| 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. |
| Click
here for circuit dia.
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