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INTRODUCTION
Long distance computer communication has become a challenge of the present day technology. Recently these computer communication networks are also termed as information networks. Presently there are many information services for information exchange like Teletex, Electronics mail, Video teleconferencing, Facsimile transmission (FAX), which are quit efficient but expensive too.
But there could be a situation where the terminal from which we want to down load or upload few lines or files or say information, may be “OFF” from the mains. Thus all the technologies are failed. And this is, where our project “Telephone Controlled Switch” comes into picture.
To establish an interactive page link with such a terminal ( which may be placed anywhere in the world ), we have to put the mains “ON” and then communicates with it using Modems on a telephone network. To materialize such a system we would require some circuitry which would receive commands on telephone line and can switch mains load “ON” or “OFF” ( i.e. the “Telephone Controlled Switch”).
The two major parts of the system are, the hardware configuration and software design. In the subsequent section we will deal with each block as well as the required software in detail.
Our project is to eliminate the problem by using such a circuitry, which can switch the remote terminal ON/OFF without anybody being present at the remote location. This aim is achieved by using TCS (Telephone Controlled Switch).
The TCS (Telephone Controlled Switch) designed here is capable of controlling up to eight main powered loads with the aid of commands received via telephone. Any tone-dialing (DTMF) telephone set or hand-held tone dialer may be used to send commands to the switching unit and remotely control a wide range of mains appliances in and around home, factories, industries etc. This system is designed to have a personal access code and with system feedback facility.
SYSTEM BLOCK DIAGRAM AND OPERATION
OPERATION:
The circuit is connected to the telephone network just like any normal telephone set. On being called, the circuit waits for predetermined number of ring signals and then answers the call (electrically it lifts the receiver). Next it waits for preprogrammed system access code, which the caller must transmit with the DTMF keypad on his telephone.
The unit is capable of switching up to light loads ON and OFF. By virtue of relays, high voltage and currents may be switched, so that loads may include mains powered ones. Reception of correct system access code is acknowledged with a short tone, which the caller can here.
UNITS OF TCS :
Ring detector Unit:
The ring detector Unit consists of a diode bridge, voltage controllers, an opto-coupler, Voltage and some other components. The main function of this unit is to detect a ringing signal and rectifies it. This rectified signal interrupts controller.
Switching Unit: -
The switching unit consists of a double pole double throw relay the . This unit switches the in coming Telephone line to the DTMF decoder after call is detected.
DTMF decoder Unit : -
This unit consists of a DTMF decoder IC 8870. Which receives signals from the tone pair and generates a 4-bit code recording to the received input. These 4-bit code are given to the  controller unit.
Micro controller Unit: -
This unit is the heart of all the circuitry. The unit controls all the operation performed by the ICS. This unit is composed of 89CSI  controller.
Assurance tone generator Unit :-
For the feedback purpose to confirm about device status to person at remote location, musical IC UM 66 is used as assurance tone generator.
Driver Unit :-
This unit consists of driver transistor and current limiting resistor. These transistors switches ON & OFF & controls the corresponding relays ON & OFF. This unit receives control signals from the part 2 of  controller.
Relay Unit: -
This unit consists of 7 relays ( single pole double through ) (12 v/2 Sov/5A). These relays are driver ON or OFF by driver unit & thus it switches the mains line ON or OFF & thus the load. The relay having different rating can be used recording to the desired application.