I want to create a simple (telephone based) intercom system in our new house. Shouting between rooms is not so good, and trying to use an IM client or Facetime doesn't have the immediacy of a good shout down the corridor.

I've got a box of old wired telephones that don't have much use any more, and will be wiring the new place with cat5e for networking so running an extra wire to each room and sticking a phone into it is no real hassle.

I've seen online plenty of sites showing me how to connect two phones together so you can talk in one, and it comes out the other. This seems pretty simple - a battery, a resistor, some phone jacks to make it neat.

What if I wanted many phones (five) and also wanted them to make them ring?

I'm not trying to create my own PBX, there's no need to ring individual phones or have more than two people talking at once. Picking up a handset and having all the others ring until any other handset is picked up is enough.

Bonus points if I can do it without touching an Arduino ;-)

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    \$\begingroup\$ Do your old wired telephones have cranking handles? :) \$\endgroup\$ – Chetan Bhargava Feb 17 '13 at 10:45
  • \$\begingroup\$ I'm not arguing you should use an Arduino (or not), but I am curious why you prefer not to use one. \$\endgroup\$ – jippie Feb 17 '13 at 12:32
  • \$\begingroup\$ Wouldn't using an Arduino add unnecessary complexity? I.e it goes from how to wire up several phones for audio and make them ring, to how to do that under software control. \$\endgroup\$ – Piku Feb 17 '13 at 13:19
  • \$\begingroup\$ Although, I just found this website which is interesting joes.com/intercom/design.html \$\endgroup\$ – Piku Feb 17 '13 at 13:19
  • \$\begingroup\$ Seriously, just buy a home PBX from Panasonic or the like and there you go. \$\endgroup\$ – InterLinked Mar 31 '17 at 1:28

Like Dave said, you need to detect the state of the phone, and you need to send ringing voltage /24 VAC should do fine, but I needed 48 VAC for some Panasonic phones/, then you have to trip this voltage off as soon as someone has picked up the phone...comes out lot of logic for what looks like simple task, hard to do without micro.

I have actually designed exactly what you need, long time ago, it was even a commercial product, but it's long history know. I was thinking of making it public anyway, but it is a bit over designed, we used the current loop for phone supply (and it was a bad choice of uC). You might find some parts useful, or try to replace the micro with some simpler logic while taking the phone supply, of / on hook detector and some other stuff from our design (us as 'me and my good friend Mario', his part was FW and analog HW design, my part was CPU HW design, PCB design, testing, and documentation).

Here you can see the circuit reponsible for powering the phone, line state detection (on/off hook, but also the pulse dialing detection), ring voltage control, everything regarding the single phone. You could build two of those circuits, and you have got yourself an intercom, but with many options you might not need :-).

enter image description here

I can post the complete schematic, code everything, if anyone was still interested for analog phones made useful. Personally, I still use similar switch at home. And yes, our exchange (two designers, remember?) has four internal extensions, one external line, phones galvanically isolated from the external line, 'do not disturb' mode, distinct ringing for internal and external line...and many more options pushed into 4K of PIC assembler code. It has got also a complete users manual (in Croatian only ;-) )

A should mentioned that phones were interconnected using the analog crosspoint switch, not relays or discrete components. Music on hold was for example also routed to the relevant phone through the crosspoint patrix, as well as the dial internal dial tone, ring tone, external party...

I should make some open hardware / open source product out of it. Bad news? I have mentioned the micro. PIC16C57, all assembler, it was nightmare /my friend has pulled it off, micro choice was mine which was as stupid as it could be considering I was already familiar with 8051, and the price of a micro was not relevant anyway. If we designed in the 8051 back than, it would have 64K Flash today in a same DIP package. May be you could do it now ;-)). The boards are separate, so even Arduino could be used for the CPU :-)


You're asking for a combination of telephone "features", to wit: "automatic ringdown" (ARD) is used on private (leased) lines between two phones, in which picking up one phone rings the other, and "party line", in which multiple phones can share a connection.

Unfortunately, the two features don't really work together. You're going to have to run wires from each of the phones to a central location, where some sort of controller recognizes an off-hook condition on any one of the phones and rings all of the other phones until (at least) one of them is picked up. In order to do this, the central controller needs to be able to sense the on-hook/off-hook condition of the phones individually.

I can't imagine any good way of doing this that doesn't involve a microcontroller, and Arduino is as good as any for this application.

If you did have phones with hand-cranked ring generators, you could simply hook them all in parallel with a battery circuit and you'd get the functionality you need.


A simple way could be this. You need a DC power supply that can supply > 40Vdc and is current limited to about 40mA and has an output imdedance that allows voice signals to develop on the line. You could use a 1kohm resistor for doing this but watch the power rating - probably need a 2W resistor from quick brain math.

All the phones are connected in parallel and there's a good chance that when you pick one phone up and tap the hookswitch it'll cause ring sounds to emit from the other phones. It may not be great but it's easy to try. If some phones don't produce a sound then maybe try a higher supply voltage and higher resistance. If this doesn't work then scrap the idea!!!!

When it comes to speaking, there may be a little sidetone issue in that you will hear your own voice in the earpiece a little louder than when using a normal phone. This is because phones tend to prefer 600 ohms and connecting two together is fine providing the pair are current fed (or via a big inductor) but this uses a further 1k ohms in the supply and more sidetone will be present - a minor issue.


All Plain Old Telephones (POTs) are wired in parallel. Somewhere along the parallel run, you need to also connect in parallel 48VDC. Individual phone line conductors are referred to as Tip & Ring. This comes from the old plugboard plugs Tip,Ring,Sleeve. Phone systems are run by -48 volts with tip being 0 and ring being -48vdc. If your wiring is isolated, you can just apply + to Tip and - to Ring. You do need a current limiter because POTs are not voltage devices, they are current devices. When all phones are hung up (on hook), you should read 48 VDC on the line. When any, or all phones are picked up (off hook) you should read around 10 to 8 VDC. Depending on your wire gauge and length, you should be able to play with some resistors to get the right value (about 600 ohms on each wire = 1200 ohms total) .

Now, here is the tricky part. To ring the phones, you need a ring generator, or build one, but be careful. POTs ring with 105 VAC at 20 to 30 HZ but ONLY at 5 to 10 milliamps. The safest bet is to look on eBay for a "ring generator" and use it. All phones must be hung up for them to ring. One phone picked up, and none will ring. At each phone, you need a way to inject the AC signal onto the Tip & Ring wires. You said you are using CAT5 wire, so your phones will use Tip (White/Blue) and Ring (Blue/White) .. That leaves the Orange pair, the Green pair and the Brown pair doing nothing. You can parallel all of the Brown pairs together and connect the ring generator to the RJ45 pins 3 and 6 - if wired 568A white/orange orange/white, or of wired 568B white/green green/white. If your phones have 4 conductor line cords, you will then bring the ring generator AC into each phone. Add a DPST momentary pushbutton to every phone, connect one side of the switch to the ring generator AC and the other side of the switch to tip & ring. Anytime any pushbutton on any phone is pressed, it will impress the 105VAC at 5mA onto the tip & ring and cause all phones to ring.

Hope this helps.


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