44-MOBILE CONTROL ELECTRICAL APPLIANCES

This unit talks about the basic definitions needed to understand the Project better and further defines the technical criteria to be implemented as a part of this project.

Why automation?

Earlier, we are looking into the face of future when we talked about automated

devices, which could do anything on instigation of a controller, but today it has

become a reality.

1. An automated device can replace good amount of human working force,

moreover humans are more prone to errors and in intensive conditions the

probability of error increases. Whereas an automated device can work with

diligence, versatility and with almost zero error.

2. This is why this project looks into construction and implementation of a system

involving hardware to control a variety of electrical and electronics instruments.

What is home/office automation?

Home/office automation is the control of any or all electrical devices in our home

or office, whether we are there or away. Home/office automation is one of the

most exciting developments in technology for the home that has come along in

decades. There are hundreds of products available today that allow us control

over the devices automatically, either by remote control; or even by voice

command!

What can be automated?

Virtually anything in the home/office that is powered by electricity can be

automated and/or controlled. We can control our electrical devices with our

cordless phone from our easy chair. We can turn our porch lights on

automatically at dark or when someone approaches and can see who is at the

front door from any nearby television, and talk to them or unlock the door from

any nearby telephone. Have the security system turn off lights, close drapes and

setback the temperature when we leave and turn on the alarm system. The

possibilities are only limited by our imagination!

How is the system controlled in this project?

This circuit enables one to utilize a telephone for remote switching ‘on’ and ‘off’ of

any electrically operated device. It does not require any physical connection to

the telephone lines or lifting of the handset. The circuit is only acoustically

coupled to the telephone instrument. Consequently, . The switching operation is

performed by making use of coded telephone ring signal.

While designing it has been ensured that the circuit is free from any false

triggering by normal telephone ring signals. Since the instrument has no

hardwired connection to the telephone line it does not affect the normal operation

of the telephone set. To avoid false triggering, suitable reset feature is

incorporated in this circuit.

Why not any other device?

In this age of automation many other devices like microprocessor or micro-

controller, infrared remote, voice controlled devices etc. are used for the

automation purposes, but they have certain limitations which are described below

1. The use of microprocessor or micro-controller involves complexities like

microprocessor operating voltages; interrupt servicing, poling, memory

access mechanism and extensive soldering. Moreover, if we use micro-

controller or a microprocessor we can’t change the working as and when

desired. The problem being, while using them we have to hardwire the code

into ROM chips and in case we need to amend we have to burn a new ROM

chip to replace the earlier one. The earlier ROM becomes useless and has to

be scraped. And this has to be done on every single time we need add

something new.

2. An infrared remote control can work for a device up to a specified range of

distance after that it cannot be used for controlling the device.

3. A voice controlled device works on a single voice and cannot be used by any

other person and have a certain range of working.

This unit talks about how the different units of the project working. How Relays and

Transistors can be interfaced to your telephone and can be used to turn ‘ON’ and ‘OFF’

your home appliances such as bulbs, tube lights, lamps or heavy-duty motors.

This project is a teleremote circuit that enables switching ‘ON’ and ‘OFF’ of appliances through telephone lines. It can be used to switch appliances from any distance, overcoming the limited range of infrared and radio remote controls. The circuit described in the project can be used to switch up to ten appliances (corresponding to the digits 0 to 9 of the telephone keypad). This circuit is based on the DTMF controller circuit. DTMF means dual tone multiple frequency. The DTMF signals on telephone instrument are used as control signals.

When we press the star button in the telephone the pulse dialing is

converted in to tone dialing. We use this tone dialing in our project. Telephone

lines are connected to the DTMF decoder circuit including a DTMF decoder IC

8870, which is a 24 pin IC. This DTMF decoder converts the DTMF pulse into a

BCD signal. It means when we press the digit no.1 then output from decoder is

0001. Circuit automatically sense and convert the DTMF signal into BCD signal.

Output from IC 8870 is connected to the next stage, which is a BCD to

Decimal converter circuit. For this purpose we use BCD to Decimal converter IC

74154. This IC converts the BCD signal into Decimal signal. It means when press

PROJECT WORKING

the digit 2 in the keypad then 2 number output of the decimal decoder is active in

this stage.

The output of the BCD to Decimal decoder circuit is active low; therefore,

we convert this low signal into high signal with the help of inverter circuit. In this

project we have used hex inverter circuit, which is IC 4049. Hex inverter circuit

converts at a time six input signal. Since we have ten outputs in this circuit,

therefore, we have used two hex inverter ICs. Output of the inverter is high now.

Output of the IC 4049 is in the form of pulse signal, now to convert the

pulse signal into toggle signal we have used a flip-flop. The flip-flop used in the

circuit is IC 4013. This IC is a dual flip-flop IC since it has two inputs and two

outputs. These flip-flops are of D-type, i.e., the input data appears at the output

at the end of the clock pulse. This means that when the output of the previous

stage is low, the output of the flip-flop is low and when the output of the previous

stage is high then the output of the flip-fop is high. If we use ten outputs of the

4049 and want all the outputs connected to flip-flop then we have to use

minimum five flip-flop ICs in the project

Output of the flip-flop is connected to the base of NPN transistor through

1k ohm resistor. Emitter of every transistor is connected to the negative voltage

and collector is connected to the relay coil. Relay is electromagnetic switch.

Relay operation voltage is 9V DC. The rectifier circuit before regulator 7805

provides this 9 V DC. Relay further ON/OFF any electrical appliance connected

to it.

Now when we press the number 1 of the telephone keypad then one

number dyle is ON and the corresponding relay is switched ON and the electrical

circuit is completed. Now again when we press the no.1 then flip-flop output is

shifted to zero and the relay is OFF and so the electrical appliance connected to

it.

When the circuit is used to switch the relays from the telephone instrument

to which it is connected physically, the mode of operation of the telephone line

can be changed by using the star button. But when switching is done from some

other telephone instrument the working of the circuit is a little different. In that

case a coupler circuit assists the working of the project. This coupler circuit

consists of an optocoupler IC 417, a timer IC 555 and a decade counter IC 4017.

When a call is established and telephone starts ringing the optocoupler IC

detects the ring. This IC is connected to the 555 timer IC and for every ring it

makes timer IC to generate a pulse. The timer IC is connected to the decade

counter and with every pulse generated by the timer IC, the decade counter

makes one of its outputs high. A relay is connected to one of the output of the

counter IC, that connects the coupler circuit with the DTMF decoder circuit,

whenever, that output is high. That means the circuit counts a preset value of

rings and then it connects the telephone line to the DTMF decoder circuit to

perform switching action.

When you press a button in the telephone set keypad, a

connection is made that generates a resultant signal of two

tones at the same time. These two tones are taken from a row

frequency and a column frequency. The resultant frequency

signal is called "Dual Tone Multiple Frequency". These tones

are identical and unique.

          A DTMF signal is the algebraic sum of two different audio

frequencies, and can be expressed as follows:           

f(t) = A0sin(2*П*fa*t) + B0sin(2*П*fb*t) + ...........    ------->(1)

Where fa and fb are two different audio frequencies with A

and B as their peak amplitudes and f as the resultant DTMF

signal. fa belongs to the low frequency group and fb belongs to

the high frequency group.

          Each of the low and high frequency groups comprise four

frequencies from the various keys present on the telephone

WHAT IS DTMF?             

keypad; two different frequencies, one from the high

frequency group and another from the low frequency group are

used to produce a DTMF signal to represent the pressed key.

The amplitudes of the two sine waves should be such

that                  

                     (0.7 < (A/B) < 0.9)V               -------->(2)

          The frequencies are chosen such that they are not the

harmonics of each other. The frequencies associated with

various keys on the keypad are shown in figure (A).

          When you send  these DTMF signals to the telephone

exchange through cables, the servers in the telephone

exchange identifies these signals and makes the connection to

the person you are calling.  

The row and column frequencies are given below: 

Fig (A)

 When you press the digit 5 in the keypad it generates a

resultant tone signal which is made up of frequencies 770Hz

and 1336Hz. Pressing digit 8 will produce the tone taken from

tones 852Hz and 1336Hz. In both the cases, the column

frequency 1336 Hz is the same. These signals are digital

signals which are symmetrical with the sinusoidal wave.

 A Typical frequency is shown in the figure below:

Figure (B)

 Along with these DTMF generator in our telephone set

provides a set of special purpose groups of tones, which is

normally not used in our keypad. These tones are identified as

'A', 'B', 'C', 'D'. These frequencies have the same column

frequency but uses row frequencies given in the table in figure

(A). These tones are used for communication signaling.

 The frequency table is as follows:

Figure (C)

    Due to its accuracy and uniqueness, these DTMF signals are

used in controlling systems using telephones. By using some

DTMF generating IC’s (UM91214, UM91214, etc) we can

generate DTMF tones without depending on the telephone set.

_ __ _!__ _ _________The MC4013

IC 4013dual type D flip–flop is constructed with MOS P–channel and

N–channel enhancement mode devices in a single monolithic

structure. Each flip–flop has independent Data, (D), Direct Set, (S),

Direct Reset, (R), and Clock (C) inputs and complementary outputs

(Q and Q). These devices may be used as shift register elements or

as type T flip–flops for counter and Static Operation Diode

Protection on All Inputs

Supply Voltage Range = 3.0 Vdc to 18 Vdc

Logic Edge–Clocked Flip–Flop Design

Logic state is retained indefinitely with clock level either high or low;

information is transferred to the output only on the positive–going

edge

of the clock pulse

Capable of Driving Two Low–power TTL Loads or One Low–power

Schottky TTL Load Over the Rated Temperature Range

Pin–for–Pin Replacement for CD4013B

54154/DM54154/DM741544-Line to 16-Line Decoders/Demultiplexers

Each or these 4-line-to-16-line decoders utilizes TTL

circuitry to decode four binary-coded inputs into one of

sixteen

mutually exclusive outputs when both the strobe inputs, G1

and G2, are low. The demultiplexing function is performed

by using the 4 input lines to address the output line, passing

data from one of the strobe inputs with the other strobe

input low. When either strobe input is high, all outputs are

high. These demultiplexers are ideally suited for

implementing

high-performance memory decoders. All inputs are buffered

and input clamping diodes are provided to minimize

transmission-line effects and thereby simplify system design.

Features

Y Decodes 4 binary-coded inputs into one of 16 mutually

exclusive outputs Y Performs the demultiplexing function by

distributing data from one input line to any one of 16 outputs

Y Input clamping diodes simplify system design

Y High fan-out, low-impedance, totem-pole outputs

Y Typical propagation delay

3 levels of logic 19 ns

Strobe 18 ns

Y Typical power dissipation 170 mW

Y Alternate Military/Aerospace device (54154) is available.

Contact a National Semiconductor Sales Office/

Distributor for specifications.

IC 4049

The MC14049B Hex Inverter/Buffer and MC14050B Noninverting Hex

Buffer are constructed with MOS P–Channel and N–Channel

enhancement mode devices in a single monolithic structure. These

complementary MOS devices find primary use where low power

dissipation and/or high noise immunity is desired. These devices

provide logic level conversion using only one supply voltage, VDD.

The input–signal high level (VIH) can exceed the VDD supply voltage

for logic level conversions. Two TTL/DTL loads can be driven when

the devices are used as a CMOS–to–TTL/DTL converter (VDD = 5.0

V, VOL _ 0.4 V,

IOL 3.2 mA).

Note that pins 13 and 16 are not connected internally on these

devices;

consequently connections to these terminals will not affect circuit

operation.

High Source and Sink Currents

High–to–Low Level Converter

Supply Voltage Range = 3.0 V to 18 V

VIN can exceed VDD

KA78XX/KA78XXA3-Terminal 1A Positive Voltage Regulator

Features• Output Current up to 1A• Output Voltages of 5, 6, 8, 9, 10, 12, 15, 18, 24V• Thermal Overload Protection• Short Circuit Protection• Output Transistor Safe Operating Area Protection

Description

The KA78XX/KA78XXA series of three-terminal positive regulator are available in the TO-220/D-PAK package and with several fixed output voltages, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible. If adequate heat sinking is provided, they can deliver over 1A output current. Although designed primarily as fixed voltage regulators, these devices can be used with external components to obtain adjustable voltages and currents.

RELAYS

In order to enable a circuit to be isolated from the system only under

faulty conditions, protective relays are used. In normal cases, it is open circuit

relay. The relay is usually provided with 4 terminals, two of which are

connected to relay winding and other two are connected to the circuit to be

controlled. It has following characteristics :

Sensitivity

Speed

Selectivity

TYPES OF RELAYS : Electromagnetic Attraction Type : These relays are actuated by DC or

AC quantities.

Electromagnetic Induction Type : It’s operation depends upon EMI

phenomena.

Thermal Relays : It’s operation depends upon the heating effect of

electric Current.

Distance Relays : It’s operation depends upon the ratio of voltage to

current.

ELECTROMAGNETIC RELAY :

These relays are electromagnetically operated. The parts of these

relays are an iron core & its surrounding coil of wire. An iron yoke

provides a low reluctance path for magnetic flux, the yoke being shaped so

that the magnetic circuit can be closed by a movable piece of iron called

the armature, and a set of contacts. The armature is hinged to the yoke

and is held by a string in such a way that there is an air gap in the

magnetic circuit. Figure shows the principle of operation of this relay.

When an electric current flows in the coil, the armature is attracted to the

iron core. Electrical switching contacts are mounted on the armature.

When the armature coil is energized, these movable contacts break their

connections with one set of fixed contacts and close a connection to a

previously open contact. When electric power is removed from the relay

coil, spring returns the armature to its original position.

Standard voltages for D.C. relay are 6,12,24,48 & 110 volts and for A.C.

relays are 6,12,24,48,120 & 240 volts.

Fig. Basic Diagram Showing the Operating Principle of a Relay

BC546; BC547NPN general purpose transistors

FEATURESLow current (max. 100 mA)Low voltage (max. 65 V).APPLICATIONSGeneral purpose switching and amplification.DESCRIPTIONNPN transistor in a TO-92; SOT54 plastic package.PNP complements: BC556 and BC557.

RECTIFIER DIODE

Philips Semiconductors Product specification

Rectifiers 1N4001G to 1N4007GFEATURES

Glass passivatedHigh maximum operatingtemperatureLow leakage currentExcellent stabilityAvailable in ammo-pack.

DESCRIPTION

Rugged glass package, using a high temperature alloyed construction. This package is hermetically sealed and fatigue free as coefficients of expansion of all used parts are matched. Fig.1 Simplified outline (SOD57) and symbol. 2/3 page (Datasheet

COMPONENTS LIST:

MOBILE CONTROL ELECTRICAL APPLIANCES. COMPONENT'S LIST

ITEM QANTITYY COSTIC 8870 1 28

74154 1 604049 2 124013 5 30

RELAY 5 60BC 548 11 11TXF 9VOLT 35

7805 1 63.58 1 15

1N4007 12 6LED 12 61000MFD 2 616 PINBASE 2 318 PIN BASE 1 324 PIN BASE 3 3MISC 20 20

304P.C.B 1 50BOARD 1 20ELECTRICAL SOCKET 5PC 30ELECTRICAL SOCKETELECTRIUCAL WIRES 20

374 50 424

LIGHT EMITTING DIODE

Light emitting diode (LED) is basically a P-N junction semiconductor diode particularly designed to emit visible light. There are infrared emitting LEDs which emit invisible light. The LEDs are now available in many colours red, green and yellow. A normal LED emits at 2.4V and consumes MA of current. The LEDs are made in the form of flat tiny P-N junction enclosed in a semi-spherical dome made up of clear coloured epoxy resin. The dome of a LED acts as a lens and diffuser of light. The diameter of the base is less than a quarter of an inch. The actual diameter varies somewhat with different makes. The common circuit symbols for the LED are shown in Fig. It is similar to the conventional rectifier diode symbol with two arrows pointing out. There are two leads- one for anode and the other for cathode.

LEDs often have leads of dissimilar length and the shorter one is the cathode. All manufacturers do not strictly adhere this to. Sometimes the cathode side has a flat base. If there is doubt, the polarity of the diode should be identified. A simple bench method is to use the ohmmeter incorporating 3-volt cells for ohmmeter function. When connected with the ohmmeter: one way there will be no deflection and when connected the other way round there will be a large deflection of a pointer. When this occurs the anode lead is connected to the negative of test lead and cathode to the positive test lead of the ohmmeter.

If low range (Rxl) of the ohmmeter is used the LED would light up in most cases because the low range of ohmmeter can pass sufficient current to light up the LED.

Another safe method is to connect the test circuit shown in Fig. 2. Use any two dry cells in series with a current limiting resistor of 68 to 100 ohms. The resistor limits the forward diode current of the LED under test to a safe value. When the LED under test is connected to the test terminals in any way: if it does not light up, reverse the test leads. The LED will now light up. The anode of the LED is that which is connected to the “A” terminal (positive pole of the battery). This method is safe, as reverse voltage can never exceed 3 volts in this test.

ELECTRICAL CHARACTERISTICS OF LEDS: -

Electrically, a LED is similar to the conventional diode in that it has relatively low forward voltage threshold. Once this is exceeded the junction has a low slope resistance and conducts current readily. An external resistor must limit this current. Forward voltage drew across red LED is nominally 1.6 V but spread with commercial diodes, it may be as high as 2 volts or so, while the Green LED drops 2.4V. This difference accounts for use of lower limiting resistor used with the Green LED.

Another important parameter of the LED is its maximum reverse voltage rating. For typical Red device it is of the order of 3 volts. But for Green LED it is somewhat higher- 5 to 10 volts.

The LED produces light only when a d.c. current is passed in the forward direction and the amount of light emitted by a LED is proportional to the forward current over a broad range. It means that light intensity increases in an approximately linear manner with increasing current.

SEVEN SEGMENT DISPLAY DECIMAL DISPLAY:-

A popular type consists of seven small, bar-shaped LED segment arranged so that depending on which combinations are energized, the numbers 0 to 9 light up. All the LED cathodes (or sometimes anodes) are joined to form a common connection. Current limiting resistors are required (e.g. 270 ohms), preferably one per segment. Common cathode method of connecting an array of display elements.

The main requirements for a suitable LED material are:-

1) It must have on energy gap of appropriate width.

2) Both P and N types must exist, preferably with low resistivities.

3) Efficient radioactive pathways must be present.

Generally, energy gaps greater than or equal to about 2 are required.

Commercial LED materials::Gallium arsenide (Ga As) doped with Si

Gallium Phosphide (GaP) doped with N & Bi

Gallium arsenide Phosphide (Ga As1-x Px)

Common Cathode

Anode Connection

Gallium aluminium arsenide (Gax Al1-x As)

LED CONSTRUCTIONS: -

To reduce reflection losses in LEDs there are two obvious ways: -

a) The first is to ensure that most rays strike the surface at less than the critical angle. This may be achieved by shaping the semiconductor /air interface into a hemisphere.

b) The second technique is to encapsulate the junction in a transparent medium of high refractive index. This is usually a plastic material with refractive index of about 1.5. Moulding the plastic into an approximately hemispherical shape can minimize the losses at the plastic lair interface.

SEVEN SEGMENT DISPLAY

A popular type consists of seven small, bar-shaped LED segment arranged so that depending on which combinations are energized, the numbers 0 to 9 light up. All the LED cathodes (or sometimes anodes) are joined to form a common connection. Current limiting resistors are required (e.g. 270 ohms), preferably one per segment.

Common cathode method of connecting an array of display elements.

The main requirements for a suitable LED material are:-

1) It must have on energy gap of appropriate width.

2) Both P and N types must exist, preferably with low resistivities.

3) Efficient radioactive pathways must be present.

Generally, energy gaps greater than or equal to about 2 are required.

Commercial LED materials::Gallium arsenide (Ga As) doped with Si

Gallium Phosphide (GaP) doped with N & Bi

Common Cathode

Anode Connection

Gallium arsenide Phosphide (Ga As1-x Px)

Gallium aluminium arsenide (Gax Al1-x As)

LED CONSTRUCTIONS: -

To reduce reflection losses in LEDs there are two obvious ways: -

a) The first is to ensure that most rays strike the surface at less than the critical angle. This may be achieved by shaping the semiconductor /air interface into a hemisphere.

b) The second technique is to encapsulate the junction in a transparent medium of high refractive index. This is usually a plastic material with refractive index of about 1.5. Moulding the plastic into an approximately hemispherical shape can minimize the losses at the plastic lair interface.

RESISTANCE

Resistance is the opposition of a material to the current. It is measured in Ohms (). All conductors represent a certain amount of resistance, since no conductor is 100% efficient. To control the electron flow (current) in a predictable manner, we use resistors. Electronic circuits use calibrated lumped resistance to control the flow of current. Broadly speaking, resistor can be divided into two groups viz. fixed & adjustable (variable) resistors. In fixed resistors, the value is fixed & cannot be varied. In variable resistors, the resistance value can be varied by an adjuster knob. It can be divided into (a) Carbon composition (b) Wire wound (c) Special type. The most common type of resistors used in our projects is carbon type. The resistance value is normally indicated by colour bands. Each resistance has four colours, one of the band on either side will be gold or silver, this is called fourth band and indicates the tolerance, others three band will give the value of resistance (see table). For example if a resistor has the following marking on it say red, violet, gold. Comparing these coloured rings with the colour code, its value is 27000 ohms or 27 kilo ohms and its tolerance is ±5%. Resistor comes in various sizes (Power rating). The bigger, the size, the more power rating of 1/4 watts. The four colour rings on its body tells us the value of resistor value as given below.

COLOURS CODE

Black----------------------------------------0Brown---------------------------------------1Red------------------------------------------2Orange-------------------------------------3Yellow---------------------------------------4Green---------------------------------------5Blue-----------------------------------------6Violet----------------------------------------7Grey-----------------------------------------8White----------------------------------------9

The first rings give the first digit. The second ring gives the second digit. The third ring indicates the number of zeroes to be placed after the digits. The fourth ring gives tolerance (gold ±5%, silver ± 10%, No colour ± 20%).

In variable resistors, we have the dial type of resistance boxes. There is a knob with a metal pointer. This presses over brass pieces placed along a circle with some space b/w each of them.

Resistance coils of different values are connected b/w the gaps. When the knob is rotated, the pointer also moves over the brass pieces. If a gap is skipped over, its resistance

is included in the circuit. If two gaps are skipped over, the resistances of both together are included in the circuit and so on.

A dial type of resistance box contains many dials depending upon the range, which it has to cover. If a resistance box has to read upto 10,000, it will have three dials each having ten gaps i.e. ten resistance coils each of resistance 10. The third dial will have ten resistances each of 100.

The dial type of resistance boxes is better because the contact resistance in this case is small & constant.

RESISTANCE

Resistance is the opposition of a material to the current. It is measured in Ohms (). All conductors represent a certain amount of resistance, since no conductor is 100% efficient. To control the electron flow (current) in a predictable manner, we use resistors. Electronic circuits use calibrated lumped resistance to control the flow of current. Broadly speaking, resistor can be divided into two groups viz. fixed & adjustable (variable) resistors. In fixed resistors, the value is fixed & cannot be varied. In variable resistors, the resistance value can be varied by an adjuster knob. It can be divided into (a) Carbon composition (b) Wire wound (c) Special type. The most common type of resistors used in our projects is carbon type. The resistance value is normally indicated by colour bands. Each resistance has four colours, one of the band on either side will be gold or silver, this is

called fourth band and indicates the tolerance, others three band will give the value of resistance (see table). For example if a resistor has the following marking on it say red, violet, gold. Comparing these coloured rings with the colour code, its value is 27000 ohms or 27 kilo ohms and its tolerance is ±5%. Resistor comes in various sizes (Power rating). The bigger, the size, the more power rating of 1/4 watts. The four colour rings on its body tells us the value of resistor value as given below.

COLOURS CODE

Black----------------------------------------0Brown---------------------------------------1Red------------------------------------------2Orange-------------------------------------3Yellow---------------------------------------4Green---------------------------------------5Blue-----------------------------------------6Violet----------------------------------------7Grey-----------------------------------------8White----------------------------------------9

The first rings give the first digit. The second ring gives the second digit. The third ring indicates the number of zeroes to be placed after the digits. The fourth ring gives tolerance (gold ±5%, silver ± 10%, No colour ± 20%).

In variable resistors, we have the dial type of resistance boxes. There is a knob with a metal pointer. This presses over brass pieces placed along a circle with some space b/w each of them.

Resistance coils of different values are connected b/w the gaps. When the knob is rotated, the pointer also moves over the brass pieces. If a gap is skipped over, its resistance is included in the circuit. If two gaps are skipped over, the resistances of both together are included in the circuit and so on.

A dial type of resistance box contains many dials depending upon the range, which it has to cover. If a resistance box has to read upto 10,000, it will have three dials each having ten gaps i.e. ten resistance coils each of resistance 10. The third dial will have ten resistances each of 100.

The dial type of resistance boxes is better because the contact resistance in this case is small & constant.

TRANSFORMER

PRINCIPLE OF THE TRANSFORMER:-

Two coils are wound over a Core such that they are magnetically coupled. The two coils are known as the primary and secondary windings.

In a Transformer, an iron core is used. The coupling between the coils is source of making a path for the magnetic flux to link both the coils. A core as in fig.2 is used and the coils are wound on the limbs of the core. Because of high permeability of iron, the flux path for the flux is only in the iron and hence the flux links both windings. Hence there is very little ‘leakage flux’. This term leakage flux denotes the part of the flux, which does not link both the coils, i.e., when coupling is not perfect. In the high frequency transformers, ferrite core is used. The transformers may be step-up, step-down, frequency matching, sound output, amplifier driver etc. The basic principles of all the transformers are same.

MINIATURE TRANSFORMER

CONVENTIONAL POWER TRANSFORMER

HOW TO SOLDER?

Mount components at their appropriate place; bend the leads slightly outwards to prevent them from falling out when the board is turned over for soldering. No cut the leads so that you may solder them easily. Apply a small amount of flux at these components leads with the help of a screwdriver. Now fix the bit or iron with a small amount of solder and flow freely at the point and the P.C.B copper track at the same time. A good solder joint will appear smooth & shiny. If all appear well, you may continue to the next solder connections.

TIPS FOR GOOD SOLDERING

1. Use right type of soldering iron. A small efficient soldering iron (about 10-25 watts with 1/8 or 1/4 inch tip) is ideal for this work.

2. Keep the hot tip of the soldering iron on a piece of metal so that excess heat is dissipated.

3. Make sure that connection to the soldered is clean. Wax frayed insulation and other substances cause poor soldering connection. Clean the leads, wires, tags etc. before soldering.

4. Use just enough solder to cover the lead to be soldered. Excess solder can cause a short circuit.

5. Use sufficient heat. This is the essence of good soldering. Apply enough heat to the component lead. You are not using enough heat, if the solder barely melts and forms a round ball of rough flaky solder. A good solder joint will look smooth, shining and spread type. The difference between good & bad soldering is just a few seconds extra with a hot iron applied firmly.

PRECAUTIONS

1. Mount the components at the appropriate places before soldering. Follow the circuit description and components details, leads identification etc. Do not start soldering before making it confirm that all the components are mounted at the right place.

2. Do not use a spread solder on the board, it may cause short circuit.

3. Do not sit under the fan while soldering.

4. Position the board so that gravity tends to keep the solder where you want it.

5. Do not over heat the components at the board. Excess heat may damage the components or board.

6. The board should not vibrate while soldering otherwise you have a dry or a cold joint.

7. Do not put the kit under or over voltage source. Be sure about the voltage either dc or ac while operating the gadget.

8. Do spare the bare ends of the components leads otherwise it may short circuit with the other components. To prevent this use sleeves at the component leads or use sleeved wire for connections.

9. Do not use old dark colour solder. It may give dry joint. Be sure that all the joints are clean and well shiny.

10. Do make loose wire connections especially with cell holder, speaker, probes etc. Put knots while connections to the circuit board, otherwise it may get loose.

What are crystal oscillators?

Crystal oscillators are oscillators where the primary frequency determining element is a

quartz crystal. Because of the inherent characteristics of the quartz crystal the crystal

oscillator may be held to extreme accuracy of frequency stability. Temperature

compensation may be applied to crystal oscillators to improve thermal stability of the

crystal oscillator.

Crystal oscillators are usually, fixed frequency oscillators where stability and accuracy

are the primary considerations. For example it is almost impossible to design a stable and

accurate LC oscillator for the upper HF and higher frequencies without resorting to some

sort of crystal control. Hence the reason for crystal oscillators.

The frequency of older FT-243 crystals can be moved upward by crystal grinding.

I won't be discussing frequency sythesisers and direct digital synthesis (DDS) here. They

are particularly interesting topics to be covered later.

A practical example of a Crystal Oscillator

This is a typical example of the type of crystal oscillators which may be used for say

converters. Some points of interest on crystal oscillators in relation to figure 1.

Figure 1 - schematic of a crystal oscillator

The transistor could be a general purpose type with an Ft of at least 150 Mhz for HF use.

A typical example would be a 2N2222A.

The turns ratio on the tuned circuit depicts an anticipated nominal load of 50 ohms. This

allows a theoretical 2K5 ohms on the collector. If it is followed by a buffer amplifier

(highly recommended) I would simply maintain the typical 7:1 turns ratio. I have

included a formula for determining L and C in the tuned circuits of crystal oscillators in

case you have forgotten earlier tutorials. Personally I would make L a reactance of

around 250 ohms. In this case I'd make C a smaller trimmer in parallel with a standard

fixed value.