Help please - circuit to control multiple relays using 3 wir

[Gaz Man]

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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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

 

[Ken Taylor]

"Gaz Man" <gaz@gaz.com> wrote in message
news:9qkp80hac9od4bpvvbfa85127v91m7rtrt@4ax.com...

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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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

You could try an RS-232 interface if you went for microcontrollers, but if
you feel this is overkill you could look up DTMF on the web and use
encoders/decoders for that. There'll be plenty of hits for that.

Cheers.

Ken

 

[John Tserkezis]

Gaz Man wrote:

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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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.

"Somehow one of the cables has been cut" is a bit vauge, were they not
protected with conduit?

Are your cables above or below ground? You will need to consider ESD
protection if they're above ground, heck, would even be a good idea for below
ground.

I was going to suggest voltage sensing, but at 150m, current would be better.
(different current levels for different functions). That would only require
one wire (well, two, the other being 0v reference). You can use the other free
wire for return communications.

If you implement something like the Philips I2C interface, you would have
more than enough comms channels for your application.
Might have to slow it down due to distance, and perhaps change from the ttl
interface to something higher and more immune to interference, but since your
application isn't time critical, it would be feasable to slow it down a lot.
Even a few seconds delay wouldn't be unacceptable in your case.

Then again, if it's going to be that elaborate, RS485 might be a better
choice. It'll need some single chip micro smarts at either end, but that will
give you virtually unlimited comms capability in that case.
You should be able to find 485 interface ICs with at least some built-in ESD
protection as well. The two comms wires plus one 0v reference is just enough
for this purpose.

--
Linux Registered User # 302622 <http://counter.li.org>

 

[Gaz Man]

The cables are 600mm below ground in a trench carrying water pipes and
telephone cables. They are in their own 25mm conduit designed for
underground electrical cables. A single cable traverses the entire
distance - there are no joins.

I really don't know how the wire got cut but I have a suspicion. The
electrician who wired the house said he saw the conduit in the pit and
became curious about what it contained. He thought I was putting
illegal electical wires in. He admits he drilled a hole into the
conduit to see the cables inside, and then sealed the hole with glue.
He swears he did not damage a cable in doing so but.......

The DTMF idea sounds hopeful. I'll look into that first...

Gaz








On Mon, 26 Apr 2004 20:09:08 +1000, John Tserkezis
<jt@techniciansyndrome.org.invalid> wrote:

Gaz Man wrote:

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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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.

"Somehow one of the cables has been cut" is a bit vauge, were they not
protected with conduit?

Are your cables above or below ground? You will need to consider ESD
protection if they're above ground, heck, would even be a good idea for below
ground.

I was going to suggest voltage sensing, but at 150m, current would be better.
(different current levels for different functions). That would only require
one wire (well, two, the other being 0v reference). You can use the other free
wire for return communications.

If you implement something like the Philips I2C interface, you would have
more than enough comms channels for your application.
Might have to slow it down due to distance, and perhaps change from the ttl
interface to something higher and more immune to interference, but since your
application isn't time critical, it would be feasable to slow it down a lot.
Even a few seconds delay wouldn't be unacceptable in your case.

Then again, if it's going to be that elaborate, RS485 might be a better
choice. It'll need some single chip micro smarts at either end, but that will
give you virtually unlimited comms capability in that case.
You should be able to find 485 interface ICs with at least some built-in ESD
protection as well. The two comms wires plus one 0v reference is just enough
for this purpose.

 

[Ken Taylor]

"Gaz Man" <gaz@gaz.com> wrote in message
news:hfqp801aik3uis96plvdq6b2b7eb832k6v@4ax.com...

The cables are 600mm below ground in a trench carrying water pipes and
telephone cables. They are in their own 25mm conduit designed for
underground electrical cables. A single cable traverses the entire
distance - there are no joins.

I really don't know how the wire got cut but I have a suspicion. The
electrician who wired the house said he saw the conduit in the pit and
became curious about what it contained. He thought I was putting
illegal electical wires in. He admits he drilled a hole into the
conduit to see the cables inside, and then sealed the hole with glue.
He swears he did not damage a cable in doing so but.......

The DTMF idea sounds hopeful. I'll look into that first...

Gaz

Another one is CTCSS tones. These may be a little more convenient in that

there are more of them to pick from and you can use the two surviving wires
(use one of the twisted pairs) as a differential cable for the audio tones.
Use the same wires for both directions, and have detectors at each end for
just the signals you want to hear at each end. There may be some issues with
doing this with DTMF (false detection, etc), less so with CTCSS. However it
may be a little harder to obtain cheap CTCSS decoder/encoder IC's as
compared to DTMF. Time for a-huntin'!

Ken

 

[Wing Fong Wong]

Gaz Man <gaz@gaz.com> wrote:

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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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
With 3 conductors you can have a tx, rx and gnd. Send six bits down the line

one to toggle each of the six switch attached to 6 d-type FF which are inturn
attached to a SIPO. On reciving a seventh control bit the device side of
things then sets to determin the state of the gate, ie either open or closed.
Hmm... on second thoughts just scrap the FFs and just use a micrcontroller.

Well anyway, if the gate is issued to change states, then wait until the gate
arrives in that state to send and ack of the pos of the gate.

On the house side of things, you have a similar microcontroller reading inputs
and sending on the info and reading the responses from the gate.

I've done this sort of thing for distances of a metre or so but not 150m. I
dunno how good the signal will be at that distance.

P.S. Sorry for the incoherent ramblings, I've been awake for 40 hours
straight
--
Wing Wong.

 

[Miro]

"Gaz Man" <gaz@gaz.com> wrote in message
news:9qkp80hac9od4bpvvbfa85127v91m7rtrt@4ax.com...

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.

Just change the intercom to a two wire version. Use C-Bus controller for the
rest of it.

 

[#HISH]

Id be finding and repairing the break.
The electrician seems suspicious and if it turns out you are right, he will
have to pay your expenses.
Word your threat the right way and he will be there with a shovel to help
you dig.
Wayno
"Gaz Man" <gaz@gaz.com> wrote in message
news:9qkp80hac9od4bpvvbfa85127v91m7rtrt@4ax.com...

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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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

 

[John Crighton]

On Mon, 26 Apr 2004 20:57:47 +1000, Gaz Man <gaz@gaz.com> wrote:

The cables are 600mm below ground in a trench carrying water pipes and
telephone cables. They are in their own 25mm conduit designed for
underground electrical cables. A single cable traverses the entire
distance - there are no joins.

I really don't know how the wire got cut but I have a suspicion. The
electrician who wired the house said he saw the conduit in the pit and
became curious about what it contained. He thought I was putting
illegal electical wires in. He admits he drilled a hole into the
conduit to see the cables inside, and then sealed the hole with glue.
He swears he did not damage a cable in doing so but.......

The DTMF idea sounds hopeful. I'll look into that first...

Gaz

Hello Gaz,
There are ways to detect breaks in cables, I would be
folowing up that course, digging and repairing the break.

Have you done resistance checks? You might be lucky
and find a couple of shorted wires. Check at both ends
of the cable run.

If the wires are cut clean and are open circuit then maybe
a slab of beer to a friendly telecom linesman might be a
good idea. They have a gadget that can indicate how far
away a break or short in a cable is. Quite accurate too.

Regards,
John Crighton
Hornsby

 

[KLR]

On Mon, 26 Apr 2004 20:57:47 +1000, Gaz Man <gaz@gaz.com> wrote:

The cables are 600mm below ground in a trench carrying water pipes and
telephone cables. They are in their own 25mm conduit designed for
underground electrical cables. A single cable traverses the entire
distance - there are no joins.

I really don't know how the wire got cut but I have a suspicion. The
electrician who wired the house said he saw the conduit in the pit and
became curious about what it contained. He thought I was putting
illegal electical wires in. He admits he drilled a hole into the
conduit to see the cables inside, and then sealed the hole with glue.
He swears he did not damage a cable in doing so but.......

The DTMF idea sounds hopeful. I'll look into that first...

Gaz

I would find it very difficult to be able to drill a hole through a
conduit without a very good chanceof hitting a wire in the process.

I would get hold of him to come and fix the damage, he had no right to
damage your private property like that, and should have asked or
minded his own business.

There is also a risk that the cable was damaged sufficiently to make
it so that the other conductors may fail or at very least become
uninsulated and corrode or short. Probably will get damp down there
in that conduit and help speed the corrosion process too, resulting
in complete failure

On Mon, 26 Apr 2004 20:09:08 +1000, John Tserkezis
jt@techniciansyndrome.org.invalid> wrote:

Gaz Man wrote:

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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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.

"Somehow one of the cables has been cut" is a bit vauge, were they not
protected with conduit?

Are your cables above or below ground? You will need to consider ESD
protection if they're above ground, heck, would even be a good idea for below
ground.

I was going to suggest voltage sensing, but at 150m, current would be better.
(different current levels for different functions). That would only require
one wire (well, two, the other being 0v reference). You can use the other free
wire for return communications.

If you implement something like the Philips I2C interface, you would have
more than enough comms channels for your application.
Might have to slow it down due to distance, and perhaps change from the ttl
interface to something higher and more immune to interference, but since your
application isn't time critical, it would be feasable to slow it down a lot.
Even a few seconds delay wouldn't be unacceptable in your case.

Then again, if it's going to be that elaborate, RS485 might be a better
choice. It'll need some single chip micro smarts at either end, but that will
give you virtually unlimited comms capability in that case.
You should be able to find 485 interface ICs with at least some built-in ESD
protection as well. The two comms wires plus one 0v reference is just enough
for this purpose.

 

[Phil]

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.

This can be done without logic circuits at all by utilising AC instead of DC
for some logic states. You have three wires. Lets label them A, B and C. On
the gate end, you can measure the difference between the following
connections and see whether there is a DC voltage, what polarity that
voltage is, and whether there is an AC voltage. This measurement can be
between conductors A and B, B and C, and C and A. This already gives you a
total of 6 independent devices that you can control (from either end too!)
without any more logic circuits than a diode (half wave filter) and
capacitor running each relay you want to control.

Eg. If cable A is positive and cable B is negative, activate lights relay
through diode 1.
If B is positive and A is negative, activate fountain relay through diode 2.
If there is AC through A and B, activate fountain and lights relay through
the same diodes. (add a small capacitor if necessary)

Repeat for cables B-C, and C-A. Use separate independent power supplies back
at the house.

Cheers,
Phil

 

[Wayne]

"Gaz Man" <gaz@gaz.com> wrote in message
news:9qkp80hac9od4bpvvbfa85127v91m7rtrt@4ax.com...

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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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

Interesting!

After reading through all the other posts I would like to suggest to you the
following.

3 wires = 8 states. ie. 000,001,010,011,100,101,110,111
Using the ground from the intercom system.

From the above, this can be run into some logic gates or a PIC.

It appears you have given up on the sparky. Possibly he won't come back and
possibly because he doesn't know where the break is.

You could as another idea connect new cable to the existing good (unbroken)
cable and pull out the old cables and pull in new cables. Just add plenty of
lube to overcome the friction.

Wayne.

 

On Mon, 26 Apr 2004 20:57:47 +1000, Gaz Man <gaz@gaz.com> wrote:

The cables are 600mm below ground in a trench carrying water pipes and
telephone cables. They are in their own 25mm conduit designed for
underground electrical cables. A single cable traverses the entire
distance - there are no joins.

I really don't know how the wire got cut but I have a suspicion. The
electrician who wired the house said he saw the conduit in the pit and
became curious about what it contained. He thought I was putting
illegal electical wires in. He admits he drilled a hole into the
conduit to see the cables inside, and then sealed the hole with glue.
He swears he did not damage a cable in doing so but.......

Reality would indicate to fuck off all the other ideas and :

Dig up where the cable was damaged and remove the glue and replace
with a proper joiner(s) and then pull through two new cat5 feeds using
lots of slippery lube.

If you need to add a joiner, then you're going to have to cut the
existing cables and rejoin to pull through the new cabling.

150m is a long feed without a pit in between, I'm not surprised that
the cable wasn't damaged during the installation process.

 

[GM]

<KLR> wrote in message news:7ltr805kheh4d0e3ho860lmb4vj77dguch@4ax.com...

On Mon, 26 Apr 2004 20:57:47 +1000, Gaz Man <gaz@gaz.com> wrote:

The cables are 600mm below ground in a trench carrying water pipes and
telephone cables. They are in their own 25mm conduit designed for
underground electrical cables. A single cable traverses the entire
distance - there are no joins.

I really don't know how the wire got cut but I have a suspicion. The
electrician who wired the house said he saw the conduit in the pit and
became curious about what it contained. He thought I was putting
illegal electical wires in. He admits he drilled a hole into the
conduit to see the cables inside, and then sealed the hole with glue.
He swears he did not damage a cable in doing so but.......

The DTMF idea sounds hopeful. I'll look into that first...

Gaz


I would find it very difficult to be able to drill a hole through a
conduit without a very good chanceof hitting a wire in the process.

I would get hold of him to come and fix the damage, he had no right to
damage your private property like that, and should have asked or
minded his own business.

There is also a risk that the cable was damaged sufficiently to make
it so that the other conductors may fail or at very least become
uninsulated and corrode or short. Probably will get damp down there
in that conduit and help speed the corrosion process too, resulting
in complete failure

Yes - the electrician was/is a dickhead!!!

 

[Gaz Man]

Thanks to all for your suggestions for my problem.

I have decided to try the DTMF option because of its simplicity and
the easy availability of decoders etc.

To respond to some of the other suggestions :

I can't dig up the cable because substantial landscaping has gone over
the top. It would involve moving tons of rock and concrete, and
wrecking alot of lawn and garden. And that's not counting the sections
under the driveway....

I can't pull a cable through because of the huge length and the fact
that the conduit turns at least 6 right-angles. It just would not pull
through.

I can't blame the electrician because without doing all the damage
referred to above, I can't prove 100% it was him that damaged the
cable. Catch-22..

I can't / won't change the intercom because I have never seen a 2 wire
intercom with the clarity and quality of the one installed. It uses
digital signal processing and is far better than any 2-wire version I
looked at.

C-Bus is too expensive and too complex for such a simple task. I think
that using it would be to over-engineer the problem.

Anyway, thanks for all your suggestions. I think now it's time for the
thread to close.

Gaz.

 

[gtrdude]

"Wayne" <nospam_onsummerhill@bigpond.com> wrote in message
news:J7rjc.370$TT.201@news-server.bigpond.net.au...

"Gaz Man" <gaz@gaz.com> wrote in message
news:9qkp80hac9od4bpvvbfa85127v91m7rtrt@4ax.com...
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 cutt, so I have only 8 conductors left. The
intercom requires 5 of these, to 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

Interesting!

After reading through all the other posts I would like to suggest to you
the
following.

3 wires = 8 states. ie. 000,001,010,011,100,101,110,111
Using the ground from the intercom system.

From the above, this can be run into some logic gates or a PIC.

It appears you have given up on the sparky. Possibly he won't come back
and
possibly because he doesn't know where the break is.

You could as another idea connect new cable to the existing good
(unbroken)
cable and pull out the old cables and pull in new cables. Just add plenty
of
lube to overcome the friction.

Wayne.

What about a microcontroller solution using shift registers?

 

[Bob M]

Hi there,

A friend of mine had a similar requirement between his main cottage
and the boathouse down by the lake (about 100 meters away).

What I built (15 years ago) for him was a simple ciruit that used a
6402 UART (Universal Asynconous Receiver Transmitter) at each end,
hardwired to transmit 8 data bits. The transmit data part of the UART
was connected to 8 toggle switches, the receive was connected to 8
relays through a 1 transisor (2N2222) amplifier. The serial output was
connected to a RS423 driver and the input to a RS423 receiver. The
range of the RS423 chips is 1000s of feet(dependent unpon the serial
data rate). The UART continuously transmitted serial characters, thus
sending the status of the 8 toggle switches. He later connected motion
sensors at the lake end to replace a toggle switch as a "monitor". The
neat thing about this circuit is all of the "logic" is done in 1 UART
chip and it creates 8 independent bydirectional "channels" of data for
you.

It is still working to this day. If you wish, drop me an email and I
will see if I can dig up the schematics for you. It runs on +5 volts.

Cheers from Canada

Bob Morgoch

Gaz Man <gaz@gaz.com> wrote in message news:<pm9v805venk3hilo8ots5o8akc5rl300g4@4ax.com>...

Thanks to all for your suggestions for my problem.

I have decided to try the DTMF option because of its simplicity and
the easy availability of decoders etc.

To respond to some of the other suggestions :

I can't dig up the cable because substantial landscaping has gone over
the top. It would involve moving tons of rock and concrete, and
wrecking alot of lawn and garden. And that's not counting the sections
under the driveway....

I can't pull a cable through because of the huge length and the fact
that the conduit turns at least 6 right-angles. It just would not pull
through.

I can't blame the electrician because without doing all the damage
referred to above, I can't prove 100% it was him that damaged the
cable. Catch-22..

I can't / won't change the intercom because I have never seen a 2 wire
intercom with the clarity and quality of the one installed. It uses
digital signal processing and is far better than any 2-wire version I
looked at.

C-Bus is too expensive and too complex for such a simple task. I think
that using it would be to over-engineer the problem.

Anyway, thanks for all your suggestions. I think now it's time for the
thread to close.

Gaz.