaker and the coupling capacitor which is arranged in the circuit as shown below,

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fig. Output stage of the tone generator

The value of capacitor chosen as 10µF is connected to the output voltage of 12V. Therefore the voltage rating of this capacitor is given as

Voltage rating=√2× output voltage

Voltage rating=√2× 12=17 V

It is therefore assumed that the capacitor voltage rating should be 17V but a 25V rating would be chosen because it is most readily available. This capacitor helps to smoothen the signal from the tone generator circuit.

The speaker used is a 16Ω, 5 W speaker.

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