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 conversation.
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 |
