How To Build Your Own Analog Phone Network

Two landline phones connected to a set of wires and boards

Analog phones may be nearly obsolete today, but having served humanity for well over a century they’re quite likely to pop up in drawers or attics now and then. If you’ve got a few of them lying around and you think it’d be cool to hook them up and make your own local telephone system, check out [Gadget Reboot]’s latest work. His video series shows all the steps towards making a fully-functional wired phone system.

Of course, dedicated phone exchanges for home or small business use are not hard to find, but [Gadget Reboot] decided it would be way more interesting to design his own system from the ground up. To begin with, he used off-the-shelf subscriber line interface circuits (SLICs) to implement the correct voltages, currents and impedances to drive analog phones. He then added a DTMF decoder chip to allow the phone to dial a number, and hooked up both systems to an ESP8266 which controls the entire system. It implements the different states of picking up, dialing, ringing and hanging up, and also generates the corresponding audio signals.

The system becomes even more interesting through the implementation of a multi-exchange layout, just like in large-scale phone systems: when a number is dialled that’s connected to a different exchange, then a connection must be made between two exchanges in order to complete the call. Large-scale systems use dedicated protocols like SS7, but [Gadget Reboot] preferred to keep things simple and used an RS-485 connection. The two ESPs check each others status and if everything’s in order, a relay connects the two lines and the circuit is completed.

The current system is a bit of a mess of wires, but it works, and [Gadget Reboot] plans to make a cleaner setup based on custom circuit boards, possibly expanding it with functions like modem support. In any case it’s already way more advanced than a simple electromechanical system. Want to know more about classic phone networks? We’ve got you covered.

16 thoughts on “How To Build Your Own Analog Phone Network

  1. This is awesome. I love the retro aspect of this build. I have wondered if this could be easily set up. Many homes have unused phone wiring that could be put to use (except for mine that has crappy CenturyLink/Brightspeed DSL…please feel sorry for me). Now to round this out there needs to be an acoustic modem to shove the handset into to connect a TRS80 to an inhouse server… or a PC with an AOL free minutes CD… ๐Ÿ˜„

    1. That’s possible, actually. Old online services like Prestel, Minitel and BTX are already being emulated. Even Quantum Link (Q-Link), Habitat and AOL are being re-created. So you can indeed connect to them again with both physical hardware, emulators or old PC/Mac software. ๐Ÿ™‚

      The software to emulate each services central computer is usually open source. But on top of that, some individuals even run them privately for others. So they can connect either over internet or old school via telephone line. ๐Ÿ˜ƒ

      Emulators for Hayes modems do exist, too. They act as if they’re serial modems, but tunnel data over internet. DOSBox has one built-in. Old software thinks it’s just an ordinary modem.

      The reverse is also available. A VOIP router that’s running a script to act as the modem of an ISP or online service, for example. Or a Raspberry Pi that simulates a DBT-03 modem/interface (for BTX service). The French have something similar for their Minitel (tรฉlรฉtel), I guess..

      That way, someone can use an old analog modem to the internet or to a simulated online service of long ago.
      – That shows how good old and new technology can coexist with each other!

      That being said, I heard some modems can also directly be wired to another modem without the need for a landline.
      That would be ideal for letting a PC act as an ISP.

      Anyway, building a simple analog telephone system is easy. Just needs an 48v (?) transformer, some diodes, electro mechanical relays, caps etc. Back in the late 20th century, they were commercial available to kids, little offices, so they could have their own little inhouse telephone using regular telephones.

      Of course, there also were toy telephones that ran off 4,5v. Sold since the 1950s or so. They were completely different, electrically. Just a few volts DC, no AC for the bell.. But fun nevertheless.

      Some links:

      https://github.com/bildschirmtext/bildschirmtext

      https://m.youtube.com/watch?v=FLRELweT17s

      https://osmocom.org/projects/retronetworking/wiki/AOL

      https://m.youtube.com/watch?v=ODq_Zo8lPRI

      1. Can confirm the modem direct link. I used it to dial up across the house. I remember one being plugged into the extension port for the the phone and dialing causing the phone in the kitchen to ring.

      2. When i was a kid back in the 50s i had something called a Zimphone. It came in pairs & used, IIRC, a pair of flashlight cells for talk battery and DC buzzer ringing. Hooked to some twisted pair and ran it thru the house. A toy, mostly, but could be used for a permanent house intercom. Very simple, carbon mics and push buttons for signalling. Before the days of consumer walkie talkies. Later on i worked for Bell on the real deal.

    1. I don’t know what’s used in this project, but in the past, I used a Silvertel Ag1171 SLIC (or r-tone clone) and connected it to an old telephone. The SLIC module contains some kind of boost converter to transform the 3.3V or 5V supply up to produce a 72V “battery voltage”. The ringing is generated by periodically turning the “battery voltage” on and off. These modules are very easy to use.

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