DTMF encoder/decoder

gaz · Jul 25, 2004

Hi all.

Regulars here may remember I asked a question a while ago about how to
send several control signals down just a pair of conductors.

After looking at several of the suggestions I have concluded that
encoding with DTMF is the most suitable (up to 16 different momentary
pulsed signals down just 2 conductors) and is likely easier than the
other suggestions using PIC's.

A couple of folks sent emails offering to send circuit diagrams or
even the finished product but none of them followed through.

So I ask again - can anyone point to a good circuit (or design one
from scratch) that would allow me to switch on and off remote relays
using 2 conductors and coded DTMF signals?

I have had trouble finding a DTMF encoding chip here in Australia. If
anyone can recommend a supplier I'd be most grateful also.

Thanks

Gaz

Email gazman at pobox dot com and don't add dot au

dmm · Jul 25, 2004

On Sun, 25 Jul 2004 20:36:49 +1000, gaz <kjh@iiweui.net> wrote:

Hi all.

Regulars here may remember I asked a question a while ago about how to
send several control signals down just a pair of conductors.

After looking at several of the suggestions I have concluded that
encoding with DTMF is the most suitable (up to 16 different momentary
pulsed signals down just 2 conductors) and is likely easier than the
other suggestions using PIC's.

A couple of folks sent emails offering to send circuit diagrams or
even the finished product but none of them followed through.

So I ask again - can anyone point to a good circuit (or design one
from scratch) that would allow me to switch on and off remote relays
using 2 conductors and coded DTMF signals?

I have had trouble finding a DTMF encoding chip here in Australia. If
anyone can recommend a supplier I'd be most grateful also.

Thanks

Gaz

Email gazman at pobox dot com and don't add dot au

Many years ago (18), I designed a circuit using an MC145026 encoder,
and a MC145027 decoder for activating remote VCRs from classrooms,
in schools in SA. Sold a few systems too. The chips are still available,
and as far as I remember, they were fairly easy to implement. They were
made by Motorola. I may even have some boards still floating around.

gaz · Jul 25, 2004

On Sun, 25 Jul 2004 22:59:22 +1000, dmm
<dmmilne_REMOVE_@ozemail.com.au> wrote:

On Sun, 25 Jul 2004 20:36:49 +1000, gaz <kjh@iiweui.net> wrote:

Hi all.

Regulars here may remember I asked a question a while ago about how to
send several control signals down just a pair of conductors.

After looking at several of the suggestions I have concluded that
encoding with DTMF is the most suitable (up to 16 different momentary
pulsed signals down just 2 conductors) and is likely easier than the
other suggestions using PIC's.

A couple of folks sent emails offering to send circuit diagrams or
even the finished product but none of them followed through.

So I ask again - can anyone point to a good circuit (or design one
from scratch) that would allow me to switch on and off remote relays
using 2 conductors and coded DTMF signals?

I have had trouble finding a DTMF encoding chip here in Australia. If
anyone can recommend a supplier I'd be most grateful also.

Thanks

Gaz

Email gazman at pobox dot com and don't add dot au

Many years ago (18), I designed a circuit using an MC145026 encoder,
and a MC145027 decoder for activating remote VCRs from classrooms,
in schools in SA. Sold a few systems too. The chips are still available,
and as far as I remember, they were fairly easy to implement. They were
made by Motorola. I may even have some boards still floating around.

Thanks, I will have a look for those chips.
Any info you can send (circuit diagrams etc) would be much
appreciated. I can solder competently and I can design circuits myself
in a very general sense, but really I need assistance to know exactly
what to solder where.

Again, gazman at pobox dot com without dot au

gaz · Jul 25, 2004

Perhaps I can make a few more details to my previous post :

My front gate is about 150 metres from the house and not visible from
there.

I'd like to be able to send signals from the gate to the house, and
back the other way to do the following :

Open the gate
Close the gate
Turn on the driveway lights
Turn off the driveway lights
Indicate at the house that the gate is open or closed
Indicate at the house that motion has been detected at the gate.

I have all the necessary circuits to detect motion, move the gates
etc.

I just need a method to pass these various signals between the two
locations using just 2 conductors.

On Sun, 25 Jul 2004 20:36:49 +1000, gaz <kjh@iiweui.net> wrote:

Hi all.

Regulars here may remember I asked a question a while ago about how to
send several control signals down just a pair of conductors.

After looking at several of the suggestions I have concluded that
encoding with DTMF is the most suitable (up to 16 different momentary
pulsed signals down just 2 conductors) and is likely easier than the
other suggestions using PIC's.

A couple of folks sent emails offering to send circuit diagrams or
even the finished product but none of them followed through.

So I ask again - can anyone point to a good circuit (or design one
from scratch) that would allow me to switch on and off remote relays
using 2 conductors and coded DTMF signals?

I have had trouble finding a DTMF encoding chip here in Australia. If
anyone can recommend a supplier I'd be most grateful also.

Thanks

Gaz

Email gazman at pobox dot com and don't add dot au

dmm · Jul 25, 2004

On Sun, 25 Jul 2004 23:07:47 +1000, gaz <kjh@iiweui.net> wrote:

On Sun, 25 Jul 2004 22:59:22 +1000, dmm
dmmilne_REMOVE_@ozemail.com.au> wrote:

<snip>

Many years ago (18), I designed a circuit using an MC145026 encoder,
and a MC145027 decoder for activating remote VCRs from classrooms,
in schools in SA. Sold a few systems too. The chips are still available,
and as far as I remember, they were fairly easy to implement. They were
made by Motorola. I may even have some boards still floating around.

Thanks, I will have a look for those chips.
Any info you can send (circuit diagrams etc) would be much
appreciated. I can solder competently and I can design circuits myself
in a very general sense, but really I need assistance to know exactly
what to solder where.

Those circuits were never converted into a CAD/image file, they're just on paper,
and still in storage in Adelaide. As I remember, the devices were fairly easy to
implement and the datasheet from Motorola shows an example. I suggest that
you use MKT capacitors with the design and utilise a frequency that's fairly
low, say in the audio range, no more than 10KHz. If I remember correctly,
you want to send signals down to your front gate, from the house.
The general idea is so that you can use cheap 2 pair telephone cable.
One pair carrying a balanced signal using a RS485 driver and receiver,
and the other pair carrying the power supply to the decoder and
associated electronics.

Again, gazman at pobox dot com without dot au

Unbeliever · Jul 25, 2004

"gaz" <kjh@iiweui.net> wrote in message
news:07c7g01n39154vvg384002vmeeadmqu14m@4ax.com...

Perhaps I can make a few more details to my previous post :

My front gate is about 150 metres from the house and not visible from
there.

I'd like to be able to send signals from the gate to the house, and
back the other way to do the following :

Open the gate
Close the gate
Turn on the driveway lights
Turn off the driveway lights
Indicate at the house that the gate is open or closed
Indicate at the house that motion has been detected at the gate.

I have all the necessary circuits to detect motion, move the gates
etc.

I just need a method to pass these various signals between the two
locations using just 2 conductors.

// Naive partial solution to wrong problem deleted

OK. So you *think* you want to design a DTMF based logic controller but
need to be told what pin to solder to what? A little like: "I need to get
from home to work, so I'll have to design and build a maglev unicycle, 'cos
I read about those somewhere, but I need someone to tell me whether to use
philips head or slotted screws". Give us a break, take the bus! Save all
of us some heartache and investigate some existing solutions.

Obviously, if you only need two wires, there's a power source available at
the gate. If it's 12 or 240V, consider an alarm panel with zone and
auxiliary expander (as a side benefit you could alarm your house). Another
possibility is a couple of the smallest of the Splat range of PLC's
(http://www.splatco.com.au/). Both solutions are overkill by a long way,
but you'll have some extra I/O stuff to play with (Mailbox full sensor?
Remote control fob? Addams Family doorbell controller?).

Cheers,
Alf

Adrian Jansen · Jul 25, 2004

Yuo might want to take a look at

http://www.circuitcellar.com/library/print/0604/RicciBitti-167/index.htm

for another possible approach.

--
Regards,

Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control
"gaz" <kjh@iiweui.net> wrote in message
news:07c7g01n39154vvg384002vmeeadmqu14m@4ax.com...

Perhaps I can make a few more details to my previous post :

My front gate is about 150 metres from the house and not visible from
there.

I'd like to be able to send signals from the gate to the house, and
back the other way to do the following :

Open the gate
Close the gate
Turn on the driveway lights
Turn off the driveway lights
Indicate at the house that the gate is open or closed
Indicate at the house that motion has been detected at the gate.

I have all the necessary circuits to detect motion, move the gates
etc.

I just need a method to pass these various signals between the two
locations using just 2 conductors.

On Sun, 25 Jul 2004 20:36:49 +1000, gaz <kjh@iiweui.net> wrote:

Hi all.

Regulars here may remember I asked a question a while ago about how to
send several control signals down just a pair of conductors.

After looking at several of the suggestions I have concluded that
encoding with DTMF is the most suitable (up to 16 different momentary
pulsed signals down just 2 conductors) and is likely easier than the
other suggestions using PIC's.

A couple of folks sent emails offering to send circuit diagrams or
even the finished product but none of them followed through.

So I ask again - can anyone point to a good circuit (or design one
from scratch) that would allow me to switch on and off remote relays
using 2 conductors and coded DTMF signals?

I have had trouble finding a DTMF encoding chip here in Australia. If
anyone can recommend a supplier I'd be most grateful also.

Thanks

Gaz

Email gazman at pobox dot com and don't add dot au

gaz · Jul 26, 2004

On Mon, 26 Jul 2004 05:00:27 +1000, "Unbeliever"
<alfkatz@remove.the.bleedin.obvious.ieee.org> wrote:

"gaz" <kjh@iiweui.net> wrote in message
news:07c7g01n39154vvg384002vmeeadmqu14m@4ax.com...
Perhaps I can make a few more details to my previous post :

My front gate is about 150 metres from the house and not visible from
there.

I'd like to be able to send signals from the gate to the house, and
back the other way to do the following :

Open the gate
Close the gate
Turn on the driveway lights
Turn off the driveway lights
Indicate at the house that the gate is open or closed
Indicate at the house that motion has been detected at the gate.

I have all the necessary circuits to detect motion, move the gates
etc.

I just need a method to pass these various signals between the two
locations using just 2 conductors.

// Naive partial solution to wrong problem deleted

OK. So you *think* you want to design a DTMF based logic controller but
need to be told what pin to solder to what? A little like: "I need to get
from home to work, so I'll have to design and build a maglev unicycle, 'cos
I read about those somewhere, but I need someone to tell me whether to use
philips head or slotted screws". Give us a break, take the bus! Save all
of us some heartache and investigate some existing solutions.

Obviously, if you only need two wires, there's a power source available at
the gate. If it's 12 or 240V, consider an alarm panel with zone and
auxiliary expander (as a side benefit you could alarm your house). Another
possibility is a couple of the smallest of the Splat range of PLC's
(http://www.splatco.com.au/). Both solutions are overkill by a long way,
but you'll have some extra I/O stuff to play with (Mailbox full sensor?
Remote control fob? Addams Family doorbell controller?).

Cheers,
Alf

Alf, I think you misunderstand my original request. I did not repeat
it in detail; perhaps I should have.

I don't need or want a micro-controller. The gate came supplied with a
micro-controller circuit to do things like shut it a minute after a
car enters, sense an obstruction, etc. etc. The micro-controller board
comes with connectors to mount a push-button open/close switch. I have
already wired this to a keypad for coded entry, also to a radio-keyfob
so I can open the gate from in my car. After finding some circuits on
the web I built and modified these myself, so am not a total amateur.
The only remaining task is to send various signals to and from the
house. These are simple pulsed signals to open or close a relay. No
logic or decision making involved - a DTMF coder, and a decoder
runningt a collection of relays and/or bistable flip-flops. I have
built timers for the lights using 555 chips *all by myself* so perhaps
I have understated my abilities.

I only have 3 conductors available. I originally installed 16 but
somehow the cables were damaged underground and I can't dig up 150M of
dirt and concrete to find the fault. So I am trying to get by with
just 3. Power (240V) is available at both ends.

I have searched the web endlessly and I either I can't find a circuit
or when I do it calls for CMOS chips that are out of production and I
can't find an equivalent. Believe me I have researched this problem. I
have looked for pre-made solutions. I have tried to educate myself. My
conclusion is that if I encode a signal using DTMF I can send it up
the wire and have the appropriate relay latch or activate momentarily.
That's all I need.

Unbeliever · Jul 26, 2004

"gaz" <kjh@iiweui.net> wrote in message
news:ber9g0loh59urhk7jfiem2f0a5tam8a424@4ax.com...

On Mon, 26 Jul 2004 05:00:27 +1000, "Unbeliever"
alfkatz@remove.the.bleedin.obvious.ieee.org> wrote:

"gaz" <kjh@iiweui.net> wrote in message
news:07c7g01n39154vvg384002vmeeadmqu14m@4ax.com...
Perhaps I can make a few more details to my previous post :

My front gate is about 150 metres from the house and not visible from
there.

I'd like to be able to send signals from the gate to the house, and
back the other way to do the following :

Open the gate
Close the gate
Turn on the driveway lights
Turn off the driveway lights
Indicate at the house that the gate is open or closed
Indicate at the house that motion has been detected at the gate.

I have all the necessary circuits to detect motion, move the gates
etc.

I just need a method to pass these various signals between the two
locations using just 2 conductors.

// Naive partial solution to wrong problem deleted

OK. So you *think* you want to design a DTMF based logic controller but
need to be told what pin to solder to what? A little like: "I need to
get
from home to work, so I'll have to design and build a maglev unicycle,
'cos
I read about those somewhere, but I need someone to tell me whether to
use
philips head or slotted screws". Give us a break, take the bus! Save
all
of us some heartache and investigate some existing solutions.

Obviously, if you only need two wires, there's a power source available
at
the gate. If it's 12 or 240V, consider an alarm panel with zone and
auxiliary expander (as a side benefit you could alarm your house).
Another
possibility is a couple of the smallest of the Splat range of PLC's
(http://www.splatco.com.au/). Both solutions are overkill by a long way,
but you'll have some extra I/O stuff to play with (Mailbox full sensor?
Remote control fob? Addams Family doorbell controller?).

Cheers,
Alf

Alf, I think you misunderstand my original request. I did not repeat
it in detail; perhaps I should have.

I don't need or want a micro-controller. The gate came supplied with a
micro-controller circuit to do things like shut it a minute after a
car enters, sense an obstruction, etc. etc. The micro-controller board
comes with connectors to mount a push-button open/close switch. I have
already wired this to a keypad for coded entry, also to a radio-keyfob
so I can open the gate from in my car. After finding some circuits on
the web I built and modified these myself, so am not a total amateur.
The only remaining task is to send various signals to and from the
house. These are simple pulsed signals to open or close a relay. No
logic or decision making involved - a DTMF coder, and a decoder
runningt a collection of relays and/or bistable flip-flops. I have
built timers for the lights using 555 chips *all by myself* so perhaps
I have understated my abilities.

I only have 3 conductors available. I originally installed 16 but
somehow the cables were damaged underground and I can't dig up 150M of
dirt and concrete to find the fault. So I am trying to get by with
just 3. Power (240V) is available at both ends.

I have searched the web endlessly and I either I can't find a circuit
or when I do it calls for CMOS chips that are out of production and I
can't find an equivalent. Believe me I have researched this problem. I
have looked for pre-made solutions. I have tried to educate myself. My
conclusion is that if I encode a signal using DTMF I can send it up
the wire and have the appropriate relay latch or activate momentarily.
That's all I need.

DTMF is a doable option. The easiest way to achieve it is using - you

guessed it - another microcontroller. DTMF is, however, a needless
complication to an otherwise simple data stream unless your gate controller
can send/receive DTMF directly. Were I going to do this I'd use RS485.

If I wanted a quick easy solution that involved no programming, I'd consider
one of these transmitters and receivers at either end
http://www.oatleyelectronics.com/kits/k065.html and rip out the IR LED and
receiver and replace them with wire. Big and clunky at the RX end compared
to a simple microcontroller, but fairly cheap and quick. There is a smaller
receiver that won't drive a relay but does have a TTL level output.

HTH,
Alf

Alan · Jul 26, 2004

On Mon, 26 Jul 2004 21:59:47 +1000, gaz <kjh@iiweui.net> wrote:

On Mon, 26 Jul 2004 05:00:27 +1000, "Unbeliever"
alfkatz@remove.the.bleedin.obvious.ieee.org> wrote:

"gaz" <kjh@iiweui.net> wrote in message
news:07c7g01n39154vvg384002vmeeadmqu14m@4ax.com...
Perhaps I can make a few more details to my previous post :

My front gate is about 150 metres from the house and not visible from
there.

I'd like to be able to send signals from the gate to the house, and
back the other way to do the following :

Open the gate
Close the gate
Turn on the driveway lights
Turn off the driveway lights
Indicate at the house that the gate is open or closed
Indicate at the house that motion has been detected at the gate.

I have all the necessary circuits to detect motion, move the gates
etc.

I just need a method to pass these various signals between the two
locations using just 2 conductors.

You could use something like our product here:
http://jenal.com/?p=9
but that may be overkill for you.

For a DTMF generator why not grab hold of a cheap telephone,
with a bit of ingenuity you could also use it as a voice
link down to the gate as well so you can talk to people!

HTH

Alan

++++++++++++++++++++++++++++++++++++++++++
Jenal Communications
Manufacturers and Suppliers of HF Selcall
P O Box 1108, Morley, WA, 6943
Tel: +61 8 9370 5533 Fax +61 8 9467 6146
Web Site: http://www.jenal.com
e-mail: http://www.jenal.com/?p=1
++++++++++++++++++++++++++++++++++++++++++

Mark Harriss · Jul 26, 2004

Three wires plus mains power and ground would
let you use a simple 3 bit binary decoder
to drive 8 outputs, something like a
74HC138.

Mark H

Ken Taylor · Jul 26, 2004

"gaz" <kjh@iiweui.net> wrote in message
news:ber9g0loh59urhk7jfiem2f0a5tam8a424@4ax.com...

On Mon, 26 Jul 2004 05:00:27 +1000, "Unbeliever"
alfkatz@remove.the.bleedin.obvious.ieee.org> wrote:

"gaz" <kjh@iiweui.net> wrote in message
news:07c7g01n39154vvg384002vmeeadmqu14m@4ax.com...
Perhaps I can make a few more details to my previous post :

My front gate is about 150 metres from the house and not visible from
there.

I'd like to be able to send signals from the gate to the house, and
back the other way to do the following :

Open the gate
Close the gate
Turn on the driveway lights
Turn off the driveway lights
Indicate at the house that the gate is open or closed
Indicate at the house that motion has been detected at the gate.

I have all the necessary circuits to detect motion, move the gates
etc.

I just need a method to pass these various signals between the two
locations using just 2 conductors.

// Naive partial solution to wrong problem deleted

OK. So you *think* you want to design a DTMF based logic controller but
need to be told what pin to solder to what? A little like: "I need to
get
from home to work, so I'll have to design and build a maglev unicycle,
'cos
I read about those somewhere, but I need someone to tell me whether to
use
philips head or slotted screws". Give us a break, take the bus! Save
all
of us some heartache and investigate some existing solutions.

Obviously, if you only need two wires, there's a power source available
at
the gate. If it's 12 or 240V, consider an alarm panel with zone and
auxiliary expander (as a side benefit you could alarm your house).
Another
possibility is a couple of the smallest of the Splat range of PLC's
(http://www.splatco.com.au/). Both solutions are overkill by a long way,
but you'll have some extra I/O stuff to play with (Mailbox full sensor?
Remote control fob? Addams Family doorbell controller?).

Cheers,
Alf

Alf, I think you misunderstand my original request. I did not repeat
it in detail; perhaps I should have.

I don't need or want a micro-controller. The gate came supplied with a
micro-controller circuit to do things like shut it a minute after a
car enters, sense an obstruction, etc. etc. The micro-controller board
comes with connectors to mount a push-button open/close switch. I have
already wired this to a keypad for coded entry, also to a radio-keyfob
so I can open the gate from in my car. After finding some circuits on
the web I built and modified these myself, so am not a total amateur.
The only remaining task is to send various signals to and from the
house. These are simple pulsed signals to open or close a relay. No
logic or decision making involved - a DTMF coder, and a decoder
runningt a collection of relays and/or bistable flip-flops. I have
built timers for the lights using 555 chips *all by myself* so perhaps
I have understated my abilities.

I only have 3 conductors available. I originally installed 16 but
somehow the cables were damaged underground and I can't dig up 150M of
dirt and concrete to find the fault. So I am trying to get by with
just 3. Power (240V) is available at both ends.

I have searched the web endlessly and I either I can't find a circuit
or when I do it calls for CMOS chips that are out of production and I
can't find an equivalent. Believe me I have researched this problem. I
have looked for pre-made solutions. I have tried to educate myself. My
conclusion is that if I encode a signal using DTMF I can send it up
the wire and have the appropriate relay latch or activate momentarily.
That's all I need.

Go to : uk.farnell.com (they sell in the US as well) and search on DTMF.

There are three responses, the last being discontinued, and the other two
conveniently a DTMF receiver and transmitter, resp. Data sheets are also
available for these devices.

Cheers.

Ken

David L. Jones · Jul 27, 2004

gaz <kjh@iiweui.net> wrote in message news:<s837g05tq1pc07epg3j1oud8iulnpab6h9@4ax.com>...

Hi all.

Regulars here may remember I asked a question a while ago about how to
send several control signals down just a pair of conductors.

After looking at several of the suggestions I have concluded that
encoding with DTMF is the most suitable (up to 16 different momentary
pulsed signals down just 2 conductors) and is likely easier than the
other suggestions using PIC's.

A couple of folks sent emails offering to send circuit diagrams or
even the finished product but none of them followed through.

So I ask again - can anyone point to a good circuit (or design one
from scratch) that would allow me to switch on and off remote relays
using 2 conductors and coded DTMF signals?

I have had trouble finding a DTMF encoding chip here in Australia. If
anyone can recommend a supplier I'd be most grateful also.

Thanks

Gaz

Email gazman at pobox dot com and don't add dot au

A left field suggestion:
Use two cascaded LM3914 bar graph driver chips in "dot" mode and then
feed the necessary DC voltage in at the other end to switch on any one
of the 20 outputs. The DC voltage could be generated with a resistor
divider and switches used on each of the taps.
Application Notes are readily available for this circuit.
Of course you'd need some filtering for noise on the line and buffers
on the LM3914 outputs to drive the relays, possibly some "deglitching"
on the output etc. Don't know how "touchy" it would be, you'd have to
experiment. Not as robust as a DTMF or software encoded solution
though.

Farnell have some DTMF chips www.farnellinone.com.au

Regards
Dave

gaz · Jul 27, 2004

On 27 Jul 2004 05:03:37 -0700, tronnort@yahoo.com (David L. Jones)
wrote:

gaz <kjh@iiweui.net> wrote in message news:<s837g05tq1pc07epg3j1oud8iulnpab6h9@4ax.com>...
Hi all.

Regulars here may remember I asked a question a while ago about how to
send several control signals down just a pair of conductors.

After looking at several of the suggestions I have concluded that
encoding with DTMF is the most suitable (up to 16 different momentary
pulsed signals down just 2 conductors) and is likely easier than the
other suggestions using PIC's.

A couple of folks sent emails offering to send circuit diagrams or
even the finished product but none of them followed through.

So I ask again - can anyone point to a good circuit (or design one
from scratch) that would allow me to switch on and off remote relays
using 2 conductors and coded DTMF signals?

I have had trouble finding a DTMF encoding chip here in Australia. If
anyone can recommend a supplier I'd be most grateful also.

Thanks

Gaz

Email gazman at pobox dot com and don't add dot au

A left field suggestion:
Use two cascaded LM3914 bar graph driver chips in "dot" mode and then
feed the necessary DC voltage in at the other end to switch on any one
of the 20 outputs. The DC voltage could be generated with a resistor
divider and switches used on each of the taps.
Application Notes are readily available for this circuit.
Of course you'd need some filtering for noise on the line and buffers
on the LM3914 outputs to drive the relays, possibly some "deglitching"
on the output etc. Don't know how "touchy" it would be, you'd have to
experiment. Not as robust as a DTMF or software encoded solution
though.

Farnell have some DTMF chips www.farnellinone.com.au

Regards
Dave

Darn good suggestion, Dave - Thanks I will plonk some components into
a breadboard and see if I can get it to work.

Gaz.

Richard Freeman · Jul 30, 2004

"gaz" <kjh@iiweui.net> wrote in message
news:9mlcg0lb9ugckmf35uiiak1uaf6j7d3p3p@4ax.com...

On 27 Jul 2004 05:03:37 -0700, tronnort@yahoo.com (David L. Jones)
wrote:

gaz <kjh@iiweui.net> wrote in message
news:<s837g05tq1pc07epg3j1oud8iulnpab6h9@4ax.com>...
Hi all.

Regulars here may remember I asked a question a while ago about how to
send several control signals down just a pair of conductors.

After looking at several of the suggestions I have concluded that
encoding with DTMF is the most suitable (up to 16 different momentary
pulsed signals down just 2 conductors) and is likely easier than the
other suggestions using PIC's.

A couple of folks sent emails offering to send circuit diagrams or
even the finished product but none of them followed through.

So I ask again - can anyone point to a good circuit (or design one
from scratch) that would allow me to switch on and off remote relays
using 2 conductors and coded DTMF signals?

I have had trouble finding a DTMF encoding chip here in Australia. If
anyone can recommend a supplier I'd be most grateful also.

Thanks

Gaz

Email gazman at pobox dot com and don't add dot au

A left field suggestion:
Use two cascaded LM3914 bar graph driver chips in "dot" mode and then
feed the necessary DC voltage in at the other end to switch on any one
of the 20 outputs. The DC voltage could be generated with a resistor
divider and switches used on each of the taps.
Application Notes are readily available for this circuit.
Of course you'd need some filtering for noise on the line and buffers
on the LM3914 outputs to drive the relays, possibly some "deglitching"
on the output etc. Don't know how "touchy" it would be, you'd have to
experiment. Not as robust as a DTMF or software encoded solution
though.

Farnell have some DTMF chips www.farnellinone.com.au

Regards
Dave

Darn good suggestion, Dave - Thanks I will plonk some components into
a breadboard and see if I can get it to work.

I have to admit that Mark Harris's 74hC138 solution sounds the easiest bet
to me. use 12V say on each line wrt Earth (from the AC mains) and maybe use
this to drive three relays at the Gate end which then switch the inouts to
the 74138 either high or low - the relays should be pretty robust in the
event of lightning strikes etc (although they will still be toast in a
direct strike ) and be reasonably immune to earth differences etc.

Regards
Richard Freeman

DTMF encoder/decoder

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