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mobilephone

Project to build a simple mobile phone (GSM). The mobile phone will handle voice calls.

I will use off-the-shelf electronic components.

The software development and electronic components is sponsored by Evidente.

Progress

Step 1 : GSM chip

I bought a SM5100B GSM chip, a simple evaluation board for this chip, and a GSM antenna. The chip understands AT commands.

I bought a normal prepaid comviq sim card.

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Step 2 : Setup

I connected everything and tried to setup the chip for the GSM card. It was quite straight forward.

By using AT commands from my PC I can send/receive SMS and dial/answer voice calls (however I have no audio yet).

Step 3 : Audio

The plan in this "step" is to connect a microphone and speaker. The datasheet for SM5100B has reference designs for connecting microphone and speaker that I will try to use.

According to the datasheet, the speaker can be connected directly to output pins on the chip.

Before I had any parts, I wanted to "dry run" the audio. When a voice call is made, it should be possible to measure something on the speaker output. I made this connection: GSM chip speaker output => voltage divider => microphone input on my PC

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Oscilloscope view when I whizzle in the calling telephone

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Audio recording when I say "test, test". It's very faint.

http://github.com/danmar/mobilephone/raw/master/images/3.wma

Step 4 : Connecting the microcontroller to the GSM chip

Here I have connected a MBED microcontroller to the UART0 port on the GSM chip:

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To start with the MBED just sends "AT" commands and then receive the response "OK".

Step 5 : Prototype box

I bought a small box. The microphone and speaker I bought was connected. The reference circuits from the datasheets worked fine!

Internal view:

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External view:

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The dialer is both used to dial numbers and to answer / hang up. By spinning the dialer all the way twice you answer and hang up.

It works when using a AC/DC power adaptor. But unfortunately it doesn't work well from a battery. I use a normal 9V battery right now. This may be too weak.