Wednesday 19 March 2014

Telephone audio interface

I work from home, and my job requires I spend a lot of time on the phone, particularly in conference calls.

I could easily go and just buy a new headset or hands free desk phone, but I already have a general purpose headset that I enjoy using, and frankly, I have accumulated a lot of headsets over the years from various phones, consoles, computers etc.

One of the items I had in my junk bin was an old wall-mounted telephone - the kind where the keypad is on the handset.

Taking the phone apart revealed three PCBs - one in the base unit of the phone, and 2 in the phone itself. One is obviously the keypad, and the other appears to be an amplifier - it was on this board that the microphone and speaker was attached.

Making the connections - the main part of the interface, is really very simply. Just cut away the microphone and speaker, and replace them with connectors.

The next step is the hang up button. Hang up buttons are press-to-break - pressing the button breaks the circuit, so as it stands connecting this to the phone line will result in your line being open.

To add extra complexity on the phone I was using the hang up button was a Double Pole Single throw switch (DPST) - this means that the switch effectively controls two circuits.
When closed, the switch connects the 4 contacts (seen de-soldered above) as follows: 1 & 2, 3 & 4

Fortunately I managed to find a 'normal' DPST switch on an old amplifier PCB in my junk bin, so wired that in place.

Also, in order to make the end product neater, I removed the coiled cable between the base and handset, and cut away the keypad - I can always use my existing phone to dial. Besides, the keypad may prove useful in a future project.

All that remained was to create a new case for it. I was able to recycle an old business card box, and 3D printed a basic front panel for the connectors.

The finished box, just connect an audio source to input, and some headphones/speaker to output, and away you go.

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