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Remote control using telephone
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Here is a teleremote 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 infrared and radio remote controls.
The circuit described here
can be used to switch up to nine appliances (corresponding to the digits 1 through
9 of the telephone key-pad). The DTMF signals on telephone instrument are used
as control signals. The digit ?0? in DTMF mode is used to toggle between the
appliance mode and normal telephone operation mode. Thus the telephone can be
used to switch on or switch off the appliances also while being used for normal
The circuit uses IC KT3170 (DTMF-to-BCD converter), 74154
(4-to-16-line demult-iplexer), and five CD4013 (D flip-flop) ICs. 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 O10 (at pin 11) of IC2 (74154). The active
low output of IC2, after inversion by an inverter gate of IC3 (CD4049), becomes
logic 1. This is used to toggle flip-flop-1 (F/F-1) and relay RL1 is energised.
Relay RL1 has two changeover contacts, RL1(a) and RL1(b). The energised RL1(a)
contacts provide a 220-ohm loop across the telephone line while RL1(b) contacts
inject a 10kHz tone on the line, which indicates to the caller that appliance
mode has been selected. The 220-ohm loop on telephone line disconnects the ringer
from the telephone line in the exchange. The line is now connected for appliance
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 appliance mode of operation, if digit ?1? is dialed,
it is decoded by IC1 and its output is ?0001?. This BCD code is then demultiplexed
by 4-to-16-line demultiplexer IC2 whose corresponding output, after inversion
by a CD4049 inverter gate, goes to logic 1 state. This pulse toggles the corresponding
flip-flop to alternate state. The flip-flop output is used to drive a relay
(RL2) which 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 and 10kHz tone needs to be removed from the telephone line.
To achieve this, digit ?0? (in DTMF mode) is dialed again to toggle flip-flop-1
to de-energise relay RL1, which terminates the loop on line and the 10kHz tone
is also disconnected. The telephone line is thus again set free to receive normal
calls.This circuit is to be connected in parallel to the telephone instrument
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