Payphone

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http://flickr.com/photos/pvck/3101832241/in/photostream/

Revision as of 15:02, 12 December 2008

Contents

Project Payphone

The Idea

There has been much talk of hooking up a payphone to a cheap VoIP account and allowing free use of it as a Noisebridge public service. A popular idea is to put said payphone outside of 83C for public use. Some of us have expressed grave concerns that this will be too much of a liability and quickly result in us no longer having said payphone. Expect bickering over the issue.

The Phone

An awesome old red payphone was serendipitously discovered abandoned on Grand Avenue in Oakland. Our local hero picked it up and brought it to the space.

We got it open to discover the following:

  • The date of manufacture seems to be a very scribbly Aug. 1988.
  • It is completely missing the coin counting and collecting mechanism.
  • The keypad and hang-up switch look intact. The metal buttons on the outside actually just press against a standard little plastic keypad on the inside.
  • The headset is not connected to anything, but terminates in four red, black, green and yellow wires that can be hooked up to a standard RJ11 (normal phone jack) connector.
  • Out of the keypad/hang-up switch part there is a DB15 connector, equally hooked up to nothing. The assumption is that both this and the headset were connected to the coin counting mechanism which was the brains of the operation.
  • We'll likely need to gut another phone for some missing parts, but how much surgery is needed will depend on what that DB15 connector can do for us. For that, we need someone to find a schematic for the pins, or someone who is a master reverse-engineer. If that's you, get to work!

Well, Seth is a master reverse-engineer, and Steen is an old school phreaker, so one long night spent on this later:

  • We decoded the standard schema for DTMF that seems to apply at least to both the gutted landline phone and the payphone internals, and determined the pinouts for both the relevant bits. It turns out, a button on the keypad doesn't simply close a circuit to make a pin hot, it activates two pins, which then correspond to inputs on the DTMF encoder chip. In other words, Dual-Tone doesn't just mean the tones that are output, it also applies to the signals inside the phone circutry. Neat! So now we can connect the output pins from the payphone keypad to the DTMF inputs on the landline guts. Seth is (I think) planning on bringing in a db15 breakout box in order to manifest this. Please no one break it further before we get a chance to do so.
  • We figured out the order of wires for the headset to operate, corresponding to the (somewhat arbitrary) aligator clip => rj11 cable we made for the purpose. We got this to connect to the guts of a functioning landline phone, and got a dialtone from the VoIP box, as well as a functioning mic. This mapping is complicated to format on the wiki, it's written on the inside of the old headset in sharpie. That counts as documentation.

Photos

I actually took more pictures but I strangely had a "Memory Card Error" (GRR!) and these were the two I recovered.

Post more if you got 'em.

Opening payphone.JPG

Opened payphone.jpg

http://flickr.com/photos/pvck/3101832241/in/photostream/

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