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PROJECT REPORTON

LAUNCHING MECHANISM OF AEROPLANE

IN PARTIAL FULFILLMENT OF THE RECORD FOR THE AWARD OF DEGREE

BACHELOR OF TECHNOLOGYIN

ELECTRONICS AND COMMUNICATION2008-2012

SUBMITTED BY :- Sthitapragyan Rout (Reg.No.0801231040)Smita Swain (Reg.No.0801231044)Sriya Halder (Reg.No.0801231051)Manorama Dash (Reg.No.0801231079)Pradipta Ranjan Nayak (Reg.No.0801231174)Srikant Sahu (Reg.No.0801231206)Manisha Parida (Reg.No.0801231214)

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

SAMANTA CHANDRASEKHAR INSTITUTE OF TECHNOLOGY & MANAGEMENT

SEMILIGUDA, KORAPUT

BIJU PATNAIK UNIVERSITY OF TECHNOLOGYROURKELA, ORISSA

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ACKNOWLEDGEMENTWe take this Opportunity to express our sincere gratitude to all those who individually as well as collectively helped us in the successful completion of this Project.

We are thankful to the Faculties of Department of Electronics and Communication for their valuable guidance, expertise and motivation for the accomplishment of this project work.

We also acknowledge the continuous encouragement rendered by our friends.

Sthitapragyan Rout (Reg. No. 0801231040)

Smita Swain (Reg. No. 0801231044)

Sriya Halder (Reg. No. 0801231051)

Manorama Dash (Reg. No. 0801231079)

Pradipta Ranjan Nayak (Reg. No. 0801231174)

Srikant Sahu (Reg. No. 0801231206)

Manisha Parida (Reg. No. 0801231214)

ABSTRACT

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A description is given of a rectractable aircraft landing gear consisting of two wheels located one behind the other, a shock absorber (used for both wheels, placed parallel to the longitudinal axis of the fuselage, and having two rods coming out the ends of the shock absorber housing), and two levers for the suspension of these wheels, each of which is hinged at its center or fastened to an eye on the shock absorber housing. To assure retraction and lowering the shock absorber is articulately connected to the fuselage by the use of two eyes located on the ends of the shock absorber housing. The mechanism for lowering and retracting the landing gear is made in the form of a plane articulated parallelogram located in a plane parallel to the longitudinal axis of the fuselage and formed by two breaking struts, each of which is articulately fastened at one end to the fuselage and at the other to the housing of the shock absorber. A crosspiece connects the center hinges of these struts and contains a telescoping lift located outside the parallelogram in its plane. The housing of the lift is connected with the fuselage and the rod is articulately connected to the fuselage by an arm of the breaking strut nearest this lift. In the retracted and lowered position the retractable landing gear can be spring-loaded in the direction corresponding to the turn axis of the landing gear, the moment of which is produced by means of a telescoping spring-loaded mechanism articulately connected on one end to the fuselage.

INTRODUCTION

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The undercarriage or landing gear in aviation, is the structure that supports an aircraft on the ground and allows it to taxi, takeoff and land. Typically wheels are used, but skids, skis, floats or a combination of these and other elements can be deployed, depending on the surface.The landing mechanism of aeroplane includes strut, shock absorber, extraction/retraction mechanism, brakes, wheel, tyre. Shock absorber and extraction/retraction mechanism may not be present in small airplanes.Landing gear usually includes wheels equipped with shock absorbers for solid ground, but some aircraft are equipped with skis for snow or floats for water, and/or skids or pontoons (helicopters).The undercarriage is a relatively heavy part of the vehicle, it can be as much as 7% of the takeoff weight, but more typically is 4-5%The landing gear is the interface of airplane to ground, so that all the ground loads are transmitted by it to the aircraft structure. There is then a high influence of the landing gear on the local structure, which must be taken into account since the initial design stage. The landing loads can reach factors of 2.5 for transport aircraft, 4.5 for small general aviation vehicles and higher for combat aircraft. The system must then have considerable mechanical resistance, which means in general that its mass is significant. Depending on aircraft category, this can range from 3 to 7% of the aircraft total mass.The main functions of the landing mechanism are energy absorption at landing, braking & taxi control.Landing is the main sizing conditions for the system and its surrounding structure; braking also determines both vertical and horizontal loads that influence structural sizing. Taxi control includes steering and taxi stability.

BLOCK DIAGRAM

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POWER SUPPLYThe power supply designed for containing a fixed demand connected in this project. The basic requirement for designing a power supply is as follows:The voltage levels required for operating the devices is +5v.Here +5v required for operating microcontroller and as well as required for drivers and amplifiers and transmitters and receivers.The current requirement of each device or load must be added to estimate the final capacity of the power supply.The power supply always specified with one or multiple voltage output along with a current capacity. As it is estimate the requirement of power is approximately as follows,

a) Output voltage = +5v b) Capacity = 1000mA

The power supply is basically consisting of three sections as follows,a) Step down sectionb) Rectifier sectionc) Regulator section

CIRCUIT CONNECTIONIn this we r using transformer (9-0-9)v/1mA,IC 7805,diods7004,LED and resisters.

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Here 230v,50Hz ac signal is given as input to the primary and secondary of the transformer is given to the bridge rectifier diode. The positive output of the bridge rectifier is given as i/p to the voltage regulator is given to the LED through resister to act as the indicator.

CIRCUIT EXPLANATIONWhen ac signal is given to the primary of the transformer, due to the magnetic effect of the coil magnetic effect of the coil magnetic flux is induced in the coil of the transformer due to the transformer action. Transformer is an electromechanical static device which transformer electrical energy from one coil to another without changing its frequency. Here the diodes are connected to the two +12v output of the transformer. The secondary coil of the transformer is given to the diode circuit for rectification purposes.During the +ve cycle of the ac signal the diode D1 conduct due to the forward bias of the diodes and diodes D2 doesn’t conduct due to the reversed bias of the diodes. Similarly during the –ve cycle of the ac signal the diodes D2 conduct due to the forward bias of the diodes and the diode D1 does not conducted due to reversed bias of the diodes. The output of the bridge rectifier is not a power dc along with rippled ac is also present. To overcome this effect, a low pass filter is connected to the output of the diodes. Which remover the unwanted ac signal and thus a pure dc is obtained. Here we need a fixed voltage, that’s for we using IC regulator(7805).”Voltage regulation is the circuit that supplies a constant voltage regardless of change in load current”. This IC’s are designed as fixed voltage regulators are with adequate heat sinking can deliver output current is excess of 1A. The o/p the full wave rectifier is given as input the IC through low pass filter with respect to GND and thus a fixed o/p is obtained. The output of the IC(7805) is given to the LED for indication purpose through resister. Due to the forward bias of the LED, the LED glows on state, and the o/p are obtain from the pin no3.

VOLTAGE REGULATOR

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A popular three pin 12 V DC voltage regulator IC.A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.Voltage Regulator (regulator), usually having three legs, converts varying input voltage and produces a constant regulated output voltage. They are available in a variety of outputs.  The most common part numbers start with the numbers 78 or 79 and finish with two digits indicating the output voltage. The number 78 represents positive voltage and 79 negative one. The 78XX series of voltage regulators are designed for positive input. And the 79XX series is designed for negative input.  EXAMPLES:   ·         5V DC Regulator Name: LM7805 or MC7805 ·         -5V DC Regulator Name: LM7905 or MC7905 ·         6V DC Regulator Name: LM7806 or MC7806 ·         -9V DC Regulator Name: LM7909 or MC7909 The LM78XX series typically has the ability to drive current up to 1A. For application requirements up to 150mA, 78LXX can be used. As mentioned above, the component has three legs: Input leg which can hold up to 36VDC Common leg (GND) and an output leg with the regulator's voltage. For maximum voltage regulation, adding a capacitor in parallel between the common leg and the output is usually recommended. Typically a 0.1MF capacitor is used. This eliminates any high frequency AC voltage that could otherwise combine with the output voltage. 

  

 IC 7805 (VOLTAGE REGULATOR IC)

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    7805 is a voltage regulator integrated circuit. It is a member of 78xx series of fixed linear voltage regulator ICs. The voltage source in a circuit may have fluctuations and would not give the fixed voltage output. The voltage regulator IC maintains the output voltage at a constant value. The xx in 78xx indicates the fixed output voltage it is designed to provide. 7805 provides +5V regulated power supply. Capacitors of suitable values can be connected at input and output pins depending upon the respective voltage levels.

MC78XX/LM78XX/MC78XXA3-TERMINAL 1A POSITIVE VOLTAGE REGULATORThe MC78XX/LM78XX/MC78XXA 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.

TO-220 D-PAK

ABSOLUTE MAXIMUM RATINGS

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Parameter Symbol Value Unit

Input Voltage (for VO = 5V to 18V)(for VO = 24V)

VI VI

35 40

V V

Thermal Resistance Junction-Cases (TO-220)

RθJC 5 oC/W

Thermal Resistance Junction-Air (TO-220)

RθJA 65 oC/W

Operating Temperature Range

TOPR 0 ~ +125 oC

Storage Temperature Range

TSTG -65 ~ +150 oC

ELECTRICAL CHARACTERISTICS (MC7805/LM7805)

Parameter Symbol Conditions MC7805/LM7805

Unit

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Min. Typ. Max.

Output Voltage VO TJ =+25 oC 4.8 5.0 5.2 V

5.0mA ≤ Io ≤ 1.0A, PO ≤ 15WVI = 7V to 20V

4.75 5.0 5.25

Line Regulation (Note1)

Regline TJ=+25 oC

VO = 7V to 25V

- 4.0 100 mV

VI = 8V to 12V

- 1.6 50

Load Regulation (Note1)

Regload TJ=+25 oC

IO = 5.0mA to1.5A

- 9 100 mV

IO =250mA to 750mA

- 4 50

Quiescent Current

IQ TJ =+25 oC - 5.0 8.0 mA

Quiescent Current Change

IQ IO = 5mA to 1.0A - 0.03 0.5 mA

VI= 7V to 25V - 0.3 1.3

Output Voltage Drift

VO/T IO= 5mA - -0.8 - mV/ oC

Output Noise Voltage

VN f = 10Hz to 100KHz,TA=+25 oC

- 42 - V/Vo

Ripple Rejection RR f = 120HzVO = 8V to 18V

62 73 - dB

Dropout Voltage VDrop IO = 1A, TJ =+25 oC

- 2 - V

Output Resistance

rO f = 1KHz - 15 - m

Short Circuit Current

ISC VI = 35V, TA =+25 oC

- 230 - mA

Peak Current IPK TJ =+25 oC - 2.2 - A

TRANSFORMER

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Three-phase step-down transformer mounted between two utility poles

Laminated core transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive coupling.If a load is connected to the secondary, current will flow in the secondary winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp) and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus enables an alternating current (AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than Np.In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.

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Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate on the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high-voltage electric power transmission, which makes long-distance transmission economically practical.

BASIC PRINCIPLES

An ideal transformer. The secondary current arises from the action of the secondary EMF on the (not shown) load impedance.The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism) and second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils. If a load is connected to the secondary winding, the load current and voltage will be in the directions indicated, given the primary current and voltage in the directions indicated (each will be alternating current in practice).

STEP DOWN TRANSFORMER

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Step down transformers are designed to reduce electrical voltage. Their primary voltage is greater than their secondary voltage. This kind of transformer "steps down" the voltage applied to it. Step down transformers convert electrical voltage from one level or phase configuration usually down to a lower level. They can include features for electrical isolation, power distribution, and control and instrumentation applications. Step down transformers typically rely on the principle of magnetic induction between coils to convert voltage and/or current levels.Step down transformers are made from two or more coils of insulated wire wound around a core made of iron. When voltage is applied to one coil (frequently called the primary or input) it magnetizes the iron core, which induces a voltage in the other coil, (frequently called the secondary or output). The turns ratio of the two sets of windings determines the amount of voltage transformation.Step down transformers can be considered nothing more than a voltage ratio device.

CIRCUIT SYMBOLS

Transformer with two windings and iron core.

Step-down or step-up transformer. The symbol shows which winding has more turns, but not usually the exact ratio.

Transformer with three windings. The dots show the relative configuration of the windings.

Transformer with electrostatic screen preventing capacitive coupling between the windings.

DIODESA Diode is the simplest two-terminal unilateral semiconductor device.  It allows current to flow only in one direction and blocks the current that flows

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in the opposite direction. The two terminals of the diode are called as anode and cathode. The symbol of diode symbol is as shown in the figure below. 

 The characteristics of a diode closely match to that of a switch. An ideal switch when open does not conduct current in either directions and in closed state conducts in both directions. The characteristic of a diode is as shown in the figure below.                                               

 Ideally, in one direction that is indicated by the arrow head diode must behave short circuited and in other one that opposite to that of the direction of arrow head must be open circuited. By ideal characteristics, the diodes is designed to meet these features theoretically but are not achieved practically. So the practical diode characteristics are only close to that of the desired.    

APPLICATION OF DIODE:Diodes are used in various applications like rectification, clipper, clamper, voltage multiplier, comparator, sampling gates and filters.

RECTIFICATION The rectification means converting AC voltage into DC voltage. The common rectification circuits are half wave rectifier (HWR), full wave rectifier (FWR) and bridge rectifier.

·    Half wave rectifier: This circuit rectifies either positive or negative pulse of the input AC. The figure is as shown below:   

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·   Full wave rectifier: This circuit converts the entire AC signal into DC. The figure is as shown below:

·  Bridge rectifier: This circuit converts the entire AC signal into DC. The figure is as shown below:

IDENTIFICATION: A diode is marked with a bar which indicates the cathode terminal of a diode which is as shown in the figure below:

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PRINCIPLE AND OPERATION: The possible configurations for a diode are:  1. Forward biased 2. Reverse biased1. FORWARD BIAS: In forward bias condition, higher or positive potential is applied at the anode and negative or lower potential is applied at the cathode of a diode. The positive potential at anode repels the holes in p-region towards n-region while negative potential at the cathode repels electrons in n-region towards p-region. Thus, the height of the potential barrier reduces. The depletion region disappears when the applied voltage equals to the potential barrier and a large current flows through the diode. The voltage required to drive the diode into a state of conduction is called as the ‘Cut in/Offset/Threshold/Firing voltage’. The current is of considerable magnitude as it is dominantly constituted by the majority charge currents that is the hole current in the p-region and the electron current in the n-region. The current that flows from anode to cathode is limited by the crystal bulk resistance, recombination of charges and the ohmic contact resistances at the two metal semiconductor junctions.  The current is restricted to mille Amperes order.   

2.REVERSE BIAS: In reverse bias condition,  the higher or positive potential is applied at the cathode and negative or lower potential is applied at the anode. The negative potential at anode attracts the holes in p-region that are away from the n-region while positive potential at the cathode attracts electrons in n-region that are away from the p-region. The applied voltage increases the height of the potential barrier. The current flows dominantly due to the minority charge currents that is the electron current in p-region and the hole current in n-region. Thus a constant current of negligible magnitude flows in the reverse direction which is called as the

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‘Reverse saturation current’. Practically, the diode remains in a state of non – conduction. The reverse saturation current is of the order of microamperes in a germanium diode or nanoamperes in a silicon diode  If the reverse voltage exceeds the limit  of ‘breakdown/zener/Peak inverse/Peak reverse voltage’, the potential breakdown that occurs leads to a large reverse current.

 TYPES OF DIODES:The other variant of diodes have different construction, characteristics and applications. The different types of diodes are: Light emitting diodes (LED) - This is the most popular kind of diode. When it works in the forward bias condition, the current flows through the junction to produce the light. Schottky diode -  These diodes are used in RF applications and clamping circuits. This diode has lower forward voltage drop as against the silicon PN junction diodes.

 GENERIC DIODES (SMALL SIGNAL AND LARGE SIGNAL): 

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A p-n junction diode is the simplest semiconductor device. It is a two-terminal, bipolar, unilateral rectifying device that conducts only in one direction. The generic diodes are used in the following fields: ·   Rectification in power supply circuits ·   Extraction of modulation from radio signals in a radio receiver and in protection circuits where large transient currents may appear on low current transistors or ICs in interfacing with relays or other high power devices. · Used in series with power inputs to electronic circuits where only one of negative or positive polarity voltage is desired.

 

RECTIFIER DIODES (LARGE CURRENT)Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where alarge current must pass through the diode. All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V.BRIDGE RECTIFIERS

Various types of Bridge Rectifiers

There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled . The diagram shows the operation of a bridge rectifier as it converts AC to DC.

CAPACITORS

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Capacitor is a passive component used to store charge. The charge (q) stored in a capacitor is the product of its capacitance (C) value and the voltage (V) applied to it. Capacitors offer infinite reactance to zero frequency so they are used for blocking DC components or bypassing the AC signals. The capacitor undergoes through a recursive cycle of charging and discharging in AC circuits where the voltage and current across it depends on the RC time constant. For this reason, capacitors are used for smoothing power supply variations. Other uses include, coupling the various stages of audio system, tuning in radio circuits etc. These are used to store energy like in a camera flash. Capacitors may be non-polarized/polarized and fixed/variable. Electrolytic capacitors are polarized while ceramic and paper capacitors are examples of non polarized capacitors. Since capacitors store charge, they must be carefully discharged before troubleshooting the circuits. The maximum voltage rating of the capacitors used must always be greater than the supply voltage.

FUNCTIONCapacitors store electric charge. They are used with resistors in timing circuits because ittakes time for a capacitor to fill with charge. They are used to smooth varying DC supplies byacting as a reservoir of charge. They are also used in filter circuits because capacitors easilypass AC (changing) signals but they block DC (constant) signals.

CAPACITANCEThis is a measure of a capacitor's ability to store charge. A large capacitance means that morecharge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large,so prefixes are used to show the smaller values.Three prefixes (multipliers) are used, μ (micro), n (nano) and p (pico):l μ means 10-6 (millionth), so 1000000μF = 1Fl n means 10-9 (thousand-millionth), so 1000nF = 1μFl p means 10-12 (million-millionth), so 1000pF = 1nF

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CIRCUIT SYMBOL

PIN DIAGRAM: 

DETERMINING CAPACITOR VALUES

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AT89C51 MICROCONTROLLER

AT89C51 is an 8-bit microcontroller and belongs to Atmel's 8051 family. ATMEL 89C51 has 4KB of Flash programmable and erasable read only memory (PEROM) and 128 bytes of RAM. It can be erased and program to a maximum of 1000 times.The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4K bytes of Flash programmable and erasable read only memory (PEROM). The device is manufactured using Atmel’s high-density nonvolatile memory technology and is compatible with the industry-standard MCS-51 instruction set and pinout. The on-chip Flash allows the program memory to be reprogrammed in-system or by a conventional nonvolatile memory programmer. By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C51 is a powerful microcomputer which provides a highly-flexible and cost-effective solution to many embedded control applications.In 40 pin AT89C51, there are four ports designated as P1, P2, P3 and P0. All these ports are 8-bit bi-directional ports, i.e., they can be used as both input and output ports. Except P0 which needs external pull-ups, rest of the ports have internal pull-ups. When 1s are written to these port pins, they are pulled high by the internal pull-ups and can be used as inputs. These ports are also bit addressable and so their bits can also be accessed individually.Port P0 and P2 are also used to provide low byte and high byte addresses, respectively, when connected to an external memory. Port 3 has multiplexed pins for special functions like serial communication, hardware interrupts, timer inputs and read/write operation from external memory. AT89C51 has an inbuilt UART for serial communication. It can be programmed to operate at different baud rates. Including two timers & hardware interrupts, it has a total of six interrupts.

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PIN DIAGRAM: 

Pin Description:

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 Pin No  Function  Name1

8 bit input/output port (P1) pins

P1.02 P1.13 P1.24 P1.35 P1.46 P1.57 P1.68 P1.79 Reset pin; Active high Reset10 I/o (rxr)for serial comm RxD

8 bit input/output port (P3) pins

P3.011 O/p(txr)for serialcomm TxD P3.112 External interrupt 1 Int0 P3.213 External interrupt 2 Int1 P3.314 Timer1 external input T0 P3.415 Timer2 external input T1 P3.516 Write to ext data mem Write P3.617 Rd from ext data mem Read P3.718 Quartz crystal oscillator (up to 24 MHz) Crystal 219 Crystal 120 Ground (0V) Ground21

8 bit input/output port (P2) pins/High-order address bits when interfacing with external memory

 P2.0/ A8

22 P2.1/ A9

23  P2.2/ A10

24 P2.3/ A11

25 P2.4/ A12

26 P2.5/ A13

27  P2.6/ A14

28 P2.7/ A15

29 Prog store enable; Read from ext program memory PSEN30 Address Latch Enable ALE

Program pulse input during Flash programming Prog31 Ext Access Enable;  Vcc for int program executions EA

Prog enable voltage; 12V (during Flash programming) Vpp32 8 bit input/output port (P0) pins

 Low-order address bits when interfacing with external memory 

 P0.7/ AD7

33  P0.6/ AD6

34  P0.5/ AD5

35 P0.4/ AD4

36 P0.3/ AD3

37 P0.2/ AD2

38  P0.1/ AD1

39 P0.0/ AD0

40 Supply voltage; 5V (up to 6.6V) Vcc

VCC(PIN 40)Supply voltage.

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GND(PIN 20)Ground.

PORT 0(PIN 32-39)Port 0 is an 8-bit open-drain bi-directional I/O port. As an output port, each pin can sink eight TTL inputs. When 1s are written to port 0 pins, the pins can be used as high impedance inputs. Port 0 may also be configured to be the multiplexed low order address/data bus during accesses to external program and data memory. In this mode P0 has internal pull ups. Port 0 also receives the code bytes during Flash programming, and outputs the code bytes during program verification. External pullups are required during program verification.

PORT 1(PIN 1-8)Port 1 is an 8-bit bi-directional I/O port with internal pull ups. The Port 1 output buffers can sink/source four TTL inputs. When 1s are written to Port 1 pins they are pulled high by the internal pullups and can be used as inputs. As inputs, Port 1 pins that are externally being pulled low will source current (IIL) because of the internal pull ups. Port 1 also receives the low-order address bytes during Flash programming and verification.

PORT 2(PIN 21-28)Port 2 is an 8-bit bi-directional I/O port with internal pull ups. The Port 2 output buffers can sink/source four TTL inputs. When 1s are written to Port 2 pins they are pulled high by the internal pullups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low will source current (IIL) because of the internal pull ups. Port 2 emits the high-order address byte during fetches from external program memory and during accesses to external data memory that use 16-bit addresses (MOVX @ DPTR). In this application, it uses strong internal pull-ups when emitting 1s. During accesses to external data memory that use 8-bit addresses (MOVX @ RI), Port 2 emits the contents of the P2 Special Function Register. Port 2 also receives the high-order address bits and some control signals during Flash programming and verification.

PORT 3(PIN 10-17)Port 3 is an 8-bit bi-directional I/O port with internal pullups. The Port 3 output buffers can sink/source four TTL inputs. When 1s are written to Port 3 pins they are pulled high by the internal pullups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low will source current (IIL) because of the pul lups.

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Port 3 also serves the functions of various special features of the AT89C51 as listed below:Port Pin Alternate

FunctionsP3.0 RXD (serial input

port)P3.1 TXD (serial

output port)P3.2 INT0 (external

interrupt 0)P3.3 INT1 (external

interrupt 1)P3.4 T0 (timer 0

external input)P3.5 T1 (timer 1

external input)P3.6 WR (external

data memory write strobe)

P3.7 RD (external data memory read strobe)

Port 3 also receives some control signals for Flash programming and verification.

RST(PIN 9)Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device.

ALE/PROG(PIN 30)Address Latch Enable output pulse for latching the low byte of the address during accesses to external memory. This pin is also the program pulse input (PROG) during Flash programming. In normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency, and may be used for external timing or clocking purposes. Note, however, that one ALEpulse is skipped during each access to external Data Memory. If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high. Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode.

PSEN(PIN 29)Program Store Enable is the read strobe to external program memory. When the AT89C51 is executing code from external program memory, PSEN is activated twice each machine cycle, except that two PSEN activations are skipped during each access to external data memory.

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EA/VPP(PIN 31)External Access Enable. EA must be strapped to GND in order to enable the device to fetch code from external program memory locations starting at 0000H up to FFFFH. Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset. EA should be strapped to VCC for internal program executions. This pin also receives the 12-volt programming enable voltage (VPP) during Flash programming, for parts that require 12-volt VPP.

XTAL1(PIN 19)Input to the inverting oscillator amplifier and input to the internal clock operating circuit.

XTAL2(PIN 18)Output from the inverting oscillator amplifier.

OSCILLATOR CHARACTERISTICSXTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier which can be configured for use as an on-chip oscillator, as shown in Figure 1. Either a quartzcrystal or ceramic resonator may be used. To drive the device from an external clock source, XTAL2 should be left unconnected while XTAL1 is driven. There are no requirements on the duty cycle of the external clock signal, since the input to the internal clocking circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high and low time specifications must be observed.

IDLE MODEIn idle mode, the CPU puts itself to sleep while all the on chip peripherals remain active. The mode is invoked by software. The content of the on-chip RAM and all the special functions registers remain unchanged during this mode. The idle mode can be terminated by any enabled interrupt or by a hardware reset. It should be noted that when idle is terminated by a hardware reset, the device normally resumes program execution, from where it left off, up to two machine cycles before the internal reset algorithm takes control. On-chip hardwareinhibits access to internal RAM in this event, but access to the port pins is not inhibited. To eliminate the possibility of an unexpected write to a port pin when Idle is terminated byreset, the instruction following the one that invokes Idle should not be one that writes to a port pin or to external memory.

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POWER-DOWN MODEIn the power-down mode, the oscillator is stopped, and the instruction that invokes power-down is the last instruction executed. The on-chip RAM and Special Function Registersretain their values until the power-down mode is terminated. The only exit from power-down is a hardware reset. Reset redefines the SFRs but does not change the on-chip RAM. The reset should not be activated before VCC is restored to its normal operating level and must be heldactive long enough to allow the oscillator to restart and stabilize.

PROGRAM MEMORY LOCK BITSOn the chip are three lock bits which can be left unprogrammed (U) or can be programmed (P) to obtain the additional features. When lock bit 1 is programmed, the logic level at the EA pin is sampled and latched during reset. If the device is powered up without a reset, the latch initializes to a random value, and holds that value until reset is activated. It is necessarythat the latched value of EA be in agreement with the current logic level at that pin in order for the device to function properly.

PROGRAMMING THE FLASHThe AT89C51 is normally shipped with the on-chip Flash memory array in the erased state (that is, contents = FFH) and ready to be programmed. The programming interface accepts either a high-voltage (12-volt) or a low-voltage (VCC) program enable signal. The low-voltage programming mode provides a convenient way to program the AT89C51 inside the user’s system, while the high-voltage programming mode is compatible with conventional third party Flash or EPROM programmers. The AT89C51 is shipped with either the high-voltage or low-voltage programming mode enabled.

PROGRAMMING ALGORITHMBefore programming the AT89C51, the address, data and control signals should be set up according to the Flash programming mode table. To program the AT89C51, take the following steps.1. Input the desired memory location on the address lines.2. Input the appropriate data byte on the data lines.3. Activate the correct combination of control signals.4. Raise EA/VPP to 12V for the high-voltage programming mode.5. Pulse ALE/PROG once to program a byte in the Flash array or the lock bits. The byte-write cycle is self-timed and typically takes no more than 1.5 ms.Repeat steps 1 through 5, changing the addressand data for the entire array or until the end of the object file is reached.

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DATA POLLING: The AT89C51 features Data Polling to indicate the end of a write cycle. During a write cycle, an attempted read of the last byte written will result in the complement of the written datum on PO.7. Once the write cycle has been completed, true data are valid on all outputs, and the next cycle may begin. Data Polling may begin any time after a write cycle has been initiated.READY/BUSY: The progress of byte programming can also be monitored by the RDY/BSY output signal. P3.4 is pulled low after ALE goes high during programming to indicate BUSY. P3.4 is pulled high again when programming is done to indicate READY.

PROGRAM VERIFY: If lock bits LB1 and LB2 have not been programmed, the programmed code data can be read back via the address and data lines for verification. The lock bits cannot be verified directly. Verification of the lock bits is achieved by observing that their features are enabled.

CHIP ERASE: The entire Flash array is erased electrically by using the proper combination of control signals and by holding ALE/PROG low for 10 ms. The code array is written with all “1”s. The chip erase operation must be executed before the code memory can be reprogrammed.

READING THE SIGNATURE BYTES: The signature bytes are read by the same procedure as a normal verification of locations 030H, 031H, and 032H, except that P3.6 and P3.7 must be pulled to a logic low. The values returned are as follows.(030H) = 1EH indicates manufactured by Atmel(031H) = 51H indicates 89C51(032H) = FFH indicates 12V programming(032H) = 05H indicates 5V programming

PROGRAMMING INTERFACEEvery code byte in the Flash array can be written and the entire array can be erased by using the appropriate combination of control signals. The write operation cycle is self timed and once initiated, will automatically time itself to completion. All major programming vendors offer worldwide support for the Atmel microcontroller series. Please contact your local programming vendor for the appropriate software revision.

AC CHARACTERISTICSUnder operating conditions, load capacitance for Port 0, ALE/PROG, and PSEN = 100 pF; load capacitance for all other outputs = 80 pF.

RESISTORS

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A resistor is a component of an electrical circuit that resists the flow of electrical current. A resistor has two terminals across which electricity must pass, and is designed to drop the voltage of the current as it flows from one terminal to the next. A resistor is primarily used to create and maintain a known safe current within an electrical component.Because resistors are often too small to be written on, a standardized color-coding system is used to identify them. The first three colors represent ohm value, and a fourth indicates the tolerance, or how close by percentage the resistor is to its ohm value. This is important for two reasons: the nature of resistor construction is imprecise, and if used above its maximum current, the value of the resistor can alter or the unit itself can burn up. • The internal resistance is usually drawn into a circuit diagram (schematic) as shown in Figure 1.

Figure 1: A schematic diagram showing internal resistance and a battery.

• The squiggly line just before the positive terminal of the battery shows the internal resistance of the circuit.• That symbol, drawn any other place in the circuit, represents an actual resistor placed in thecircuit.• A resistor is a device found in circuits that has a certain amount of resistance.• The most common reason to add resistance to a circuit by using a resistor is that we need to be able to adjust the current flowing through a particular part of the circuit.• If voltage is constant, then we can change the resistor to change the current. I=V R If “V” is constant and we change “R”, “I” will be different.

ACTUAL RESISTORS The colored lines tell you the resistance and error range (tolerance) for a resistor according to the following rules and table of numbers. You do NOT have to memorize this table… it will be given to you if you need it.• To use the table you need to remember the following rules:1. The first line is the first digit2. The second line is the second digit3. The third line is the multiplier4. The last line (if any) is the tolerance• Some resistors may have additional colored bands, but we will ignore them here.• They usually have something to do with measuring things like failure rates or temperature coefficients.

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Figure 2: Three resistors used in electronic devices

Color Number Multiplier ToleranceBlack 0 100Brown 1 101 +_ 1%Red 2 102 +_ 2%�Orange 3 103Yellow 4 104Green 5 105Blue 6 106Violet 7 107Grey 8 108whiteite 9 109Gold 10-1 +_5%�Silver 10-2 +_ 10%�No Color � +_ 20%

CRYSTAL OSCILLATOR

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A miniature 4 MHz quartz crystal enclosed in a hermetically sealed HC-49/US package, used as the resonator in a crystal oscillator.A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits designed around them became known as "crystal oscillators."

MODELINGELECTRICAL MODEL

Electronic symbol for a piezoelectric crystal resonatorA quartz crystal can be modeled as an electrical network with a low impedance (series) and a high impedance (parallel) resonance point spaced closely together.

Mathematically (using the Laplace transform) the impedance of this network can be written as:

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or,

where s is the complex frequency( ), is the series resonant frequency in radians per second and is the parallel resonant frequency in radians per second.Adding additional capacitance across a crystal will cause the parallel resonance to shift downward. This can be used to adjust the frequency at which a crystal oscillates. Crystal manufacturers normally cut and trim their crystals to have a specified resonance frequency with a known 'load' capacitance added to the crystal. For example, a crystal intended for a  6 pF load has its specified parallel resonance frequency when a 6.0 pF capacitor is placed across it. Without this capacitance, the resonance frequency is higher.

Schematic symbol and equivalent circuit for a quartz crystal in an oscillator

PUSH TO ON SWITCH

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A push switch is is a momentary or non-latching switch which causes a temporary change in the state of a electrical circuit only while the switch is physically actuated. An automatic mechanism (ie a spring) returns the switch to its default position immediately afterwards, restoring the initial circuit condition. There are two types:

A push to make switch allows electricity to flow between its two contacts when held in. When the button is released, the circuit is broken.

A push to break switch does the opposite, i.e when the button is not pressed, electricity can flow, but when it is pressed the circuit is broken.

(ON)-OFF (PUSH-TO-MAKE = SPST MOMENTARY)

A push-to-make switch returns to its normally open (off) position when you release the button, this is shown by the brackets around ON. This is the standard doorbell switch.

CIRCUIT SYMBOL

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L293D MOTOR DRIVER IC

L293D is a dual H-bridge motor driver integrated circuit (IC). Motor drivers act as current amplifiers since they take a low-current control signal and provide a higher-current signal. This higher current signal is used to drive the motors.L293D contains two inbuilt H-bridge driver circuits. In its common mode of operation, two DC motors can be driven simultaneously, both in forward and reverse direction. The motor operations of two motors can be controlled by input logic at pins 2 & 7 and 10 & 15. Input logic 00 or 11 will stop the corresponding motor.Logic 01 and 10 will rotate it in clockwise and anticlockwise directions, respectively. Enable pins 1 and 9 (corresponding to the two motors) must be high for motors to start operating. When an enable input is high, the associated driver gets enabled. As a result, the outputs become active and work in phase with their inputs. Similarly, when the enable input is low, that driver is disabled, and their outputs are off and in the high-impedance state.

PIN DIAGRAM:

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PIN DESCRIPTION: 

Pin No  Function  Name1 Enable pin for Motor 1; active high Enable 1,22 Input 1 for Motor 1 Input 13 Output 1 for Motor 1 Output 14 Ground (0V) Ground5 Ground (0V) Ground6 Output 2 for Motor 1 Output 27 Input 2 for Motor 1 Input 28 Supply voltage for Motors; 9-12V

(upto 36V) Vcc 2

9 Enable pin for Motor 2; active high Enable 3,410 Input 1 for Motor 1 Input 311 Output 1 for Motor 1 Output 312 Ground (0V) Ground13 Ground (0V) Ground14 Output 2 for Motor 1 Output 415 Input2 for Motor 1 Input 416 Supply voltage; 5V (up to 36V) Vcc 1

CODE; Motor1 connected with the p1.0 and p1.1; Motor2 connect with the p1.2 and p1.3

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; Motor 3 connect with the p1.4 and p1.5;Seven segment is conneccted with the port 0 org 00hmain:acall display; ------------------ Motor operation from this point -----------setb p1.0clr p1.1 ; Motor 1 forward Acall delayAcall delayAcall delayAcall delayAcall delayAcall delayCLR P1.0CLR P1.1Acall delayAcall delayAcall delaysetb p1.2 clr p1.3 ; Motor 2 forward Acall delayAcall delayAcall delayAcall delayAcall delayAcall delayCLR P1.2CLR P1.3acall delayAcall delayAcall delaysetb p1.4clr p1.5 ; Motor 3 forward Acall delayAcall delayAcall delayAcall delayAcall delayAcall delay

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CLR P1.4CLR P1.5acall delayAcall delayAcall delay; -------------------------- display -----------------------acall display ; ---------------------------- Motor operation ------------------setb p1.5clr p1.4 ;motor 3 reversedAcall delayAcall delayAcall delayAcall delayAcall delayAcall delayclr p1.5clr p1.4acall delayAcall delayAcall delayAcall delayAcall delayAcall delaysetb p1.3clr p1.2 ;Motor 2 reversedAcall delayAcall delayAcall delayAcall delayAcall delayAcall delayclr p1.3clr p1.2acall delayAcall delayAcall delaysetb p1.1clr p1.0 ;Motor 1 reversedAcall delay

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Acall delayAcall delayAcall delayAcall delayAcall delayclr p1.1clr p1.0acall delayAcall delayAcall delayLjmp main; ------------------------------ TIME DELAY ---------------------delay:mov R1,#255here1:mov R2,#255here:Djnz R2,hereDjnz R1,here1ret; ----------------------------- SEVEN SEGMENT DISPLAY -------------------display: MOV P0,#00Hmov p0,#01011111b ;Display9Acall delayAcall delayAcall delayAcall delayAcall delaymov p0,#01111111b ;Display 8Acall delayAcall delayAcall delayAcall delayAcall delaymov p0,#00001110b ; Display 7Acall delayAcall delayAcall delayAcall delayAcall delaymov p0,#01111011b ;Display 6Acall delay

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Acall delayAcall delayAcall delayAcall delaymov p0,#01011011b ;Display 5Acall delayAcall delayAcall delayAcall delayAcall delaymov p0,#01001101b ;Display 4Acall delayAcall delayAcall delayAcall delayAcall delaymov p0,#01011110b ;Display 3Acall delayAcall delayAcall delayAcall delayAcall delaymov p0,#01110110b ;Display 2Acall delayAcall delayAcall delayAcall delayAcall delaymov p0,#00001100b ;Display 1Acall delayAcall delayAcall delayAcall delayAcall delaymov p0,#00111111b ;Display 0Acall delayAcall delayAcall delayAcall delayAcall delay

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MOV P0,#00Hret; --------------------------------------------------------------------------------------end

SEVEN-SEGMENT DISPLAY

A typical 7-segment LED display component, with decimal pointA seven-segment display (SSD), or seven-segment indicator, is a form of electronic display device for displaying decimal numerals that is an alternative to the more complex dot-matrix displays. Seven-segment displays are widely used in digital clocks, electronic meters, and other electronic devices for displaying numerical information. The idea of the seven-segment display is quite old. In 1910, for example, a seven-segment display illuminated by incandescent bulbs was used on a power-plant boiler room signal panel.

CONCEPT AND VISUAL STRUCTURE

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The individual segments of a seven-segment display

16x8-grid showing the 128 states of a seven-segment displayA seven segment display, as its name indicates, is composed of seven elements. Individually on or off, they can be combined to produce simplified representations of the arabic numerals. Often the seven segments are arranged in an oblique (slanted) arrangement, which aids readability. In most applications, the seven segments are of nearly uniform shape and size (usually elongated hexagons, though trapezoids and rectangles can also be used), though in the case of adding machines, the vertical segments are longer and more oddly shaped at the ends in an effort to further enhance readability.

IMPLEMENTATIONS

An incandescent light type early seven-segment display

A mechanical seven-segment display for displaying automotive fuel prices

Seven-segment displays may use a liquid crystal display (LCD), arrays of light-emitting diodes (LEDs), or other light-generating or controlling techniques such as cold cathode gas discharge, vacuum fluorescent,

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incandescent filaments, and others. In a simple LED package, typically all of the cathodes (negative terminals) or all of the anodes (positive terminals) of the segment LEDs are connected and brought out to a common pin; this is referred to as a "common cathode" or "common anode" device. Hence a 7 segment plus decimal point package will only require nine pins (though commercial products typically contain more pins, and/or spaces where pins would go, in order to match industry standard pinouts).Multiple-digit LED displays as used in pocket calculators and similar devices used multiplexed displays to reduce the number of IC pins required to control the display. DISPLAY PATTERN TABLES.HINDU-ARABIC NUMERAL

0 1 2 3 4 5 6 7 8 9

THE LED

Light emitting diodes (LEDs) are semiconductor light sources. The light emitted from LEDs varies from visible to infrared and ultraviolet regions. They operate on low voltage and power. LEDs are one of the most common electronic components and are mostly used as indicators in circuits. They are also used for luminance and optoelectronic applications.Based on semiconductor diode, LEDs emit photons when electrons recombine with holes on forward biasing. The two terminals of LEDs are anode (+) and cathode (-) and can be identified by their size. The longer leg is the positive terminal or anode and shorter one is negative terminal.The forward voltage of LED (1.7V-2.2V) is lower than the voltage supplied (5V) to drive it in a circuit. Using an LED as such would burn it because a high current would destroy its p-n gate. Therefore a current limiting resistor is used in series with LED. Without this resistor, either low input voltage (equal to forward voltage) or PWM (pulse width modulation) is used to drive the LED.

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Besides red, they can also be yellow, green and blue. The letters LED stand for Light Emitting Diode.. The important thing to remember about diodes (including LEDs) is that current can only flow in one direction.

PIN DIAGRAM:

FUNCTIONLEDs emit light when an electric current passes through them.CONNECTING AND SOLDERING

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method). LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow.No special precautions are needed for soldering most LEDs.

TESTING AN LED

Never connect an LED directly to a battery or power supply! It will be destroyed almost instantly because too much current will pass through and

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burn it out. LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less.

FLASHING LEDSFlashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, typically 3Hz (3 flashes per second). They are designed to be connected directly to a supply, usually 9 - 12V, and no series resistor is required. Their flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED, for example our Flashing LED project which uses a 555 astable circuit.

TRANSISTORSTransistors are basic components in all of today's electronics. They are just simple switches that we can use to turn things on and off. Even though they are simple, they are the most important electrical component. For example, transistors are almost the only components used to build a Pentium processor. A single Pentium chip has about 3.5 million transistors. The ones in the Pentium are smaller than the ones we will use but they work the same way. Transistors that we will use in projects look like this:

The transistor has three legs, the Collector (C), Base (B), and Emitter (E). Sometimes they are labeled on the flat side of the transistor. Transistors always have one round side and one flat side. If the round side is facing you, the Collector leg is on the left, the Base leg is in the middle, and the Emitter leg is on the right. TRANSISTOR BC548

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BC548 is general purpose silicon, NPN, bipolar junction transistor. It is used for amplification and switching purposes. The current gain may vary between 110 and 800. The maximum DC current gain is 800.Its equivalent transistors are 2N3904 and 2SC1815. These equivalent transistors however have different lead assignments. The variants of BC548 are 548A, 548B and 548C which vary in range of current gain and other characteristics.The transistor terminals require a fixed DC voltage to operate in the desired region of its characteristic curves. This is known as the biasing. For amplification applications, the transistor is biased such that it is partly on for all input conditions. The input signal at base is amplified and taken at the emitter. BC548 is used in common emitter configuration for amplifiers. The voltage divider is the commonly used biasing mode. For switching applications, transistor is biased so that it remains fully on if there is a signal at its base. In the absence of base signal, it gets completely off.

TRANSISTOR SYMBOL The following symbol is used in circuit drawings (schematics) to represent a transistor.

 Basic Circuit

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The Base (B) is the On/Off switch for the transistor. If a current is flowing to the Base, there will be a path from the Collector (C) to the Emitter (E) where current can flow (The Switch is On.) If there is no current flowing to the Base, then no current can flow from the Collector to the Emitter. (The Switch is Off.) Below is the basic circuit we will use for all of our transistors.

FUNCTIONTransistors amplify current, for example they can be used to amplify the small output current from a logic chip so that it can operate a lamp, relay

or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage. A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on). The amount of current amplification is called the current gain, symbol hFE.

TYPES OF TRANSISTORThere are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are Transistor circuit

symbols

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NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors. The leads are labelled base (B), collector (C) and emitter (E).These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels! A Darlington pair is two transistors connected together to give a very high current gain. In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

CONNECTINGTransistors have three leads which must be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on. If you are lucky the orientation of the transistor will be clear from the PCB or stripboard layout diagram, otherwise you will need to refer to a supplier's catalogue to identify the leads. The drawings on the right show the leads for some of the most common case styles. Please note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which show the view from above.

SOLDERINGTransistors can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body. A standard crocodile clip can be used as a heat sink. Do not confuse this temporary heat sink with the permanent heat sink (described below) which may be required for a power transistor to prevent it overheating during operation.

HEAT SINKSWaste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for power transistors because they pass large currents. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air. .

CHOOSING A TRANSISTORMost projects will specify a particular transistor, but if necessary you can usually substitute an equivalent transistor from the wide range available. The most important properties to look for are the maximum collector current IC

Crocodile clip.

Heat sink

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and the current gain hFE. To make selection easier most suppliers group their transistors in categories determined either by their typical use or maximum power rating. To make a final choice you will need to consult the tables of technical data which are normally provided in catalogues. They contain a great deal of useful information but they can be difficult to understand if you are not familiar with the abbreviations used. The table below shows the most important technical data for some popular transistors, tables in catalogues and reference books will usually show additional information but this is unlikely to be useful unless you are experienced.

The quantities shown in the table are explained below.

Code Case ICMax

VCE.max

hFEmin

Ptotmax.

Category(typicaluse)

Possiblesubstitutes

Structure

BC107 TO18 100mA

45V 110 300mW

Audio,Low Power BC182 BC547 NPN

BC108 TO18 100mA

20V 110 300mW

Generalpurpose,Low Power

BC108C BC183 BC548

NPN

BC108C

TO18 100mA

20V 420 600mW

Audio(low noise),Low Power

NPN

BC109 TO18 200mA

20V 200 300mW

General purpose,Low Power

BC184 BC549 NPN

BC182 TO92C

100mA

50V 100 350mW

General purpose,Low Power

BC107 BC182L NPN

BC182L

TO92A

100mA

50V 100 350mW

Audio,Low Power BC107 BC182 NPN

BC547B

2C 100mA

45V 200 500mW

General purpose,Low Power

BC107B NPN

BC548B

2C 100mA

30V 220 500mW

Audio(low noise),Low Power

BC108B NPN

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BC549B

TO39 100mA

30V 240 625mW

General purpose,Low Power

BC109 NPN

2N3053

TO39 700mA

40V 50 500mW

General purpose,Medium Power

BFY51 NPN

BFY51 TO39 1A 30V 40 800mW

General purpose,Medium Power

BC639 NPN

BC639 TO92A

1A 80V 40 800mW

General purpose,Medium Power

BFY51 NPN

TIP29A

TO220

1A 60V 40 30W Genpurpose,High Power

NPN

TIP31A

TO220

3A 60V 10 40W General purpose,High Power

TIP31 TIP41A NPN

TIP31C

TO220

3A 100V 10 40W General purpose,High Power

TIP31A TIP41A NPN

TIP41A

TO220

6A 60V 15 65W General purpose,High Power

NPN

2N3055

TO3 15A 60V 20 117W General purpose,High Power

NPN

This shows the type of transistor, NPN or PNP. The polarities of the two types are different, so if you are looking for a substitute it must be the same type. IC max. Maximum collector current. VCE max. Maximum voltage across the collector-emitter junction. hFE This is the current gain (strictly the DC current gain). The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! Ptot max. Maximum total power which can be developed in the transistor, note that a heat sink will be required to achieve the maximum rating. This rating is important for transistors operating as amplifiers, the power is roughly IC × VCE. For transistors operating as switches the maximum collector current (IC max.) is more important.

DARLINGTON PAIRThis is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000. Darlington pairs are sold as complete packages containing the two

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transistors. They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor. You can make up your own Darlington pair from two transistors. For example:

For TR1 use BC548B with hFE1 = 220. For TR2 use BC639 with hFE2 = 40.

The overall gain of this pair is hFE1 × hFE2 = 220 × 40 = 8800.The pair's maximum collector current IC(max) is the same as TR2.Absolute Maximum Ratings Ta=25C unless otherwise note Symbol Parameter Value Units

VCBO Collector-Base Voltage : BC546 : BC547/550 : BC548/549

805030

VVV

VCEO Collector-Emitter Voltage : BC546 : BC547/550 : BC548/549

654530

VVV

VEBO Emitter-Base Voltage : BC546/547 : BC548/549/550

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VV

IC Collector Current (DC) 100 mA

PC Collector Power Dissipation 500 mW

TJ Junction Temperature 150 °C

TSTG Storage Temperature -65 ~ 150 °C

MECHANISM OF AEROPLANE

GEAR ARRANGEMENTS

Wheeled undercarriages normally come in two types: conventional or "taildragger" undercarriage, where there are two main wheels towards the front of the aircraft and a single, much smaller, wheel or skid at the rear; or tricycle undercarriage where there are two main wheels (or wheel assemblies) under the wings and a third smaller wheel in the nose. The taildragger arrangement was common during the early propeller era, as it allows more room for propeller clearance. Most modern aircraft have tricycle undercarriages. Taildraggers are considered harder to land and take off

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(because the arrangement is unstable, that is, a small deviation from straight-line travel is naturally amplified by the greater drag of the mainwheel which has moved farther away from the plane's centre of gravity due to the deviation), and usually require special pilot training. Sometimes a small tail wheel or skid is added to aircraft with tricycle undercarriage, in case of tail strikes during take-off. The Concorde, for instance, had a retractable tail "bumper" wheel, as delta winged aircraft need a high angle when taking off. The Boeing 727 also had a retractable tail bumper. Some aircraft with retractable conventional landing gear have a fixed tailwheel, which generates minimal drag (since most of the airflow past the tailwheel has been blanketed by the fuselage) and even improves yaw stability in some cases.

EXTRACTION AND RETRACTION A retractable landing gear is installed whenever a drag improvement is worthy. This means in all aircraft with exception of agricultural and small general aviation airplanes, where the installation of a movable landing gear would increase the costs beyond the requirements of the aircraft category. Landing gear extraction is a primary operation and always its actuation has high redundancy. There are different solutions for the mechanism to obtain suitable landing gear movement. Some are schematically shown in fig.1. Many solutions are based on the four bar linkage (cases A to C), where one bar is represented by the aircraft frame. In other solutions (case D) one bar end can slide along a slot. More complex kinematics include three-dimensional motion and the deflection of the bogie, that for the main landing gear of large airplanes is made of double tandem wheels. Actuators, normally of the hydraulic type, control the extraction/retraction

operation. In general the mechanism should be designed in such a way that gravity and aerodynamic drag favour extraction; if the conditions on gravity and drag are satisfied, the extraction is possible with no power from the hydraulic system; a diagram reporting piston load vs. stroke will be of the type shown in fig.2, with a constant sign: this means that retraction is obtained by applying a force to contrast drag and movable equipment weight, while extraction can initiate by gravity and be completed by drag. The area under the load line represents the necessary work. If this is divided by the area of the rectangle defined by the max load and stroke, one obtains the efficiency of the kinematic mechanism, which commonly is in the range

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Fig1.In both extracted and retracted configurations, the mechanism must be

blocked (downlock and uplock respectively). A kinematic lock at extraction can be obtained by making the four bar linkage to reach its dead centre at full extraction. In any case a downlock based on a hydraulic or electric device is activated to prevent any movement of the strut when the aircraft is taxiing. An uplock is also activated when the landing gear is fully retracted, to prevent non-intentional extraction during flight, which also could be a dangerous operation at high velocity. Uplocks and downlocks are normally provided for the landing gear doors too.

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Fig2.Load Vs Stroke

RETRACTABLE GEAR

Fig3. Main and nosewheel undercarriage of an Airbus A330

To decrease drag in flight some undercarriages retract into the wings and/or fuselage with wheels flush against the surface or concealed behind doors; this is called retractable gear.If the wheels rest protruding and partially exposed to the air stream after being retracted, the system is called semi-retractable.

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Schematic showing hydraulically operated landing gear, with the wheel stowed into the wing root of the aircraft

Most retraction systems are hydraulically operated, though some are electrically operated or even manually operated. This adds weight and complexity to the design. In retractable gear systems, the compartment where the wheels are stowed are called wheel wells, which may also diminish valuable cargo or fuel space.

A Boeing 737-700 with main undercarriage retracted in the wheel wells without landing gear doors

Pilots confirming that their landing gear is down and locked refer to "three green" or "three in the green.", a reference to electrical indicator lights from the nosewheel and the two main gears. Amber lights indicate the gears are in the up-locked position; red lights indicates that the landing gear is in transit (neither down and locked nor fully retracted).

  

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CONCLUSION

Malfunctions or human errors (or a combination of these) related to retractable landing gear have been the cause of numerous accidents and incidents throughout aviation history. In the event of a failure of the aircraft's landing gear extension mechanism a back-up is provided. This may be an alternate hydraulic system, a hand-crank, compressed air (nitrogen), pyrotechnic or free-fall system. A free-fall or gravity drop system uses gravity to deploy the landing gear into the down and locked position. To accomplish this the pilot activates a switch or mechanical handle in the cockpit, which releases the up-lock. Gravity then pulls the landing gear down and deploys it. Once in position the landing gear is mechanically locked and safe to use and land on. Some aircraft have a stiffened fuselage bottom or added firm structures, designed to minimise structural damage in a wheels-up landing.

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 REFERENCEwww.google.com

www.wikipedia.com

www.reynoldelectronics.com

www.engineersgarage.com

www.datasheetcatalogue.com

www.electricityforum.com