31-05-2012, 05:06 PM
Telephone-based Messaging System
Telephone-based Messaging System.pdf (Size: 579.66 KB / Downloads: 72)
Abstract
This report gives details of implementation of the telephone-based messaging system. The text message can be sent across the telephone lines. Whenever the user at the transmitter end telephone, calls the receiver end telephone, a line is established by the telephone exchange. The transmitter end telephone line is attached to the set up, that consists of an AT89C52 controller, to which a keyboard, an LCD and a DTMF encoder chip is interfaced. Keyboard and LCD are used to type the text message and view it at the transmitter end respectively.DTMF encoder is used to send the text message information across the telephone line. The receiver end telephone line is attached to the set up that consists of an AT89C52 controller, to which an isolating transformer, optocoupler, DTMF decoder chip and LCD is interfaced. After five rings, Rx-end controller sets the phone in off-hook condition. Now the DTMF encoded text message data sent from the transmitter end is decoded by DTMF decoder chip and the text message gets displayed on the LCD at receiver end.
Introduction
Whenever the Tx-end telephone calls the Rx-end telephone, the optocoupler converts the 25 Hz sinusoidal ringing signal into square pulses. Optocoupler also provides electrical isolation between the telephone lines and the Rx-end circuitry. The internal counter of the microcontroller counts the predefined number of pulses at the output of optocoupler and sets the phone in off-hook condition by placing a low value resistor across the telephone lines. The Tx-end and the Rx-end lines are now connected and data can be exchanged across them. The bandwidth of the channel of communication is the audio range frequency (<4KHz). The DTMF signals are in the same range. DTMF signals corresponding to the text message, typed by the user by pressing the keys and displayed at LCD at the Tx-end, are generated by MT8880 DTMF transceiver and sent across the telephone lines.
4N25 Optocoupler and 6V relay:
Optocoupler is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, keeping them electrically isolated. In our application, it electrically isolates a part of the Rx-end circuitry from telephone lines. Whenever the Tx-end telephone calls the Rx-end telephone, the exchange sends a 100Vpp 25 Hz ringing signal superimposed on -48V DC to the Tx-end. When a telephone is set in the off-hook condition (that is, telephone is picked up), a low impedance (typically 300-1500ohms) is placed across the telephone lines on both, Tx-end and Rx-end and a line is established between them.
Data Send-Receive Protocol
Each burst of data to be sent is sent using asynchronous communication mode. When the send button is pressed on the keyboard, firstly a start is sent and then data from data space is being sent in reverse order of typing, by decrementing the data pointer continuously. After the end of each 4 bit code, a separator is sent so that only the valid codes are received. Each 4 bit code goes to encoder with delay of 200 ms. When the data space is empty a 4 bit stop code is sent which is same as the start bit. Thus, the valid characters will be between the start and stop.
Conclusion and Future Work
The telephone-based messaging system was implemented and tested successfully on the telephone simulator. The project can be extended to, sending the message to mobile phone through GSM network. For this, it will be required to interface the GSM module at the transmitter end telephone.