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Wireless Led Dot Matrix Notice Board Essay

POWER SUPPLY

Power supply section power the whole circuitry. Different section of this system requires different voltage levels. This requirement cannot be achieved using a single power adapter or battery. So a power supply consist of dc-dc converter is used in our project. The DC-DC converted dc power supply can provide 12v dc, 9v dc and 5 v dc simultaneously for display, discrete IC and micro controller respectively. The power supply section is also included with short circuit protection to prevent system being damaged due to the careless usage or electrical malfunctions.

CONTROLLER

The controller used here is PIC 16f877a, which is an 8 bit micro controller used for general purpose medium scale applications. In our project this device retrieves signals from the RF receiver as serial data through its URAT port on a baud rate of 2400 bps and stores in its internal data memory. From there it sends the data one by one to display section to display it. The scrolling algorithm is also done through the program written in the controller. This controller also communicate with the RFID reader to fetch the ID of RFID card brought to its proximity and then retrieves the corresponding registry entry to display it in the display board. The scrolling message mode and personal message mode is switched with help of two way switch attached in the display board end. When the switch is in its normal position the message from the PC is displayed and when the switch is in other position the board will display the personal information of the student whose card is in the proximity of the RFID reader.

DISPLAY

The display used here are dot matrix LED. The whole panel consist of 32 displays each one of 5×7 dot matrix LED. Each character in this display is formed using four such display units. That means the whole display can show a total of 8 characters at a time. The display is drived and controlled using a ring counter IC 4017. By this IC we select each display independently and their column.

It uses persistence of vision to let you drive the 32 led matrixes with only 10 microcontroller outputs! Normally you would need 32outputs for 32 LEDs but by using multiplexing and a helper chip you can get away with 10. LED Display multiplexing simply means turning on one led for a short period of time and doing this repeatedly for each LED. If you do this fast enough then your eye will not notice any flicker.The LEDs are no different to any other LEDs but it saves a huge amount of soldering as all the wiring has been done for you..Or you could wire it up yourself if you can’t get hold of the module. Still only need 10 control wires (just wire you leds the same as shown in the module diagram). Your eye reacts slowly to changes in light intensity so that if a light is turned on and off quickly enough then it does not notice that the light is off. Basically your eye remembers a light pulse for a short time. The approximate time is 20ms so if the light is turned on at a frequency >50Hz (1/20ms) then your eye will not notice any flicker at all. Multiplexing uses this fact to reduce the number of pins needed to drive an LED display.

You can do this by splitting the 32led displays into 7 rows and 5columns which lets you drive it using 7 row outputs and 5 column outputs. In fact the 5×7 led matrix block used here has all the leds arranged in this way already. Each row is driven in turn and as long as all of the rows are driven within a time period of 20ms it will appear as though the LEDs are on continuously. To turn a specific led ‘ON’, data is output to the column drivers when a row is driven. To save more pins it is common to use a helper chip and in this project it is a Johnson counter (a 4017). This generates a walking one every time that it’s clocked. Since you only want one row on at a time it is the ideal chip for this application. In this project when the 4017 has been reset it outputs logic high at Q0 – which is not connected – so during reset the 4017 does nothing.

This allows you to use the column driver port for something else if you want to when you are not driving the LEDs.To drive the 4017 all you need is two pins one for reset and one for clock. So to fully drive the 32 matrix led display you need only 4 4017. The most difficult thing about using the dot matrix LED display is defining the characters. Basically for ASCII characters you need an array of 128 blocks each having 8 column data numbers. The usual way is to get out a piece of graph paper and define your characters by drawing blocks where a pixel is on. You then translate each line into hex (binary to hex is very easy) and then transfer this information to your program source code.

RFID READER

RFID is used here to implement the personal information display. RFID technique helps the system for identifying each student with the help of a unique ID given to each student. This reader is capable of powering and reading data from an RFID card which is brought ot its proximity. It has a transmission section through which it sends the received data to controller. The range of this RFID is approximately about 10 cm so that it can avoid unwanted detection of RF signals.

In our system RFID system consists of a reader and one or more tags. The reader’s antenna is used to transmit radio frequency (RF) energy. Depending on the tag type, the energy is “harvested” by the tag’s antenna and used to power up the internal circuitry of the tag. The tag will then modulate the electromagnetic waves generated by the reader in order to transmit its data back to the reader. The reader receives the modulated waves and converts them into digital data. In the case of the Parallax RFID Reader Module, correctly received digital data is sent serially through the SOUT pin. There are two major types of tag technologies. “Passive tags” are tags that do not contain their own power source or transmitter. When radio waves from the reader reach the chip’s antenna, the energy is converted by the antenna into electricity that can power up the microchip in the tag (known as “parasitic power”).

The tag is then able to send back any information stored on the tag by reflecting the electromagnetic waves as described above. “Active tags” have their own power source and transmitter. The power source, usually a battery, is used to run the microchip’s circuitry and to broadcast a signal to a reader. Due to the fact that passive tags do not have their own transmitter and must reflect their signal to the reader, the reading distance is much shorter than with active tags. However, active tags are typically larger, more expensive, and require occasional service.

The RFID Reader Module is designed specifically for low-frequency (125 kHz) passive tags. Frequency refers to the size of the radio waves used to communicate between the RFID system components. Just as you tune your radio to different frequencies in order to hear different radio stations, RFID tags and readers have to be tuned to the same frequency in order to communicate effectively. RFID systems typically use one of the following frequency ranges: low frequency (or LF, around 125 kHz), high frequency (or HF, around 13.56 MHz), ultra-high frequency (or UHF, around 868 and 928 MHz), or microwave (around 2.45 and 5.8 GHz).

Here we are using an RFID Reader Module with a single TTL-level. The current consumption of the module will increase dramatically when the module is active. A visual indication of the state of the RFID Reader Module is given with the on-board LED. When the module is successfully powered-up and is in an idle state, the LED will be GREEN. When the module is in an active state and the antenna is transmitting, the LED will be RED. The face of the RFID tag should be held parallel to the front or back face of the antenna (where the majority of RF energy is focused).

If the tag is held sideways (perpendicular to the antenna) you’ll either get no reading or a poor reading. Only one transponder tag should be held up to the antenna at any time. The use of multiple tags at one time will cause tag collisions and confuse the reader. The two tags available in the Parallax store have a read distance of approximately 3 inches. Actual distance may vary slightly depending on the size of the transponder tag and environmental conditions of the application. When a valid RFID transponder tag is placed within range of the activated reader, the unique ID will be transmitted as a 12-byte ASCII string via the TTL-level SOUT (Serial Output) pin in the following format:

RF RECEIVER

The RF receiver is used to receive RF wireless signals which are transmitted fro the pc section and feed it to the controller. This receiver works in the principle of ASK modulation, an analogue modulation technique to implement digital transmission. This receiver has an active antenna. It works with power of 5V and sink a current of 0.1 mA. The maximum baud rate allowable with this receiver is 2400 bps. For efficient operation we uses 1200 bps. The range of this receiver is with in 100 mtrs. But it can be extended to kms be increasing the power of transmitter .

PC SECTION

PC section contains application software which is used to interface with the computer. This application has a text box where we can enter the message to be displayed in the notice board. On hitting the send button the data in the text box is transmitted to the display board wirelessly.

RS 232 – TTL CONVERTER

The signal obtained from th PC is of RS 232 standard. Which means the signal from the PC will be in the range of +12 to -12V level. For the transmitter and the controller to understand the signal it should be in the range of 0 – +5 V. so the signal should be level converted before transmission. For this level conversion we use MAX 232 IC. This IC is dedicated for RS 232 to TTL level conversion. With the help of some discrete components this IC can convers the level to ttl logic.

RF TRANSMITTER

RF transmitter transmits the digital signal generated by the level converter through space as electromagnetic waves. The transmitter uses ASK modulation technique. The range varies up to 100mtrs. The power varies from 5V to 12v. The more the power supply voltage the more will be its quality and range.


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