A PROJECT REPORT

ON

LASER COMMUNICATION

Bachelor of Technology

In

Electronics & Communication

2011-2012

Project Incharge : Submitted By:Mr. Saurabh Sharma Akshat Mittal (0829231006)Miss. Swati Singh Neha Singh (0829231025) Shrey Agarwal (0829231039) Mudit Rander (0829231404) Bharti Joshi (0829221010)

Department of Electronics & Communication

MEERUT INSTITUTE OF TECHNOLOGY,MEERUT (U.P.)

CERTIFICATE

This is to certify that work which is being presented in the project entitled LASER COMMUNICATION submitted by Mr. Akshat Mittal Miss. Neha Singh, Mr. Shrey Agarwal, Mr. Mudit Rander, Miss. Bharti Joshi student of final year B.Tech. In ELECTRONICS & COMMUNICATION in partial fulfillment of the requirement for award of the degree of B.Tech in ELECTRONICS & COMMUNICATION is a record of students work carried out by them under my guidance and supervision.

As per the candidates declaration this work has not been submitted elsewhere for the award of any other degree.

Dated: 27 April 2012 Signature of Project GuidePlace: Meerut Name: Miss. Swati Singh

Designation: LECTURER

Signature of Project Incharge Signature of H.O.D

ACKNOWLEDGEMENT

“Enthusiasm is the feet of all progresses, with it there is accomplishment and without it there are only slits alibis.”

Acknowledgment is not a ritual but is certainly an important thing for the successful completion of the project. At the time when we were made to know about the project, it was really very tough to proceed further as we were to develop the same on a platform, which was new to us. More so, the coding part seemed so tricky that it seemed to be impossible for us to complete the work within the given duration.

We really feel indebted in acknowledging the organizational support and encouragement received from the management of our college.

The task of developing this system would not have been possible without the constant help of our mentors. We take this opportunity to express our profound sense of gratitude and respect to those who helped us throughout the duration of this project.

We express our gratitude to Mr. Saurabh Sharma (H.O.D,MIT) Miss. Swati Singh(Lecturer,MIT). We would again like to thank all of them for giving their valuable time to us in developing this project.

Dated:27 April 2012 Mr. Akshat MittalPlace: Meerut Miss. Neha Singh Mr. Shrey Agarwal

Mr. Mudit Rander Miss. Bharti Joshi

TABLE OF CONTENTS

• INTRODUCTION

• PLATFORM USED

• AIM OF THE PROJECT

• BLOCK DIAGRAM

• WORKING OF THE PROJECT

• CIRCUIT DIAGRAM

• COMPONENT LIST

• CIRCUIT DESCRIPTION

• PCB LAYOUT

• STEPS FOR MAKING PCB

• PROGRAMMING

• SENSING UNIT DESCRIPTION

• COMPONENTS DESCRIPTION

• APPLICATION

• CONCLUSION

• REFERENCE

Introduction

INTRODUCTION :

Laser communications systems are wireless connections through the

atmosphere. They work similarly to fiber optic links, except the beam is

transmitted through free space. While the transmitter and receiver must

require line-of-sight conditions, they have the benefit of eliminating the need

for broadcast rights and buried cables. Laser communications systems can be

easily deployed since they are inexpensive, small, low power and do not

require any radio interference studies. The carrier used for the transmission

signal is typically generated by a laser diode. Two parallel beams are

needed, one for transmission and one for reception. Due to budget

restrictions, the system implemented in this project is only one way.

This project is microcontroller based Laser communication system

used for the successful transmission of data. In this project using two

microcontroller ,we are transmitting data from one end using laser

transmitter and at other end received by laser receiver which is connected to

the pin of microcontroller ,here the transmitted as well as received data

displayed on lcd .

Platform used

Hardware requirements :

1) Microcontroller AT89C51

2) LDR

3) LM7805 Regulator

4) Power Supply

5) Resistors

6) Capacitors

7) Transistors

8) LIQUID CRYSTAL DISPLAY

9) Transformer

10) Connectors

11) Laser Transmitter

12) Laser Receiver

13) Switch

Software requirements :

1) Assembler of ATMEL microcontroller series

2) PADS for PCB designing

AIM OF THE PROJECT

The Aim of this project is to design a communication system through

Laser,a laser diode at the transmitting end act as a transduser to convert the

digital data into laser form and transmitted ,at the receiving end a laser

transistor convert the laser data into digital form .

Here the motive of using Laser is that While the transmitter and

receiver must require line-of-sight conditions, they have the benefit of

eliminating the need for broadcast rights and buried cables. Laser

communications systems can be easily deployed since they are inexpensive,

small, low power and do not require any radio interference studies.

Block diagram

Laser communication

TRANSMITTER

TRANSMITTER

LASER DIODE

SUPPLYSECTION

MICRO

CONTROLLER 89C51

Communication

DISPLAY SECTIONUSING

LCD

Receiver

MICRO CONTROLLER 89C51

RECEIVER

LDR

DISPLAY SECTIONUSING

LCD SUPPLYSECTION

WORKING OF THE PROJECT

There are two microcontroller one at sending end and the other at

receiving end .Laser transmitter is connected to the pin of the

microcontroller at the sending end and the LASER receiver is

connected to microcontroller at receiver end.whenever a person is

wishing to send the data the microcontroller make the laser transmitter

to send the frequency corresponding to that data and at receiver end

that frequency can change to the original data form which will display

on the lcd connected to the pin of the microcontroller.in this way the

function of transmitting the data through laser receiver and

transmitter have been completed.

CIRCUIT DIAGRAM

Attach the hard copy of the ckt diagram

Component list

Attach hard copy ofComponent list

CIRCUIT DESCRIPTION

POWER SUPPLY SECTION:

Consists of:

1. RLMT Connector --- It is a connector used to connect the step down transformer to the bridge rectifier.

2. Bridge Rectifier --- It is a full wave rectifier used to convert ac into dc , 9-15v ac made by transformer is converted into dc with the help of rectifier.

3. Capacitor: -----It is an electrolytic capacitor of rating 1000M/35V used to remove the ripples. Capacitor is the component used to pass the ac and block the dc.

4. Regulator: ----LM7805 is used to give a fixed 5v regulated supply.

5. Capacitor: -----It is again an electrolytic capacitor 10M/65v used for filtering to give pure dc.

6. Capacitor: ----- It is an ceramic capacitor used to remove the spikes generated when frequency is high(spikes).

So the output of supply section is 5v regulated dc.

MICROCONTROLLER SECTION:

Requires three connections to be successfully done for it’s operation to begin.

1. +5v supply: This +5v supply is required for the controller to get start which is provided from the power supply section. This supply is provided at pin no.31and 40 of the 89c51 controller.

2. Crystal Oscillator: A crystal oscillator of 12 MHz is connected at pin no.19,x1 and pin no.18,x2 to generate the frequency for the controller. The crystal oscillator works on piezoelectric effect.The clock generated is used to determine the processing speed of the controller. Two capacitors are also connected one end with the oscillator while the other end is connected with the ground. As it is recommended in the book to connect two ceramic capacitor of 20 pf—40pf to stabilize the clock generated.

3. Reset section: It consists of an rc network consisting of 10M/35V capacitor and one resistance of 1k. This section is used to reset the controller connected at pin no.9 of AT89c51.

DISPLAY SECTION:

LCD(LIQUID CRYSTAL DISPLAY)

“MICROCONTROLLER BASED LCD DISPLAY” ,this project is an embedded project . Embedded is the combination of software and hardware before designing any embedded project it is the first step to design the proper hardware for the desired application. Here we are interfacing the LCD, LIQUID CRYSTAL DISPLAY with the Microcontroller, we are using ATMEL series 51 controller 89c51 controller. It is a 40 pin IC, the first step while designing hardware is to design the required power supply as the controller operates on +5 v supply so first we have to design the regulated supply with the help of transformer, regulator and filtering capacitor.Next step is the necessary connections of the controller like reset and the crystal oscillator for resetting and speed respectively.Then comes the LCD interfacing ,we are using 16x2 LCD for display, pin no. 7 to 14 are the data lines of the LCD which has to be interfaced with the microcontroller input/output pins. Port p0 has been used for the interfacing of data lines.Since the display becomes very easy when we use microcontroller hence we have made this project and we have tried to show different display using the switch.

RELAY SECTION:

RELAY is an isolator and an electrical switch. The relay used is 12V-5A.To control the operation of relay an NPN transistor BC547 has been used. Whenever high signal comes at the base of NPN transistor it is switched on and whenever low arrives it is switched off. Base of the transistor is connected with the I/O pin of the microcontroller. Base resistance of 1k5 is connected at the base of the transistor. Whenever low is sensed at the pin of microcontroller transistor gets off and the output of the collector becomes high and the relay which is connected at the output of the collector becomes off. The reverse action of it takes place when high is sensed at the pin of microcontroller. This section also consists of pull up & pull down resistance. A 2k2 resistance is used as pull up. In any case when more than 5v comes then pull up resistance sinks the excess voltage & maintains 5v. If pull up is not used then the 12v of

relay can damage the processor when the transistor BC547 is on. A pull down resistor of value 2k2 is also used.

PCB LAYOUT

Attach the hard copy of the

component layout

Attach the hard copy of the pcb layout

STEPS FOR MAKING PCB

• Prepare the layout of the circuit (positive).

• Cut the photofilm (slightly bigger) of the size of the layout.

• Place the layout in the photoprinter machine with the photofilm above it. Make sure that the bromide (dark) side of the film is in contact with the layout.

• Switch on the machine by pressing the push button for 5 sec.

• Dip the film in the solution prepared (developer) by mixing the chemicals A & B in equal quantities in water.

• Now clean the film by placing it in the tray containing water for 1 min.

• After this, dip the film in the fixer solution for 1 min. now the negative of the

Circuit is ready.

• Now wash it under the flowing water.

• Dry the negative in the photocure machine.

• Take the PCB board of the size of the layout and clean it with steel wool to make the surface smooth.

• Now dip the PCB in the liquid photoresist, with the help of dip coat machine.

• Now clip the PCB next to the negative in the photo cure machine, drying for approximate 10-12 minute.

• Now place the negative on the top of the PCB in the UV machine, set the timer for about 2.5 minute and switch on the UV light at the top.

• Take the LPR developer in a container and rigorously move the PCB in it.

• After this, wash it with water very gently.

• Then apply LPR dye on it with the help of a dropper so that it is completely covered by it.

• Now clamp the PCB in the etching machine that contains ferric chloride solution for about 10 minutes.

• After etching, wash the PCB with water, wipe it a dry cloth softly.

• Finally rub the PCB with a steel wool, and the PCB is ready.

Programming

Attach hard copy of

programming

SENSING UNITDESCRIPTION

Laser transmitter

Laser receiver

A laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common and practical type of laser diode is formed from a p-n junction and powered by injected electric current. These devices are sometimes referred to as injection laser diodes to distinguish them from (optically) pumped laser diodes, which are more easily manufactured in the laboratory.

•

Theory of operation

A laser diode, like many other semiconductor devices, is formed by doping a very thin layer on the surface of a crystal wafer. The crystal is doped to produce an n-type region and a p-type region, one above the other, resulting in a p-n junction, or diode.

Laser diodes form a subset of the larger classification of semiconductor p-n junction diodes. As with any semiconductor p-n junction diode, forward electrical bias causes the two species of charge carrier – holes and electrons – to be "injected" from opposite sides of the p-n junction into the depletion region, situated at its heart. Holes are injected from the p-doped, and electrons from the n-doped, semiconductor. (A depletion region, devoid of any charge carriers, forms automatically and unavoidably as a result of the difference in chemical potential between n- and p-type semiconductors wherever they are in physical contact.)

As charge injection is a distinguishing feature of diode lasers as compared to all other lasers, diode lasers are traditionally and more formally called "injection lasers." (This terminology differentiates diode lasers, e.g., from flashlamp-pumped solid state lasers, such as the ruby laser. Interestingly, whereas the term "solid-state" was extremely apt in differentiating 1950s-era semiconductor electronics from earlier generations of vacuum electronics, it would not have been adequate to convey unambiguously the unique characteristics defining 1960s-era semiconductor lasers.) When an electron and a hole are present in the same region, they may recombine or "annihilate" with the result being spontaneous emission — i.e., the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron and hole states involved. (In a conventional semiconductor junction diode, the energy released from the recombination of electrons and holes is carried away as phonons, i.e., lattice vibrations, rather than as photons.) Spontaneous emission gives the laser diode below lasing threshold similar properties to an LED. Spontaneous

emission is necessary to initiate laser oscillation, but it is one among several sources of inefficiency once the laser is oscillating.

The difference between the photon-emitting semiconductor laser and conventional phonon-emitting (non-light-emitting) semiconductor junction diodes lies in the use of a different type of semiconductor, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called "direct bandgap" semiconductors. The properties of silicon and germanium, which are single-element semiconductors, have bandgaps that do not align in the way needed to allow photon emission and are not considered "direct." Other materials, the so-called compound semiconductors, have virtually identical crystalline structures as silicon or germanium but use alternating arrangements of two different atomic species in a checkerboard-like pattern to break the symmetry. The transition between the materials in the alternating pattern creates the critical "direct bandgap" property. Gallium arsenide, indium phosphide, gallium antimonide, and gallium nitride are all examples of compound semiconductor materials that can be used to create junction diodes that emit light.

Diagram (not to scale) of a simple laser diode, such as shown above.

In the absence of stimulated emission (e.g., lasing) conditions, electrons and holes may coexist in proximity to one another, without recombining, for a certain time, termed the "upper-state lifetime" or "recombination time" (about a nanosecond for typical diode laser materials), before they recombine. Then a nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. This generates another photon of the same frequency, travelling in the same direction, with the same polarization and phase as the first photon. This means that

stimulated emission causes gain in an optical wave (of the correct wavelength) in the injection region, and the gain increases as the number of electrons and holes injected across the junction increases. The spontaneous and stimulated emission processes are vastly more efficient in direct bandgap semiconductors than in indirect bandgap semiconductors; therefore silicon is not a common material for laser diodes.

As in other lasers, the gain region is surrounded with an optical cavity to form a laser. In the simplest form of laser diode, an optical waveguide is made on that crystal surface, such that the light is confined to a relatively narrow line. The two ends of the crystal are cleaved to form perfectly smooth, parallel edges, forming a Fabry–Pérot resonator. Photons emitted into a mode of the waveguide will travel along the waveguide and be reflected several times from each end face before they are emitted. As a light wave passes through the cavity, it is amplified by stimulated emission, but light is also lost due to absorption and by incomplete reflection from the end facets. Finally, if there is more amplification than loss, the diode begins to "lase".

Some important properties of laser diodes are determined by the geometry of the optical cavity. Generally, in the vertical direction, the light is contained in a very thin layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the lateral direction, if the waveguide is wide compared to the wavelength of light, then the waveguide can support multiple lateral optical modes, and the laser is known as "multi-mode". These laterally multi-mode lasers are adequate in cases where one needs a very large amount of power, but not a small diffraction-limited beam; for example in printing, activating chemicals, or pumping other types of lasers.

In applications where a small focused beam is needed, the waveguide must be made narrow, on the order of the optical wavelength. This way, only a single lateral mode is supported and one ends up with a diffraction-limited beam. Such single spatial mode devices are used for optical storage, laser pointers, and fiber optics. Note that these lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously.

The wavelength emitted is a function of the band-gap of the semiconductor and the modes of the optical cavity. In general, the maximum gain will occur for photons with energy slightly above the band-gap energy, and the modes nearest the gain peak will lase most strongly. If the diode is driven strongly

enough, additional side modes may also lase. Some laser diodes, such as most visible lasers, operate at a single wavelength, but that wavelength is unstable and changes due to fluctuations in current or temperature.

Due to diffraction, the beam diverges (expands) rapidly after leaving the chip, typically at 30 degrees vertically by 10 degrees laterally. A lens must be used in order to form a collimated beam like that produced by a laser pointer. If a circular beam is required, cylindrical lenses and other optics are used. For single spatial mode lasers, using symmetrical lenses, the collimated beam ends up being elliptical in shape, due to the difference in the vertical and lateral divergences. This is easily observable with a red laser pointer.

The simple diode described above has been heavily modified in recent years to accommodate modern technology, resulting in a variety of types of laser diodes, as described below.

MICROCONTROLLER AT89C51

Features

• Compatible with MCS-51™ Products• 8K Bytes of In-System Re programmable Flash Memory• Endurance: 1,000 Write/Erase Cycles• Fully Static Operation: 0 Hz to 24 MHz• Three-level Program Memory Lock• 256 x 8-bit Internal RAM• 32 Programmable I/O Lines•Three 16-bit Timer/Counters• Eight Interrupt Sources• Programmable Serial Channel• Low-power Idle and Power-down Modes

DESCRIPTION

The AT89C52 is a low-power, high-performance CMOS 8-bit microcomputer 8Kbytes 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 80C51 and 80C52 instruction set and pin out.

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 AT89C52 is a powerful microcomputer that provides a highly flexible and cost-effective solution to many embedded control application.

The AT89C52 provides the following standard features: 8K bytes of Flash, 256 bytes of RAM, 32 I/O lines, three 16-bit timer/counters, a six-vector two-level interrupt architecture, a full-duplex serial port, on-chip oscillator, and clock

circuitry. In addition, the AT89C52 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode tops the CPU while allowing the RAM; timer/counters, serial port, and

interrupt system to continue functioning.

The Power-down mode saves the RAM contents but Freezes the oscillator, disabling all other chip functions until the next hardware reset

.

Pin Description

VCCSupply voltage.

GND

Ground.

Port 0

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 can 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 pull-ups are required during program verification.

Port 1

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 pull-ups 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.

In addition, P1.0 and P1.1 can be configured to be the timer/counter 2 external count input (P1.0/T2) and the timer/counter 2 trigger input (P1.1/T2EX), respectively, as shown in the following table.

Port 1 also receives the low-order address bytes during

Port 2

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 pull-ups 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, Port 2 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

Port 3 is an 8-bit bi-directional I/O port with internal pull-ups. 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 pull-ups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low will source current (IIL) because of the pull-ups. Port 3 also serves the functions of various special features of the AT89C51, as shown in the following table. Port 3 also receives some control signals for Flash programming.

RST

Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device.

ALE/PROG

Address Latch Enable is an 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 ALE pulse 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 micro controller is in external execution mode.

PSEN

Program Store Enable is the read strobe to external program memory. When the AT89C52 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.

EA/VPP

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 when 12-volt programming is selected.

XTAL1

Input to the inverting oscillator amplifier and input to the internal clock operating circuit.

XTAL2

Output from the inverting oscillator amplifier .

Special Function Registers

A map of the on-chip memory area called the Special Function Register (SFR) space is shown in Table 1.

Note that not all of the addresses are occupied, and unoccupied addresses may not be implemented on the chip. Read accesses to these addresses will in general return random data, and write accesses will have an indeterminate effect. User software

should not write 1s to these unlisted locations, since they may be used in future prod new features. In that case, the reset or inactive values of the new bits will always be

0.

Timer 2 Registers

Control and status bits are contained in registers T2CON (shown in Table 2) and T2MOD (shown in Table 4) for Timer 2. The register pair (RCAP2H, RCAP2L) are the Capture/Reload registers for Timer 2 in 16-bit capture mode or 16-bit auto-reload mode.

Interrupt Registers

The individual interrupt enable bits are in the IE register. Two priorities can be set for each of the six interrupt sources in the IP register. Instructions that use indirect addressing access the upper 128 bytes of RAM. For example, the following indirect addressing instruction, where R0 contains 0A0H, accesses the data byte at address 0A0H, rather than P2 (whose address is 0A0H).MOV @R0, #dataNote that stack operations are examples of indirect addressing, so the upper 128 bytes of data RAM are avail available as stack space.

Timer 0 and 1

Timer 0 and Timer 1 in the AT89C52 operate the same way as Timer 0 and Timer 1 in the T89C51.

Timer 2

Timer 2 is a 16-bit Timer/Counter that can operate as either a timer or an event counter. The type of operation is selected by bit C/T2 in the SFR T2CON (shown in Table 2).Timer 2 has three operating modes: capture, auto-reload (up or down counting), and baud rate generator. The modes are selected by bits in T2CON, as shown in Table 3.Timer 2 consists of two 8-bit registers, TH2 and TL2. In the Timer function, the TL2 register is incremented every machine cycle. Since a machine cycle consists of 12

oscillator periods, the count rate is 1/12 of the oscillator input pin, T2. In this function, the external input is sampled during S5P2 of every machine cycle. When the samples show a high in one cycle and a low in the next cycle, the count is incremented. The new count value appears in the register during S3P1 of the cycle following the one in whichthe transition was detected. Since two machine cycles (24 oscillator periods) are required to recognize a 1-to-0 transition, the maximum count rate is 1/24 of the oscillator frequency. To ensure that a given level is sampled at least once before it changes, the level should be held for at least one full machine cycle.

Capture Mode

In the capture mode, two options are selected by bit EXEN2 in T2CON. If EXEN2 = 0, Timer 2 is a 16-bit timer or counter which upon overflow sets bit TF2 in T2CON.This bit can then be used to generate an interrupt. If EXEN2 = 1, Timer 2 performs the same operation, but a 1-to-0 transition at external input T2EX also causes the current value in TH2 and TL2 to be captured into CAP2H and RCAP2L, respectively. In addition, the transition at T2EX causes bit EXF2 in T2CON to be set. The EXF2 bit, like TF2, can generate an interrupt. The capture mode is illustrated in Figure 1.

Auto-reload (Up or Down Counter)

Timer 2 can be programmed to count up or down when configured in its 16-bit auto-reload mode. This feature is invoked by theDCEN (Down Counter Enable) bit located in the SFR T2MOD (see Table 4). Upon reset, the DCEN bit is set to 0 so that timer 2 will default to count up. When DCEN is set, Timer 2 can count up or down, depending on the value of the T2EX pin.

Figure 2 shows Timer 2 automatically counting up when DCEN = 0. In this mode, two options are selected by bitEXEN2 in T2CON. If EXEN2 = 0, Timer 2 counts up to 0FFFFH and then sets the TF2 bit upon overflow. The overflow also causes the timer registers to be reloaded with the 16-bit value in RCAP2H and RCAP2L. The values in Timer in Capture ModeRCAP2H and RCAP2L are preset by software. If EXEN2 = 1, a 16-bit reload can be triggered either by an overflow or by a 1-to-0 transition at external input T2EX. This transition also sets the EXF2 bit. Both the TF2 and EXF2 bits can generate an interrupt if enabled. Setting the DCEN bit enables Timer 2 to count up or down, as shown in Figure 3. In this mode, the T2EX pin controls

the direction of the count. A logic 1 at T2EX makes Timer 2 count up. The timer will overflow at 0FFFFH and set the TF2 bit. This overflow also causes the 16-bit value in RCAP2H and RCAP2L to be reloaded into the timer registers, TH2 and TL2, respectively. A Logic 0 at T2EX makes Timer 2 count down. The timer underflows when TH2 and TL2 equal the values stored in RCAP2H and RCAP2L. The underflow sets the TF2 bit and causes 0FFFFH to be reloaded into the timer Registers. The EXF2 bit toggles whenever Timer 2 overflows or underflows and can be used as a 17th bit of resolution. In this operating mode, EXF2 does not flag an interrupt.

Baud Rate Generator

Timer 2 is selected as the baud rate generator by setting TCLK and/or RCLK in T2CON (Table 2). Note that the baud rates for transmit and receive can be different if Timer 2 is used for the receiver or transmitter and Timer 1 is used for the other function. Setting RCLK and/or TCLK puts Timer 2 into its baud rate generator mode, as shown in Figure4. The baud rate generator mode is similar to the auto-reload mode, in that a rollover in TH2 causes the Timer 2 registers to be reloaded with the 16-bit value in registers RCAP2H and RCAP2L, which are preset by software.The baud rates in Modes 1 and 3 are determined by Timer2’s overflow rate according to the following equation.

The Timer can be configured for either timer or counter operation. In most applications, it is configured for timer operation (CP/T2 = 0). The timer operation is different for Timer 2 when it is used as a baud rate generator. Normally, as a timer, it increments every machine cycle (at 1/12 the oscillator frequency). As a baud rate generator, however, it increments every state time (at 1/2 the oscillator frequency). The baud rate formula is given below.

where (RCAP2H, RCAP2L) is the content of RCAP2H and RCAP2L taken as a 16-bit unsigned integer. Timer 2 as a baud rate generator is shown in Figure 4. This figure is valid only if RCLK or TCLK = 1 in T2CON. Note that a rollover in TH2 does not set TF2 and will not generate an interrupt. Note too, that if EXEN2 is set, a 1-to-0 transition in T2EX will set EXF2 but will not cause a reload from (RCAP2H, RCAP2L) to (TH2, TL2). Thus when Timer 2 is in use as a baud rate generator, T2EX can be used as an extra external interrupt.

Note that when Timer 2 is running (TR2 = 1) as a timer in the baud rate generator mode, TH2 or TL2 should not be read from or written to. Under these conditions, the Timer is incremented every state time, and the results of a read or write may not be accurate. The RCAP2 registers may be read but should not be written to, because a write might overlap a reload and cause write and/or reload errors. The timer should be turned off (clear TR2) before accessing the Timer 2 or RCAP2 registers.

Programmable Clock Out

A 50% duty cycle clock can be programmed to come out on P1.0, as shown in Figure 5. This pin, besides being a regular I/O pin, has two alternate functions. It can be programmed to input the external clock for Timer/Counter 2 or to output a 50% duty cycle clock ranging from 61 Hz to 4 MHz at a 16 MHz operating frequency. To configure the Timer/Counter 2 as a clock generator, bit C/T2 (T2CON.1) must be cleared and bit T2OE (T2MOD.1) must be set. Bit TR2 (T2CON.2) starts and stops the timer. The clock-out frequency depends on the oscillator frequency and the reload value of Timer 2 capture registers (RCAP2H, RCAP2L), as shown in the following equation.

In the clock-out mode, Timer 2 roll-overs will not generate an interrupt. This behavior is similar to when Timer 2 is used as a baud-rate generator. It is possible to use Timer 2 as a baud-rate generator and a clock generator simultaneously. Note, however, that the baud-rate and clock-outFrequencies cannot be determined independently from one another since they both use RCAP2H and RCAP2L.

UART

The UART in the AT89C52 operates the same way as the UART in the AT89C51.

Interrupts

The AT89C52 has a total of six interrupt vectors: two external interrupts (INT0 and INT1), three timer interrupts (Timers 0, 1, and 2), and the serial port interrupt. These interrupts are all shown in Figure 6.Each of these interrupt sources can be individually enabled or disabled by setting or clearing a bit in Special Function Register IE. IE also contains a global disable bit, EA, which disables all interrupts at once.Note that Table shows that bit position IE.6 is unimplemented. In the AT89C51, bit position IE.5 is also unimplemented. User software should not write 1s to these bit positions, since they may be used in future AT89 products. Timer 2 interrupt is generated by the logical OR of bits TF2 and EXF2 in register T2CON. Neither of these flags is cleared by hardware when the service routine is vectored to. In fact, the service routine may have to determine whether it was TF2 or EXF2 that generated the interrupt, and that bit will have to be cleared in software. The Timer 0 and Timer 1 flags, TF0 and TF1, are set at S5P2 of the cycle in which the timers overflow. The values are then polled by the circuitry in the next cycle. However, the Timer 2 flag, TF2, is set at S2P2 and is polled in the same cycle in which the timer overflows.

Oscillator Characteristics

XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier that can be configured for use as an on-chip oscillator, as shown in Figure 7. Either

a quartz crystal or ceramic resonator may be used. To drive the device from an external clock source, XTAL2 should be left

Un connected while XTAL1 is driven, as shown in Figure 8.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 Mode

In 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.

Note that when idle mode 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

hardware inhibits 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 mode is terminated by a reset, the instruction following the one that invokes idle mode should not write to a port pin or to external memory.

Power-down Mode

In 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 Registers retain their values until the power-down mode is terminated. The only exit from power-down is a hardware reset. Reset redefines the SFR s but does not change the on-chip RAM. The reset should not be cultivated before VCC is restored to its normal operating level and must be held active long enough to allow the oscillator to restart and stabilize.

AC Characteristics

Under operating conditions, load capacitance for Port 0, ALE/PROG, and PSEN = 100 pF; load capacitance for all otheroutputs = 80 pF.

Note: 1. AC Inputs during testing are driven at VCC - 0.5Vfor a logic 1 and 0.45V for a logic 0. Timing measurementsare made at VIH min. for a logic 1 and VIL max.for a logic 0.

Float Waveforms(1)

Note: 1. For timing purposes, a port pin is no longer floatingwhen a 100 mV change from load voltage occurs. Aport pin begins to float when a 100 mV change fromthe loaded VOH/VOL level occurs.

Component description

Transformers

A transformer is a device that transfers electrical energy from one circuit to another by magnetic coupling without requiring relative motion between its parts. It usually comprises two or more coupled windings, and, in most cases, a core to concentrate magnetic flux. A transformer operates from the application of an alternating voltage to one winding, which creates a time-varying magnetic flux in the core. This varying flux induces a voltage in the other windings. Varying the relative number of turns between primary and secondary windings determines the ratio of the input and output voltages, thus transforming the voltage by stepping it up or down between circuits.

2.8.1Basic principle The principles of the transformer are illustrated by consideration of a hypothetical ideal transformer consisting of two windings of zero resistance around a core of negligible reluctance. A voltage applied to the primary winding causes a current, which develops a magnetomotive force (MMF) in the core. The current required to create the MMF is termed the magnetising current; in the ideal transformer it is considered to be negligible. The MMF drives flux around the magnetic circuit of the core.

Figure 26: The ideal transformer as a circuit element

An electromotive force (EMF) is induced across each winding, an effect known as mutual inductance. The windings in the ideal transformer have no resistance and so the EMFs are equal in magnitude to the measured terminal voltages. In accordance with Faraday's law of induction, they are proportional to the rate of change of flux:

and

Equation 7: EMF induced in primary and secondary windings

where:

and are the induced EMFs across primary and secondary windings,

and are the numbers of turns in the primary and secondary windings,

and are the time derivatives of the flux linking the primary and secondary windings.

In the ideal transformer, all flux produced by the primary winding also links the secondary, and so , from which the well-known transformer equation follows:

Equation 8: Transformer Equation

The ratio of primary to secondary voltage is therefore the same as the ratio of the number of turns; alternatively, that the volts-per-turn is the same in both windings. The conditions that determine Transformer working in STEP UP or STEP DOWN mode are:

Ns > Np

Equation 9: Conditon for STEP UP

Ns < Np

Equation 10: Conditon for STEP DOWN

Rectifier

A bridge rectifier is an arrangement of four diodes connected in a bridge circuit as shown below, that provides the same polarity of output voltage for any polarity of the input voltage. When used in its most common application, for conversion of alternating current (AC) input into direct current (DC) output, it is known as a bridge rectifier. The bridge rectifier provides full wave rectification from a two wire AC input (saving the cost of a center tapped transformer) but has two diode drops rather than one reducing efficiency over a center tap based design for the same output voltage.

Figure 9: Schematic of a bridge rectifier

The essential feature of this arrangement is that for both polarities of the voltage at the bridge input, the polarity of the output is constant.

2.2.1 Basic Operation

When the input connected at the left corner of the diamond is positive with respect to the one connected at the right hand corner, current flows to the right along the upper colored path to the output, and returns to the input supply via the lower one.

When the right hand corner is positive relative to the left hand corner, current flows along the upper colored path and returns to the supply via the lower colored path.

Figure 10: AC, half-wave and full wave rectified signals

In each case, the upper right output remains positive with respect to the lower right one. Since this is true whether the input is AC or DC, this circuit not only produces DC power when supplied with AC power: it also can provide what is sometimes called "reverse polarity protection". That is, it permits normal functioning when batteries are installed backwards or DC input-power supply wiring "has its wires crossed" (and protects the circuitry it powers against damage that might occur without this circuit in place).Prior to availability of integrated electronics, such a bridge rectifier was always constructed from discrete components. Since about 1950, a single four-terminal component containing the four diodes connected in the bridge configuration became a standard commercial component and is now available with various voltage and current ratings.

2.2.2 Output Smoothing

For many applications, especially with single phase AC where the full-wave bridge serves to convert an AC input into a DC output, the addition of a capacitor may be

important because the bridge alone supplies an output voltage of fixed polarity but pulsating magnitude.

Figure 11: Bridge Rectifier with smoothen output

The function of this capacitor, known as a 'smoothing capacitor' (see also filter capacitor) is to lessen the variation in (or 'smooth') the raw output voltage waveform from the bridge. One explanation of 'smoothing' is that the capacitor provides a low impedance path to the AC component of the output, reducing the AC voltage across, and AC current through, the resistive load. In less technical terms, any drop in the output voltage and current of the bridge tends to be cancelled by loss of charge in the capacitor. This charge flows out as additional current through the load. Thus the change of load current and voltage is reduced relative to what would occur without the capacitor. Increases of voltage correspondingly store excess charge in the capacitor, thus moderating the change in output voltage / current.

The capacitor and the load resistance have a typical time constant = τ RC where C and R are the capacitance and load resistance respectively. As long as the load resistor is large enough so that this time constant is much longer than the time of one ripple cycle, the above configuration will produce a well smoothed DC voltage across the load resistance. In some designs, a series resistor at the load side of the

capacitor is added. The smoothing can then be improved by adding additional stages of capacitor–resistor pairs, often done only for sub-supplies to critical high-gain circuits that tend to be sensitive to supply voltage noise.Voltage Regulators

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages. With the exception of shunt regulators, all voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element. This forms a negative feedback servo control loop. If the output voltage is too low, the regulation element is commanded to produce a higher voltage. For some regulators if the output voltage is too high, the regulation element is commanded to produce a lower voltage; however, many just stop sourcing current and depend on the current draw of whatever it is driving to pull the voltage back down. In this way, the output voltage is held roughly constant. The control loop must be carefully designed to produce the desired tradeoff between stability and speed of response.

2.4.1 LM317 (3-Terminal Adjustable Regulator)

DescriptionThe LM317 is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.2 V to 37 V. It is exceptionally easy to use and requires only two external resistors to set the output voltage. Furthermore, both line and load regulation are better than standard fixedregulators. The LM317 is packaged in the KC (TO-220AB) and KTE packages, which are easy to handle and use. In addition to having higher performance than fixed regulators, this device includes on-chip current limiting, thermal overload protection, and safe-operating-area protection. All overload

protection remains fully functional, even if the ADJUST terminal is disconnected.

Figure 16: TOP IC view of LM 317

The LM317 is versatile in its applications, including uses in programmable output regulation and local on-card regulation. Or, by connecting a fixed resistor between the ADJUST and OUTPUT terminals, the LM317 can function as a precision current regulator. An optional output capacitor can be added to improve transient response. The ADJUST terminal can be bypassed to achieve very high ripple-rejection ratios, which are difficult to achieve with standard three-terminal regulators. The LM317 is characterized for operation over thevirtual junction temperature range of 0°C to 125°C.

Figure 17: Adjustable Voltage Regulator

2.4.2 LM7805 (3-Terminal Fixed Voltage Regulator)

The MC78XX/LM78XX/MC78XXA series of three terminal positive regulators are available in theTO-220/D-PAK package and with several fixed output voltages, making them useful in a wide range ofapplications. 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.

Figure 18: Internal block Diagram

Figure 19 : Fixed Output Regulator

Features

• Output Current up to 1A

• Output Voltages of 5, 6, 8, 9, 10, 12, 15, 18, 24V

• Thermal Overload Protection

• Short Circuit Protection

• Output Transistor Safe Operating Area Protection

Liquid crystal display(LCD)

A liquid crystal display (commonly abbreviated LCD) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector. It is prized by engineers because it uses very small amounts of electric power, and is therefore suitable for use in battery-powered electronic devices. Each pixel of an LCD consists of a layer of perpendicular molecules aligned between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. With no liquid crystal between the polarizing filters, light passing through one filter would be blocked by the electrodes. The surfaces of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using a cloth (the direction of the liquid crystal alignment is defined by the direction of rubbing). Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces. In a twisted nematic device (the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular, and so the molecules arrange themselves in a helical structure, or twist. Because the liquid crystal material is birefringent, light passing through one polarizing filter is rotated by the liquid crystal helix as it passes through the liquid crystal layer, allowing it to pass through the second polarized filter. Half of the light is absorbed by the first polarizing filter, but otherwise the entire assembly is transparent. When a voltage is applied across the electrodes, a torque acts to align the liquid crystal molecules parallel to the electric field, distorting the helical structure (this is resisted by elastic forces since the molecules are constrained at the surfaces). This reduces the rotation of the polarization of the incident light, and the device appears gray. If the applied voltage is large enough, the liquid crystal molecules are completely untwisted and the polarization of the incident light is not rotated at all as it passes through the liquid crystal layer. This light will then be polarized perpendicular to the second filter, and thus be completely blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts, correspondingly illuminating the pixel. With a twisted nematic liquid crystal device it is usual to operate the device between crossed polarizers, such that it appears bright with no applied voltage. With this setup, the dark voltage-on state is uniform. The device can be operated

between parallel polarizers, in which case the bright and dark states are reversed.

Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided by applying either an alternating current, or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field). When a large number of pixels is required in a display, it is not feasible to drive each directly since then each pixel would require independent electrodes. Instead, the display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically in rows), with each group getting a voltage sink. The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.

Figure 20:LCD Pictorial View

2.5.1 LCD Standards

Frequently, an 8051 program must interact with the outside world using input and output devices that communicate directly with a human being. One of the most common devices attached to an 8051 is an LCD display. Some of the most common LCDs connected to the 8051 are 16x2 and 20x2 displays. This means 16 characters per line by 2 lines and 20 characters per line by 2 lines, respectively. Fortunately, a very popular standard exists which allows us to communicate with the vast majority of LCDs regardless of their manufacturer. The standard is referred to as HD44780U, which refers to the controller chip which receives data from an external source (in this case, the 8051) and communicates directly with the LCD.

2.5.2 44780 Standard

The 44780 standard requires 3 control lines as well as either 4 or 8 I/O lines for the data bus. The user may select whether the LCD is to operate with a 4-bit data bus or an 8-bit data bus. If a 4-bit data bus is used the LCD will require a total of 7 data lines (3 control lines plus the 4 lines for the data bus). If an 8-bit data bus is used the LCD will require a total of 11 data lines (3 control lines plus the 8 lines for the data bus).The three control lines are referred to as EN, RS, and RW.

The EN line is called "Enable." This control line is used to tell the LCD that you are sending it data. To send data to the LCD, your program should make sure this line is low (0) and then set the other two control lines and/or put data on the data bus. When the other lines are completely ready, bring EN high (1) and wait for the minimum amount of time required by the LCD datasheet (this varies from LCD to LCD), and end by bringing it low (0) again.The RS line is the "Register Select" line. When RS is low (0), the data is to be treated as a command or special instruction (such as clear screen, position cursor, etc.). When RS is high (1), the data being sent is text data which sould be displayed on the screen. For example, to display the letter "T" on the screen you would set RS high.

The RW line is the "Read/Write" control line. When RW is low (0), the information on the data bus is being written to the LCD. When RW is high (1), the program is effectively querying (or reading) the LCD. Only one instruction ("Get LCD status") is a read command. All others are write commands--so RW will almost always be low.Finally, the data bus consists of 4 or 8 lines (depending on the mode of operation selected by the user). In the case of an 8-bit data bus, the lines are referred to as DB0, DB1, DB2, DB3, DB4, DB5, DB6, and DB7.

2.5.3 An Example Hardware Configuration

As we've mentioned, the LCD requires either 8 or 11 I/O lines to communicate with. For the sake of this tutorial, we are going to use an 8-bit data bus--so we'll be using 11 of the 8051's I/O pins to interface with the LCD.A sample psuedo-schematic of how the LCD will be connected to the 8051.

Figure 21: Schematic Of LCD interfacing with microcontroller

As you can see, we've established a 1-to-1 relation between a pin on the 8051 and a line on the 44780 LCD. Thus as we write our assembly program to access the LCD, we are going to equate constants to the 8051 ports so that we can refer to the lines by their 44780 name as opposed to P0.1, P0.2, etc. Let's go ahead and write our initial equates:DB0 EQU P1.0DB1 EQU P1.1DB2 EQU P1.2DB3 EQU P1.3DB4 EQU P1.4DB5 EQU P1.5DB6 EQU P1.6

DB7 EQU P1.7EN EQU P3.7RS EQU P3.6RW EQU P3.5DATA EQU P1Having established the above equates, we may now refer to our I/O lines by their 44780 name. For example, to set the RW line high (1), we can execute the following insutrction:SETB RW

2.5.4 Handling the EN Control Line

As we mentioned above, the EN line is used to tell the LCD that you are ready for it to execute an instruction that you've prepared on the data bus and on the other control lines. Note that the EN line must be raised/lowered before/after each instruction sent to the LCD regardless of whether that instruction is read or write, text or instruction. In short, you must always manipulate EN when communicating with the LCD. EN is the LCD's way of knowing that you are talking to it. If you don't raise/lower EN, the LCD doesn't know you're talking to it on the other lines.Thus, before we interact in any way with the LCD we will always bring the EN line low with the following instruction:CLR ENAnd once we've finished setting up our instruction with the other control lines and data bus lines, we'll always bring this line high: SETB EN

The line must be left high for the amount of time required by the LCD as specified in its datasheet. This is normally on the order of about 250 nanoseconds, but check the datasheet. In the case of a typical 8051 running at 12 MHz, an instruction requires 1.08 microseconds to execute so the EN line can be brought low the very next instruction. However, faster microcontrollers (such as the DS89C420 which executes an instruction in 90 nanoseconds given an 11.0592 Mhz crystal) will require a number of NOPs to create a delay while EN is held high. The number of NOPs that must be inserted depends on the microcontroller you are using and the crystal you have selected. The instruction is executed by the LCD at the moment the EN line is brought low with a final CLR EN instruction.

Programming Tip: The LCD interprets and executes our command at the instant the EN line is brought low. If you never bring EN low, your instruction will never be executed. Additionally, when you bring EN low and the LCD executes your instruction, it requires a certain amount of time to execute the command. The time it requires to execute an instruction depends on the instruction and the speed of the crystal which is attached to the 44780's oscillator input.

2.5.5 Checking the Busy Status of the LCD

As previously mentioned, it takes a certain amount of time for each instruction to be executed by the LCD. The delay varies depending on the frequency of the crystal attached to the oscillator input of the 44780 as well as the instruction which is being executed. While it is possible to write code that waits for a specific amount of time to allow the LCD to execute instructions, this method of "waiting" is not very flexible. If the crystal frequency is changed, the software will need to be modified. Additionally, if the LCD itself is changed for another LCD which, although 44780 compatible, requires more time to perform its operations, the program will not work until it is properly modified. A more robust method of programming is to use the "Get LCD Status" command to determine whether the LCD is still busy executing the last instruction received.The "Get LCD Status" command will return to us two tidbits of information; the information that is useful to us right now is found in DB7. In summary, when we issue the "Get LCD Status" command the LCD will immediately raise DB7 if it's still busy executing a command or lower DB7 to indicate that the LCD is no longer occupied. Thus our program can query the LCD until DB7 goes low, indicating the LCD is no longer busy. At that point we are free to continue and send the next command.Since we will use this code every time we send an instruction to the LCD, it is useful to make it a subroutine.

Let's write the code:WAIT_LCD:CLR EN ; Start LCD commandCLR RS ; It's a commandSETB RW ; It's a read commandMOV DATA,#0FFh ; Set all pins to FF initially

SETB EN ; Clock out command to LCDMOV A,DATA ; Read the return valueJB ACC.7,WAIT_LCD ; If bit 7 high, LCD still busyCLR EN ; Finish the commandCLR RW ; Turn off RW for future commandsRET

Thus, our standard practice will be to send an instruction to the LCD and then call our WAIT_LCD routine to wait until the instruction is completely executed by the LCD. This will assure that our program gives the LCD the time it needs to execute instructions and also makes our program compatible with any LCD, regardless of how fast or slow it is.

Programming Tip: The above routine does the job of waiting for the LCD, but were it to be used in a real application a very definite improvement would need to be made: as written, if the LCD never becomes "not busy" the program will effectively "hang," waiting for DB7 to go low. If this never happens, the program will freeze. Of course, this should never happen and won't happen when the hardware is working properly. But in a real application it would be wise to put some kind of time limit on the delay--for example, a maximum of 256 attempts to wait for the busy signal to go low. This would guarantee that even if the LCD hardware fails, the program would not lock up.

2.5.6 Initializing the LCD

Before you may really use the LCD, you must initialize and configure it. This is accomplished by sending a number of initialization instructions to the LCD. The first instruction we send must tell the LCD whether we'll be communicating with it with an 8-bit or 4-bit data bus. We also select a 5x8 dot character font. These two options are selected by sending the command 38h to the LCD as a command. As you will recall from the last section, we mentioned that the RS line must be low if we are sending a command to the LCD. Thus, to send this 38h command to the LCD we must execute the following 8051 instructions:CLR RSMOV DATA, #38hSETB ENCLR ENLCALL WAIT_LCD

Programming Tip: The LCD command 38h is really the sum of a number of option bits. The instruction itself is the instruction 20h ("Function set"). However, to this we add the values 10h to indicate an 8-bit data bus plus 08h to indicate that the display is a two-line display.

We've now sent the first byte of the initialization sequence. The second byte of the initialization sequence is the instruction 0Eh. Thus we must repeat the initialization code from above, but now with the instruction.

Thus the the next code segment is:CLR RSMOV DATA, #0EhSETB ENCLR ENLCALL WAIT_LCD

Programming Tip: The command 0Eh is really the instruction 08h plus 04h to turn the LCD on. To that an additional 02h is added in order to turn the cursor on.

The last byte we need to send is used to configure additional operational parameters of the LCD. We must send the value 06h.CLR RSMOV DATA, #06hSETB ENCLR ENLCALL WAIT_LCD

Programming Tip: The command 06h is really the instruction 04h plus 02h to configure the LCD such that every time we send it a character, the cursor position automatically moves to the right.

So, in all, our initialization code is as follows:INIT_LCD:CLR RSMOV DATA, #38hSETB EN

CLR ENLCALL WAIT_LCDCLR RSMOV DATA, #0EhSETB ENCLR ENLCALL WAIT_LCDCLR RSMOV DATA, #06hSETB ENCLR ENLCALL WAIT_LCDRETHaving executed this code the LCD will be fully initialized and ready for us to send display data to it.

2.5.7 Clearing the Display

When the LCD is first initialized, the screen should automatically be cleared by the 44780 controller. However, it's always a good idea to do things yourself so that you can be completely sure that the display is the way you want it. Thus, it's not a bad idea to clear the screen as the very first opreation after the LCD has been initialiezd.

An LCD command exists to accomplish this function. Not suprisingly, it is the command 01h. Since clearing the screen is a function we very likely will wish to call more than once, it's a good idea to make it a subroutine:CLEAR_LCD:CLR RSMOV DATA, #01hSETB ENCLR ENLCALL WAIT_LCDRETHow that we've written a "Clear Screen" routine, we may clear the LCD at any time by simply executing an LCALL CLEAR_LCD.

Programming Tip: Executing the "Clear Screen" instruction on the LCD also positions the cursor in the upper left-hand corner as we would expect.

2.5.8 Writing Text to the LCD

Now we get to the real meat of what we're trying to do: All this effort is really so we can display text on the LCD. Really, we're pretty much done. Once again, writing text to the LCD is something we'll almost certainly want to do over and over--so let's make it a subroutine.WRITE_TEXT:SETB RSMOV DATA, ASETB ENCLR ENLCALL WAIT_LCDRET

The WRITE_TEXT routine that we just wrote will send the character in the accumulator to the LCD which will, in turn, display it. Thus to display text on the LCD all we need to do is load the accumulator with the byte to display and make a call to this routine. Pretty easy, huh?

2.5.9 A Program: "HELLO WORLD"

Now that we have all the component subroutines written, writing the classic "Hello World" program--which displays the text "Hello World" on the LCD is a relatively trivial matter. Consider:LCALL INIT_LCDLCALL CLEAR_LCD

MOV A,#'H'LCALL WRITE_TEXTMOV A,#'E'LCALL WRITE_TEXTMOV A,#'L'LCALL WRITE_TEXTMOV A,#'L'LCALL WRITE_TEXTMOV A,#'O'LCALL WRITE_TEXTMOV A,#' 'LCALL WRITE_TEXTMOV A,#'W'LCALL WRITE_TEXTMOV A,#'O'LCALL WRITE_TEXTMOV A,#'R'LCALL WRITE_TEXTMOV A,#'L'LCALL WRITE_TEXTMOV A,#'D'LCALL WRITE_TEXT

The above "Hello World" program should, when executed, initialize the LCD, clear the LCD screen, and display "Hello World" in the upper left-hand corner of the display.

2.5.10 Cursor Positioning

The above "Hello World" program is simplistic in the sense that it prints its text in the upper left-hand corner of the screen. However, what if we wanted to display the word "Hello" in the upper left-hand corner but wanted to display the word "World" on the second line at the tenth character? This sounds simple--and actually, it is simple. However, it requires a little more understanding of the design of the LCD.The 44780 contains a certain amount of memory which is assigned to the display. All the text we write to the 44780 is stored in this memory, and the 44780 subsequently reads this memory to display the text on the LCD itself. This memory can be represented with the following "memory map":

Figure 22: Memory Mapping in LCD

In the above memory map, the area shaded in blue is the visible display. As you can see, it measures 16 characters per line by 2 lines. The numbers in each box is the memory address that corresponds to that screen position. Thus, the first character in the upper left-hand corner is at address 00h. The following character position (character #2 on the first line) is address 01h, etc. This continues until we reach the 16th character of the first line which is at address 0Fh. However, the first character of line 2, as shown in the memory map, is at address 40h. This means if we write a character to the last position of the first line and then write a second character, the second character will not appear on the second line. That is because the second character will effectively be written to address 10h--but the second line begins at address 40h. Thus we need to send a command to the LCD that tells it to position the cursor on the second line. The "Set Cursor Position" instruction is 80h. To this we must add the address of the location where we wish to position the cursor. In our example, we said we wanted to display "World" on the second line on the tenth character position. Referring again to the memory map, we see that the tenth character position of the second line is address 4Ah. Thus, before writing the word "World" to the LCD, we

must send a "Set Cursor Position" instruction--the value of this command will be 80h (the instruction code to position the cursor) plus the address 4Ah. 80h + 4Ah = CAh. Thus sending the command CAh to the LCD will position the cursor on the second line at the tenth character position:

CLR RSMOV DATA,#0CAhSETB ENCLR ENLCALL WAIT_LCD

The above code will position the cursor on line 2, character 10. To display "Hello" in the upper left-hand corner with the word "World" on the second line at character position 10 just requires us to insert the above code into our existing "Hello World" program.

This results in the following:

LCALL INIT_LCDLCALL CLEAR_LCDMOV A,#'H'LCALL WRITE_TEXTMOV A,#'E'LCALL WRITE_TEXTMOV A,#'L'LCALL WRITE_TEXTMOV A,#'L'LCALL WRITE_TEXTMOV A,#'O'LCALL WRITE_TEXTCLR RSMOV DATA,#0CAhSETB ENCLR ENLCALL WAIT_LCDMOV A,#'W'LCALL WRITE_TEXTMOV A,#'O'

LCALL WRITE_TEXTMOV A,#'R'LCALL WRITE_TEXTMOV A,#'L'LCALL WRITE_TEXTMOV A,#'D'LCALL WRITE_TEXT

RELAYS

Circuit symbol for a relay

Relays Photographs © Rapid Electronics

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field, which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches. Relays allow one circuit to switch a second circuit that can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical. The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification. Relays are usually SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches. Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay. The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils

produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil. The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT.

The relay's switch connections are usually labeled COM, NC and NO: • COM = Common, always connect to this, it is the moving part of the

switch. • NC = Normally Closed, COM is connected to this when the relay coil

is off. • NO = Normally Open, COM is connected to this when the relay coil is

on. • Connect to COM and NO if you want the switched circuit to be on

when the relay coil is on. • Connect to COM and NC if you want the switched circuit to be on

when the relay coil is off.

Choosing a relay

You need to consider several features when choosing a relay:

1. Physical size and pin arrangement If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue.

2. Coil voltage The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.

3. Coil resistance The circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current:

Relay coil current = Supply voltage Coil resistance

For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current.

4. Switch ratings (voltage and current) The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".

5. Switch contact arrangement (SPDT, DPDT etc) Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). COMPARISON BETWEEN TRANSISTORS & RELAYS

Advantages of relays:

• Relays can switch AC and DC, transistors can only switch DC.

• Relays can switch high voltages, transistors cannot. • Relays are a better choice for switching large currents (> 5A). • Relays can switch many contacts at once. •

Disadvantages of relays:

• Relays are bulkier than transistors for switching small currents. • Relays cannot switch rapidly (except reed relays), transistors can

switch many times per second. • Relays use more power due to the current flowing through their coil.

Relays require more current than many chips can provide, so a low power transistor may be needed to switch the current for the relay's coil.

Crystal Oscillator

It is often required to produce a signal whose frequency or pulse rate is very stable and exactly known. This is important in any application where anything to do with time or exact measurement iscrucial. It is relatively simple to make an oscillator that produces some sort of a signal, but another matter to produce one of relatively precise frequency and stability. AM radio stations must have a carrier frequency accurate within 10Hz of its assigned frequency, which may be from 530 to 1710 kHz. SSB radio systems used in the HF range (2-30 MHz) must be within 50 Hz of channel frequency for acceptable voice quality, and within 10 Hz for best results. Some digital modes used in weak signal communication may require frequency stability of less than 1 Hz within a period of several minutes. The carrier frequency must be known to fractions of a hertz in some cases. An ordinary quartz watch must have an oscillator accurate to better than a few parts per million. One part per million will result in an error of slightly less than one half second a day, which would be about 3 minutes a year. This might not sound like much, but an error of 10 parts per million would result in an error of about a half an hour per year. A clock such as this would need resetting about once a month, and more often if you are the punctual type. A programmed VCR with a clock this far off could miss the recording of part of a TV show. Narrow band SSB communications at VHF and UHF frequencies still need 50 Hz frequency accuracy. At 440 MHz, this is slightly more than 0.1 part per million.Ordinary L-C oscillators using conventional inductors and capacitors can achieve typically 0.01 to 0.1 percent frequency stability, about 100 to 1000 Hz at 1 MHz. This is OK for AM and FM broadcast receiver applications

and in other low-end analog receivers not requiring high tuning accuracy. By careful design and component selection, and with rugged mechanical construction, .01 to 0.001%, or even better (.0005%) stability can be achieved. The better figures will undoubtedly employ temperature compensation components and regulated power supplies, together with environmental control (good ventilation and ambient temperature regulation) and “battleship” mechanical construction. This has been done in some communications receivers used by the military and commercial HF communication receivers built in the 1950-1965 era, before the widespread use of digital frequency synthesis. But these receivers were extremely expensive, large, and heavy. Many modern consumer grade AM, FM, and shortwave receivers employing crystal controlled digital frequency synthesis will do as well or better from a frequency stability standpoint.An oscillator is basically an amplifier and a frequency selective feedback network (Fig 1). When, at a particular frequency, the loop gain is unity or more, and the total phaseshift at this frequency is zero, or some multiple of 360 degrees, the condition for oscillation is satisfied, and the circuit will produce a periodic waveform of this frequency. This is usually a sine wave, or square wave, but triangles, impulses, or other waveforms can be produced. In fact, several different waveforms often are simultaneously produced by the same circuit, at different points. It is also possible to have several frequencies produced as well, although this is generally undesirable.

CAPACITOR

A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When a potential difference (voltage) exists across the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the conductors. The effect is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called plates.

An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce an equivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage.

Capacitors are widely used in electronic circuits to block the flow of direct current while allowing alternating current to pass, to filter out interference, to smooth the output of power supplies, and for many other purposes. They are used in resonant circuits in radio frequency equipment to select particular frequencies from a signal with many frequencies.

Theory of operationMain article: Capacitance

Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric (orange) reduces the field and increases the capacitance.

A simple demonstration of a parallel-plate capacitor

A capacitor consists of two conductors separated by a non-conductive region.The non-conductive substance is called the dielectric medium,

although this may also mean a vacuum or a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from an external electric field. The conductors thus contain equal and opposite charges on their facing surfaces, and the dielectric contains an electric field. The capacitor is a reasonably general model for electric fields within electric circuits.

An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of charge ±Q on each conductor to the voltage V between them

Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to vary. In this case, capacitance is defined in terms of incremental changes:

In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage of one volt across the device.

Energy storage

Work must be done by an external influence to move charge between the conductors in a capacitor. When the external influence is removed, the charge separation persists and energy is stored in the electric field. If charge is later allowed to return to its equilibrium position, the energy is released. The work done in establishing the electric field, and hence the amount of energy stored, is given by:

Current-voltage relation

The current i(t) through a component in an electric circuit is defined as the rate of change of the charge q(t) that has passed through it. Physical charges cannot pass through the dielectric layer of a capacitor, but rather build up in equal and opposite quantities on the electrodes: as each electron accumulates

on the negative plate, one leaves the positive plate. Thus the accumulated charge on the electrodes is equal to the integral of the current, as well as being proportional to the voltage (as discussed above). As with any antiderivative, a constant of integration is added to represent the initial voltage v (t0). This is the integral form of the capacitor equation,

.

Taking the derivative of this, and multiplying by C, yields the derivative form,[12]

.

The dual of the capacitor is the inductor, which stores energy in the magnetic field rather than the electric field. Its current-voltage relation is obtained by exchanging current and voltage in the capacitor equations and replacing C with the inductance L.

DC circuits

A simple resistor-capacitor circuit demonstrates charging of a capacitor.

A series circuit containing only a resistor, a capacitor, a switch and a constant DC source of voltage V0 is known as a charging circuit. If the capacitor is initially uncharged while the switch is open, and the switch is closed at t = 0, it follows from Kirchhoff's voltage law that

Taking the derivative and multiplying by C, gives a first-order differential equation,

At t = 0, the voltage across the capacitor is zero and the voltage across the resistor is V0. The initial current is then i (0) =V0 /R. With this assumption, the differential equation yields

where τ0 = RC is the time constant of the system.

As the capacitor reaches equilibrium with the source voltage, the voltage across the resistor and the current through the entire circuit decay exponentially. The case of discharging a charged capacitor likewise demonstrates exponential decay, but with the initial capacitor voltage replacing V0 and the final voltage being zero.

RESISTORResistors are used to limit the value of current in a circuit. Resistors offer opposition to the flow of current. They are expressed in ohms for which the symbol is ‘Ω’. Resistors are broadly classified as

(1)Fixed Resistors(2)Variable Resistors

Fixed Resistors :

The most common of low wattage, fixed type resistors is the molded-carbon composition resistor. The resistive material is of carbon clay composition. The leads are made of tinned copper. Resistors of this type are readily available in value ranging from few ohms to about 20MΩ, having a tolerance range of 5 to 20%. They are quite inexpensive. The relative size of all fixed resistors changes with the wattage rating.

Another variety of carbon composition resistors is the metalized type. It is made by deposition a homogeneous film of pure carbon over a glass, ceramic or other insulating core. This type of film-resistor is sometimes called the precision type, since it can be obtained with an accuracy of ± 1%.

Lead Tinned Copper Material

Colour Coding Molded Carbon Clay Composition

Fixed Resistor

A Wire Wound Resistor :

It uses a length of resistance wire, such as nichrome. This wire is wounded on to a round hollow porcelain core. The ends of the winding are attached to these metal pieces inserted in the core. Tinned copper wire leads are attached to these metal pieces. This assembly is coated with an enamel coating powdered glass. This coating is very smooth and gives mechanical protection to winding. Commonly available wire wound resistors have resistance values ranging from 1Ω to 100KΩ, and wattage rating up to about 200W.

Coding Of Resistor :

Some resistors are large enough in size to have their resistance printed on the body. However there are some resistors that are too small in size to have numbers printed on them. Therefore, a system of colour coding is used to indicate their values. For fixed, moulded composition resistor four colour bands are printed on one end of the outer casing. The colour bands are always read left to right from the end that has the bands closest to it. The first and second band represents the first and second significant digits, of the

resistance value. The third band is for the number of zeros that follow the second digit. In case the third band is gold or silver, it represents a multiplying factor of 0.1to 0.01. The fourth band represents the manufacture’s tolerance.

RESISTOR COLOUR CHART

For example, if a resistor has a colour band sequence: yellow, violet, orange and gold

Then its range will be—

Yellow=4, violet=7, orange=10³, gold=±5% =47KΏ ±5% =2.35KΏ

Most resistors have 4 bands:• The first band gives the first digit.• The second band gives the second digit.• The third band indicates the number of zeros.• The fourth band is used to show the tolerance (precision) of the

resistor.

5 green

0 black

1 brown2 red3 orange

4 yellow

6 blue

7 purple

8 silver

9 white

0 black

1 brown2 red3 orange

4 yellow

6 blue

7 purple

8 silver

9 white

5 green

5 green

0 black

1 brown2 red3 orange

4 yellow

6 blue

7 purple

8 silver

9 white

5 green

0 black

1 brown2 red3 orange

4 yellow

6 blue

7 purple

8 silver

9 white

This resistor has red (2), violet (7), yellow (4 zeros) and gold bands. So its value is 270000 = 270 k .

The standard colour code cannot show values of less than 10 . To show these small values two special colours are used for the third band: gold, which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal.

For example:

red, violet, gold bands represent 27 × 0.1 = 2.7 blue, green, silver bands represent 56 × 0.01 = 0.56

The fourth band of the colour code shows the tolerance of a resistor. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390 resistor with a tolerance of ±10% will have a value within 10% of 390 , between 390 - 39 = 351 and 390 + 39 = 429 (39 is 10% of 390).

A special colour code is used for the fourth band tolerance:silver ±10%, gold ±5%, red ±2%, brown ±1%. If no fourth band is shown the tolerance is ±20%.

VARIABLE RESISTOR:In electronic circuits, sometimes it becomes necessary to adjust the values of currents and voltages. For n example it is often desired to change the volume of sound, the brightness of a television picture etc. Such adjustments can be done by using variable resistors.

Although the variable resistors are usually called rheostats in other applications, the smaller variable resistors commonly used in electronic circuits are called potentiometers.

Resistor shorthand:Resistor values are often written on circuit diagrams using a code system which avoids using a decimal point because it is easy to miss the small dot. Instead the letters R, K and M are used in place of the decimal point. To read the code: replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter R means multiply by 1.

For example:560R means 560 2K7 means 2.7 k = 2700 39K means 39 k 1M0 means 1.0 M = 1000 k

Power Ratings of Resistors

Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this.Power ratings of resistors are rarely quoted in parts lists because for most circuits the standard power ratings of 0.25W or 0.5W are suitable. For the rare cases where a higher power is required it should be clearly specified in the parts list, these will be circuits using low value resistors (less than about 300 ) or high voltages (more than 15V).The power, P, developed in a resistor is given by:

P = I² × Ror P = V² / R

where: P = power developed in the resistor in watts (W) I = current through the resistor in amps (A) R = resistance of the resistor in ohms ( ) V = voltage across the resistor in volts (V)

Examples:

• A 470 resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W. In this case a standard 0.25W resistor would be suitable.

• A 27 resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W. A high power resistor with a rating of 5W would be suitable.

TRANSISTORS

A transistor is an active device. It consists of two PN junctions formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types.

High power resistors(5W top, 25W bottom)Photographs © Rapid Electronics

There are two types of transistor:1. n-p-n transistor 2. p-n-p transistor

An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of p-type. However a p-n-p type semiconductor is formed by two p-sections separated by a thin section of n-type.

Transistor has two pn junctions one junction is forward biased and other is reversed biased. The forward junction has a low resistance path whereas a reverse biased junction has a high resistance path.

The weak signal is introduced in the low resistance circuit and output is taken from the high resistance circuit. Therefore a transistor transfers a signal from a low resistance to high resistance.

Transistor has three sections of doped semiconductors. The section on one side is emitter and section on the opposite side is collector. The middle section is base.

Emitter : The section on one side that supplies charge carriers is called emitter. The emitter is always forward biased w.r.t. base.

Collector : The section on the other side that collects the charge is called collector. The collector is always reversed biased.

Base : The middle section which forms two pn-junctions between the emitter and collector is called base.

A transistor raises the strength of a weak signal and thus acts as an amplifier. The weak signal is applied between emitter-base junction and output is taken across the load Rc connected in the collector circuit. The collector current flowing through a high load resistance Rc produces a large voltage across it. Thus a weak signal applied in the input appears in the amplified form in the collector circuit.

Heat sink

Waste 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.

CONNECTORS

Connectors are basically used for interface between two. Here we use connectors for having interface between PCB and 8051 Microprocessor Kit.

There are two types of connectors they are male and female. The one, which is with pins inside, is female and other is male.These connectors are having bus wires with them for connection.For high frequency operation the average circumference of a coaxial cable must be limited to about one wavelength, in order to reduce multimodal propagation and eliminate erratic reflection coefficients, power losses, and signal distortion. The standardization of coaxial connectors during World War II was mandatory for microwave operation to maintain a low reflection coefficient or a low voltage standing wave ratio.Seven types of microwave coaxial connectors are as follows:1.APC-3.52.APC-73.BNC4.SMA5.SMC6.TNC7.Type N

LED (LIGHT EMITTING DIODE)

A junction diode, such as LED, can emit light or exhibit electro luminescence. Electro luminescence is obtained by injecting minority carriers into the region of a pn junction where radiative transition takes place. In radiative transition, there is a transition of electron from the conduction band to the valence band, which is made possibly by emission of a photon. Thus, emitted light comes from the hole electron recombination. What is required is that electrons should make a transition from higher energy level to lower energy level releasing photon of wavelength corresponding to the energy difference associated with this transition. In LED the supply of high-energy electron is provided by forward biasing the diode, thus injecting electrons into the n-region and holes into p-region.

The pn junction of LED is made from heavily doped material. On forward bias condition, majority carriers from both sides of the junction cross the potential barrier and enter the opposite side where they are then minority carrier and cause local minority carrier population to be larger than

normal. This is termed as minority injection. These excess minority carrier diffuse away from the junction and recombine with majority carriers.

In LED, every injected electron takes part in a radiative recombination and hence gives rise to an emitted photon. Under reverse bias no carrier injection takes place and consequently no photon is emitted. For direct transition from conduction band to valence band the emission wavelength.

In practice, every electron does not take part in radiative recombination and hence, the efficiency of the device may be described in terms of the quantum efficiency which is defined as the rate of emission of photons divided by the rate of supply of electrons. The number of radiative recombination, that take place, is usually proportional to the carrier injection rate and hence to the total current flowing.

LED Materials:

One of the first materials used for LED is GaAs. This is a direct band gap material, i.e., it exhibits very high probability of direct transition of electron from conduction band to valence band. GaAs has E= 1.44 eV. This works in the infrared region.

GaP and GaAsP are higher band gap materials. Gallium phosphide is an indirect band gap semiconductor and has poor efficiency because band to band transitions are not normally observed. Gallium Arsenide Phosphide is a tertiary alloy. This material has a special feature in that it changes from being direct band gap material.Blue LEDs are of recent origin. The wide band gap materials such as GaN are one of the most promising LEDs for blue and green emission. Infrared LEDs are suitable for optical coupler applications.

ADVANTAGES OF LEDs:

1. Low operating voltage, current, and power consumption makes Leds compatible with electronic drive circuits. This also makes easier interfacing as compared to filament incandescent and electric discharge lamps.

2. The rugged, sealed packages developed for LEDs exhibit high resistance to mechanical shock and vibration and allow LEDs to be used in severe environmental conditions where other light sources would fail.

3. LED fabrication from solid-state materials ensures a longer operating lifetime, thereby improving overall reliability and lowering maintenance costs of the equipment in which they are installed.

4. The range of available LED colours-from red to orange, yellow, and green-provides the designer with added versatility.

5. LEDs have low inherent noise levels and also high immunity to externally generated noise.

6. Circuit response of LEDs is fast and stable, without surge currents or the prior “warm-up”, period required by filament light sources.

7. LEDs exhibit linearity of radiant power output with forward current over a wide range.

LEDs have certain limitations such as:

1. Temperature dependence of radiant output power and wave length.

2. Sensitivity to damages by over voltage or over current.3. Theoretical overall efficiency is not achieved except in special cooled or pulsed conditions.Buzzer

It is an electronic signaling device which produces buzzing sound. It is

commonly used in automobiles, phone alarm systems and household

appliances. Buzzers work in the same manner as an alarm works. They are

generally equipped with sensors or switches connected to a control unit and

the control unit illuminates a light on the appropriate button or control panel,

and sound a warning in the form of a continuous or intermittent buzzing or

beeping sound.

The word "buzzer" comes from the rasping noise that buzzers made when

they were electromechanical devices, operated from stepped-down AC line

voltage at 50 or 60 cycles.

Typical uses of buzzers and beepers include alarms, timers and confirmation

of user input such as a mouse click or keystroke.

2.9.1Types of Buzzers

The different types of buzzers are electric buzzers, electronic buzzers,

mechanical buzzers, electromechanical, magnetic buzzers, piezoelectric

buzzers and piezo buzzers.

(i) Electric buzzers –

A basic model of electric buzzer usually consists of simple circuit

components such as resistors, a capacitor and 555 timer IC or an integrated

circuit with a range of timer and multi-vibrator functions. It works through

small bits of electricity vibrating together which causes sound.

(ii) Electronic buzzers –

An electronic buzzer comprises an acoustic vibrator comprised of a circular

metal plate having its entire periphery rigidly secured to a support, and a

piezoelectric element adhered to one face of the metal plate. A driving

circuit applies electric driving signals to the vibrator to vibrationally drive it

at a 1/N multiple of its natural frequency, where N is an integer, so that the

vibrator emits an audible buzzing sound. The metal plate is preferably

mounted to undergo vibration in a natural vibration mode having only one

nodal circle. The drive circuit includes an inductor connected in a closed

loop with the vibrator, which functions as a capacitor, and the circuit applies

signals at a selectively variable frequency to the closed loop to accordingly

vary the inductance of the inductor to thereby vary the period of oscillation

of the acoustic vibrator and the resultant frequency of the buzzing sound.

(iii) Mechanical Buzzer-

A joy buzzer is an example of a purely mechanical buzzer.

(iv) Piezo Buzzers/ Piezoelectric Buzzers –

A piezo buzzer is made from two conductors that are separated by Piezo

crystals. When a voltage is applied to these crystals, they push on one

conductor and pull on the other. The result of this push and pull is a sound

wave. These buzzers can be used for many things, like signaling when a

period of time is up or making a sound when a particular button has been

pushed. The process can also be reversed to use as a guitar pickup. When a

sound wave is passed, they create an electric signal that is passed on to an

audio amplifier.

Piezo buzzers are small electronic devices that emit sounds when driven by

low voltages and currents. They are also called piezoelectric buzzers. They

usually have two electrodes and a diaphragm. The diaphragm is made from a

metal plate and piezoelectric material such as a ceramic plate.

(v) Magnetic Buzzers –

Magnetic buzzers are magnetic audible signal devices with built-in

oscillating circuits. The construction combines an oscillation circuit unit

with a detection coil, a drive coil and a magnetic transducer. Transistors,

resistors, diodes and other small devices act as circuit devices for driving

sound generators. With the application of voltage, current flows to the drive

coil on primary side and to the detection coil on the secondary side. The

amplification circuit, including the transistor and the feedback circuit, causes

vibration. The oscillation current excites the coil and the unit generates an

AC magnetic field corresponding to an oscillation frequency. This AC

magnetic field magnetizes the yoke comprising the magnetic circuit. The

oscillation from the intermittent magnetization prompts the vibration

diaphragm to vibrate up and down, generating buzzer sounds through the

resonator.

In this project, a magnetic buzzer has been used.

2.9.2 Circuit of buzzer –

2.9.3 Role of buzzer in this project

Buzzer in this system gives the beep when car moves inside cutting the

infrared light. Basically it generates the signal to indicate that car has entered

in the parking space.

2.10 Pressure Sensor/Switch

A pressure sensor or switch measures pressure. Pressure is usually expressed

in terms of force per unit area. A pressure sensor usually acts as a

transducer; it generates a signal as a function of the pressure imposed.

Pressure sensors can be classified in term of pressure ranges they measure, temperature

ranges of operation, and most importantly the type of pressure they measure. In terms of

pressure type, pressure sensors can be divided into five categories:

1) Absolute pressure sensor

This sensor measures the pressure relative to perfect vaccum pressure.

2) Gauge pressure sensor

This sensor is used in different applications because it can be calibrated to measure the

pressure relative to a given atmospheric pressure at a given location.

3)Vaccum pressure sensor

This sensor is used to measure pressure less than the atmospheric pressure at a given

location.

4) Differential pressure sensor

This sensor measures the difference between two or more pressures introduced as inputs to

the sensing unit.

5) Sealed pressure sensor

This sensor is the same as the gauge pressure sensor except that it is previously calibrated

by manufacturers to measure pressure relative to sea level pressure.

Fig: Operation of pressure switch

1.10.1 Pressure Sensing Technology

There are two basic categories of analog pressure sensors:

(i) Force collector types - These types of electronic pressure sensors generally use a force

collector (such a diaphragm, piston, bourdon tube, or bellows) to measure strain (or

deflection) due to applied force (pressure) over an area.

(ii) Other types - These types of electronic pressure sensors use other properties (such as

density) to infer pressure of a gas, or liquid.

Here we’ll discuss only about Force collector type of pressure sensors. Force collecting

pressure sensors are of following types:

Piezoresistive Strain Gauge-

Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to

applied pressure. Generally, the strain gauges are connected to form a wheat stone bridge

circuit to maximize the output of the sensor. This is the most commonly employed sensing

technology for general purpose pressure measurement.

Capacitive - Uses a diaphragm and pressure cavity to create a variable capacitor to detect

strain due to applied pressure. Common technologies use metal, ceramic, and silicon

diaphragms. Generally, these technologies are most applied to low pressures (Absolute,

Differential and Gauge)

Electromagnetic - Measures the displacement of a diaphragm by means of changes in

inductance (reluctance), LVDT, Hall Effect, or by eddy current principal.

Piezoelectric - Uses the piezoelectric effect in certain materials such as quartz to measure

the strain upon the sensing mechanism due to pressure. This technology is commonly

employed for the measurement of highly dynamic pressures.

Optical - Uses the physical change of an optical fiber to detect strain due to applied

pressure.

Potentiometric - Uses the motion of a wiper along a resistive mechanism to detect the

strain caused by applied pressure .

DIODE

ACTIVE COMPONENT-

Active component are those component for not any other component

are used its operation. I used in this project only function diode, these

component description are described as bellow.

SEMICONDUCTOR DIODE-

A PN junctions is known as a semiconductor or crystal diode.A

crystal diode has two terminal when it is connected in a circuit one thing is

decide is weather a diode is forward or reversed biased. There is a easy rule

to ascertain it. If the external CKT is trying to push the conventional current

in the direction of error, the diode is forward biased. One the other hand if

the conventional current is trying is trying to flow opposite the error head,

the diode is reversed biased putting in simple words.

1. If arrowhead of diode symbol is positive W.R.T Bar of the

symbol, the diode is forward biased.

2.The arrowhead of diode symbol is negative W.R.T bar , the diode is

the reverse bias.

When we used crystal diode it is often necessary to know

that which end is arrowhead and which end is bar. So

following method are available.

1.Some manufactures actually point the symbol on the

body of the diode e. g By127 by 11 4 crystal diode

manufacture by b e b.

2. Sometimes red and blue marks are on the body of the crystal

diode. Red mark do not arrow where’s blue mark indicates bar e .g

oa80 crystal diode.

ZENER DIODE-

It has been already discussed that when the reverse bias on a crystal diode is increased a critical voltage, called break down voltage. The break down or zener voltage depends upon the amount of doping. If the diode is heavily doped depletion layer will be thin and consequently the break down of he junction will occur at a lower reverse voltage. On the other hand, a lightly doped diode has a higher break down voltage, it is called zener diode

A properly doped crystal diode, which has a sharped break down voltage, is

known as a zenor diode.

APPLICATION:

Laser data transfer is used to transfer the data successfully via two microcontrollers while displaying the data using the LCD screen so anywhere where data is to be transfer we can use this technology

But due to its range limitation we cannot send the data for a long range , only when receiver and transmitter are close to each otherthen only data can be transmitted.

CONCLUSION: This project gave us the understanding of microcontroller and made

us realize the power of microcontroller and help us understand how

to use the laser.

The developing of this project has been a learning experience for all team

members and would prove as a milestone in their academic career. The

achievement of this project are

i. The project has achieved its set target well in “Time” and

“Budget”.

ii. Based on cutting edge technology called Embedded development

which is niche in the market today and its future is much bright.

iii. The product developed is ready for implementation and can bring

financial benefits too by sale in the market.

So, we conclude that the LASER communicationis still far away from the

perfect, but we believe we have laid the groundwork to enable it to improve

out of sight.

References

1. Mazedi, The 8051 Microcontroller and Embedded Systems, Prentice

Hall, 1ST Edition

2. Kenneth J. Ayala, The 8051 Microcontroller, Penram International

Publishing,1996, 2nd Edition

3. Some Websites :

www.alldatasheets.com

www.datasheetcatalog.com

www.electronicscircuits.com

www.scielectronics.com

www.parallax.com