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Using telephones as an intercom

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Hello all.

First post so apologise if it is in the wrong area.


Has anyone tried using telephones as a low budget alternative to intercom systems? I saw a thread a while back about a PBX for theatre use, however I was thinking more along the lines of about 3 telephones in parallel with a 24v DC supply (possibly with current limiting if needed). In the telephones themselves, replace the speaker and microphone with some 3.5mm jack sockets, and place the whole thing in a plastic project box, possibly with volume controls. In theory, this could also be extended to a DECT cordless phone, provided that it doesn't have any form of dial tone detection.


Also, if this were to work, would it be possible / safe to run a couple of stations on one channel of a multicore?


At the moment we use some PMR446's, which work OK but can be annoying, as we have earpieces but not microphones.


No call / signalling is needed on these, as I only plan to use about 3.



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Well, back in the old days when I were a lad.... :)


...circa 1978 that is; we used to use GPO bakelite headsets, which were basically a carbon disc telephone earpiece with an acoustic horn mouthpiece arrangement. They could be connected quite happily with a twisted pair cable of moderate gauge and were powered by a 9V battery, and they worked a treat. Sure, they were a little bit noisy and they were always on; but the only maintenance they ever needed was to replace the battery every so often, and maybe to add another one in parallel if the followspots were in use. :unsure:

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Setting up telephones is a simple process which I've played around with.


For sometime I've tried to get a good cheap DECT system working for contacting my sound team. I got 6 individual DECT phones working together which all had headsets. Problems were mainly due to the crew inadvertantly pressing buttons on the phones and not being able to mute the mics. They worked really well as far as range goes. I could talk at the back of Malvern Theatre with the sound crew running about all over the back of the theatre and 2 floors up.


I've got all the other alternative forms of communication including radios etc.... but I still won't be beaten once I've got an idea.

Phase 2 is about to be put into place. The Plantronics C351N Gap Wireless Headset will work with the base stations I've already got. They include their own mute switch / volume and have about 9 hours talktime. So should have a wireless DECT system which is truely wireless by about Wednesday this week!


If you are looking for the simple wiring for a telephone hookup a quite search will return some helpful results.

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Wouldn't some sort of impedance matching be required from what's needed for the telephone speaker and microphone to whatever's in the headset?



By the looks of things yes.


I have just tried with two telephones, and these work fine when connected to a power supply through a resistor. However, when using an external headset, the speaker was very quiet. I was not able to test the microphone.


The DC resistance of the telephone's speaker is 122 ohms, and for the computer headset seems to be about 32 ohms.

Do most electret microphones have a similar impedance?. I am partly aware of dynamic microphones with different impedances, but presumed as an electret mic has a buffer amplifier this would not matter so much?


I think a preamp may be needed on the circuit, would it be better in the microphone or the earphone side?

I will try to build an op-amp preamp and see how things go.

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I have built a working system using just the method described above.


I took the guts of some desktop phones (my work means I end up binning quite a few of these for simple cable faults - the PCB's are normally fine) and fitted them into beltpacks. The power supply also contains a decent size toroidal transformer to act as an inductor, it appears that the system works much better with an inductive element in the power supply.




The lower schematic shows my power supply setup.


You have to switch the mic circuit using an inline switch, using the on hook switch also mutes the earphone which isn't helpful.


I did toy with building a dtmf decoder into some of the units so it would be possible to send cue signals or trigger relay outputs using the tone generator in the handset.


As for mics, luckily in my case, I could fit the original mic from the handset, into the boom of my headsets with little problem. Seemed easier than having a pre amp.


The only thing I've struggled with is call lights. I though this would be easier, but as it's basically a balanced audio line with DC sat on top, it's very difficult to get a cue light working without making quite a loud audible pop on the audio side of things.


The other downsides of this method are that you have very little control over the earphone level and there seems to be a limit of 5 beltpacks on my system before levels start getting really quite low.


The system also needs labelling up pretty well - it's definately not compatible with other systems.


I think it would be possible to interface another brand intercom into this system using transformer isolation and if you want to do things really easily, you could also try some off the shelf telaphone recording adapters.


It's a pretty easy way into having your own intercom, I've certainly had more success than any DIY solutions like comclone.


Have fun, enjoy!

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For some reason or another, when I tried the two telephones in parallel, no audio passed between them today. I thought it could be the decoupling capacitor on the power supply, although still nothing with a battery. It is entirely possible that I have damaged one of the phones, although this seems unlikely. I will be able to borrow a few other phones in a few days, so in the meantime I decided to try another approach.


As I would probably have to use a few ICs anyway for impedance matching, would it be possible just to use, for example, two op amps and a LM386?

One op-amp would be a microphone preamp, the other would be a kind of buffer amplifier, to provide just the microphone signal without the audio from the other stations. This would be then fed into the inverting input of the LM386. The output of the op-amp would go to the input of the LM386, and the rest of the headsets.


However, I struggled to get the inverting input of the LM386 to work. All of the sample circuits that I have seen just connect the non-inverting input to ground. I may also try using the other op-amp as another inverting stage. I will try to convert the hand drawn diagram to a computer drawn one.


I would prefer not to have to use any ICs at all, but if I need to, this solution looks quite good.



This is just a little project really, although if it is finished by december ( :( ) it could be used in the next school play (A play about a play of grease).

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Plantonics headsets have arrived. Connected up 2 using the "telephones into intercom idea" by linking to the old DECT Gap Base Stations, no problem. These headsets are amazing, really comfortable. Own volume control and mutes - Really loud if you need it. Whispering is spot on. 9 hours battery life. Truely wireless intercom for a fraction of the cost. Testing out at show tonight (with traditional backup just in case!). If all goes well will extend to 4 headset system and make up a few wired systems for "fixed" crew (Lighting and Sound).


If you want to grab a look at the system (in it's very basic state!) Follow this link

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Very interesting idea indeed.


Just to confirm then, each headset has to have its' own base station. ese base stations are simply connected together in terms of the telephone pair running to them? Is there anything else clever to interface the two so they can talk to each other or is it simply a wire?


Any joy or thoughts on interfacing to an exisiting cans system? I was thinking some form of transformer isolation on the line. Would be intrigued to hear your results.

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Yes. A DECT base station for each. The headsets have no way to dial each other. Just on or off hook by pressing a button on the headset. Worked fine for 3 hours tonight!


Basically this works using a Master Phone socket. On the reverse using connections 2 and 5. Using a 12 volt supply to power the thing. Other items I used are a couple of 12 volt transformers with the 12 volt cables cut off and the 240v cables stripped! You then put +12 volts from the power to the "2" of the master socket. Then on the "5" of the socket the live (brown) from one cut up transformer. The "blue" neutral from this to the live (brown) of another transformer then the brown of the final one to the negative of the 12 volts supply (It's late and I've just finished a show so may not make sense!). You can use one cut up transformer and a resister I think. With the one "cut up" transformed I had a bit of hum, with 2 nothing.


I'll rip apart and photo it tomorrow! In the picture you see that there is a normal "yellow" phone. I'm going to play around with this to adapt into a beltpack when I've got some time. For the time being the Stage Manager was able to grab it to talk to me rather than flashing me for hours on the wired beltpack! You can put in additional phones for a wired system although I noticed the volume dropping after about 6. Of course the volume on these headsets can go so loud you may not notice!

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And with that, I may well be sold on the idea!


I guess the only functionality you're loosing is a call lamp.


Another thought, if you're getting range problems (I know you're not in this case), you could locate both base sations seperate to each other, connected by a cable. For instance, stage left and stage right. This way you keep the flexibility of a no wires system, but each person is much nearer to their respective base station.


Somewhere else in this thread, I've linked to an epanorama site showing a power supply for a wired telephone power supply. It sounds very similair to what you've described, although only uses one transformer and also has a resistor in place for line termination.


If these will interface easily with an off the shelf wired comms system (I think they will with some transformers and possibly some amplification/attenuation as required) then this suddenly becomes a very cheap party line wireless comms system that will interface directly to a standard Theatre system.


What base stations are you using?

Do you get sidetone from the headsets?

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You are right about the relocation if you need to; although I have had no problems. I have run normal telephone on it by plugging in a normal telephone extension lead in. In the final version I'll modify the ariels in the base units for optimum transmission. I popped the whole thing in a corner of the stage tonight. Worked fine in the dressings rooms whilst I sat at the back of the auditorium.


Base stations are from a Doro 530. I bought a load of "nearly new" for about £12 each complete with the handset.


No sidetone whatsoever.


Regarding the call lamp, I've been thinking about another option. You can pickup remote units which can transmit to a receiver which works about the same sort of range. With a bit of playing around it's possible to have a wireless call lamp or even better a vibrating unit! The project continues.

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Couple of thoughts on the call lights, thinking about it, the possibility of somebody being on stage with a wireless headset during a blackout is pretty high, so it could lead to a nice light show mid-blackout. If somebody is away from their station, they are now likely to be wearing their headset as it's wireless, so perhaps a call lamp on the base station would suffice, perhaps next to the charging cradle.


As for the mute, does it just mute the microphone or do you loose the audio from other stations as well? That could be a show stopper if you have to have your mic open to hear other people and may require a quick bodge with a switch.


DECT seems to be a pretty open protocal, I guess any base station would do that was GAP compatible. I have a couple available through a supplier at work that have long range antennae and seem pretty hardy.


Just to check then, the headset replaces the handset, the handset is redundant and no longer needed?

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For some reason or another, when I tried the two telephones in parallel, no audio passed between them today. I thought it could be the decoupling capacitor on the power supply, although still nothing with a battery. It is entirely possible that I have damaged one of the phones, although this seems unlikely. I will be able to borrow a few other phones in a few days, so in the meantime I decided to try another approach.


The basic telephone circuit was designed for one-to-one communications. The two telephones and the DC power supply are all run in series. That is your basic 'field telephone' setup. (An actual field telephone has a battery included in it.) Well in to the 1970's telephones had carbon microphones and, because they have to work on existing telephone lines, modern analogue telephones still emulate the carbon mic technology, in other words the mic is actually seen by the phone line as a variable resistor which varies its resistance in response to the audio (speech) that hits it. Special transformers in older phones - known as ASTIC (anti side tone induction coil)- were wired so that the mic at the far end was mostly in series with the earphone at the near end and vice versa with the windings arranged so as to reduce, but not completely eliminate, the level of your own voice in your own ear (sidetone). In modern phones the ASTIC function is emulated by electronics. Commonly a single chip will provide all the necessary circuitry to interface an electret mic and a dynamic earphone to an analogue telephone line, all powered from the ~50V DC supply on the telephone line.


If you connect telephones and DC power supply directly in parrallel you end up with no audio because the audio is shorted out by the internal, very low, AC impedance of the power supply (battery or smoothing C's). To get any audio using a parrallel arrangement you need either a series AC impedance i.e. a very large inductor, to block the AC from reaching the power supply or a series resistance to develop the audio voltage across.

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