Closed system intercom between 2 746's ??

[richwk1]

Opinions welcome as to how i go about wiring two 746 phone together to make a closed system intercom. If i breifly explain my circumstances it may give people a clearer idea of what i need to do. I have a workshop at the end of my garden, it has mains wired internet and wired phone line. I want my wife to be able to contact me without walkig up the garden. She sometimes jiggles one of the house phones which makes my 746 in the shop tinkle but this is not idea.
I'm thinking can i connect my 746 inthe shop directly to my other in the house and if so some advice as to wiring requirments. Or does somebody have an alterantive solution. Any help or idea's would be appreciated.

Thanks everyone Rich

[julie_m]

Two FXS interfaces, an old PC, and the free and Open Source Asterisk software?

[Dave Moll]

Making them talk to each other is easy - simply connect them in parallel (as originally wired for two-wire operation) and apply a DC voltage somewhere in the order of 10V between the A (white) and B (red) terminals (a 9V battery will suffice). Signalling is rather more tricky, as you will need some way of generating the ringing current. If the ringers in your 'phones will operate at 50c/s, you may simply be able to use a transformer (fully isolated) to inject around 75VAC onto the line - which should make the phones at both ends of the line ring. But no doubt someone will think of a reason why this arrangement won't work!

Please note that these 'phones should NOT be connected to the telephone line when sest up in this manner.

Alternatively, if you wish to have a combined telephone and intercom, the way to go would be to obtain a planset - which was specifically designed for this purpose.

[russell_w_b]

Originally Posted by richwk1

I'm thinking can i connect my 746 inthe shop directly to my other in the house and if so some advice as to wiring requirments. Or does somebody have an alterantive solution. Any help or idea's would be appreciated.

Here's a way that can be done with manual signalling, but you'll need four wires:

1) Connect your telephones back-to-back first of all, so you have a loop between both instruments' terms: 8 and 18.

2) Connect one end of a 9V battery to a large choke (PO 3000 relay would do, dependent on type).

3) Take the other end of the choke and the other end of the battery, and connect these wires across your telephone loop, so you're effectively feeding each telephone with mic excitation current via a choke. You can locate this at whichever end of the line you choose.

That's the speech circuits taken care of...

4) Connect a push-button and a 9V buzzer in series at either end of your line.

5) At one end of the line, connect the centre-point between buzzer and push button to terminal 18 of your 746.

6) At the other end of your line, connect the centre point between buzzer and push button to terminal 8 of your 746.

Voila!

Of course, this system must remain independent from your telephone system - it's an intercom: nothing more, nothing less!

[russell_w_b]

If you connect as an intercom like I've shown, you could arrange a switch or a transfer-socket at the house-end so that, upon 'buzzing' your wife, she could plug you up to BT and you use your telephone as normal. A sort of plan-set, if you like.

[Dave Moll]

Whoops. Having read Russell's design, I realise that I should also have included a choke - otherwise I would be feeding AC back to the battery!

[AndiiT]

Originally Posted by Dave Moll

Making them talk to each other is easy - simply connect them in parallel (as originally wired for two-wire operation) and apply a DC voltage somewhere in the order of 10V between the A (white) and B (red) terminals (a 9V battery will suffice).

Hi,
Wired in the fashion mentioned above you will only get speech current flowing in the right direction on one of the telephones, you either need to connect the battery in series with the 'B' Legs (red of line cord in conventional wiring) or reverse the 'A' and 'B' legs of one of the instruments if connecting the battery in parallel.

The method using a choke is probably best as the transmitter current would flow in the same direction in both telephone instruments (not that this seems critical). Also if a relay was exploited as the "transmission" choke the contacts could possibly be used to operate a signalling device of some kind when the calling end loops the line, as it were.

As has also been mentioned to obtain Hold and transfer facilities a Planset might be a better option, these are often available on eBay and still meet BTs approval requirements.

Regards
Andrew

[richwk1]

Originally Posted by russell_w_b

Here's a way that can be done with manual signalling, but you'll need four wires:

1) Connect your telephones back-to-back first of all, so you have a loop between both instruments' terms: 8 and 18.

2) Connect one end of a 9V battery to a large choke (PO 3000 relay would do, dependent on type).

3) Take the other end of the choke and the other end of the battery, and connect these wires across your telephone loop, so you're effectively feeding each telephone with mic excitation current via a choke. You can locate this at whichever end of the line you choose.

That's the speech circuits taken care of...

4) Connect a push-button and a 9V buzzer in series at either end of your line.

5) At one end of the line, connect the centre-point between buzzer and push button to terminal 18 of your 746.

6) At the other end of your line, connect the centre point between buzzer and push button to terminal 8 of your 746.

Voila!

Of course, this system must remain independent from your telephone system - it's an intercom: nothing more, nothing less!

thanks Russel your idea sounds the easiest to put into practice to me, due to this not being my area of expertise but could you explain the need for a choke and where i could get one, is there an alternative item that could be more easily obtained (or cobbled together) that could be used.

Thanks to everyone as usual for the helpful replies. Rich

[russell_w_b]

Originally Posted by AndiiT

Also if a relay was exploited as the "transmission" choke the contacts could possibly be used to operate a signalling device of some kind when the calling end loops the line, as it were.

If you had a couple of P.O.3000 relays and, say, a 36V d.c. supply, you could use the fancier 'automatic' version (attached).

One end of the line has the transmission bridge formed by the two relays and the capacitor. The other end is connected - using separate d.c. buzzers - with only three wires.

Station 'A' lifts their handset and energises relay 'A'. Contact 'A' (closed) opens and contact 'A' (open) closes, sounding buzzer at extension 'B'.

Station 'B' lifts their handset and energises relay 'B'. Contact 'B' (closed) opens, cutting off the buzzer, and contact 'B' (open) closes - doing absolutely nothing as it is in series with relay contact 'A' (closed) that is currently open.

The same in reverse, of course. Sorry - I should have labelled the relays more appropriately, but you'll get what I mean from the drawing; pictures paint one thousand words and all that... Relays act as speech blocking chokes with the speech from tele to tele passing through the capacitor.

[russell_w_b]

Originally Posted by richwk1

'but could you explain the need for a choke and where i could get one'

A choke is just an inductance, allowing d.c. to pass but not a.c. A type 59A bell-motor would do, offering, as it does, an impedance of about 17 kilohms to a 1000Hz speech current. All the choke does is prevent the a.c. speech components from being shunted by the battery. You could try the primary winding of an old mains transformer if you haven't a spare bell-motor kicking about.

The d.c. from the battery passes through the choke to energise the microphones, whilst the a.c. component will pass around the telephone-to-telephone loop, being prevented from passing through the battery (or, at least, attenuated sufficiently) by the presence of the choke.

[richwk1]

Thanks for the explantion Russel i understand a bit better now. I think i have a bell set motor of a 746 somewhere not sure if it's th 1k or 4k. I will have a bash in the morning, i think i have some 4 strand exterior phone extesion cable which should fit the bill. Thanks for the help.

Rich

[dagskarlsen]

I have used this simple intercom design, it may be you need mecanical tuning of the clapper and bells to get it ringing on 50Hz, but i works for mee with Norwegian phones. http://img.photobucket.com/albums/v4...com_simple.jpg

It works this way, when you lit off the handset, and replace it all (2) telephones rings once. When the canversation is ended and both has putted the handset back, you get one more ring. Allmoast as on the old manual exanges.

dsk

[ex 2 Base]

I use a baby alarm,one in the house the other in the shed, run off Ni Cads.Very cheap second hand from car boot sale, together with a very long length of phone type multi cored cable, only need a pair so parallel up the spare cores.I ran the cable inside the hedge 15 years ago, charge the Ni Cads occasionally.Ted

[Station X]

Paralleling cores does not reduce transmission loss. It does lower the signalling resistance though.

[russell_w_b]

Originally Posted by russell_w_b

'If you had a couple of P.O.3000 relays and, say, a 36V d.c. supply, you could use the fancier 'automatic' version .'

A refinement (untried and untested, but why shouldn't it)?

1). Tele 'A' handset is lifted, pulling in RLA (part of transmission bridge) via battery.

2). RLA 2 opens, breaking circuit to its own bell, and RLA 1 closes, putting 50V 50Hz onto bell of Tele 'B' via standard socket connections 3 and 5. Tele 'B' bell rings.

3). Tele 'B' handset is lifted, pulling in RLB (part of transmission bridge) via battery.

4). RLB 1 opens, cutting off ringing by breaking energised bell circuit, being in series with contact RLA 1. RLB 2 closes, doing nothing, as RLA 2 is already open.

Ringing has been interrupted, and speech is allowed to flow around loop between terms. 2, 2, 5, 5 of telephones.

It is possible to adjust a 59A bell-motor to work on 50Hz. Secondary LJU boxes would be needed, as there would be no need for a capacitor, the 50Hz going directly to the bell-motors themselves via terms. T4 and T16-17-18 of the telephones on the system. It may prove better to use a series capacitor within the telephone to 'tune' the bells, but this is easily achieved by connection between links inside the telephone.

It's a basis for experimentation, if nothing else.

[DAVEHALL]

I know that this is not strictly answering the question, as it doesn't use the specified 746's -but considered using two cordless phones , base station in house ( with paging facility ) .

But ,from memory ,and to be used independently ,without line current from BT line, both would need to be in LB mode - something i don't believe 746 supports-so a transmission bridge would need to be included - perhaps a way of signalling ( and lets not forget earth signalling) .But , there's always the old favourite -ring back ,although you'd need to talk over dial tone .On the topic of bells working on 50hz - many of the non BT private telephone systems ( i.e Panasonic for ex) use 50 hz, and problems only occur on things like fax machines .

[peetleetech]

Maybe I can add something here. The problem is an old one, my dad once tackled it this way. Consider the modern little loudspeaker, it is sensitive to noise both ways, so, in the shack you have an amp, you listen. When the wife calls you depress a changeover switch and talk into your 'speaker. And so on. The problem is finding just the right speaker.

[russell_w_b]

I could be wrong, but I don't think the intercom bit is at issue here. The OP wished to connect two 746 telephone instruments as an intercom. The circuits shown would work with other telephones, of course, and the pure d.c. powered circuit in post #9 would activate the ringers on the likes of a BT Viscount or similar, the ringer of which is fed via a bridge rectifier and doesn't need an a.c. ringing supply to hoot.

[cmjones01]

There is another approach, which isn't exactly vintage...since Rich mentions he's already got internet in the shed, what about one or a pair of VoIP adapters, so the phones can talk to each other across the network connection? A web search for 'SIP adapter' brings up lots of things for around £30 or less. Not the cheapest solution, but may be worth a thought.

Chris

[DAVEHALL]

As I mentioned in my musings - have a look at the old switchboard designs- with a transmission bridge in the house ,supplying transmission current to both phones, and on lift off of the distant one , giving an alarm to the house .
Old designs sometimes are the best . OR DO I NEED TO SHOUT .