Vintage Intercom Re-purposing

I bought a beautiful old intercom in the local car boot sale and thought it would be good to use it as the door intercom for our "stair" (as Victorian apartment blocks are called in Edinburgh).

It's a GEC K7867 and looks looks pre First World War. Our local museum has an almost identical model from 1904. This one looks like it may have been re-furbished in the early 1930s - GEC went on making them for 30 years. I like to imagine it was in a big country house in the highlands so you could ask the butler to bring up another bottle of port.

It's an intercom (rather than a public telephone) and was part of a system with 5 stations.

The way it worked was: you turn the knob at the top to point at one of the other station numbers, lift the earphone then press the "call" button. The bell at the other station rings and a latching mechanism locks you into not receiving other calls. When you've finished, you replace the earphone and the mechanism unlatches.

So it contains all the parts needed for a door intercom: a microphone, a speaker, a bell and a button. Any similar telephone-like object can work as a door intercom.

As it's a somewhat historical object, I only made changes to it that could easily be undone.

This Hack gives general principles that could be applied to other door intercom conversions. It also shows how to replace a carbon microphone of an old telephone with a modern electret microphone.

You'll need the usual handtools and a soldering iron; plus a few electronic components it you have to replace the microphone or internal switches.

The Current Intercom

The current intercom was installed in the 1980s and is rather ugly. The circuit is very simple but it has worked reliably for over 30 years.

The handset unit contains:

    a carbon microphone
    a speaker
    a hook switch
    a buzzer
    a lock button
It's a typical design for door intercoms. The microphones, speakers and buttons are wired in parallel for all the flats. Each flat has its own buzzer wire so only one buzzer sounds when a visitor presses a button.

Make a note of how yours is wired up and the colours used.

Many door intercoms are constructed that way. Some modern ones just have two wires and all the cleverness is done with digital electronics. If your intercom is like that, there will be a circuit board inside the handset station. You'll need to port it into the new intercom you're building.

The Old Microphone

The old microphone of the K7867 was broken. As you can see from the photo, the brass mouthpiece had been squashed and that had shattered the diaphragm.

A carbon microphone works with a chamber filled with powdered carbon. a separate diaphragm is mechanically coupled to the chamber and the changes in pressure due to sounds change the resistance through the carbon.

A DC voltage (5V to 7V) is applied to the chamber. The changing resistance changes the current drawn from the supply.

Early carbon microphones like this one used the diaphragm itself as the lid of the chamber. The diaphragm is made of some sort of carbon and is very brittle. That's what had broken.

Carbon microphones were never very good quality and when you restore an old telephone it's usual to replace the carbon microphone with a modern electret microphone.

The New Microphone

The modern intercom also used a carbon microphone. That seems a bit old fashioned nowadays but probably made sense in the 1980s. It is powered from a 7V source.

You can tell that your intercom has carbon microphone if the microphone is big and its resistance is exactly the same in both directions (i.e. swap the leads round on your multimeter). A typical electret microphone is much smaller: 9mm dia and 6mm high. Look on the web at pictures of electret and carbon microphones if you're not sure what you've got.

All the replacement carbon microphones I could find...

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