Wireless electronic notice board using zigbee

8 August 2016

Notice boards play a vital role mostly in educational institutions. The events, occasions or any news, which has to be passed to the students, will be written on the notice boards present in every floor in the colleges or schools. The present system is like, a person will be told the news and he has to update this news on all the notice boards present in the college or school. This will be seen mostly during the examination seasons.

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The time table or the schedule of the exams has to be given to the students. This will be done by writing the details on the notice boards. But this process consumes a lot time to update the news on all the notice boards and there may be chances that the person responsible may commit some mistakes or he may be absent sometimes. So, this may create disturbances and the entire schedule may be disturbed. To avoid all these, Wireless Notice Board have been designed which completely eliminates the manual work. Zigbee is PAN technology based on the IEEE 802. 15. 4 standard.

Unlike Bluetooth or wireless USB devices, zigbee devices have the ability to form a mesh network between nodes. Meshing is a type of daisy chaining from one device to another. This technique allows the short range of an individual node to be expanded and multiplied, covering a much larger area. The project is built around the AT89S52 microcontroller from Atmel. This microcontroller provides all the functionality of the display and wireless control. It also takes care of creating different display effects for given text. Alphanumerical keypad is interfaced to the transmitter to type the data and transmit.

The message can be transmitted to multipoint receivers. After entering the text, the user can disconnect the keyboard. At anytime the user can add or remove or after the text according to his requirement. This project uses regulated 5V, 1A power supply. 7805 three terminal voltage regulator is used for voltage regulation. Bridge type full wave rectifier is used to rectify the ac o/p of secondary of 230/12V step down transformer. 1. 2 Proposed model:- Our proposed model consists of two modules i. e. one transmitter and one or more receiver module.

The transmitter module consists of interfacing computer via serial interface to the zig- bee module. The receiver module placed at the remote end consists of zigbee module interfacedwith a microcontroller for displaying messages on LCD. Password based authentication is employed on the transmitter side in order to provide access control to only authorized users. Primarily 16×2 LCD is been used for displaying messages which we can further extend to larger LCD. 1. 3 Block diagram for the proposed model:- 1. 4 HARDWARE AND SOFTWARE:- Hardware components required: AT89S52 based our own developed board Power supply Zigbee Max232 Lcd

Pc Software required: Embedded ‘c’ RIDE to write code ISP to burn the chip CHAPTER-2 2. 1 INTRODUCTION TO EMBEDDED SYSTEMS: Embedded systems are designed to do some specific task, rather than be a general-purpose computer for multiple tasks. Some also have real time performance constraints that must be met, for reason such as safety and usability; others may have low or no performance requirements, allowing the system hardware to be simplified to reduce costs. An embedded system is a combination of hardware and software which is custom built for system specific purpose, as the requirements of each system vary considerably.

Depending on the quantity and functionality needed, the embedded hardware is cost customized and software that goes into it also varies widely to meet desired functionality. 2. 1. 1 Variety of embedded systems: Embedded systems are commonly found in consumer, cooking, industrial, automotive, medical, commercial and military applications. Telecommunications systems employ numerous embedded systems from telephone switches For the network to cell phones at the end-user. Computer networking uses dedicated routers and network bridges to route data.

Consumer electronics include personal digital assistants (PDAs), mp3 players, mobile phones, videogame consoles, digital cameras, DVD players, GPS receivers, and printers. Household appliances, such as microwave ovens, washing machines and dishwashers, include embedded systems to provide flexibility, efficiency and features. Advanced HVAC systems use networked thermostats to more accurately and efficiently control temperature that can change by time of day and season. Home automation uses wired- and wireless-networking that can be used to control lights, climate, security, audio/visual, surveillance, etc.

All of which use embeddeddevices for sensing and controlling. Transportation systems from flight to automobiles increasingly use embedded systems. New airplanes contain advanced avionics such as inertial guidance systems and GPS receivers those also have considerable safety requirements. Various electric motors — brushless DC motors,induction motors and DC motors — use electric/electronic motor controllers. electric vehicles, and hybrid vehicles increasingly use embedded systems to maximize efficiency and reduce pollution.

Other automotive safety systems include antilock breaking systems. Medical equipment uses embedded systems for vitalsigns monitoring, electronic stethoscopes for amplifying sounds, and various medical imaging (PET, SPECT, CT, MRI) for non-invasive internal inspections. Embedded systems within medical equipment are often powered by industrial computers. Embedded systems are used in transportation, fire safety, safety and security, medical applications and life critical systems, as these systems can be isolated from hacking and thus, be more reliable.

For fire safety, the systems can be designed to have greater ability to handle higher temperatures and continue to operate. In dealing with security, the embedded systems can be Self-sufficient and be able to deal with cut electrical and communication systems. 2. 1. 2 Characteristics: Embedded systems are application specific & single functioned. Embedded systems are not always standalone devices Efficiency is of paramount importance for embedded systems. They are optimized for energy, code size, execution time, weight & dimensions, and cost.

Embedded systems are typically designed to meet real time constraints; a real time system reacts to stimuli from the controlled object/ operator within the time interval dictated by the environment. For real time systems, right answers arriving too late (or even too early) are wrong. Embedded systems often interact (sense, manipulate & communicate) with external world through sensors and actuators and hence are typically reactive systems; a reactive system is in continual interaction with the environment and executes at a pace determined by that environment.

Many embedded systems consists of small, computerized parts with in a larger device that serves a more general purpose. The program instructions written for embedded systems are referred to as firm ware, and are stored in read only memory or flash memory chips. They generally have minimal or no user interface. 2. 1. 3 Advantages of embedded systems: There are many advantages of embedded systems out of which some are as fallows Design and efficiency:

The central processing core in embedded systems is generally less complicated, making it easier to maintain. The limited function required of embedded systems allows them to be designed to most efficiently perform their functions. Cost: The streamlined make-up of most embedded systems allows their parts to be smaller less expensive to produce. Accessibility: Embedded systems are difficult to service because they are inside another machine, so a greater effort is made to carefully develop them. However, if something does go wrong with certain embedded systems they can be too inaccessible to repair.

This concern is sometimes addressed in the design stage, such as by programming an embedded system so that it will not affect related systems negatively when malfunctioning. Maintenance: Embedded systems are easier to maintain because the supplied power is embedded in the system and does not require remoter maintenance. Redundancies: Embedded systems do not involve the redundant programming and maintenance involved in other system models. They allow the system hardware to be simplified so that the costs can be reduced.

They are designed to do a specific task and have real time performance costaints which are to be met They usually in the form of small computerized parts in place of larger devices which serve a general purpose, so that size can be reduced Enables real-time, deterministic, scheduling and task prioritization Abstracts away the complexities of the processor Provides a solid infrastructure constructed of rules and policies. Simplifies development and improves developer productivity Integrates and manages resources needed by communications stacks and middleware. Optimizes use of system resources.

Improves product reliability, maintainability and quality. Promotes product evolution and scaling. 2. 1. 4 Disadvantages of embedded systems: Just like that of a head and tail of a coin, there are also some disadvantages for the embedded systems, they are They aren’t very scalable and in most cases are limited to the components they came with. Though they are only designed to fill a very specific role and I wouldn’t use them in a dynamic, constantly changing environment. They have very few resources at their disposal They are very expensive to repair They do not have any user interface(more often than not)

A compiler is necessary on the target and due to compilation during run time, execution times are not predictable. 2. 1. 5 Applications: Embedded system applications describe the latest techniques for embedded system design in a variety of applications. This includes some of the latest software tools for embedded system design. Applications of embedded system design in avionics, satellites, and radio astronomy, space and control systems are illustrated best practice in embedded system design. Embedded system applications will be of interest to researchers and designers working in the design of embedded systems for industrial applications.

Military and aerospace embedded software applications: from in-orbit embedded systems to jumbo jets to vital battlefield networks, designers of mission-critical aerospace and defense systems requiring real-time performance, scalability and high availability facilities consistently turn t the Lynx OS-178 RTOS for software certification to DO-178B. Medical electronics technology: five-nine “availability, compact PCI hot swap support, and hard real-time response-Lynx OS delivers on these key requirements and more for today’s carrier-class systems.

Scalable kernel configurations, distributed computing capabilities, integrated communications stacks, and fault-management facilities make Lynx OS the ideal choice for companies looking for a single operating system for all embedded telecommunications applications-from complex central controllers to simple line/trunk cards Electronics applications and consumer devices: As the number of powerful embedded processors in consumer devices continues to rise, the Lynx OS real-time operating system provides a highly reliable option for systems designers.

Industrial automation and process control software: Designers of industrial and process control systems know from experience that Linux work’s operating systems provide the security and reliability that their industrial applications require. 2. 2 INTRODUCTION TO WIRELESS TECHNOLOGY:- Wireless technology has been making tremendous progress over the past few years. The ever increasing use of wireless networks serves as an indicator of the progress in the area of wireless networks.

The demand for wireless technology is increasing not only in industrial applications but also for domestic purposes. Some benefits of wireless technology are: Completes the access technology portfolio: customers commonly use more than one access technology to service various parts of their network and during the migration phase of their networks, when upgrading occurs on a scheduled basis. Wireless enables a fully comprehensive access technology portfolio to work with existing dial, cable, and DSL technologies.

Goes where cable and fiber cannot: the inherent nature of wireless is that it doesn’t require wires or line to accommodate the data/voice/video pipeline. As such the system will carry information across geographical areas that are prohibitive in terms of distance, cost, access, or time. It also sidesteps the numerous issues of ILEC collocation. Involves reduced time to revenue: companies can generate revenue in less time through the deployment of wireless solutions than with comparable access technologies because a wireless system can be assembled and brought online in as little as two to three hours.

Provides broadband access extension: wireless commonly both competes with and complements existing broadband access. Wireless technologies play a key role in extending the reach of cable, fiber, and DSL markets, and it does so quickly and reliably. It also commonly provides a competitive alternative to broadband wireline or provides access in geographies that don’t qualify for loop access. CHAPTER-3 3. 1 A BRIEF HISTORY OF 8051 In 1981, Intel Corporation introduced an 8 bit microcontroller called 8051. This microcontroller had 128 bytes of RAM, 4K bytes of chip ROM, two timers, one serial port, and four ports all on a single chip.

At the time it was also referred as “A SYSTEM ON A CHIP” The 8051 is an 8-bit processor meaning that the CPU can work only on 8 bits data at a time. Data larger than 8 bits has to be broken into 8 bits pieces to be processed by the CPU. The 8051 has a total of four I\O ports each 8 bit wide. There are many versions of 8051 with different speeds and amount of on-chip ROM and they are all compatible with the original 8051. This means that if you write a program for one it will run on any of them. The 8051 is an original member of the 8051 family. There are two other members in the 8051 family of microcontrollers.

They are 8052 and 8031. All the three microcontrollers will have the same internal architecture, but they differ in the following aspects. 8031 has 128 bytes of RAM, two timers and 6 interrupts. 8051 has 4K ROM, 128 bytes of RAM, two timers and 6 interrupts. 8052 has 8K ROM, 256 bytes of RAM, three timers and 8 interrupts. Of the three microcontrollers, 8051 is the most preferable. Microcontroller supports both serial and parallel communication. In the concerned project 8052 microcontroller is used. Here microcontroller used is AT89S52, which is manufactured by ATMEL laboratories.

Microprocessors brought the concept of programmable devices and made many applications of intelligent equipment. Most applications, which do not need large amount of data and program memory, tended to be costly. The microprocessor system had to satisfy the data and program requirements so, sufficient RAM and ROM are used to satisfy most applications . The peripheral control equipment also had to be satisfied. Therefore, almost all-peripheral chips were used in the design. Because of these additional peripherals cost will be comparatively high. 8085 chip needs:

An Address latch for separating address from multiplex address and data. 32-KB RAM and 32-KB ROM to be able to satisfy most applications. As also Timer / Counter, Parallel programmable port, Serial port, and Interrupt controller are needed for its efficient applications. In comparison a typical Micro controller 8051 chip has all that the 8051 board has except a reduced memory as follows. 4K bytes of ROM as compared to 32-KB, 128 Bytes of RAM as compared to 32-KB. Bulky: On comparing a board full of chips (Microprocessors) with one chip with all components in it (Microcontroller). Debugging:

Lots of Microprocessor circuitry and program to debug. In Micro controller there is no Microprocessor circuitry to debug. Slower Development time: As we have observed Microprocessors need a lot of debugging at board level and at program level, whereas, Micro controller do not have the excessive circuitry and the built-in peripheral chips are easier to program for operation. So peripheral devices like Timer/Counter, Parallel programmable port, Serial Communication Port, Interrupt controller and so on, which were most often used were integrated with the Microprocessor to present the Micro controller .

RAM and ROM also were integrated in the same chip. The ROM size was anything from 256 bytes to 32Kb or more. RAM was optimized to minimum of 64 bytes to 256 bytes or more. Microprocessor has following instructions to perform: Reading instructions or data from program memory ROM. Interpreting the instruction and executing it. Microprocessor Program is a collection of instructions stored in a Nonvolatile memory. Read Data from I/O device Process the input read, as per the instructions read in program memory. Read or write data to Data memory.

Write data to I/O device and output the result of processing to O/P device. 3. 2 Introduction to AT89S52: The system requirements and control specifications clearly rule out the use of 16, 32 or 64 bit micro controllers or microprocessors. Systems using these may be earlier to implement due to large number of internal features. They are also faster and more reliable but, the above application is satisfactorily served by 8-bit micro controller. Using an inexpensive 8-bit Microcontroller will doom the 32-bit product failure in any competitive market place. Coming to the question of why to use 89S52 of all the 8-bit Microcontroller available in the market the main answer would be because it has 8kB Flash and 256 bytes of data RAM32 I/O lines, three 16-bit timer/counters, a Eight-vector two-level interrupt architecture, a full duplex serial port, on-chip oscillator, and clock circuitry. In addition, the AT89S52 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode stops 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. The Flash program memory supports both parallel programming and in Serial In-System Programming (ISP). The 89S52 is also In-Application Programmable (IAP), allowing the Flash program memory to be reconfigured even while the application is running. By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89S52 is a powerful microcomputer which provides a highly flexible and cost effective solution to many embedded control applications.

Compatible with MCS-51® Products 8K Bytes of In-System Programmable (ISP) Flash Memory Endurance: 1000 Write/Erase Cycles 4. 0V to 5. 5V Operating Range Fully Static Operation: 0 Hz to 33 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 Full Duplex UART Serial Channel Low-power Idle and Power-down Modes Interrupt Recovery from Power-down Mode Watchdog Timer Dual Data Pointer Power-off Flag 3. 2. 2 AT89S52 BLOCL DIAGRAM:

Flash programming and verification Port 2: Port 2 is an 8-bit bi-directional I/O port with internal pullups. The Port 2 output buffers can sink/source four TTL inputs. When 1s are written to Port 2 pins, they are pulled high by the internal pullups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low will source current (IIL) because of the internal pullups. Port 2 emits the high-order address byte during fetches from external program memory and during accesses to external data memory that uses 16-bit addresses (MOVX @ DPTR).

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 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 microcontroller is in external execution mode 3. 2. 4 MEMORIES: Types of memory: The 8052 have three general types of memory. They are on-chip memory, external Code memory and external Ram. On-Chip memory refers to physically existing memory on the micro controller itself. External code memory is the code memory that resides off chip. This is often in the form of an external EPROM. External RAM is the Ram that resides off chip.

This often is in the form of standard static RAM or flash RAM. a) Code memory Code memory is the memory that holds the actual 8052 programs that is to be run. This memory is limited to 64K. Code memory may be found on-chip or off-chip. It is possible to have 8K of code memory on-chip and 60K off chip memory simultaneously. If only off-chip memory is available then there can be 64K of off chip ROM. This is controlled by pin provided as EA b) Internal RAM The 8052 have a bank of 256 bytes of internal RAM. The internal RAM is found on-chip. So it is the fastest Ram available.

And also it is most flexible in terms of reading and writing. Internal Ram is volatile, so when 8051 is reset, this memory is cleared. 256 bytes of internal memory are subdivided. The first 32 bytes are divided into 4 register banks. Each bank contains 8 registers. Internal RAM also contains 256 bits, which are addressed from 20h to 2Fh. These bits are bit addressed i. e. each individual bit of a byte can be addressed by the user. They are numbered 00h to FFh. The user may make use of these variables with commands such as SETB and CLR. Special Function registered memory:

Special function registers are the areas of memory that control specific functionality of the 8052 micro controller. Accumulator (0E0h): As its name suggests, it is used to accumulate the results of large no of instructions. It can hold 8 bit values. B registers (0F0h): The B register is very similar to accumulator. It may hold 8-bit value. The b register is only used by MUL AB and DIV AB instructions. In MUL AB the higher byte of the product gets stored in B register. In div AB the quotient gets stored in B with the remainder in A. Stack pointer (81h): The stack pointer holds 8-bit value.

This is used to indicate where the next value to be removed from the stack should be taken from. When a value is to be pushed onto the stack, the 8052 first store the value of SP and then store the value at the resulting memory location. When a value is to be popped from the stack, the 8052 returns the value from the memory location indicated by SP and then decrements the value of SP. Data pointer: The SFRs DPL and DPH work together work together to represent a 16-bit value called the data pointer. The data pointer is used in operations regarding external RAM and some instructions code memory.

It is a 16-bit SFR and also an addressable SFR. Program counter: The program counter is a 16 bit register, which contains the 2 byte address, which tells the 8052 where the next instruction to execute to be found in memory. When the 8052 is initialized PC starts at 0000h. And is incremented each time an instruction is executes. It is not addressable SFR. PCON (power control, 87h): The power control SFR is used to control the 8051’s power control modes. Certain operation modes of the 8051 allow the 8051 to go into a type of “sleep mode” which consumes much lee power.

TCON (timer control, 88h): The timer control SFR is used to configure and modify the way in which the 8051’s two timers operate. This SFR controls whether each of the two timers is running or stopped and contains a flag to indicate that each timer has overflowed. Additionally, some non-timer related bits are located in TCON SFR. These bits are used to configure the way in which the external interrupt flags are activated, which are set when an external interrupt occurs. TMOD (Timer Mode, 89h): The timer mode SFR is used to configure the mode of operation of each of the two timers.

Using this SFR your program may configure each timer to be a 16-bit timer, or 13 bit timer, 8-bit auto reload timer, or two separate timers. Additionally you may configure the timers to only count when an external pin is activated or to count “events” that are indicated on an external pin. TO (Timer 0 low/high, address 8A/8C h): These two SFRs taken together represent timer 0. Their exact behavior depends on how the timer is configured in the TMOD SFR; however, these timers always count up. What is configurable is how and when they increment in value. T1 (Timer 1 Low/High, address 8B/ 8D h):

These two SFRs, taken together, represent timer 1. Their exact behavior depends on how the timer is configured in the TMOD SFR; however, these timers always count up.. P0 (Port 0, address 90h, bit addressable): This is port 0 latch. Each bit of this SFR corresponds to one of the pins on a micro controller. Any data to be outputted to port 0 is first written on P0 register. For e. g. , bit 0 of port 0 is pin P0. 0, bit 7 is pin p0. 7. Writing a value of 1 to a bit of this SFR will send a high level on the corresponding I/O pin whereas a value of 0 will bring it to low level. P1 (port 1, address 90h, bit addressable):

This is port latch1. Each bit of this SFR corresponds to one of the pins on a micro controller. Any data to be outputted to port 0 is first written on P0 register. For e. g. , bit 0 of port 0 is pin P1. 0, bit 7 is pin P1. 7. Writing a value of 1 to a bit of this SFR will send a high level on the corresponding I/O pin whereas a value of 0 will bring it to low level P2 (port 2, address 0A0h, bit addressable): This is a port latch2. Each bit of this SFR corresponds to one of the pins on a micro controller. Any data to be outputted to port 0 is first written on P0 register. For e. g.

An interrupt may interrupt interrupts. For e. g. , if we configure all interrupts as low priority other than serial interrupt. The serial interrupt always interrupts the system, even if another interrupt is currently executing. However, if a serial interrupt is executing no other interrupt will be able to interrupt the serial interrupt routine since the serial interrupt routine has the highest priority. PSW (Program Status Word, 0D0h): The program Status Word is used to store a number of important bits that are set and cleared by 8052 instructions.

The PSW SFR contains the carry flag, the auxiliary carry flag, the parity flag and the overflow flag. Additionally, it also contains the register bank select flags, which are used to select, which of the “R” register banks currently in use. SBUF (Serial Buffer, 99h): SBUF is used to hold data in serial communication. It is physically two registers. One is writing only and is used to hold data to be transmitted out of 8052 via TXD. The other is read only and holds received data from external sources via RXD. Both mutually exclusive registers use address 99h. 3. 2. 5 I/O ports:

One major feature of a microcontroller is the versatility built into the input/output (I/O) circuits that connect the 8052 to the outside world. The main constraint that limits numerous functions is the number of pins available in the 8051 circuit. The DIP had 40 pins and the success of the design depends on the flexibility incorporated into use of these pins. For this reason, 24 of the pins may each used for one of the two entirely different functions which depend, first, on what is physically connected to it and, then, on what software programs are used to “program” the pins.

PORT 0: Port 0 pins may serve as inputs, outputs, or, when used together, as a bi directional low-order address and data bus for external memory. To configure a pin as input, 1 must be written into the corresponding port 0 latch by the program. When used for interfacing with the external memory, the lower byte of address is first sent via PORT0, latched using Address latch enable (ALE) pulse and then the bus is turned around to become the data bus for external memory. PORT 1: Port 1 is exclusively used for input/output operations. PORTS 1 pin have no dual function.

When a pin is to be configured as input, 1 is to be written into the corresponding Port 1 latch. PORT 2: Port 2 may be used as an input/output port. It may also be used to supply a high –order address byte in conjunction with Port 0 low-order byte to address external memory. Port 2 pins are momentarily changed by the address control signals when supplying the high byte a 16-bit address. Port 2 latches remain stable when external memory is addressed, as they do not have to be turned around (set to 1) for data input as in the case for Port 0. PORT 3: Port 3 may be used to input /output port.

The input and output functions can be programmed under the control of the P3 latches or under the control of various special function registers. Unlike Port 0 and Port 2, which can have external addressing functions and change all eight-port b se, each pin of port 3 maybe individually programmed to be used as I/O or as one of the alternate functions. The Port 3 alternate uses are: 3. 2. 6 INTERRUPTS: The AT89S52 has a total of six interrupt vectors: two external interrupts (INT0 and INT1), three timer interrupts (Timers0, 1, and 2), and the serial port interrupt. These interrupts are all shown in Figure 10.

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 5 shows that bit position IE. 6 is unimplemented. In the AT89S52, 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. CHAPTER-4 4. 1 POWER SUPPLY: All digital circuits require regulated power supply. In this article we are going to learn how to get a regulated positive supply from the mains supply.

So The secondary voltage of the transformer depends on the number of turns in the Primary as well as in the secondary. 4. 1. 2 Rectifier: A rectifier is a device that converts an AC signal into DC signal. For rectification purpose we use a diode, a diode is a device that allows current to pass only in one direction i. e. when the anode of the diode is positive with respect to the cathode also called as forward biased condition & blocks current in the reversed biased condition. Rectifier can be classified as follows: 1)      Half Wave rectifier.

This is the simplest type of rectifier as you can see in the diagram a half wave rectifier consists of only one diode. When an AC signal is applied to it during the positive half cycle the diode is forward biased & current flows through it. But during the negative half cycle diode is reverse biased & no current flows through it. Since only one half of the input reaches the output, it is very inefficient to be used in power supplies. 2)      Full wave rectifier. Half wave rectifier is quite simple but it is very inefficient, for greater efficiency we would like to use both the half cycles of the AC signal.

This can be achieved by using a center tapped transformer i. e. we would have to double the size of secondary winding & provide connection to the center. So during the positive half cycle diode D1 conducts & D2 is in reverse biased condition. During the negative half cycle diode D2 conducts & D1 is reverse biased. Thus we get both the half cycles across the load. One of the disadvantages of Full Wave Rectifier design is the necessity of using a center tapped transformer, thus increasing the size & cost of the circuit. This can be avoided by using the Full Wave Bridge Rectifier.

Bridge Rectifier As the name suggests it converts the full wave i. e. both the positive & the negative half cycle into DC thus it is much more efficient than Half Wave Rectifier & that too without using a center tapped transformer thus much more cost effective than Full Wave Rectifier. Full Bridge Wave Rectifier consists of four diodes namely D1, D2, D3 and D4. During the positive half cycle diodes D1 & D4 conduct whereas in the negative half cycle diodes D2 & D3 conduct thus the diodes keep switching the transformer connections so we get positive half cycles in the output.

If we use a center tapped transformer for a bridge rectifier we can get both positive & negative half cycles which can thus be used for generating fixed positive & fixed negative voltages. 4. 1. 3 FILTER CAPACITOR: Even though half wave & full wave rectifier give DC output, none of them provides a constant output voltage. For this we require to smoothen the waveform received from the rectifier. This can be done by using a capacitor at the output of the rectifier this capacitor is also called as “FILTER CAPACITOR” or “SMOOTHING CAPACITOR” or “RESERVOIR CAPACITOR”.

Even after using this capacitor a small amount of ripple will remain. We place the Filter Capacitor at the output of the rectifier the capacitor will charge to the peak voltage during each half cycle then will discharge its stored energy slowly through the load while the rectified voltage drops to zero, thus trying to keep the voltage as constant as possible. If we go on increasing the value of the filter capacitor then the Ripple will decrease. But then the costing will increase. The value of the Filter capacitor depends on the current consumed by the circuit, the frequency of the waveform & the accepted ripple.

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