Payphone Back Pack

Overview

Nullstring's Prison Clear Payphone Back Pack is the best of ideas come up with in response to the question “what is the dopest thing you could make Prison Clear?”

Do you remember the Conair landline phones from the 80s and 90s? The clear ones, with all the colors inside?

Envision that, but its a fully wireless, battery-powered, functioning payphone on a back pack frame!

I Want One!

You can't have one! BUT! You CAN have a BananaPhone! A BananaPhone is the core and heart of the Payphone Back Pack! It is a USB-C adapter to connect landline phones to your cellphone!

COTS Iterations – Gross, Bluetooth!

There are a couple of commercial cellphone adapters which enable the use of normal landlines by a cellphone, but they are primarily marketed to old folks and fairly expensive and difficult to obtain. On top of all that, these devices rely exclusively on Bluetooth to connect to a cellphone which is a real drag. Opening these devices up they are both ESP32-based and have some other stuff like what look like DACs.

Using these devices, I set up a little demonstration at DEF CON 33 with a clear Conair landline phone, though most people were wary to try it out even if interested in seeing an old clear neon phone connected by wire to a fanny pack carried around.

Not good enough! While this was fun I would prefer to keep everything hardwired and leave the Bluetooth out of it, so I must go bespoke.

Bespoke Iterations

Housing Design

My initial plan was to buy a payphone and then cast a mold from it and use that to cast a resin housing. This turned out to be extremely difficult as the housing was in bad shape and very hard to get a good mold from.

I gave up on this and decided to pursue 3D modeling for large scale SLA resin printing instead.

3D Modeling

To accomplish this I downloaded some payphone models for reference and spent weeks calipering and measuring my own housing until I could FDM print sections to match perfectly with the mounting holes and shape and spirit of the real housing. Using clear resin also sheds the enormous 60lb weight of the steel housing. I started the CAD work in Tinkercad thinking it was going to be fairly simple, but soon had to switch to regular Autodesk.

I modified the original payphone model to be slightly larger to accommodate both a thicker shell as well as interior mounting plates both for the original payphone parts as well as the boards and lights and such for my modifications. In addition, I also designed the housing to assemble a bit differently in order to better accommodate the big scary battery required for truly wireless operation.

Oh no! it turns out none of the coin mechs I have fit when the battery is installed, so I have to make my own using optical endstop switches and CAD. If any consolation, at least this makes mounting the RFID reader easier.

Printing

FDM Test Printing

Large-Scale SLA Printing

Tariffs make it insanely expensive and I would like to only do it once. There are no photos of this as I have not ordered it yet.

BOM Generalized

Original Payphone Parts Used

Controller Board

Prototyping Previews

The main controller board will feature a ProSLIC chip and a high-speed USB isolator for the connection to a cellphone.

The ProSLIC test boards have arrived!

picture

Displays

coming soon, it takes forever to put graphics up on this

Power Board

Features

When Can I See It?!

Project State

Upcoming Appearances