Gaz Man writes:
...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.
....
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.

IF you could still find an old dumb uart chip, one that didn't need
all the programming registers configured, that just woke up and
presented the last received serial byte on the 8 output lines...
THEN you could use one line for the clock and a second line for the
serial data. You are beyond the specificaitons for RS232 to go that
far but it might work anyway. Or you could just use beefier drivers.
I remember a little circuit that used a 74H00 nand gate that was
claimed to drive signals hundreds of meters.

So, the circuit for that is just to find a really really old uart,
maybe on Ebay, that is so dumb that all it knows how to do is work.
Clock in, data in, and you just keep sending the same byte to it
with the appropriate bits set. To avoid needing crystals and all
the mess of getting the frequency just right you could use the
sending uart's clock signal sent down the second wire. Usually
this clock signal is something like 16x the actual data rate and
you feed the same clock to both uarts. The actual clock rate is
probably a don't-care in your situation, just don't have it jumping
around. You might be able to configure a PC to deliver the clock
out, I don't remember whether there is any way to get that out of
new uart's today or not.