Cellphone to Arduino

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You can find a Motorola C168i on Ebay for less than 10 dollars. The simplest way to get service for it is to buy a pay-as-you-go SIM card, at Radio Shack for example. While you're there you can also pick up the 3/32 plug; unfortunately, they don't have any information on the packaging but I've included a figure here for your reference (see left).

Here's a step-by-step on how to make your "breakout wire" and plug it into your Arduino.

Tools: soldering iron, wire clippers, wire strippers, a soldering stand is helpful

Bill of Materials:

3/32 Mini-jack
Three lengths of wire
Two resistors to make a voltage divider. I used resistor values of R1=3.3kOhms and R2=5.6kOhms but plenty of other ratios will work (just be careful that you don't reduce current too much).

In general, for any R1, ideal R2 = (3.3/5*R1) / (1-3.3/5)

Steps

Solder one wire that will be the TX for the phone or the RX on the Arduino end to the lower-most part of the connector.Be careful! The connector is made of plastic so you'll want to avoid the need to resolder.
And another right above it (RX for phone, TX for Arduino)
Finally add the ground wire to large metal tab
Twist the wires and make them look pretty (I like to first twist two and wrap additional wires one-by-one)
Plug things into your breadboard! Choose a resistor ratio such that you divide 3.3V out of the Arduino's 5V.

Note the resistors I used are 3.3kOhms (R1/Left/Horizontal) and 5.6kOhms (R2/Right/Vertical).

Now try out the GSMSerial example library to make sure everything works fine (in Arduino IDE, go to File>Examples>GSMSerial>GSMSerial_Example