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1. Introduction
Telephone is one of the most common and useful consumer electronics gadgets in the world today. It incorporate both Communication & Digital Electronics filed. They provide us with countless conveniences in our daily lives.
As the device is so much popular and part of every-day-activities, many add-on circuits are constructed to make it still more feature rich instrument. And one such add-on feature is this system which allows user to switch ON/OFF loads from remote end. It finds applications in homes, offices and even in industries.
Here is a telephone operated device control circuit which enables switching ‘ON’ and ‘OFF’ of appliances through telephone lines. It can be used to switch appliances from any distance, overcoming the limited range of infra-red and radio remote control.
Basically this system performs the following operations:
1. It senses the “real” telephone ring.
2. Automatically switches the call to this system, after predefined rings.
3. Switches ON/OFF the load depends upon the key pressed by the caller.
MAIN FEATURES OF THE PROJECT:
1. Effective in implementation.
2. Low power consumption, and compact in size.
3. High reliability, due to the usage of semiconductor devices.
4. Easy to use.
2. BLOCK DIAGRAM & ITS EXPLANATION
The explanation of this system goes like this:
In Brief: This system is connected to the telephone lines in parallel to the user’s Telephone Set. Whenever the call comes, detects it and starts counting the number of rings. If the rings are equal to pre-defined number, say five, it simply means user is not present and the system has to attend the call. So the system switches the call to this unit, so that it can detect the commands send by caller. Depends upon the key pressed by the caller, the respective load goes ON/OFF or latches.
The Block Diagram, as shown in Fig 2.1, of this system is explained in these blocks: RC Coupling, Ring Detector, Schmitt Trigger, Ring Counter, Switching Unit, DTMF Decoder, BCD to Binary Converter, Latch Unit, Driver Unit and finally Power Supply. Let us see the details of each part in the following explanation.
RC NETWORK: This is nothing but a resistor, capacitor & diode combination across the Telephone line to match the impedance & reduce input voltage to required level.
RING DETECTOR: The ringing signal of 75 Vrms, 15 Hz from the Telephone line is simultaneously given to Telephone Set and this Ring Detector block. This block identifies that the call is coming to the Telephone Set, and thus alerts this system. The output of this block is given to Schmitt Trigger for conversion.
SCHMITT TRIGGER: This block converts ring signal [which is AC voltage of about 75Vrms, with ringing tone duration of 0.4 sec ON – 0.2 OFF] into digital pulses for further processing. This is required as rest of the system needs digital pulses for manipulation and taking appropriate decisions. The output of this block is given to Ring Counter.
RING COUNTER: This block counts the number of ringing pulses, so that after predefined rings [say 5 rings] this system must lift the cradle automatically to attend the incoming call. The output of this block is given to Switching Unit for decision making step.
SWITCHING UNIT: This block takes the information, the number of rings, from Ring Counter block and decides whether to attend the call automatically or leave it for humans hanging around Telephone Set. The number of rings crosses the predefined rings [say 5 rings] this block concludes that there are NO humans to attend the call, hence lifts the call for itself.
DTMF DECODER: This block acts as Demultiplexer and receives the command [key pressed] sent by caller, decodes Dual Tone Multiple Frequency coded commands into BCD form and fed to next block for further processing.
BCD TO BINARY CONVERTER: This block converts the BCD [Binary Coded Decimal] form commands into Binary form and thus gets eight output points for manipulation.
LATCH UNIT: This block latches the condition of the load to any one form: ON or OFF. This is basically a Flip Flop, which changes to High or Low state whenever any trigger pulse is given.
DRIVER UNIT: This block drives the low impedance relay with the help of high current and high voltage Darlington transistor pair.
LOAD: To this system user can connect any mains operated loads, depend upon one’s requirement. For example home user can connect Lights, Washing Machine, Air-conditioner, Fan as loads and Industrial users can connect Motors, Security Cameras, Lights,
POWER SUPPLY: This Power Supply Unit provides all necessary working voltages to circuits gathered to make this project.