When SparkFun had a SM5100B module on sale I thought it would be fun to make some sort of device that can provide feedback by way of a cell phone.
But having looked around a bit most people seemed to recommend skipping the SM5100B in favor of one of the Telit modules (GM862, GE864 or GE865). Apparently, the Telit modules have better documentation and there are more information about projects using those. Unsurprisingly, they are also more expensive...
Both the GE864 and GE865 needs an external SIM card holder. The GM862 has an internal SIM card holder and also includes a GPS (there is also a non-GPS version).
Most projects out there seem to use the GM862, but I settled on a GE865. The reasons: it's cheaper and smaller due to its having fewer connection pins (most of which I won't use anyway).
There are a few issues to pay special attention to when using this module (or any of the other Telit modules). Two main concerns are: 1) power supply and 2) logic levels
- Power supply
The power supply is ciritial. Firstly, the module requires 3.8V (will accept 3.4V - 4.2V). Secondly, the power supply has to be able to supply 2 Amps in peaks. This is way more than most simple power supplies can handle. This seems to be the single most common source of problems people have when the module does not work properly.
I will power the module from a single 3.7V LiPo battery. The hardware guide recommends a battery capacity of 500-1000 mAh in order to handle an output of 2A. Therefore [this] battery looks ideal.
In order to handle the current spikes a capacitor must be fitted. It is crucial that the capacitor is a "low ESR" type. The hardware guide recommends a 100 µF 10V tantalum capacitor.
- Logic levels
The module uses 2.8V CMOS logic levels. That means that logical high is nominally 2.8V (specified as between ~2.2V and 3.0V). This is different from the ATmega168 levels where output high is close to Vcc (required input high when supplied at 3.7V is ok at approx. 2.0V).
Off to click the "put in basket" button a few times...