Simple Intercom - Where to Start?

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jrlittle86
 
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Simple Intercom - Where to Start?

Post by jrlittle86 »

My 8 year old daughter would like me to help her build a simple intercom that we can use during our upcoming road trip. I have a PC headset with a mic (3.5mm connector) and an old telephone and could even pick up another PC headset. There has to be an easy way to do this. Where do I start?

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Renate
 
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Re: Simple Intercom - Where to Start?

Post by Renate »

Hmm, this comes down to how to do it a few different ways.
If you said that you only have 2 or more old telephones, there is one solution.
If you said that you only have 2 PC headsets, there is another solution.
If you said that you want it to work with a dozen headsets, then, aw, you get the idea.

The basic problem is the question of 2 wire vs. 4 wire.
"2 wire" means a single signal path that carries the audio in both directions.
"4 wire" means a signal path for each direction of the audio.
(Note that this does not always mean the exact count of wires.)
Old telephones inherently use 2 wire.

With two old phones, hook up a power supply through a choke and connect the phones in parallel.

For two PC headsets, hook a small amplifier with a bias resistor on the input to the mic of user A and the output to the earpiece of user B.
Then do the same for the other direction with another amplifier.
If you wanted to use an old phone for one side, you could cannibalize the phone and separate the mic and the earpiece electrically to make it about the same as a PC headset.

If you wanted to hook up 12 PC headsets you are getting into something like an RTS intercom system.
This is a professional device, not a recommendation: http://www.rtsintercoms.com/us/rts/product/BP-319/1443

It uses a "2 wire/4 wire hybrid" inside.
There are hybrids using transformers and hybrids that use balancing circuits with op amps.

For a simple, two person intercom, I'd stick to a 4 wire system.

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jrlittle86
 
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Re: Simple Intercom - Where to Start?

Post by jrlittle86 »

I grabbed two basic phones, added power and a little resistor....easy peasy. Decided I don't want to have a phone up to my ear while driving. Besides, it's illegal in some states. It's official - headset up front and keeping the telephone in the rear of the van.

Ok, time to put something in the 'black box' to make this work. So far I have:
  • Headset w/mic - 3.5mm connector.
    Basic 2 wire phone
    project box w 3.5mm and rj11 connectors
    Power supply
I'm not sure what else I need to add 'in the box' to amplify the audio between the headset and phone...
I'd be happy to post a solution once I get it put together for future reference.

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Renate
 
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Re: Simple Intercom - Where to Start?

Post by Renate »

The first step is amplifying the PC headset microphone.
To make life easier, I'd start with: https://www.adafruit.com/product/1063
You can unsolder the microphone capsule that is on the board and connect your headset to those points.
One of the points is ground, the other is signal.
You'll also need a 5V regulator to power that module.

You know, life would be easier if you just used the handset off the telephone.
If you wanted to retain the base you could just wire the four conductor cable directly through to the handset.
If you don't want to modify the telephone you'll have to be homebrewing your own 2 wire/4 wire hybrid.

You could also make life simpler by just using another PC headset or else one of those joke, retro handsets for cell phones.
You could also gut the telephone mouthpiece and install another Adafruit board with the on-board mic.

Throw in a couple of inexpensive 386 amplifiers and you are done.
http://www.ebay.com/itm/140894919328

If the point of this project is to be amusing, you can add a ring generator to have the phone mechanically ring.
That will be a circuit with 2 transistors and a transformer.

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jrlittle86
 
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Re: Simple Intercom - Where to Start?

Post by jrlittle86 »

Renate - You nailed it! I totally forgot about those old retro phones for cell phones. This should be easier than I thought. Thanks so much. Now it's time to come up with some other projects for the trip. You're brilliant!

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