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Teleremote
Control
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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 |
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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 conversation.
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| 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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