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.
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.
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).
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
Oscilloscope view when I whizzle in the calling telephone
Audio recording when I say "test, test". It's very faint.
http://github.com/danmar/mobilephone/raw/master/images/3.wma
Here I have connected a MBED microcontroller to the UART0 port on the GSM chip:
To start with the MBED just sends "AT" commands and then receive the response "OK".
I bought a small box. The microphone and speaker I bought was connected. The reference circuits from the datasheets worked fine!
Internal view:
External view:
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.