Project synopsis

Dissertation submitted in partial fulfillment of the Btech. course.

HOT  case

Under the Guidence of

Arun Bansal

Innovative Project solutions


Submitted By

Abhishek Kaushik


Table of Contents

  1. Acknowledgements.
  2. Certificate.
  3. Introduction to the Project.
  4. Circuit Diagram.
  5. Component List.
  6. Hardware
    1.     Power Supply for the circuit.
    1.     Integrated Circuits.
    1.     Transistors.
    1.     Diode.
    1.     Relays.
    1.     Transformer.
    1.     Resistors.
    1.     Capacitors.
  • Project Working.
  • Project Synopsis.
  • Bibliography.


                             Introduction & working

 In this project we will make a hot case.  We will design a hot case which will automatically heat our case on time. We will set on time and it will work for fixed time duration. After that duration our hot cae will switch off. During the on interval our food in hot case will remain hot and after fixed duration it will get off.  It will save electricity and will save any typical situation. In this project we will use project we will use

Hot case will be time based control system, So for this project we use four input switches with lcd display. When we press switch no . As we switch on the circuit clock is on and display the 00.00 time. 

Relay drive circuit is uded to control relay 12 v .

for relay drive circuit we used transistors


Programmable logic control for 4 outputs by using this project we control and program tim zone . In this project we design one digital clock in addition with output . On this output we feed a time for switch on and separate time for switch off the unit. Main part of this project is 89c51 Arduino. One LCD display and opto-coupler drive triac control


Transformer works on the principle of mutual inductance. We know that if two coils or windings are placed on the core of iron, and if we pass alternating current in one winding, back emf or induced voltage is produced in the second winding. We know that alternating current always changes with the time. So if we apply AC voltage across one winding, a voltage will be induced in the other winding. Transformer works on this same principle. It is made of two windings wound around the same core of iron. The winding to which AC voltage is applied is called primary winding. The other winding is called as secondary winding.

Voltage and current relationship:

Let V1 volts be input alternating voltage applied to primary winding. I1 Amp is input alternating current through primary winding. V2 volt is output alternating voltage produced in the secondary. I2 amp be the current flowing through the secondary.

Then relationship between input and output voltages is given by

V1/V2 = N1/N2

Relationship between input and output currents is

I1/I2 = N2/N1

(Where N1 is no. of  turns of coil in primary and N2 is number of turns in secondary )

We know that Power = Current X Voltage. It is to be noted that input power is equal to output power. Power is not changed. If V2 is greater than V1, then I2 will be less than I1. This type of transformer is called as step up transformer. If V1 is

greater than V2, then I1 will be less than I2. This type of transformer is called as step down transformer.

For step up transformer, N2>N1, i.e., number of turns of secondary winding is more than those in primary.

For step down transformer, N1>N2, i.e., numbers of turns of primary winding is more than those in secondary.


resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm’s law:

V = IR

Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).

The primary characteristics of a resistor are the resistance, the tolerance, maximum working voltage and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance is determined by the design, materials and dimensions of the resistor.

Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size, and position of leads (or terminals) are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power


The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. Commonly used multiples and submultiples in electrical and electronic usage are the milliohm (1×10−3), kilohm (1×103), and megohm (1×106).

Theory of operation

Ohm’s law

The behavior of an ideal resistor is dictated by the relationship specified in Ohm’s law:

Ohm’s law states that the voltage (V) across a resistor is proportional to the current (I) through it where the constant of proportionality is the resistance (R).

Equivalently, Ohm’s law can be stated:

This formulation of Ohm’s law states that, when a voltage (V) is maintained across a resistance (R), a current (I) will flow through the resistance.

This formulation is often used in practice. For example, if V is 12 volts and R is 400 ohms, a current of 12 / 400 = 0.03 amperes will flow through the resistance R.

Series and parallel resistors

Main article: Series and parallel circuits

Resistors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent resistance (Req):

The parallel property can be represented in equations by two vertical lines “||” (as in geometry) to simplify equations. For two resistors,

The current through resistors in series stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total resistance:

A resistor network that is a combination of parallel and series can be broken up into smaller parts that are either one or the other. For instance,

However, many resistor networks cannot be split up in this way. Consider a cube, each edge of which has been replaced by a resistor. For example, determining the resistance between two opposite vertices requires additional transforms, such as the Y-Δ transform, or else matrix methods must be used for the general case. However, if all twelve resistors are equal, the corner-to-corner resistance is 56 of any one of them.

The practical application to resistors is that a resistance of any non-standard value can be obtained by connecting standard values in series or in parallel.

Power dissipation

The power dissipated by a resistor (or the equivalent resistance of a resistor network) is calculated using the following:

All three equations are equivalent. The first is derived from Joule’s first law. Ohm’s Law derives the other two from that.

The total amount of heat energy released is the integral of the power over time:

If the average power dissipated is more than the resistor can safely dissipate, the resistor may depart from its nominal resistance and may become damaged by overheating. Excessive power dissipation may raise the temperature of the resistor to a point where it burns out, which could cause a fire in adjacent components and materials. There are flameproof resistors that fail (open circuit) before they overheat dangerously.

Note that the nominal power rating of a resistor is not the same as the power that it can safely dissipate in practical use. AIR circulation and proximity to a circuit board, ambient temperature, and other factors can reduce acceptable dissipation significantly. Rated power dissipation may be given for an ambient temperature of 25 °C in free air. Inside an equipment case at 60 °C, rated dissipation will be significantly less; a resistor dissipating a bit less than the maximum figure given by the manufacturer may still be outside the safe operating area and may prematurely fail.

Resistor marking

Most axial resistors use a pattern of colored stripes to indicate resistance. Surface-mount resistors are marked numerically, if they are big enough to permit marking; more-recent small sizes are impractical to mark. Cases are usually tan, brown, blue, or green, though other colors are occasionally found such as dark red or dark gray.

Early 20th century resistors, essentially uninsulated, were dipped in paint to cover their entire body for color coding. A second color of paint was applied to one end of the element, and a color dot (or band) in the middle provided the tthird digit. The rule was “body, tip, dot”, providing two significant digits for value and the decimal multiplier, in that sequence. Default tolerance was ±20%. Closer-tolerance resistors had silver (±10%) or gold-colored (±5%) paint on the other end.

Four-band resistors

Four-band identification is the most commonly used color-coding scheme on resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The first three bands are equally spaced along the resistor; the spacing to the fourth band is wider. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits.

For example, green-blue-yellow-red is 56×104 Ω = 560 kΩ ± 2%. An easier description can be as followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and is counted as 56. The tthird band, yellow, has a value of 104, which adds four 0’s to the end, creating 560,000 Ω at ±2% tolerance accuracy. 560,000 Ω changes to 560 kΩ ±2% (as a kilo- is 103).

Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in the chart below.

Color1st band2nd band3rd band (multiplier)4th band (tolerance)Temp. Coefficient
Brown11×101±1% (F)100 ppm
Red22×102±2% (G)50 ppm
Orange33×103 15 ppm
Yellow44×104 25 ppm
Green55×105±0.5% (D) 
Blue66×106±0.25% (C) 
Violet77×107±0.1% (B) 
Gray88×108±0.05% (A) 
Gold  ×10−1±5% (J) 
Silver  ×10−2±10% (K) 
None   ±20% (M) 

 Variable Resistors


Text Box:

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available. Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment.

Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm.

The resistance and type of track are marked on the body:
    4K7 LIN means 4.7 k linear track.
    1M LOG means 1 M logarithmic track.

Some variable resistors are designed to be mounted directly on the circuit board, but most are for mounting through a hole drilled in the case containing the circuit with stranded wire connecting their terminals to the circuit board.


A capacitor (formerly known as condenser) is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When there is a potential difference (voltage) across the conductors a static electric field develops in the dielectric that stores energy and produces a mechanical force between the conductors. An ideal capacitor is characterized by a single constant value, capacitance, measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them.

Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass, in filter networks, for smoothing the output of power supplies, in the resonant circuits that tune radios to particular frequencies and for many other purposes.

The effect is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called “plates”, referring to an early means of construction. In practice the dielectric between the plates passes a small amount of leakage current and also has an electric field strength limit, resulting in a breakdown voltage, while the conductors and leads introduce an equivalent series resistance

Ripple current

Ripple current is the AC component of an applied source (often a switched-mode power supply) whose frequency may be constant or varying. Certain types of capacitors, such as electrolytic tantalum capacitors, usually have a rating for maximum ripple current (both in frequency and magnitude). This ripple current can cause damaging heat to be generated within the capacitor due to the current flow across resistive imperfections in the materials used within the capacitor, more commonly referred to as equivalent series resistance (ESR). For example electrolytic tantalum capacitors are limited by ripple current and generally have the highest ESR ratings in the capacitor family, while ceramic capacitors generally have no ripple current limitation and have some of the lowest ESR ratings.

Capacitor types

Practical capacitors are available commercially in many different forms. The type of internal dielectric, the structure of the plates and the device packaging all strongly affect the characteristics of the capacitor, and its applications.

Values available range from very low (picofarad range; while arbitrarily low values are in principle possible, stray (parasitic) capacitance in any circuit is the limiting factor) to about 5 kF supercapacitors.

Above approximately 1 microfarad electrolytic capacitors are usually used because of their small size and low cost compared with other technologies, unless their relatively poor stability, life and polarised nature make them unsuitable. Very high capacity supercapacitors use a porous carbon-based electrode material.

Dielectric materials

Capacitor materials. From left: multilayer ceramic, ceramic disc, multilayer polyester film, tubular ceramic, polystyrene, metalized polyester film, aluminum electrolytic. Major scale divisions are in centimetres.

Most types of capacitor include a dielectric spacer, which increases their capacitance. These dielectrics are most often insulators. However, low capacitance devices are available with a vacuum between their plates, which allows extremely high voltage operation and low losses. Variable capacitors with their plates open to the atmosphere were commonly used in radio tuning circuits. Later designs use polymer foil dielectric between the moving and stationary plates, with no significant air space between them.

In order to maximise the charge that a capacitor can hold, the dialectric material needs to have as high a permittivity as possible, while also having as high a breakdown voltage as possible.

Several solid dielectrics are available, including paper, plastic, glass, mica and ceramic materials. Paper was used extensively in older devices and offers relatively high voltage performance. However, it is susceptible to water absorption, and has been largely replaced by plastic film capacitors. Plastics offer better stability and aging performance, which makes them useful in timer circuits, although they may be limited to low operating temperatures and frequencies. Ceramic capacitors are generally small, cheap and useful for high frequency applications, although their capacitance varies strongly with voltage and they age poorly. They are broadly categorized as class 1 dielectrics, which have predictable variation of capacitance with temperature or class 2 dielectrics, which can operate at higher voltage. Glass and mica capacitors are extremely reliable, stable and tolerant to high temperatures and voltages, but are too expensive for most mainstream applications. Electrolytic capacitors and supercapacitors are used to store small and larger amounts of energy, respectively, ceramic capacitors are often used in resonators, and parasitic capacitance occurs in circuits wherever the simple conductor-insulator-conductor structure is formed unintentionally by the configuration of the circuit layout.

Electrolytic capacitors use an aluminum or tantalum plate with an oxide dielectric layer. The second electrode is a liquid electrolyte, connected to the circuit by another foil plate. Electrolytic capacitors offer very high capacitance but suffer from poor tolerances, high instability, gradual loss of capacitance especially when subjected to heat, and high leakage current. Poor quality capacitors may leak electrolyte, which is harmful to printed circuit boards. The conductivity of the electrolyte drops at low temperatures, which increases equivalent series resistance. While widely used for power-supply conditioning, poor high-frequency characteristics make them unsuitable for many applications. Electrolytic capacitors will self-degrade if unused for a period (around a year), and when full power is applied may short circuit, permanently damaging the capacitor and usually blowing a fuse or causing arcing in rectifier tubes. They can be restored before use (and damage) by gradually applying the operating voltage, often done on antique [[vacuum tube] equipment over a period of 30 minutes by using a variable transformer to supply AC power. Unfortunately, the use of this technique may be less satisfactory for some solid state equipment, which may be damaged by operation below its normal power range, requiring that the power supply first be isolated from the consuming circuits. Such remedies may not be applicable to modern high-frequency power supplies as these produce full output voltage even with reduced input.

Tantalum capacitors offer better frequency and temperature characteristics than aluminum, but higher dielectric absorption and leakage.[21] OS-CON (or OC-CON) capacitors are a polymerized organic semiconductor solid-electrolyte type that offer longer life at higher cost than standard electrolytic capacitors.

Several other types of capacitor are available for specialist applications. Supercapacitors store large amounts of energy. Supercapacitors made from carbon aerogel, carbon nanotubes, or highly porous electrode materials offer extremely high capacitance (up to 5 kF as of 2010[update]) and can be used in some applications instead of rechargeable batteries. Alternating current capacitors are specifically designed to work on line (mains) voltage AC power circuits. They are commonly used in electric motor circuits and are often designed to handle large currents, so they tend to be physically large. They are usually ruggedly packaged, often in metal cases that can be easily grounded/earthed. They also are designed with direct current breakdown voltages of at least five times the maximum AC voltage.


In this project, 0.01 microfarad capacitor is a ceramic capacitor.                                                                   The basis of the ceramic material is mainly barium titanate or a similar material, but other ceramic substance including hydrous silicate of magnesia or talc are also used. The electrodes are applied in the form of silver which is either spread or plated on to the opposite faces of a thin tube, wafer or disc made from the ceramic material. Connecting wires are then soldered to this deposit and the whole capacitor dipped in for a suitable coating.

Fig. Tabular and disc type ceramic capacitors


In this project, 10mf capacitor is an electrolytic capacitor. In this type of capacitors, the dielectric consists of an extremely thin film of aluminum oxide formed on one of its aluminum foil plates. Intimate contact with the other plate is achieved by impregnating the paper between the foils with an electrolyte in the form of viscous substance, such as ammonium borate. The sandwich is then rolled into a cylindrical element and housed in either metallic cardboard, plastic or ceramic protective tube.

Fig. Electrolytic and Tantalum capacitor


Capacitors have many uses in electronic and electrical systems. They are so common that it is a rare electrical product that does not include at least one for some purpose.

Energy storage

A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed. (This prevents loss of information in volatile memory.)

Conventional electrostatic capacitors provide less than 360 joules per kilogram of energy density, while capacitors using developing technologies can provide more than 2.52 kilojoules per kilogram[22].

In car audio systems, large capacitors store energy for the amplifier to use on demand. Also for a flash tube a capacitor is used to hold the high voltage. In ceiling fans, capacitors play the important role of storing electrical energy to give the fan enough torque to start spinning.

Pulsed power and weapons

Groups of large, specially constructed, low-inductance high-voltage capacitors (capacitor banks) are used to supply huge pulses of current for many pulsed power applications. These include electromagnetic forming, Marx generators, pulsed lasers (especially TEA lasers), pulse forming networks, radar, fusion research, and particle accelerators.

Large capacitor banks (reservoir) are used as energy sources for the exploding-bridgewire detonators or slapper detonators in nuclear weapons and other specialty weapons. Experimental work is under way using banks of capacitors as power sources for electromagnetic armour and electromagnetic railguns and coilguns.

Variable capacitor

A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefore they are sometimes called tuning capacitors), or as a variable reactance, e.g. for impedance matching in antenna tuners.

Mechanically controlled

In mechanically controlled variable capacitors, the distance between the plates, or the amount of plate surface area which overlaps, can be changed.

The most common form arranges a group of semicircular metal plates on a rotary axis (“rotor”) that are positioned in the gaps between a set of stationary plates (“stator”) so that the area of overlap can be changed by rotating the axis. Air or plastic foils can be used as dielectric material. By choosing the shape of the rotary plates, various functions of capacitance vs. angle can be created, e.g. to obtain a linear frequency scale. Various forms of reduction gear mechanisms are often used to achieve finer tuning control, i.e. to spread the variation of capacity over a larger angle, often several turns. A vacuum variable capacitor uses a set of plates made from concentric cylinders that can be slid in or out of an opposing set of cylinders[1] (sleeve and plunger). These plates are then sealed inside of a non-conductive envelope such as glass or ceramic and placed under a high vacuum. The movable part (plunger) is mounted on a flexible metal membrane that seals and maintains the vacuum. A screw shaft is attached to the plunger, when the shaft is turned the plunger moves in or out of the sleeve and the value of the capacitor changes. The vacuum not only increases the working voltage and current handling capacity of the capacitor it also greatly reduces the chance of arcing across the plates. The most common usage for vacuum variables are in high powered transmitters such as those used for broadcasting, military and amateur radio as well as high powered RF tuning networks. Vacuum variables can also be more convenient since the elements are under a vacuum the working voltage can be higher than an air variable the same size, allowing the size of the vacuum capacitor to be reduced.

Very cheap variable capacitors are constructed from layered aluminium and plastic foils that are variably pressed together using a screw. These so-called squeezers can’t provide a stable and reproducible capacitance, however. A variant of this structure that allows for linear movement of one set of plates to change the plate overlap area is also used and might be called a slider. This has practical advantages for makeshift or home construction and may be found in resonant loop antennas or crystal radios.

Small variable capacitors operated by screwdriver (for instance, to precisely set a resonant frequency at the factory and then never be adjusted again) are called trimmer capacitors. In addition to air and plastic, trimmers can also be made using a ceramic dielectric.

Electronically controlled

The thickness of the depletion layer of a reverse-biased semiconductor diode varies with the DC voltage applied across the diode. Any diode exhibits this effect (including p/n junctions in transistors), but devices specifically sold as variable capacitance diodes (also called varactors or varicaps) are designed with a large junction area and a doping profile specifically designed to maximize capacitance.

Their use is limited to low signal amplitudes to avoid obvious distortions as the capacitance would be affected by the change of signal voltage, precluding their use in the input stages of high-quality RF communications receivers, where they would add unacceptable levels of intermodulation. At VHF/UHF frequencies, e.g. in FM Radio or TV tuners, dynamic range is limited by noise rather than large signal handling requirements, and varicaps are commonly used in the signal path.

Varicaps are used for frequency modulation of oscillators, and to make high-frequency voltage controlled oscillators (VCOs), the core component in phase-locked loop (PLL) frequency synthesizers that are ubiquitous in modern communications equipment.

Digitally Tuned Capacitor

A digitally tuned capacitor is a type of chip-form variable capacitor patented by Peregrine Semiconductor in the form of DuNE™ technology using UltraCMOS™ process and HaRP™ design innovation.[1]. The DuNE digitally tunable capacitor (DTC) chip contains five capacitors switched by MOSFETs that operate from a serial input bus with a 5-bit code providing 32 possible capacitor values.

The capacitor values can range from 0.5 to 10 pF with typical tuning ratios of 3:1 to 6:1, or 10:1 in some cases. Typical switching speed is less than 5 µs. Capacitor Q’s greater than 100 are possible. The frecuency range is up to 3 GHz, and power handling is up to 40 dBm. The chip operates with a supply voltage of 2.4 to 3.0 V with current consumption in the 20- to 100-µA range, unlike other . The device comes in a 2- by 2-mm dual flat no-lead (DFN) 8L flip-chip or plastic package.

It is ideal for antenna impedance matching in multi-band GSM/WCDMA cellular handsets and mobile TV recivers that must operate over wide frequency ranges such as the European DVB-H and Japanese ISDB-T mobile TV systems, due to its small size, high Q factor, low voltage operation and current consumption.[2]



1N4001 – 1N4007

General Purpose Rectifiers (Glass Passivated)

Absolute Maximum Ratings* TA = 25°C unless otherwise noted


• Low forward voltage drop.

• High surge current capability.



A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. Semiconductor diodes are symbolized in schematic diagrams such as Figure below. The term “diode” is customarily reserved for small signal devices, I ≤ 1 A. The term rectifier is used for power devices, I > 1 A.

Semiconductor diode schematic symbol: Arrows indicate the direction of electron current flow.

When placed in a simple battery-lamp circuit, the diode will either allow or prevent current through the lamp, depending on the polarity of the applied voltage. (Figure below)

Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased.

When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. Conversely, when the battery is “backward” and the diode blocks current, the diode is said to be reverse-biased. A diode may be thought of as like a switch: “closed” when forward-biased and “open” when reverse-biased. Oddly enough, the direction of the diode symbol’s “arrowhead” points against the direction of electron flow. This is because the diode symbol was invented by engineers, who predominantly use conventional flow notation in their schematics, showing current as a flow of charge from the positive (+) side of the voltage source to the negative (-). This convention holds true for all semiconductor symbols possessing “arrowheads:” the arrow points in the permitted direction of conventional flow, and against the permitted direction of electron flow.

Diode behavior is analogous to the behavior of a hydraulic device called a check valve. A check valve allows fluid flow through it in only one direction as in Figure below.

Hydraulic check valve analogy: (a) Electron current flow permitted. (b) Current flow prohibited.

Check valves are essentially pressure-operated devices: they open and allow flow if the pressure across them is of the correct “polarity” to open the gate (in the analogy shown, greater fluid pressure on the right than on the left). If the pressure is of the opposite “polarity,” the pressure difference across the check valve will close and hold the gate so that no flow occurs. Like check valves, diodes are essentially “pressure-” operated (voltage-operated) devices. The essential difference between forward-bias and reverse-bias is the polarity of the voltage dropped across the diode. Let’s take a closer look at the simple battery-diode-lamp circuit shown earlier, this time investigating voltage drops across the various components in Figure below.

ucts current and drops a small voltage across it, leaving most of the battery voltage dropped across the lamp. If the battery’s polarity is reversed, the diode becomes reverse-biased, and drops all of the battery’s voltage leaving none for the lamp. If we consider the diode to be a self-actuating switch (closed in the forward-bias mode and open in the reverse-bias mode), this behavior makes sense. The most substantial difference is that the diode drops a lot more voltage when conducting than the average mechanical switch (0.7 volts versus tens of millivolts).

This forward-bias voltage drop exhibited by the diode is due to the action of the depletion region formed by the P-N junction under the influence of an applied voltage. If no voltage applied is across a semiconductor diode, a thin depletion region exists around the region of the P-N junction, preventing current flow. (Figure below (a)) The depletion region is almost devoid of available charge carriers, and acts as an insulator:

Diode representations: PN-junction model, schematic symbol, physical part.

The schematic symbol of the diode is shown in Figure above (b) such that the anode (pointing end) corresponds to the P-type semiconductor at (a). The cathode bar, non-pointing end, at (b) corresponds to the N-type material at (a). Also note that the cathode stripe on the physical part (c) corresponds to the cathode on the symbol.

If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it. (Figure below)

Depletion region expands with reverse bias.


The reed switch contains a pair (or more) of magnetizable, flexible, metal reeds whose end portions are separated by a small gap when the switch is open. The reeds are hermetically sealed in opposite ends of a tubular glass envelope. A magnetic field (from an electromagnet or a permanent magnet) will cause the reeds to come together, thus completing anelectrical circuit. The stiffness of the reeds causes them to separate, and open the circuit, when the magnetic field ceases. Another configuration contains a non-ferrous normally-closed contact that opens when the ferrous normally-open contact closes. Good electrical contact is assured by plating a thin layer of non-ferrous precious metal over the flat contact portions of the reeds; low-resistivity silver is more suitable than corrosion-resistant gold in the sealed envelope. There are also versions of reed switches with mercury “wetted” contacts. Such switches must be mounted in a particular orientation otherwise drops of mercury may bridge the contacts even when not activated.

Since the contacts of the reed switch are sealed away from the atmosphere, they are protected against atmospheric corrosion. The hermetic sealing of a reed switch make them suitable for use in explosive atmospheres where tiny sparks from conventional switches would constitute a hazard.

One important quality of the switch is its sensitivity, the amount of magnetic field necessary to actuate it. Sensitivity is measured in units of Ampere-turns, corresponding to the current in a coil multiplied by the number of turns. Typical pull-in sensitivities for commercial devices are in the 10 to 60 AT range. The lower the AT, the more sensitive the reed switch. Also, smaller reed switches, which have smaller parts, are more sensitive to magnetic fields, so the smaller the reed switch’s glass envelope is, the more sensitive it is.

In production, a metal reed is inserted in each end of a glass tube and the end of the tube heated so that it seals around a shank portion on the reed. Infrared-absorbing glass is used, so an infrared heat source can concentrate the heat in the small sealing zone of the glass tube. The thermal coefficient of expansion of the glass material and metal parts must be similar to prevent breaking the glass-to-metal seal. The glass used must have a high electrical resistance and must not contain volatile components such as lead oxide and fluorides. The leads of the switch must be handled carefully to prevent breaking the glass envelope.



In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another.[1][2] The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts. Each set of contacts can be in one of two states: either ‘closed’ meaning the contacts are touching and electricity can flow between them, or ‘open’, meaning the contacts are separated and nonconducting.

A switch may be directly manipulated by a human as a control signal to a system, such as a computer keyboard button, or to control power flow in a circuit, such as a light switch. Automatically-operated switches can be used to control the motions of machines, for example, to indicate that a garage door has reached its full open position or that a machine tool is in a position to accept another workpiece. Switches may be operated by process variables such as pressure, temperature, flow, current, voltage, and force, acting as sensors in a process and used to automatically control a system. For example, a thermostat is an automatically-operated switch used to control a heating process. A switch that is operated by another electrical circuit is called a relay. Large switches may be remotely operated by a motor drive mechanism. Some switches are used to isolate electric power from a system, providing a visible point of isolation that can be pad-locked if necessary to prevent accidental operation of a machine during maintenance, or to prevent electric shock.

IC 7805 :

Three Terminal Positive Fixed Voltage Regulators

            These voltage regulators are monolithic integrated circuits designed as fixed voltage. These regulators employ internal current limiting, thermal shutdown, and safe-area compensation. With adequate heat sinking they can deliver output currents in excess of 1.0A. Although designed primarily as a fixed voltage regulator, these devices can be used with external components to obtain adjustable voltages and currents.

Output Current in Excess of 1.0A

No external components required

Internal thermal overload protection

Internal short circuit current limiting

Output transistor safe – area compensation

Output voltage offered in 2% and 4% tolerance

Available in surface mount D2pAK and standard 3-lead transistor packages

Previous commercial temperature range has been extended to a junction temperature range of –40°C to +125°C


            The 7805 series of three terminal positive regulators 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.

Fig. Block Diagram of IC7805

Absolute Maximum Rating :

Input Voltage (for VO=5V to 18V) (for VO =24V)VI VI35 40V V
Thermal Resistance, Junction to Cases (TO-220)RqJC5°C/W
Thermal Resistance, Junction to Air (TO-220)RqJC65°C/W
Operating Temp. RangeTOPR0 – +125°C
Storage Temp. RangeTSTG-65 – +150°C

Electrical Characteristics (TA = 25°C unless otherwise noted)

Output Voltage TJ =+25°CVO4.85.05.2V
Line Regulation (Note 1) VO =7V to 25VRegline4.0100MV
Load Regulation (Note 1) IO = 5.0mA to 1.5ARegload9100MV
Quiescent Current TJ =+25°CIQ5.08.0mA
Quiescent Current Change IO = 5.0mA to 1.0ADIQ0.030.5mA
Output Voltage Drift IO = 5.0mADVO/DT-0.8MV/°C
Output Noise Voltage f=10Hz to 100MHz TA=+25°CVN42mV/VO
Ripple Rejection f=120Hz, VO=8V to 18VRR6273dB
Dropout Voltage IO = 1A, TA=+25°CVDrop2V
Output Resistance f=1KHzrO15mW
Short Circuit Current VI = 35V, TA=+25°CISC230MA
Peak Current TA=+25°CIPK2.2A

NOTE : Load and line regulation are specified at constant junction temperature. Changes in VO due to heating effects must be taken into account separately. Pulse testing with low duty is used


A relay is an electrically operated switch. The relay contacts can be made to operate in the pre-arranged fashion. For instance, normally open contacts close and normally closed contacts open. In electromagnetic relays, the contacts however complex they might be, they have only two position i.e. OPEN and CLOSED, whereas in case of electromagnetic switches, the contacts can have multiple positions.


The reason behind using relay for switching loads is to provide complete electrical isolation. The means that there is no electrical connection between the driving circuits and the driven circuits. The driving circuit may be low voltage operated low power circuits that control several kilowatts of power. In our circuit where a high fan could be switched on or off depending upon the output from the telephone.

Since the relay circuit operated on a low voltage, the controlling circuit is quite safe. In an electromagnetic relay the armature is pulled by a magnetic force only. There is no electrical connection between the coil of a relay and the switching contacts of the relay. If there are more than one contact they all are electrically isolated from each other by mounting them on insulating plates and washers. Hence they can be wired to control different circuits independently.

Some of the popular contacts forms are described below:

     1.  Electromagnetic relay

  • Power Relay.
  • Time Delay Relay.
  • Latching Relay.
  • Crystal Can Relay.
  • Co-axial Relay.

1. Electromagnetic relay:

An electromagnetic relay in its simplest form consists of a coil, a DC current passing through which produces a magnetic field. This magnetic field attracts an armature, which in turn operates the contacts. Normally open contacts close and normally closed contacts open. Electromagnetic relays are made in a large variety of contacts forms.

2. Power relays:

Power relays are multi-pole heavy duty lapper type relays that are capable of switching resistive loads of upto 25amp.. These relays are widely used for a variety of industrial application like control of fractional horse power motors, solenoids, heating elements and so on. These relays usually have button like silver alloy contacts and the contact welding due to heavy in rush current is avoided by wiping action of the contacts to quench the arc during high voltage DC switching thus avoiding the contact welding.

3. Time Delay Relay:

A time delay relay is the one in which there is a desired amount of time delay between the application of the actuating signal and operation of the load switching devices.

4. Latching Relay:

 In a Latching Relay, the relay contacts remain in the last energized position even after removal of signal in the relay control circuit. The contacts are held in the last relay-energized position after removal of energisation either electrically or magnetically. The contacts can be released to the normal position electrically or mechanically.

5. Crystal Can Relay:

They are so called, as they resemble quartz crystal in external shapes. These are high performance hermeticallysealed miniature or sub-miniature relay widely used in aerospace and military application. These relays usually have gold plated contacts and thus have extremely low contact resistance. Due to low moment of inertia of the armature and also due to statically and dynamically balanced nature of armature, these relays switch quite reliably even under extreme condition of shock and vibration.

6. Co-axial Relay:

A Co-axial Relay has two basic parts, an actuator which is nothing but some kind of a coil and a cavity, housing the relay contacts. The co-axial relay are extensively used for radio frequency switching operations of equipment


Soldering is the process of joining two metallic conductors the joint where two metal conductors are to be joined or fused is heated with a device called soldering iron and then as allow of tin and lead called solder is applied which melts and converse the joint. The solder cools and solidifies quickly to ensure is good and durable connection between the jointed metal converting the joint solder also present oxidation.


There are basically two soldering techniques:

Manual soldering with iron.

Mass soldering.

The iron consist of an insulated handle connected via a metal shank to the bit the function of bit is to

Stare host & convey it to the component

To store and deliver molten solder 7 flux.

To remove surplus solder from joints.

Soldering bit are made of copper because it has good heat capacity & thermal conductivity. It may erode after long term use to avoid it coating of nickel or tin is used.


The surface to be soldered must be cleaned & fluxed. The soldering iron switched on & bellowed to attain soldering temperature. The solder in form of wire is allied hear the component to be soldered &b heated with iron. The surface to be soldered is filled, iron is removed & the joint is cold without disturbing.

Solder joint are supposed to

Provide permanent low resistance path

Make a robust mechanical link between PCB & leads of components.

Allow heat flow between component, joining elements & PCB.

Retain adequate strength with temperature variation.

The following precaution should be taken while soldering.

Use always an iron plated copper core tip for soldering iron.

Slightly fore the tip with a cut file when it is cold.

Use a wet sponge to wipe out dirt from the tip before soldering instead of asking the iron.

Tighten the tip screw if necessary before iron is connected to power supply.

Clean component lead & copper pad before soldering.

Use proper tool for component handling instead of direct handling.

Apply solder between component leads, PCB pattern & tip of soldering iron.

Iron should be kept in contact with the joint s for 2-3 second s only instead of keeping for very long or very small time.

Use optimum quantity of solder.

Use multistoried wire instead of single strands solvent like isopropyl alcohol.

Every time soldering is over, put a little clean solder on the tip.

Material required :

  • Arduino UNO Board – 1nos – Rs 500
  • Connecting Cables – Rs 50
  • LCD display 16×2 Rs 150
  • Switches RS 30
  • Transformer Rs 150
  • Relay – 12v – Rs  40
  • Voltage regulator 7805
  • Transistor  BC548 – Rs 2 each
  • Transistor BC557 rs 2 Each
  • Soldering wire
  • Soldering Iron
  • Multimeter
  • Screwdriver
  • wire cutter


1. HAND BOOK OF ELECTRONICS                                                                                                            A.K. MAINI.

2.HAND BOOK OF ELECTRONICS                                                                                                GUPTA & KUMAR.

3.LET US C                                                                                                                              YASHWANT KANITKAR.

 4.SHYAM SERIES                                                                                                                                     TATA MC GRILL.      

5.DIGITAL SYSTEMS PRINCIPLES AND APPLICATION                                                            RONALD LTOCCI.

   (Sixth addition)

6.ELECTRONICS FOR YOU                                                                                                                    (MARCH 1998).

7.DIGITAL DESIGN                                                                                                                                  MORIS MANO.

    (Second addition)

8.RELAYS AND ITS APPLICATION                                                                                                       SHARMA, MC.


9.MODERN ALL ABOUT MOTHERBOARD                                                                                              LOTHIA, M.


                    10.POWER SUPPLY FOR ALL OCCASION                                                                                            SHARMA, MC.


               11.CMOS DATA BOOK (74SERIES)                                                                                                                         ECA.


               12.PRACTICAL VALUE AND TRANSISTOR DATA                                                                                            POPE.  


               13.PRACTICAL TRANSFORMER DESIGN HAND BOOK                                                                           LABON. E.


                     14 MODERN  IC                                                                                                                              MANAHAR  LOTIA.


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