Use old phones as an intercom

Very simple intercom circuit

Get two phone jacks and a 680 ohm, 1/2 W resistor. Connect the components to make the circuit below:

reen              680 1/2W    +   12V    -          Red
To Phone                                          To Phone
Red                                                  Green
You may wish to use a 12 to 24V battery, like a Gel Cell, since the car power or cheap power wall wart style supply is probably too noisy. You may have to adjust the resistor value (1.2K for 24V). Whatever gets the phones working and does not use more than 30mA total loop current.

If you can manage it, get two speakerphone-type phones. Leave them in "speakerphone" mode continuously and pound on the keypad to attract the attention of the other end.

Intercom with ring tone

From: (Markus Wandel)

I have recently thought about this and come up with a kludgy but workable scheme.

Talking over the phones is easy. You put DC current through the phone and it transmits and receives audio. So two phones and a current source (about 25mA) all in series will give you a talking circuit. A suitable current source can be as simple as a 9V battery and a series resistor whose value is adjusted (with both phones offhook) till about 25mA flows. You can then bypass the battery and the resistor with a capacitor to couple the audio straight across and get a loud and clear connection.

What is much harder is signaling the other end. To ring the bell you need to put 90V (RMS) 20Hz AC into the phone (nominally). Lower voltages will work (down to about 40V) but different frequencies won't. You can't ring the phone at 60Hz. I have a ringing circuit in a PBX I built but it consists of a 20Hz sinewave generator, a push-pull power booster and a big transformer. Much too elaborate for a simple 2-phone intercom circuit, and anyway the ringing voltage could painfully zap a kid.

So forget the bell and look into other forms of signaling. This is what I have come up with:

                              +  | | -
      +-------+------ - - --+---||||---/\/\/--+---- - -----+-------+
      |       |             |    | |     R    |            |       |
      |       |             | 24V             |            |       |
      |      ---            |                 |           ---      |
      |     |   |           +---||------------+          |   |     |
      |      --- Sonalert       C                Sonalert ---      |
      |   C   |                                            |   C   |
      +---||--+                                            +--||---+
      |      _|_,                                         _|_      |
      |      / \  15V                               15V   \ /      |
    PHONE    -+- Zener                             Zener `-+-    PHONE
      |       |                                            |       |
      |       |                                            |       |
      +-------+------------------ - - - -------------------+-------+

As before, set R to give you a talking current (both phones offhook) of about 25mA. Start with 1K ohm. Leave it in if the phones work well enough; the current is not very critical. The capacitors C are audio bypass capacitors and should be about 0.47uF. When the phones are onhook they present an open circuit, and the 24V battery voltage is not enough to overcome the 30V series drop of the Zeners and no current flows. When both phones are offhook they present a very low resistance and the talking current (determined by R) flows.

When only one phone is offhook it places its low DC resistance across the Zener diode on its side so that the full 24V supply is applied to the other side. This overcomes the voltage drop of the other Zener diode so the other Sonalert beeps. The wonderful thing about Sonalerts is that they make a loud noise with only a few milliamps of current so the series resistor R doesn't matter. Especially nice is a pulsing Sonalert which goes "Beep beep beep" automatically. While the far-end Sonalert is beeping, you hear the beeping in the near-end receiver (at low volume thanks to the bypass capacitor across the far-end Sonalert) to confirm that the line is working and the other end is being signaled.

The power supply can be three 9V batteries in series but since 80% of the power is lost in series resistor R rather than in powering the phones it seems a little wasteful. A 24V wall wart with clean filtering would be better.

The signaling components can be mounted inside the phones. Only two wires are needed to go to each phone, and the power supply can be mounted centrally, out of harm's way. If R is adequately big (1/2 watt) and has enough ventilation then both lines can be indefinitely shorted out without any fire hazard and there is not enough voltage anywhere to hurt anyone.

I have tested this with 500-type phones and two different types of piezo buzzers (pulsing sonalerts and non-pulsing brand X ones) and it works great. You should be able to get all the needed parts including piezo buzzers at Radio Shack. I love telephones. Too bad I don't have any kids who want an intercom line.

Threatre intercom circuit

From: (Ben Mehlman)
Subject: Here's a Circuit for a Cheap Intercom
Date: 11 Nov 1996 20:14:01 GMT
Organization: Prodigy Services Company

Many times on this group I've seen people asking how to build a cheap theatrical intercom. I have built several different systems but that was a while ago (before I got my hands on a cheap used Clear Com system), but I always used random parts that I had lying around, and not stuff I could explain to a non-electronics-experienced person.

Well, this weekend I needed a system in a hurry, for a community theatre with moderate means.. so I built them a system out of stuff entirely from Radio Shack (I don't love Radio Shack but they're the only place you can get this moderate means.. so I built them a system out of stuff entirely from Radio Shack (I don't love Radio Shack but they're the only place you can get this stuff on a moments notice, and they are all over the place). It works very well, and I thought perhaps someone else can use this, because even if they don't know a lot about electronics they can go into Radio Shack and get absolutely everything they need. Here's the circuit.

  |                |        50 ohm 10W               120V:12V 3A
  |  12V DC 1.2A   |+ -------/\/\/\/\----+-----------
  |  Power Supply  |                     |+         ) # (--
  |                |           4700 uf -----        ) # (
  |                |              35V  /---\        ) # (--
  |                |                     |          ) # (
  |                |- -------------------+         _) # (__
  |________________|                     |         |
                                         |         |
                                         |         |
                                         ^         ^
                                        Red      Green

                                       To Telephone Jacks
The power supply is the type sold to run your CB or car stereo inside the house. It costs around $29. In the past I built the power supply portion myself (usually 24V) but this is a neat solution, safe, saves time, and works fine. The resistor and capacitor have a dual purpose: 1) Filter residual hum from the (cheesy) power supply, and 2) protect the system from short circuits (the resistor limits the current to around 250 MA). The transformer is actually being used as a choke; Radios Shack doesn't sell a regular choke that does the job. You use the (thick black) primary wires, and just tape off the secondary wires. I suggest soldering all connections, for reliability. If you want a portable system you can probably fit everything into the case of the 12V supply. It's pretty empty in there. I used a terminal strip and screwed the whole thing to the wall of their tech-booth (in community theatre, that's the only way to keep stuff from walking).

The red and green outputs connect to ordinary modular telephone jacks. I used shielded twisted-pair wire because I thought I might want to plug in my clear-com system one day, but regular unshielded telephone wire will work too. The jacks are all in parallel, daisy chaining is ok. You can use a modular Y adapter to make one jack into two. Re telephones, I'm running the system now with four headset telephones- ie a regular wired telephone except that it has a headset instead of a handset and can clip to your belt etc. Also purchased from Radio Shack. I also have a regular "500" style telephone in the system (by the dimmers for patching), and a regular cordless phone. Use any phones you like. The headset phones work well- It's not quite as good as a professional system but it's darn close. They even have volume controls- no mute button though.. You have to 'hang up' when you sneeze ;) There is a definite limit to how many phones you can plug in- I have six now and it's fine, but I'm guessing about 10 is where it'll start to be a problem.

Anyway, the whole system as described, including wire, jacks, power supply and other parts, and four headset phones, cost around $350, ie around the cost of one clearcom station... Enjoy!