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I am hacking some old phones into my arduino project. I am currently controlling all of the ringers, dials, and hooks through my board.

At a certain point, I would like to connect the two handsets and let them function kind of like a basic intercom. There are four wires into each handset, two for the microphone, and two for the speaker. As I understand it they function much like any cheap microphone or headphones you could plug into your computer.

My initial thought is that I could just connect the mic of one to the speaker of the other, but I don't know audio circuits very well and am not sure if additional circuitry or op-amps would be required. What is the simplest way to do this?

Also, what would be the best way to toggle this connection, so I can control if the lines are open or closed?

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3 Answers 3

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Your microphone will need to have a simple amplifier in order to power the speaker. This might depend a little bit on what speaker is actually being used in the headset but an op-amp circuit should work for you. You will just have to make sure that the op-amp can output enough power to power the headset. If standard op-amps are not powerful enough to power to speaker you will need to go with a more complicated amplifier.

As for the toggling the connection, are you wanting to just have a dedicated 1-1 connection that can either be on or off? If so, you could do this by powering off the amps with a simple switch. If you are wanting some switch capabilities, such as 1 phone can be routed to 2 different locations, you should look into analog multiplexers.

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  • \$\begingroup\$ Also, as a note, many phones provide a little bit of a loop back to your own speaker in order for you to feel like you aren't talking to nothing. To do this you will just need to do a combination of your mic plus the other persons mic, but keep it variable gain on each one separately in order to get the balance correct. \$\endgroup\$
    – Kellenjb
    Commented Oct 28, 2010 at 3:29
  • \$\begingroup\$ I have an LM741 op-amp, but I am having trouble wiring it up to through the arduino, which only has +5v and 0v. I have two input wires and two output wires, but I am not sure how to hook them all up or what resistors to use where. Could you point me at a good circuit to use? \$\endgroup\$
    – captncraig
    Commented Oct 28, 2010 at 4:29
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About 10 years ago, I bought a device called "Party Line" from a company whose name escapes me. This device has six telephone jacks on it, and simulates the telephone main office. You could plug in up to six regular phones, and they could call one another. Think of it as a very simple PBX (with no connection to the real phone network).

Any phone can dial any of the other five by dialing a 7-digit number where one of the digits indicates the line to connect to. It only supports one connection at a time, though. But for two phones, that's all you need.

I bought it as a kit, and assembled it myself. Inside it has a PIC and some discrete circuitry. I used it while developing some software that would transfer data over a modem connection. I had two computers with 56K modems, each connected to the Party Line. It worked very well, and I made thousands of calls from one system to the other without tying up my real line or incurring any charges.

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  • \$\begingroup\$ nice tool, it should have just been a few audio muxes, as 56k modems and voice just operate in the audio frequencies. \$\endgroup\$
    – Kortuk
    Commented Mar 1, 2011 at 4:11
  • \$\begingroup\$ I found the web site for this: digitalproductsco.com/partyln.htm. It's more expensive than I remember, but it looks like it has new features. This may be way more than you care to spend, and not as much fun as building something yourself. \$\endgroup\$ Commented Mar 1, 2011 at 23:50
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If those are truly old fashioned phones, (e.g., like 1950's phones) there is probably a way to interconnect things without needing any active elements. After all, at the time when they were designed (1920's?), there were no active elements that would fit inside the phone case and which also would not require bulky power supplies. Basically, the whole phone was passive elements and ran off batteries in the local office, the voltages being surprisingly low. I don't know the exact levels off the top of my head (google is your friend here) but want to say that while off-the-hook, the voltage is somewhere around 7 volts.

The only significant part you find inside an old phone, other than the mic and the speaker, (and disregarding the mechanical ringer and dialer) is some kind of a multi-tap transformer. Not sure what the transformer accomplished. The microphones used a finely divided carbon powder, under a diaphragm, and were basically variable power resistors that respond to sound. I know, from fooling around with them as a kid, that you can put a 6V lantern battery, a carbon mic, and an 8 ohm speaker in series, and you can get a signal through. If you put two such microphones in series with two speakers, and of course, the battery, you should get at least some signal back and forth. The transformer in the phone was (speculation!) probably there to get a better signal into the ear piece by doing an impedance match.

At some point (1960's maybe, or 70's?), they stopped using those carbon microphone elements in favor of more modern microphone technology, at which point there would have to have been an amplifier involved. The carbon mics I've referred to can be recognized by the fact that when you unscrewed the mouthpiece cover, this nearly 2 inch diameter mic element would fall out in your hand. If your old phones are newer than that, just disregard everything I said.

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  • \$\begingroup\$ I'm pretty sure I have the old style carbon mic. I took the network apart and there was a pretty nice hefty little coil pack under there that I assumed was a transformer of some kind as you describe. Alas, I threw it away, so I am back to square 1 with a mic and a speaker only. My bad. \$\endgroup\$
    – captncraig
    Commented Oct 28, 2010 at 17:02
  • \$\begingroup\$ I was thinking about the transformer more today. I think it would have kept the DC operating voltage off the ringer, and there may have been some trick that kept the AC ringing signal from clobbering the earpiece. \$\endgroup\$
    – JustJeff
    Commented Oct 28, 2010 at 23:53
  • \$\begingroup\$ pondered some more, and recalled that actually, the 'switch hook' switch disconnects the handset and connects the ringer when the handset it is 'on the hook'. conversely, off the hook the handset is connected, and the local office circuits sense this and just don't send the 90V ringing signal. \$\endgroup\$
    – JustJeff
    Commented Oct 30, 2010 at 2:28
  • \$\begingroup\$ I remember those big 1.5v batteries connected to it, and reading about the transformer as to how the connections allowed the signal to go to the lines, but not through the speaker. \$\endgroup\$
    – f p
    Commented Nov 13, 2013 at 13:20

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