Wednesday, December 2, 2009

BlueTooth Ericofon

Wouldn't be cool sitting at your favorite coffee shop with this phone on the table, and while chatting with your friends the Ericofon rings and it is a call for you? Or picking it up, hearing a dial tone, cranking up the rotary dial and dialing home to say that you will be soon in your way? Well, this is what this is about. This is an open source project on how to retrofit an extraordinary phone to make it a special, functional piece of conversation. Literally.

The idea started when several months ago when I read about the Port-o-Rotary phone. The cool guys from SparkFun converted an ordinary rotary dial telephone into a telephone that can make cellular phone calls. My plan is to come up with a retrofit kit (open source) for a cool phone that is easy to carry, and adding some nice features to the original idea. And what better to show off the project than an Ericofon.

After doing some research on the internet about where to get an Ericofon, I found this amazing site I contacted the owner, Richard Rose and told him about what I was planning to do. He promptly sent me a very nice email explaining how tight it is in there and how quickly I will run into space problems. He also said: "You're not the first to think of this, although I've never heard of anyone actually going through with the idea. I've been inside these phones for a while and I don't see being able to maintain rotary dialing with the design you have in mind." I emailed him back: "You just dared my inner engineer and geek! Game on!"

Getting an Ericofon
Certainly, there are many places where you can get Ericofons, specially eBay. However, if you really want to get an excellent phone, probably NOS (new old stock), try first Richard's website. His website has an amazing collection of these and other sort of classic telephones. He has been a phone collector since 1990. What impressed me most is his knowledge about these phones. His website has tons of great, detailed information about how to fix and maintain the phones and other useful and interesting information. If you really appreciate this classic phone and want to get one, I seriously recommend to contact him first.

Inside the Ericofon
The color of the phone I got to work on the prototype is called "Candle Glow", which is an ivory color. The model I have is a K7 without a ringer.

Richard was right. These phones are very dense and don't have room for much inside. They don't even have a ring bell. The Swedish phones use an internal buzzer (here its sound) and the American phones use an electronic wobbler called Ericotone. The Ericotone is placed inside the neck of the shell and the ring noise is produced modulating the earphone (here its sound).

If you ever wondered how the designers crammed all the circuitry and a rotary dial inside the little shell, you can see what is inside mine (click on the pics to see the details).

Here are the original schematic diagrams (posted on of the phone without ringer, with ringer (Ericotone) and seeing from the Ericotone perspective (click to enlarge).

I decided to use the BlueGiga Bluetooth module WT32. This module packs a lot of features in a very small package. It supports the required HFP protocol to interface via Bluetooth to cell phones and some of its neat features include analog input and output (for voice), programmable current and voltage source for an electret microphone, Bluetooth antenna and a battery charger/management for Lithium Polymer (LiPo) batteries. The WT32 interfaces include USB, SPI and serial asynchronous (UART) protocols.

The goal of this retrofit was to keep the phone as original as possible with minimum modifications. I am very familiar with Microchip PICs, so I decided to build the retrofit circuit around the 16F688 micro-controller and programming in C using the CCS compiler.

The design (HW and SW) in its initial version should:
  • Generate (fake) dial tone: 440 Hz
  • Generate (fake) busy tone: 440 Hz @ 1.5 Hz rate
  • Mimic Ericotone ring: 2,300 Hz and 2,700 Hz, 10 Hz (wobble) rate
  • Support dial speed: 10 pps to 20 pps
  • Support voice recognition dialing (through 'flashing' the hook)
  • Use and recharge a Lithium-Polymer (LiPo) battery
  • Use original earphone
Other features may be added later, such as user programmable volume levels and other features using the rotary dial. Suggestions are welcome! Post your wish list!

In order to mimic the Ericotone, first I needed to determine the wobble frequencies. I used the original .wav recording on Richard's page and an Agilent 33120A frequency generator applying the tones to the original earphone. I checked the amplitude and waveform of the applied signal with an oscilloscope and played with an RC circuit to smooth the signal and get an acceptable, similar tone effect.

Although I could have used a differential OP amplifier adder to provide the tones and voice to the earphone, I decided to isolate the unbalanced ring signal from the balanced output signal from the WT32.

I designed the schematic and the PCBs using Eagle software. Note that I really didn't have too much space to cram all inside the Ericofon, so the final printed circuit board (PCB) is only 2"x1.5" (5x3.8 cms)!!. Here are the printed circuit board designs. I will post the schematic, PCB and software Eagle files for download soon.

                           Both Layers                                        Top Layer                                           Bottom Layer

Next stop: PCB


  1. Beautiful work.

    -Jerzee (HackaDay)

  2. Amazing. I just ordered me a WT32. I can't wait to get started on this.

  3. If you were to sell these as kits, I would litterally buy two, at least! 330-333-2739(google voice)

  4. Thanks! :)

    Sure, for more info please refer to the Q&A. I will give you a call.

  5. BlueTooth Ericofon..
    its amazing and very nice to see ..
    i am waiting from a long time for this kind of feature in ericofone ...


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