Gaz Man wrote in message . ..
Hi all. Thanks in advance for any help you can give. Please note my
email address is invalid - this is to stop spam. Reply to gazman at
pobox dot com to email me directly.

Here's my problem :

My front gate is 150M from the house and I will have an intercom and
system to open the gates, turn on the lights, etc etc.

I ran two Cat 5E cables (total 16 conductors) underground. Somehow one
of the cables has been cut, so I have only 8 conductors left. The
intercom requires 5 of these, so I have just 3 conductors to control
the gates, lights, fountain etc as well as send a signal back to the
house to indicate what position the gate is in.

So I need a circuit that can send a coded signal down the wire, which
is decoded at the other end to open or close a relay.

My suggestion is this : At the house, each of 6 control buttons sends
a signal down the wire at a unique frequency. At the gate a circuit
detects the signal, and according to its frequency, toggles one of 6
relays. That way, multiple relays can be activated using just a pair
of wires.

It would NEVER occur that two control buttons are pressed at the same
time.

There are other ways of coding the signal (pulse count for example)
and I have used frequency just as an example.

Does anybody know where I can find a circuit to do this sort of thing?
I have searched the web without success.

Thanks in advance.

Gaz

If you have 6 distinct control signals, and only one can be sent at a
time, you could use the 3 conductors as a simple digital link. With 3
bits, there are 8 possible combinations. If you use a 3-8 decoder (a
readily available IC), you can tie 6 of the outputs to relays (be sure
the relay doesn't exceed the decoder's max output current) and use one
of the remaining two combinations as a "no action" signal. At the
house end, you could use a priority-encoder IC (such as the 74F148) to
generate these signals.

It would also be possible to use tone encoding as you described, and
this would have the advantage that you could press more than one
button at a time. Simply connect 6 tone generators (maybe built from
555/556 timer IC's) to a summing amplifier at the input end, and 6
tone decoders at the output. This would, however, be much more
difficult than the digital method.