HELP PLEASE - how to connect two old telephones to form intercom
Giles Moss
Hello.
I have two old style telephones (With bells, as opposed to piezo
buzzers), and I was wondering if it was possible to connect them
together to form an intercom system.
Each telephone is in full working order, and has a four core cable
leading into the wall jack. The core colours are: red, blue, green and
white. This is the same for both telephones.
Basically, I want to know how to connect them together so that, by
picking up the handset on both phones, a two way conversation can be
had.
Also, would it be possible to wire them up so when the handset is picked
up on one phone, the ringer rings on the other phone, until the handset
is picked up?
I realise that some sort of power input will be required - what voltage,
applied across whick two wires.. etc.
If there is anyone reading this who can help (maybe you are a telephone
systems engineer, or just know about phones), I would appreciate it very
much if you could help me out.
Thanks in advance,
Giles Moss
***********************************
This e-mail was from Giles Moss
gi...@mossy.demon.co.uk
***********************************
Alan Macdonald
In article <$WqElDAb...@mossy.demon.co.uk>, Giles Moss
<gi...@mossy.demon.co.uk> writes
together (possibly with a strap), if not,connect them. connect the
white/green cores of each phone together. Connect the red cores one to
each side of a 6 or 9 volt battery. Ignore the blue core. This should do
it, let me know if it doesn't.
--
It's time to ring the changes!
Jay Hennigan
Giles Moss (gi...@mossy.demon.co.uk) wrote:
: Hello.
:
: I have two old style telephones (With bells, as opposed to piezo
: buzzers), and I was wondering if it was possible to connect them
: together to form an intercom system.
:
: Each telephone is in full working order, and has a four core cable
: leading into the wall jack. The core colours are: red, blue, green and
: white. This is the same for both telephones.
This is probably a country-specific color code. If they are telephones
designed to be connected to a conventional line, there will be a
strapping arrangement in which you wind up with two wires. In the US
that wouuld be the green and red wires.
: Basically, I want to know how to connect them together so that, by
: picking up the handset on both phones, a two way conversation can be
: had.
:
: Also, would it be possible to wire them up so when the handset is picked
: up on one phone, the ringer rings on the other phone, until the handset
: is picked up?
What you need is a "ringdown circuit". There's a US-made one from a
company named Viking, their model DLE-200B. It works with a "wall bug"
transformer, so powering it in the UK should be doable. Proctor also
makes one, not sure of the model. What these units do is supply DC
voltage to both phones. When one goes off-hook, it applies ringing
voltage to the other. When the second phone goes off-hook, the ringing
voltage is removed and a talk path established. This type of arrangement
is of course totally automatic, and requires only a single pair of wires
to each phone.
: I realise that some sort of power input will be required - what voltage,
: applied across whick two wires.. etc.
A cheaper solution would be to identify the two wires for each phone,
connect the two phones together in series with each other and a battery of
about 12 volts. A 12-volt battery is obviously much cheaper than a
ringdown box, but not automatic. You would need a separate auxiliary
buzzer arrangement for signalling, requiring additional wiring, etc.
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Giles Moss
>
>: Each telephone is in full working order, and has a four core cable
>: leading into the wall jack. The core colours are: red, blue, green and
>: white. This is the same for both telephones.
>
>This is probably a country-specific color code. If they are telephones
>designed to be connected to a conventional line, there will be a
>strapping arrangement in which you wind up with two wires. In the US
>that wouuld be the green and red wires.
The master wall jack (Ie. the one that is connected to the line) has a
small circuit in it which converts (I don't know how) the two conductor
telephone line into the four cores to feed the phone.
>
>
>: Basically, I want to know how to connect them together so that, by
>: picking up the handset on both phones, a two way conversation can be
>: had.
>:
>: Also, would it be possible to wire them up so when the handset is picked
>: up on one phone, the ringer rings on the other phone, until the handset
>: is picked up?
>
>What you need is a "ringdown circuit".
mail order, or where I could find a circuit diagram for one?
Alan Macdonald
In article <o1WdeBA3...@mossy.demon.co.uk>, Giles Moss
<gi...@mossy.demon.co.uk> writes
>>
resistor to provide a current path for the exchange when all the phones
are unplugged. 3. a capacitor to pass the ac ringing current to the
bell. In the older phones like the ones in question, the bell capacitor
is inside the phone anyway. In the modern phones the capacitor is
ommitted due to its physical size, thats why its in the master
socket.don't forget , the phones you have were about long before master
sockets were even thought of. the capacitor passes ac to pin 3. the line
pair is on pins 2 & 5. this corresponds to white, green and red wires.
the blue wire is not connected. the simple circuit i suggested allows no
signalling but the voice path is perfect and a 6 or 9 volt battery is
cheap. if signalling is needed a more sophisticated circuit is required
as the bells require about 70 volts at 25 Hz. Not all that easy. try the
simple approach first. BTW would ex GPO phones be ok on a US ringdown
unit?
Tomi Holger Engdahl
Giles Moss <gi...@mossy.demon.co.uk> writes:
> I have two old style telephones (With bells, as opposed to piezo
> buzzers), and I was wondering if it was possible to connect them
> together to form an intercom system.
From: mwa...@bnr.ca (Markus Wandel)
I have recently thought about this and come up with a kludgy but workable scheme.
Talking over the phones is easy. You put DC current through the phone and it transmits and receives audio. So two
phones and a current source (about 25mA) all in series will give you a talking circuit. A suitable current source can be
as simple as a 9V battery and a series resistor whose value is adjusted (with both phones offhook) till about 25mA
flows. You can then bypass the battery and the resistor with a capacitor to couple the audio straight across and get a
loud and clear connection.
What is much harder is signaling the other end. To ring the bell you need to put 90V (RMS) 20Hz AC into the phone
(nominally). Lower voltages will work (down to about 40V) but different frequencies won't. You can't ring the phone
at 60Hz. I have a ringing circuit in a PBX I built but it consists of a 20Hz sinewave generator, a push-pull power
booster and a big transformer. Much too elaborate for a simple 2-phone intercom circuit, and anyway the ringing
voltage could painfully zap a kid.
So forget the bell and look into other forms of signaling. This is what I have come up with:
+ | | -
+-------+------ - - --+---||||---/\/\/--+---- - -----+-------+
| | | | | R | | |
| | | 24V | | |
| --- | | --- |
| | | +---||------------+ | | |
| --- Sonalert C Sonalert --- |
| C | | C |
+---||--+ +--||---+
| _|_, _|_ |
| / \ 15V 15V \ / |
PHONE -+- Zener Zener `-+- PHONE
| | | |
| | | |
+-------+------------------ - - - -------------------+-------+
As before, set R to give you a talking current (both phones offhook) of about 25mA. Start with 1K ohm. Leave it in if
the phones work well enough; the current is not very critical. The capacitors C are audio bypass capacitors and
should be about 0.47uF. When the phones are onhook they present an open circuit, and the 24V battery voltage is
not enough to overcome the 30V series drop of the Zeners and no current flows. When both phones are offhook they
present a very low resistance and the talking current (determined by R) flows.
When only one phone is offhook it places its low DC resistance across the Zener diode on its side so that the full 24V
supply is applied to the other side. This overcomes the voltage drop of the other Zener diode so the other Sonalert
beeps. The wonderful thing about Sonalerts is that they make a loud noise with only a few milliamps of current so the
series resistor R doesn't matter. Especially nice is a pulsing Sonalert which goes "Beep beep beep" automatically.
While the far-end Sonalert is beeping, you hear the beeping in the near-end receiver (at low volume thanks to the
bypass capacitor across the far-end Sonalert) to confirm that the line is working and the other end is being signaled.
The power supply can be three 9V batteries in series but since 80% of the power is lost in series resistor R rather
than in powering the phones it seems a little wasteful. A 24V wall wart with clean filtering would be better.
The signaling components can be mounted inside the phones. Only two wires are needed to go to each phone, and
the power supply can be mounted centrally, out of harm's way. If R is adequately big (1/2 watt) and has enough
ventilation then both lines can be indefinitely shorted out without any fire hazard and there is not enough voltage
anywhere to hurt anyone.
I have tested this with 500-type phones and two different types of piezo buzzers (pulsing sonalerts and non-pulsing
brand X ones) and it works great. You should be able to get all the needed parts including piezo buzzers at Radio
Shack. I love telephones. Too bad I don't have any kids who want an intercom line.
> Also, would it be possible to wire them up so when the handset is picked
> up on one phone, the ringer rings on the other phone, until the handset
> is picked up?
You need more electronics to do that. You need some type of circuit which
generates the ring signal and some logic to select when to ring those
telephones (few relays should do this quit easily).
You can find information how to generate the ring signals from my web page at
http://www.hut.fi/~then/circuits/telephone_ringer.html
> I realise that some sort of power input will be required - what voltage,
> applied across whick two wires.. etc.
Simple telephone test circuit
I have used this type of circuits for quick testing of telephone equipments. This is a quick
hack that does not exactly resemble the characteristics of the real telephone netwotk, but is
enough for this kind if applications. If you want better simulation, change the resistor to 600
ohm and increase the power supply voltage. And by the way you can also feed signals to
telehone if you connect the circuit output wires to small amplifier output.
+12V (well filtered power supply or batteries)
|
|
---
| |
| | 220 Ohm 2W resistor
| |
---
| ||
+---------||---------------------------> audio input signal
| || Capacitor 470 nF
|
|
Telephone
|
|
+--------------------------------------> audio input ground
|
|
power supply ground
> If there is anyone reading this who can help (maybe you are a telephone
> systems engineer, or just know about phones), I would appreciate it very
> much if you could help me out.
I am still studying, but I have already designed circuits to be connected
to telephone network. If that was not enough, for more information
check my telephone technology links and articles at
http://www.hut.fi/~then/electronics/telecom.html#telephone
--
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You can find more info and links at http://www.hut.fi/~then/index.html
Jock Mackirdy
Giles Moss <gi...@mossy.demon.co.uk> wrote:
>I have two old style telephones (With bells, as opposed to piezo
>buzzers), and I was wondering if it was possible to connect them
>together to form an intercom system.
What is the model designation on the label on the bottom of the phones
(typically 332 or 706 or 746)? The circuit diagram number is
designated N plus the model number incremented by 100. So a telephone
746 is to diagram N846.
>Each telephone is in full working order, and has a four core cable
>leading into the wall jack. The core colours are: red, blue, green and
>white. This is the same for both telephones.
Relevance of wire colours depends on what model it is. It's not clear
from your description whether they are plug-in or hard-wired.
>Basically, I want to know how to connect them together so that, by
>picking up the handset on both phones, a two way conversation can be
>had.
Easy. Connect the speech circuits in series. For a hard-wired 706 or
746, connect red to red and white to white, with a 9 volt dry battery
in series with this loop.
>Also, would it be possible to wire them up so when the handset is picked
>up on one phone, the ringer rings on the other phone, until the handset
>is picked up?
In short, no. The bell works on a.c. current and you would need
additional circuitry (normally situated in the telephone exchange) to
generate 17Hz a.c. and also provide the "ring trip" function when the
line is looped by picking up a phone. However, if you can get hold of
a magneto generator (as used on manual switchboards) you can give a
burst of ringing current and then wait for a reply. There are also
several designs of "Ringing Convertors" designed to generate 17Hz or
25Hz from 50Hz (*not* mains voltage) but you need to know what you are
doing.
Suggested background reading (possibly available in the reference
section of a decent library) is Atkinson's "Telephony", published in 2
volumes around 1950 but reprinted many times up to the mid-1960's. It
doesn't cover anything more modern than black bakelite phones.
Jock Mackirdy
Business Advisory Services, Luton (UK)
E-mail: jo...@basluton.demon.co.uk
Independent telecomms. and business advice
Tom Thiel
In article <84143773...@basluton.demon.co.uk>,
jo...@basluton.demon.co.uk (Jock Mackirdy) writes:
>>Basically, I want to know how to connect them together so that, by
>>picking up the handset on both phones, a two way conversation can be
>>had.
>
>>Also, would it be possible to wire them up so when the handset is picked
>>up on one phone, the ringer rings on the other phone, until the handset
>>is picked up?
You might want to try Viking Electronics, Hudson WI, USA. They make a
"black-box" with two modular jacks, provides interrupted ringing when you
go off hook, and talk battery when the other extension is answered. The
only conflict might be the Power Supplly being made for US; but if you
contact them, their tech assistance dept has been very helpful in the past
modifying their stuff for special applications.
An e-mail address I lifted from this newsgroup some time back lists
te...@VikingElectronics.com; they might also have a Web page (I havent
tried it yet)
VIKINGELEC
In article <50i9m9$5...@newsbf02.news.aol.com>, tomt...@aol.com (Tom
Thiel) writes:
Viking makes a device called the DLE-200B Line Simulator / Ring-down
Circuit that will do exactly what you need. They are also available with
230V AC international power supplies. For more information...
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