July/August 2018
We use our cell phones as our main phone service, so our “land line” is no longer being used. Is there a way to use the house phone wiring along with the old phones as a whole house intercom system? I have a detached garage/workshop away from the house where this would be especially useful, not to mention almost every room is wired. Would I need to disconnect from the phone company "grid" to do this?
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Yes, you can use your landline phone as an intercom.
1st option: In our part of Canada, for example, if you dial your own phone number, then hangup when you hear a short “Beep” on the line, the phone company’s automated CO equipment will call you back, and ring all the phones on your line. When you pickup the phone, you will hear the same short “Beeps” on the line, BUT, you can also talk on the line to anyone else in the house/shop, that has also picked up the line.
Try it, it might work in your part of the world too.
2nd option: Most if not all house wiring has 4 conductors in the phone cable, but only 2 wires are used for the actual phone line (usually colour coded Red and Green), the other two (usually black and yellow) are spare. You can use the other two wires as a signalling pair to ring buzzers at other phones.
In the past, I’ve used a low voltage AC source in series with 2 diodes and two buzzers to make a signalling circuit ON THE SPARE PAIR (BLACK AND YELLOW wires).
The AC source was placed at a convenient mid point location, in series with one of the signalling wires (yellow or black). At each phone I installed a diode, in series with a DC buzzer, then placed a momentary push switch across the diode. Note, that the cathodes of each diode must be facing the AC source.
The diodes block any current flow through both buzzers, but if one diode is bypassed by the switch being pushed, the current (now DC) will flow through the both buzzers, signalling attention.
By picking up the phone and dialing any single digit (except 0), you turn off the dial tone, and have about 15 seconds of talk time before a warning tone comes on to signal you do do something, like dial another number. This is as simple as it gets, but there are many more options.
Back in the last century, phone companies didn’t like you messing with their equipment, that probably still applies today.
Using Google you can find many articles on using old style phones as intercoms (http://www.instructables.com/id/How-to-connect-two-phones-at-home-for-an-intercom-/).
A phone works just fine on 12 to 18 volts DC (my magneto phone operates on two 1.5 volt batteries), but then, you MUST, be disconnected from the phone company’s service.
You could use existing phones but you’d need a way to generate a ring signal. Hand-crank ring generators from old phones are for sale on eBay, but an electronic intercom costs less per station. I suggest you use your phone wires for a commercial wired intercom, or try a wireless unit instead.
After disconnecting the line from the telco at the demarc (box most likely on the side of your house), you can do it with a telephone line simulator but for the same money you can just buy a cordless phone system with intercom built in.
As a bonus you can buy models that have bluetooth so you can connect two cellphones and have them ring throughout the house while they sit in their chargers.
Unless you are willing to set up your own switchboard and 24 volt power supply, you cannot reuse the phone for your intercom. It is perfectly possible, however, to use the wiring to connect your own wired intercoms which I have done for years. You should have 2 pairs in your home’s phone lines. YES, you must disconnect the pair you are repurposing from the phone system. You can leave one pair connected to your existing phones as a backup.
