GPO phones - ringing intercom project

[Kilders]

Hello all and thank you in advance for any help!

I am trying to make an intercom circuit using 2 old phones between the kids play house and the living room (~25m). I have found a YouTube video of someone doing this with old soviet phones and it seems relatively simple even for a novice like myself (i’m assuming it’s not though!) I’m looking to buy old GPO phone from eBay and create the rest of the circuit from modern components.

The video is at https://youtu.be/0nVkSxVJ2kk?si=33W6_v2JQ1JMW24r

The circuit schematic is attached.

Is this possible with UK GPO phones? Is there a better way or modified way to achieve this?

Thanks again in advance, I’m looking forward to learning a lot with this project.

[AndiiT]

Hi, your system looks fine.
There are details in this issue of Television magazine which describes a system I designed some years ago, it may be of use of give you some ideas.

Theres also information here that shows how to exploit a Linksys PAP2 voip ATA as an intercom system.

Andrew

[dagskarlsen]

I have made something similar, and it worked, but 50Hz ringing is not good. another "hack" may be something like this: https://www.instructables.com/Hack-a-VOIP-Box-Into-a-Telephone-Intercom/
Then you may get UK dial tone, UK ring cadence.. if you want.

[Kilders]

Originally Posted by AndiiT

Hi, your system looks fine.
There are details in this issue of Television magazine which describes a system I designed some years ago, it may be of use of give you some ideas.

Theres also information here that shows how to exploit a Linksys PAP2 voip ATA as an intercom system.

Andrew

That’s a brilliant article Andrew. Almost exactly what I’m looking for. I’ll try this as option one and the other as option two. I assume this works for 1990's type equipment and not the older stuff?

[AndiiT]

.... I assume this works for 1990's type equipment and not the older stuff?......

There's no reason why my design shouldn't work with anything from the GPO 300 series onwards, providing that they have been wired for three-wire working (eg converted to be used with a "PST" cord) and have a suitable auxiliary button/switch fitted.

Details for converting older phones can be found at Bobs Telephone Files

Similarly the "Hack a VOIP box into an intercom" design should work with any phone which is wired for two-wire working, using the phones internal bell capacitor for the ringing circuit.

Andrew

[dagskarlsen]

3 wires systems are easily converted to 2 wires if it has the capcitor, or you ad a capacitor. Buildin a 3 wire system may even be easier.
50 Hz ringing may need an adjustment to of the ringer, but possible with most the of ringers.

[dagskarlsen]

If the phones disconnect the ringer when going off hook this should be an easy well working solution for 3 wires. No need for extra/internal capacitors.

You just use a transformer with an output voltage between 35 and 50V
The capacitor named Large needs to be as large as you can find (in your storage) just to reduce the hum when the rectifying is just half wave.
The 2 lower resistors has to be there to limit the current to the phones to be something between 25 and 40 mA. This may be depending on the transformer voltage, and the relay (+line)resistance. (my guess is that the line resistance in a house is quite ignore-able).

[julie_m]

Why not build yourself a proper telephone exchange? A scrap PC, with the free and Open Source Asterisk software and the addition of a multi-way FXS interface card (example eBay link, just to give you an idea of the sort of thing. Note that you have to specify the desired combination of FXO and FXS ports. FXS is for connecting to a phone, FXO is for connecting to an exchange line) will let you do just that. If you have an FXO port, it can be connected to an exchange line, and thence the rest of the world's telephone network; or alternatively, it can be connected to a SIP trunk, saving a position for another FXS port. An FXS port is still powerful enough to ring the bells in most classic phones, and it can still decode the pulses from a rotary dial as easily as DTMF tones -- even a phone with no dial can trigger a fixed action, such as calling another extension, as soon as the handset goes off-hook.

And that's before you get onto anything exotic, like programming your dialplan so, for example, dialling 846 (=VINdaloo) connects you straight through to your local curry house .....

One disadvantage with this method is, it could leave you wanting a real, clicky-clicky exchange .....

[dagskarlsen]

Exsample of phoen for my scetch:

[Kilders]

Originally Posted by julie_m

Why not build yourself a proper telephone exchange? A scrap PC, with the free and Open Source Asterisk software and the addition of a multi-way FXS interface card (example eBay link, just to give you an idea of the sort of thing. Note that you have to specify the desired combination of FXO and FXS ports. FXS is for connecting to a phone, FXO is for connecting to an exchange line) will let you do just that. If you have an FXO port, it can be connected to an exchange line, and thence the rest of the world's telephone network; or alternatively, it can be connected to a SIP trunk, saving a position for another FXS port. An FXS port is still powerful enough to ring the bells in most classic phones, and it can still decode the pulses from a rotary dial as easily as DTMF tones -- even a phone with no dial can trigger a fixed action, such as calling another extension, as soon as the handset goes off-hook.

And that's before you get onto anything exotic, like programming your dialplan so, for example, dialling 846 (=VINdaloo) connects you straight through to your local curry house .....

One disadvantage with this method is, it could leave you wanting a real, clicky-clicky exchange .....

Oh my, I’m a total beginner with electronics and this is too complex for me! I think keeping it as simple as possible is the way forward!

[Graham G3ZVT]

Originally Posted by Kilders

And that's before you get onto anything exotic, like programming your dialplan so, for example, dialling 846 (=VINdaloo) connects you straight through to your local curry house .....

I can't do that Julie, as 846, better known as TIM connects me to Pat Simmons, who's still got that gig on my Asterisk.

[Kilders]

Thank you all for your help so far. My first attempt will be for Andrew's published circuit from Television Magazine. I now have most of the components (the transformers were the trickiest to get!) and did a bit of reading up on the basics. The last issue is what phones to buy? Will the cheap 1990's BT models (eg converse 100) have an earth recall loop or should I look older than this to make the circuit work?

[Nickthedentist]

Personally, I would just visit your local charity shops and ask. Most of the house-clearance type outfits will have dozens of phones which are hard to shift these days, and may just end up dumped.

Otherwise, choose what you like the look of.

I rather the like the BT Viscount from the early 1980s. Unlike the Statesman from the same era, they seem to have survived the test of time very well, and the styling is quite striking and fresh even 40+ years later. Most have earth recall (some require a switch underneath to be moved to select it).

N.

[Kilders]

Thanks nick, just secured a couple of viscount models 9515AR from eBay which AI assures me has the option of earth recall loop….

[198 kHz]

AI sometimes makes things up, but I'd trust
https://www.britishtelephones.com/t9515.htm

[AndiiT]

Originally Posted by Kilders

....... The last issue is what phones to buy? Will the cheap 1990's BT models (eg converse 100) have an earth recall loop or should I look older than this to make the circuit work?

Originally Posted by 198 kHz

AI sometimes makes things up, but I'd trust
https://www.britishtelephones.com/t9515.htm

The Viscount phones should be fine, also check for the Statesman phones on the same site, both are readily available from eBay from give away to delusional prices.

The transformer used in my intercom circuit was from a scrap VHS VCR which used a VFD display, as they require around 40/50 volts to illuminate.

If using standard 700 series phones the bells won't sound quite right due to the 50Hz ringing current, I think they can be adjusted to sound a bit better though, once again all the information can be found on Bob Freshwater's Site

Andrew

[Kilders]

I have had my first failed attempt at the circuit! Can anyone confirm the correct connections to the viscount phones for pins 2-5? In the image the wires attached into the phone are white, green, blue and red in that order. After some research I think I had decided it was blue is 2, 3 is green, 4 is red and 5 is white?

You can also see the chaos that is my project box if there are any other obvious errors which can be decipherable! The bridge rectifier is hidden behind the capacitor and I can already see I am missing C2 by the single diode. The transformers are 2x12v 10VA on the right and 2x24v 2.5VA on the right.

Thanks again in advance!

[julie_m]

Usually, on a phone with an RJ-431 ("BT") plug, red is pin 2 (B-wire), blue is 3 (ringing), green is 4 (earth; often not connected in single-line residential installations) and 5 is white (A-wire).

You sometimes see phone leads with the internal insulation colour-coded yellow, red, green, black; in this case, yellow is usually 2, red is 3, green is 4 and black is 5 (think: green is always green). There are a few rarities with the opposite wiring scheme, though!

For the benefit of anyone who may find this thread in a future search: Phones not meant for a BT line may use just two wires (or sometimes, just the middle two of four). In this case, they will be the A- and B-wires, and the phone will have its own internal ringing capacitor.

[Nickthedentist]

I think the Viscounts will work (less earth recall) with just the outer two wires connected.

[AndiiT]

Originally Posted by Nickthedentist

I think the Viscounts will work (less earth recall) with just the outer two wires connected.

I have a few Viscounts and can confirm that they definitely need the third wire for ringing

Andrew