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circuit diagram for the dtmf based device switching
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 Circuit diagram for the DTMF based device switching

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CIRCUIT DIAGRAM

 

 

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

WORKING DETAILS OF THE CIRCUIT

 

The working of the circuit is as follows:

Once a call is established (after hearing ring-back tone), dial '0' in DTMF mode. IC1 decodes this as '1010,' which is further demultiplexed by IC2 as output 10 (at pin 11) of IC2 (74154). The active low output of IC2, RL10 is energised. The after inversion by an inverter gate of IC3 (74LS04), becomes logic 1. This is used to toggle flip-flop (IC9 (B)) and

relay RL10 is energised. The energized RL10 contact provides a 220-ohm loop across the telephone line. The

220-ohm loop on telephone line disconnects the ringer from the telephone line in the exchange. The line is now

connected for control mode of operation. If digit '0' is not dialed (in DTMF) after establishing the call, the ring continues and the telephone can be used for normal conversation. After selection of the control mode of operation, if digit '1' is dialed, it is decoded by IC1 and its output is '0001'. This BCD code is then de-multiplexed by 4-to-16-line demultiplexer IC2 whose corresponding output, after inversion by a 74LS04 inverter gate, goes to logic 1 state. This

pulse toggles the corresponding flipflop to alternate state. The flip-flop output is used to drive a relay (RL1) that

can switch on or switch off the appliance connected through its contacts. By dialing other digits in a similar way,

other appliances can also be switched 'on' or 'off. Once the switching operation is over, the 220-ohm loop resistance needs to be removed from the telephone line. To achieve this, digit '0' (in DTMF mode) is dialed again to toggle flip-flop-10 to de-energise relay RL10, which terminates the loop on line. The telephone line is thus again set free to receive normal calls. This circuit is to be connected in parallel to the existing telephone instrument.