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8051 base GSM based messages display board


Submitted By:
1.         LIMBASIYA ABHISHEK L.            136120324019
2.         MAKWANA PRAKASH B.             136120324020
3.         MARATHE SACHIN U.                   136120324022
4.         MORE KIRTI S.                               136120324023
5.         NAI JIGAR R.                                  136120324024

In Fulfillment for the Award Of The
Diploma In
Gujarat Technological University,
May, 2016



Downlod PDF file....    

The scrolling message display will allow the user to type in any alphanumeric message and see it displayed through an array of red LED‘s (Light Emitting Diodes). A block diagram illustrating this process is depicted.

            This project will be an embedded system, so the display will be able to scroll messages anywhere there is a wall outlet. It will be dependent on a PC for user input. Messages can be created and saved on the display‘s microcomputer by use of a terminal program on a PC. The PC and microcomputer will communicate via serial port. After the message is created and saved, the display can be detached from the PC and then plugged in elsewhere to scroll the message. The array of LED‘s will be configured together on a single board called a module.
The circuit board was designed with the microcontroller located on the back of the circuit board leaving just the display and reset button on the front. Your message is hard coded into code space within the microcontroller.


With the development of cellular networks in the 1970‘s for increasing the lack of frequencies in the radiotelephone services which in turn lead to introduction of AMPS (Advanced Mobile Phone System) where the transmission was analog based. This was known to be the first generation in cellular networks. The second generation was based on digital transmission and was called with various abbreviations as GSM (Global System for Mobile communications), ERMES (European Radio Messaging System). Various Cordless telephone standards were also introduced during this time only. The third generation has risen with the unification of different technologies; some of them which are popularly known are FPLMTS (Future Public Land Mobile Telecommunications System), UMTS (Universal Mobile Telecommunication System), and IMT-2000(International Mobile elecommunication).
To realize the proposed wireless GSM Based Display unit the following prototype model has been developed in the laboratory. It consists of Micro controller, GSM Modem, One cellphone and LED display board. LED display board is used for testing the proposed methodology. The interfacing of a GSM modem with a normal PC is quite easy with help of the AT commands sent to it from the HyperTerminal window. But we must take into fact that the modem requires a wired connection at one end and wireless at the other. As it is too expensive to use a dedicated general purpose computer at each and every site of the display boards, the possibility of performing the objective with a dedicated computer is not feasible practically on cost factors. Hence we employ Atmel microcontroller with 1024 bytes EEROM storage memory. The complexity of coding considerably intensifies as compared with PC, but once programmed the micro controller works at its best since it is a committed embedded system. The design procedure involves identifying the different components and assembling all of them and ensuring safe interfacing

between all these components. Then coding process has to be done, which has to take care of the deferrals between two successive communications and most importantly the authentication of the sender‘s number. The number of authenticated mobile numbers can be more than one. This enables the multiple users can operate the digital display. The main limiting constraint is the RAM of the microcontroller.


A dot-matrix display is a display device used to display information on machines, clocks, railway departure indicators and many other devices requiring a simple display device of limited resolution.
The display consists of a dot matrix of lights or mechanical indicators arranged in a rectangular configuration (other shapes are also possible, although not common) such that by switching on or off selected lights, text or graphics can be displayed. A dot matrix controller converts instructions from a processor into signals which turns on or off lights in the matrix so that the required display is produced

In a dot matrix display, multiple LEDs are wired together in rows and columns. This is done to minimize the number of pins required to drive them. For example, a 8×8 matrix of LEDs (shown below) would need 64 I/O pins, one for each LED pixel. By wiring all the anodes together in rows (R1 through R8), and cathodes in columns (C1 through C8), the required number of I/O pins is reduced to 16. Each LED is addressed by its row and column number. In the figure below, if R4 is pulled high and C3 is pulled low, the LED in fourth row and third column will be turned on. Characters can be displayed by fast scanning of either rows or columns.

The LED matrix used in this experiment is of size 5×7. We will learn how to display still characters in a standard 5×7 pixel format. The figure below shows which LEDs are to be turned on to display the English alphabet ‗A‘. The 7 rows and 5 columns are controlled through the microcontroller pins.
Suppose, we want to display the alphabet A. We will first select the column C1 (which means C1 is pulled low in this case), and deselect other columns by blocking their ground paths (one way of doing that is by pulling C2 through C5 pins to logic high). Now, the first column is active, and you need to turn on the LEDs in the rows R2 through R7 of this column, which can be done by applying forward bias voltages to these rows. Next, select the column C2 (and deselect all other columns), and apply forward bias to R1 and R5, and so on. Therefore, by scanning across the column quickly (> 100 times per second), and turning on the respective LEDs in each row of that column, the persistence of vision comes in to play, and we perceive the display image as still.


The table below gives the logic levels to be applied to R1 through R7 for each of the columns in order to display the alphabet ‗A‘.

Row values of each column for displaying the alphabet A

Scanning across the columns and feeding with appropriate row values


Project Power supply:-

As shown in circuit this is 5vdc conversion circuit from AC main line power supply. Ac main line is step down by using 0-12 transformer and this is give 12v ac output and 500mA power supply. This AC supply is convert in DC by using rectifier bridge. This is give 12VDC supply this supply is filter by 1000uf capacitor is giving pure dc supply. This power supply is convert 5VDC by using Regulator power. In this power supply we have to use 7805 power regulator which is convert 24vdc to 5vdc up to 1A current limiting.

Microcontroller & LCD interfacing:
Microcontroller is heart of the project and this is working with 8-bit architecture system. We have to use 8051 microcontroller family Atmel company controller AT89s52 and this is 8-bit microcontroller. We have to use LCD interfacing with microcontroller in its port connection.
LCD 16x2 have total 16-pin which have many pin with RS, RW, En and also Data pin its DATA pin is connected with PORT0 and command pin RS, RW, En is connected with microcontroller P2.5, P2.6, P2.7 respectively.
There is 8-bit mode programming with microcontroller system. Lcd 1 number pin is gnd pin 2 number pin is power supply and 3 number pin is contrast pin. Its interfacing circuit diagram is shown in figure.

Microcontroller & Serial Conversion:-
8051 microcontroller have some features like Interrupt, Serial communication, External Interrupt this all features are available on PORT3. In our project we have to use serial communication module with RX and TX pin which is working with specific frequency. This frequency is providing by microcontroller programming and also depend on its machine cycle. We have to use external crystal frequency 11.0592MHz for serial communication with other devices.
Other serial communication device is connect with microcontroller serial pin is directly or indirectly. Computer interfacing with microcontroller we have to compulsory use to MAX232 conversion circuit which is use to convert TTL conversion system. GSM modem and RFID module is also directly communicate with serial conversion pin. Because it is signal level is same as microcontroller.

This project is aimed at developing a system that will display the message received by the cell or a modem. Microcontroller then will control the system by doing authentication and AT commands. And the messages that is been displayed depends upon the LCD type used, that is if we use graphical LCD we can even display the images with interfacing it with AT89S52 . This system is easy, robust to use in normal life by any-one at any place.
We propose a mobile-based notice board system that consists of a MOTOROLLA C-168 cell phone, and a microcontroller. The notice is been displayed by the microcontroller, which operates according to the user commands received from the mobile phone via MOTOROLLA C-168 cell phone. In our project system is built upon an SMS (short message service) mobile cell module and a microcontroller, allowing a user to display real-time messages. This report presents the design and implementation of MOTOROLLA C-168 AT modem using AT commands and I2C Serial protocol. Microcontroller used in our project uses AT commands to monitor MOTOROLLA C-168 mobile. It receives incoming message, manipulate it and take appropriate actions. It also informs the user via creating the SMS that contains information about actions which is to be performed and sending it to the MOTOROLLA C-168 mobile via the serial port.

For the first step , we designed one circuit to connect mobile to PC and we created a communication link via HYPERTERMINAL . For circuit and hyper terminal see Fig.

  The AT89S52 is a low-power, high-performance CMOS 8-bit microcomputer with 4k bytes of flash programmable and erasable read only memory (EPROM).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 and pin out. 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 [1-9]. We also are using SIM300 GSM module with following features.
TTL UART interface for connection to microcontroller
RS232 interface for connection to PC/Laptop
12V power supply option
5V power supply option
LED indicating network status

we have interfaced microcontroller with GSM Modem to decode the received message and do the required action. The protocol used for the communication between the two is AT command. RS-232 standards are used for the serial communication of binary bits. Various AT commands of call control, data card control, phone control, computer data interface, service, network communication parameter, SMS text mode and SMS PDU mode are used for the communication purpose from microcontroller to the GSM module. Raisonance 8051 Integrated Development Environment (RIDE) is the software used from editing to compiling, linking and debugging along with a simulator which conveniently manages all aspects of the embedded systems development with a single user interface.
Operation with serial port:
The unit is connected to the display and hyper terminal settings are configured. Shift# is typed to enter the menu mode where you will be asked to choose to view message (type 1), format (type 2), set time (type 3) and exit (type 4). On choosing 1 the message is displayed on the board.
The message has the following format:
<M message><DEF 1><S 5><D L2> on choosing 2 the memory is formatted and 3 can be used to set the time and date accordingly.
Sending messages through serial port:
Connect the unit to serial port and configure hyper terminal to default settings and Press Shift * to feed the new messages. Then the display unit will print ********.
<M Hello world><DEF 1><S 4><D R2>In the above format <M defines that this is message, Hello world is the message<DEF 1> defines the message number, <S 4>
defines the speed is 4, <D R2> defines the message should scroll from left to right 2 times. After pressing enter the message saved in the memory.
The microcontroller, GSM module and display board are powered by AC to DC adapter with input 100-240V AC, 50/60Hz and Output 12V DC, 1A.

Figure 2 shows the GSM module
It consists of a SIM card of MIN ―8686400268‖. The message transmitted by any number to this MIN is received and saved in the memory of the SIM card. This module works with the AT-Commands set as mentioned in earlier sections. The RxD and TxD pins of this GSM module are connected to the TxD and RxD of the microcontroller respectively.


In this project module circuit, following components were used.
 Transformer
 AT89S52 Microcontroller
 LCD Display
 20 MHz & 12 MHz Crystal Oscillator
 Resistor
 Push button
 Capacitor
 Voltage Regulator IC 78XX
 IN 4007 Diode
 Gsm modem

 Max 232

The AT89S52 is a low-power, high-performance CMOS 8-bit microcontroller with 8K bytes of in-system programmable Flash memory. The device is manufactured using Atmel‘s high-density nonvolatile memory technology and is compatible with the industry-standard 80C51 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 in-system programmable Flash on a monolithic chip, the Atmel AT89S52 is a powerful microcontroller which provides a highly-flexible and cost-effective solution to many embedded control applications.In RPM measurement and display circuit, this microcontroller is used.
 Compatible with MCS®-51 Products
 8K Bytes of In-System Programmable (ISP) Flash Memory – Endurance:
         10,000 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
 Dual Data Pointer
 Flexible ISP Programming (Byte and Page Mode)


VCC (Pin no. 40) - Supply voltage.
GND (Pin no.20) - Ground.

Port0 (32 to 39) -Port 0 is an 8-bit open drain bidirectional 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.

Port1 (1 to 8)- Port 1 is an 8-bit bidirectional 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 Flash programming and verification.

Port2 (21 to 28) -Port 2 is an 8-bit bidirectional 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 uses 16-bit addresses (MOVX @ DPTR). In this application, Port 2 uses strong internal pull-ups when emitting 1s. During accesses to external data memories 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.

Port3 (11 to 17) -Port 3 is an 8-bit bidirectional 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. Asinputs, Port 3 pins that are externally being pulled low will source current (IIL) because of the pull-ups. Port 3 receives some control signals for Flash programming
and verification. Port 3 also serves the functions of various special features of the AT89S52..
RST - Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device. This pin drives high for 98 oscillator periods after the
Watchdog times out. The DISRTO bit in SFR AUXR (address 8EH) can be used to disable this feature. In the default state of bit DISRTO, the RESET HIGH out feature is enabled.
ALE/PROG - Address Latch Enable (ALE) 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 microcontroller is in external execution mode.
PSEN -Program Store Enable (PSEN) is the read strobe to external program memory. When the AT89S52 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.
XTAL1 (Pin no. 19)– Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
XTAL2 (Pin no. 18) -Output from the inverting oscillator amplifier.

A liquid crystal display is flat-panel display or other electronics visual display that uses light modulating properties of liquid crystals. A 16x2 LCD means it can display 16 characters per line and there are 2 such lines. In this LCD each character is displayed in 5x7 pixel matrix. This LCD has two registers, namely, Command and Data.The command register stores the command instructions given to the LCD. A command is an instruction given to LCD to do a predefined task like initializing it, clearing its screen, setting the cursor position, controlling display etc. The data register stores the data to be displayed on the LCD. The data is the ASCII value of the character to be displayed on the LCD. Click to learn more about internal structure of a LCD. To display the RPM of motor, this LCD display is used.


A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a precise frequency. This frequency is commonly used to keep track of time, as in quartz wristwatches, to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits incorporating them became known as crystal oscillators. A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric resonator, a crystal, as its frequency-determining element. Crystal is the common term used in electronics for the frequency-determining component, a wafer of quartz crystal or ceramic with
electrodes connected to it. A more accurate term for it is piezoelectric resonator. Crystals are also used in other types of electronic circuits, such as crystal filters.
In our circuit, two crystals is used in variant value. In AT89S52, 12 MHz crystal is used and In PIC18F4331, 20 MHz crystal is used.


A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. Resistors may be used to reduce current flow, and, at the same time, may act to lower voltage levels within circuits. In electronic circuits, resistors are used to limit current flow, to adjust signal levels, bias active elements, and terminate transmission lines among other uses. High-power resistors, that can dissipate many watts of electrical power as heat, may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity. Resistors are common elements of electrical networks and electronic circuits and are ubiquitous in electronic equipment. Practical resistors

as discrete components can be composed of various compounds and forms. Resistors are also implemented within integrated circuits. The electrical function of a resistor is specified by its resistance: common commercial resistors are manufactured over a range of more than nine orders of magnitude. The nominal value of the resistance will fall within a manufacturing tolerance.


A push button is a momentary or non-latching switch which causes a temporary change in the state of an electrical circuit only while the switch is physically actuated. An automatic mechanism (i.e. a spring) returns the switch to its default position immediately afterwards, restoring the initial circuit condition..push button is used in start button, reset button and stop button in our circuit.


A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store electrical energy temporarily in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e. an insulator that can store energy by becoming polarized). The conductors can be thin films, foils or sintered beads of metal or conductive electrolyte, etc.A capacitor consists of two metal plates which are separated by a non-conducting substance or dielectric. Take a look at the figure given below to know about dielectric in a capacitor.Though any non-conducting substance can be used as a dielectric, practically some special materials like porcelain, mylar, teflon, mica, cellulose and so on. A capacitor is defined by the type of dielecric selected. It also defines the application of the capacitor.According to the size and type of dielectric used, the capacitor can be used for high-voltage as well as low-voltage applications.


A voltage regulator is one of the most widely used electronic circuitry in any device. A regulated voltage (without fluctuations & noise levels) is very important for the smooth functioning of many digital electronic devices. A common case is with micro controllers, where a smooth regulated input voltage must be supplied for the micro controller to function smoothly. The 78xx is a family of self-contained fixed linear voltage regulator integrated circuits. The 78xx family is commonly used in electronic circuits requiring a regulated power supply due to their ease-of-use and low cost. For ICs within the family, the xx is replaced with two digits, indicating the output voltage (for example, the 7805 has a 5-volt output, while the 7812 produces 12 volts). The 78xx line are positive voltage regulators: they produce a voltage that is positive relative to a common ground.


A potentiometer, informally a pot, is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrument called a potentiometer is essentially a voltage divider used for measuring electric potential (voltage); the component is an implementation of the same principle, hence its name. Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick. Potentiometers are rarely used to directly control significant power (more than a watt), since the power dissipated in the potentiometer would be comparable to the power in the controlled load.

9. Light Emitting Diode:-

A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode, which emits light when activated.[4] When a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.LEDS displays are made to display numbers from segments. One such design is the seven-segment display as shown below. Any desired numerals from 0-9 can be displayed by passing current through the correct segments. To connect such segment a common anode or common cathode cathode configuration can be used. Both the connections are shown below. The LED‘s are switched ON and OFF by using transistors.

10. PN Junction Diode:-

A diode is a device which only allows unidirectional flow of current if operated within a rated specified voltage level. A diode only blocks current in the reverse direction while the reverse voltage is within a limited range otherwise reverse barrier breaks and the voltage at which this breakdown occurs is called reverse breakdown voltage. The diode acts as a valve in the electrical and electronic circuit. A Pn junction is the simplest form of the diode which behaves as ideally short circuit when it is in forward biased and behaves as ideally open circuit when it is in the reverse biased. Beside simple PN junction diodes, there are different types of diodes although the fundamental principle is more or less same. So a particular arrangement of diodes can convert AC to pulsating DC, and hence, it is sometimes also called as a rectifier. The name diode is derived from "di-ode" which means a device having two electrodes. A simple PN junction diode can be created by doping donor impurity in one portion and acceptor impurity in other portion of a silicon or germanium crystal block. These make a p n junction at the middle portion of the block beside which one portion is p type (which is doped by trivalent or acceptor impurity) and other portion is n type (which is doped by pentavalent or donor impurity). It can also be formed by joining a p-type (intrinsic semiconductor doped with a trivalent impurity) and n-type semiconductor (intrinsic semiconductor doped with a pentavalent impurity) together with a special fabrication technique such that a p-n junction is formed. Hence, it is a device with two elements, the p-type forms anode and the n-type forms the cathode. These terminals are brought out to make the external connections.


Transformers convert AC electricity from one voltage to another with a little loss of power. Step-up transformers increase voltage, step-down transformers reduce voltage. Most power supplies use a step-down transformer to reduce the dangerously high voltage to a safer low voltage.

The input coil is called the primary and the output coil is called the secondary. There is no electrical connection between the two coils; instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core. Transformers waste very little power so the power out is (almost) equal to the power in. Note that as voltage is stepped down and current is stepped up.
The ratio of the number of turns on each coil, called the turn‘s ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.
TURNS RATIO = (Vp /Vs) = ( Np / Ns )
Vp = primary (input) voltage.
Vs = secondary (output) voltage
Np = number of turns on primary coil
Ns = number of turns on secondary coil
Ip = primary (input) current
Is = secondary (output) current.

Ideal power equation:-

The ideal transformer as a circuit element

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power:
giving the ideal transformer equation
Transformers normally have high efficiency, so this formula is a reasonable approximation.
If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turns ratio. For example, if an impedance Zs is attached across the terminals of the secondary coil, it

appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2Zp.

12. GSM Modem:-

This GSM Modem can accept any GSM network operator SIM card and act just like a mobile phone with its own unique phone number. Advantage of using this modem will be that you can use its RS232 port to communicate and develop embedded applications. Applications like SMS Control, data transfer, remote control and logging can be developed easily.
The modem can either be connected to PC serial port directly or to any microcontroller. It can be used to send and receive SMS or make/receive voice calls. It can also be used in GPRS mode to connect to internet and do many applications for data logging and control. In GPRS mode you can also connect to any remote FTP server and upload files for data logging.
This GSM modem is a highly flexible plug and play quad band GSM modem for direct and easy integration to RS232 applications. Supports features like Voice, SMS, Data/Fax, GPRS and integrated TCP/IP stack.


Use DC Power Adaptor with following ratings
· DC Voltage: 12V
· DC Current Rating at least: 1A
· DC Socket Polarity: Centre +ve& Outside –ve
· Current consumption in normal operation 250mA, can rise up to 1Amp peak while transmission so your power supply should be able to handle at least 1Amp current. Power supply is included in the packaging of this product.
The RS-232(X) is a communication cable, commonly used for transferring and receiving the serial data between two devices. It supports both synchronous and asynchronous data transmissions. Many devices in the industrial environment are still using RS-232 communication cable. Rs-232 cable is used to identify the difference of two signal levels between logic 1 and logic 0. The logic 1 is represented by the -12V and logic 0 is represented the +12V. The RS-232 cable works at different baud rates like 9600 bits/s, 2400bits/s, 4800bits/s etc. The RS-232 cable has two terminal devices namely Data Terminal Equipment and Data
communication Equipment. Both device will sends and receives the signals. The data terminal equipment is computer terminal and data communication Equipment is modems, or controllers etc.

Now a day‘s most of personal computers have two serial ports and one parallel port (RS232). These two types of ports are used for communicating with external devices and they work in different ways. The parallel port sends and receives the 8-bit data at a time over eight separate wires and this transfers the data very quickly, the parallel ports are typically used to connect a printer to a PC.
A serial port sends and receives one bit data at a time over one wire and it transfer data very slowly. The RS-232 stands for recommended slandered and 232 is a number X is indicates the latest version like RS-232c, RS232s.
The most commonly used type of serial cable connectors are 9-pin connector DB9 and 25-pin connector DB-25. Each of them may be male or female type. Now a days most of computers use DB9 connector for asynchronous data exchange. The maximum length of RS-232 cable is 50ft

   How to make a working RS232 – TTL level converter using MAX232
May 12, 2013Kasun Siriwardena
Today, most of the latest microcontrollers are integrated with at least one USART (Universal Synchronous Asynchronous Receiver Transmitter) peripheral. A well set module is very useful not only to establish a communication medium (receive or transmit data) between microcontroller and PC but also to program the device via preloaded boot-loader. Here is how to make proper level converter using MAX232. Before that, it is better to have a simple idea about RS232 & TTL levels.
RS232 & TTL
In RS232 protocol, +12V indicates the Space state or Zero and -12V indicates the Mark state or 1. In TTL, +5V indicates the Mark state and 0 indicates Space state. Most importantly, TTL level is not accepting negative voltage levels. So for the RS232 to TTL interface translation is not only a task with 12V to 5V level shifting, and it includes a level inversion too. To do that, MAX232 is more often used in microcontroller applications.
MAX232 is an IC that operates on single 5V supply and it is integrated with two drivers and two receivers and also that meets all specifications under EIA/TIA 232-F standards. All receivers can be used to convert RS232 levels to TTL/CMOS levels and all drivers can be used to convert TTL/CMOS level inputs to RS232 levels. Since it functions with capacitive voltage generator, external charge-pump
capacitors used in module to support internal voltage doubler and inverter. Some ICs in this series may not ask for external capacitors.(MAX225)

Let‘s take a closer look at for MAX232s‘ internal block diagram.8 pins of this 16 pin DIP are already reserved for 2 drivers and 2 receivers(as 2 receiver-inputs, 2 receiver-outputs, 2 driver-inputs and 2 driver-outputs). VCC and GND are for 5V single supply. 6 Other pins reserved for connect external capacitor network consist of 5 capacitors. 4 out of them are directly connected to the IC via 1-6 pins. 5th capacitor is connected between VCC and GND to bypass power rail noises. Presence or absence of this won‘t be enormously affect to the data communication. For a good practice, include all 5 capacitors to the design.

Fig.2: Basic internal block diagram of MAX232
As I stated earlier, those external charge-pump capacitors are to support ICs‘ internal voltage doubler and inverter. These 4 capacitors are musts for proper operation of this circuit(see fig.2). As I seen in a data sheet, non-polarized ceramic capacitors are also acceptable for this circuit. But I have several times tested with electrolytic capacitors. Each capacitor should be value of 1μF(105 if non-polarized). But additional care must be taken on their polarity if using electrolytics.
Connectivity precautions for electrolytic capacitors
 First capacitor should be connected between ICs‘ pin #1 & 3(C1+& C1-). Attach Positive to the pin #1 and negative to the pin #3.
 Second capacitor should be connected between ICs‘ pin #4 & 5(C2+& C2-). Attach Positive to the pin #4 and negative to the pin #5.
 Third capacitor should be connected between ICs‘ pin #2 & positivesupply(VS+& VCC). Make sure that, capacitors’ positive end connected to be Pin #2(VS+) and negative to the VCC.
 Fourth capacitor should be connected between ICs‘ pin #6 and ground(VS-& GND). Make sure That, capacitors’ negative end connected to be pin #6(VS-) and positive to the supply ground.
 Next (5th) capacitor may connected between supply VCC and GND.
That‘s all about external capacitors network. Now I‘m going to discuss about driver/receiver connections.
Understanding driver/receiver interface between RS232 – TTL
Take a look at figure 2. It has two TTL/CMOS inputs on pin #11 & 10. Inputs of those are internally pulled up for VCC through 400kΩ resistors. Their respective RS232 outputs are pin #14 & 7.

When input is open circuited, input for the inverter is 5V due to the pull-up. After the inversion, the output of inverter should be 0. But output of IC should be -12V for represent the ‖ IDLE‖ at Mark state according to RS232 standards. When Input is Low for TXin(X = 1,2), inverter input is 0 and inverter output will be 1. But output of the IC should be +12V for represent the Space state. If input goes High, for TXin, inverter input may left in High and inverter output will be 0. then output of the IC should be -12V for represent Mark state again. Simply,Jumping into receivers, This IC has two RS232 receiving inputs on pin #13 & 8. Those are internally pulled-down through 5kΩ resistors. Their respective TTL/CMOS outputs are pin #12 & 9.
When input is open circuited, input of the inverter remains Low. The output of the inverter should be High and output of the IC is 5V. When input is -12V, inverter input should be 0, and after inversion it should be 5V. same result occurs at ICs‘
output. When input is getting +12V, inverter input should be 5V, and after inversion it should be 0.

Understanding interconnections
Now I want to discuss about interconnections to PC to the μC(microcontroller). According to the EIA232 standards, PC is named as DTE(Data Terminal Equipment). So the device is named as DCE(Data Circuit-Terminating Equipment). Generally, DTE holds the Male port(DB9) and DCE holds the Female port. So it is very important to include a Female DB9 port, when designing a device. Even If you are using an serial extension cable to connect your device and PC, it should be consist one end with DB9 female and other end with DB9 male port and its‘ internal links should be parallel and straight through with no crossovers.
Let us start the connection from μC. μC has USART TX and RX pins at one port that may or may not be multiplexed with GPIO or other. TX is data transmitter and RX is data receiver. Both are working at TTL level. To proper data transmission TX should be connected to one.



  • Educational Institutions and Organizations: Currently we rely on putting up papers on notice boards to inform people of events. This method can be discarded by using wireless notice boards to display information in real time.
  • Crime Prevention: Display boards put up on roads will display tips on public security, accident prevention, information on criminals on the run. The board will help flash messages such as vehicle thefts as and when they occur.
  • Managing Traffic: In metropolitan cities we frequently come across traffic jams. One way to avoid this would be inform people beforehand to take alternate routes.
  • wireless notice board serves well for this purpose.
  • Advertisement: In shopping malls we get to hear the offers on various products from time to time. Instead we continuously display the information regarding the products and related offers on electronic display boards.
  • Railway Station: Instead of announcing the delay in arrival of trains we can display the information.


  • User friendly: Messages are only to be typed on a mobile or a computer, which in turn are displayed wireless on the display unit.
  • Eliminates use of printers: Since we don‘t use papers to display information, printers are also of no use in this system.
  • Faster means of transferring information: There is no delay in transmission of information. Messages are displayed in a matter of seconds after typing.
  • Long Range: As long as we have the required network coverage we can send messages from any part of the world.



sbitrs = P2^0;
sbitrw = P2^1;
sbit en = P2^2;
sfrlcd = 0x80;
void delay(int time)
void lcdcmd(unsigned char val)
lcd = val;
void lcddata(unsigned char val)
lcd = val;
void lcdstring(unsigned char *ptr)
unsigned char a=0;
void lcdini()
void serialini()
void main()
unsigned char g=0,mychar=0,c=0;
lcdstring("XBEE BASE LED");
lcdstring("MESSAGE DISPLAY");
if(g>=16 && c==0)



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