Phone design
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Julian Rogers Home

Design considerations

I need to consider what services and facilities the phone to should provide within my goal of largely providing a retro experience for the user. There will be no keyboard so sending an SMS (apart from perhaps pre-recorded messages will not be possible. One question is whether there should be an LCD screen so that, for example SMS could be received and phone status could be displayed. I feel it would be in order to conceal this in the drawer at the base of the phone. It would then not normally be seen.

A lot of the technical possibilities will be dictated by the electronic modules available. (My own electronic design will almost certainly be limited to providing interconnections between the modules.)

I am going to go with the Adafruit Fona 3G module which, it is claimed can send and receive voice calls, SMS , data and GPS. It also has a charging circuit for a 3.7 volt lithium-polymer battery. I want to provide an authentic bell-type ring tone. I’m going to use another Adafruit product, the Music Maker Arduino shield. This can play mp3 and other format files and has an audio amplifier and a slot for an SD card. I will be able to store all sorts of bell sounds. Youtube is a good source for these.

As implied above I am going to control the whole shebang with an Arduino (with which I am reasonably familiar - however, I expect the control program will be no walk in the park!) For the first time I’m going to use the Mega 2560 (more pins, hardware serial ports, and a lot more memory which with the Uno, (even with a relatively straightforward project such as my central heating programmer), gets rapidly used up.

The Mega runs at 5 volts so, if I want to run it off the 3.7 volt li-poly, I’ll need a step-up regulator. At this stage I don’t know whether the Fona charging circuit will be able to keep up with the power demand if the phone is on continuously. At this stage I’ll hope for the best!

I’ll need to fit all this stuff and a couple of loudspeakers into the phone. The case looks big but the dial mechanism and the cradle switch take up quite a bit of room.

I’ll also need to fit a small speaker and an electret microphone in the handset. I want to keep a plaited cord. This looks to be four-core but one “core” is just a non-conductor. The speaker and the electret don’t share a common ground so I need four core. I have located a multi-core plaited cable which might do. Whether the electret will work over three feet of unscreened wire without picking up hum and suffering other degradation remains to be seen. I may need an amplifier in the handset with a low impedance output.

Adafruit Music Maker

Arduino Mega 2560

Adafruit Fona

Lithium-polymer battery with WRONG connector!

20mm speaker

Next, initial testing…