Archive for July, 2010


16. July 2010 13:36 by Jens Willy Johannsen
Categories: Arduino | Telit GE865

I connected everything – that is: battery, SIM card, push button, LED and transistor and level shifting for 2.8V <-> 5V logic. And considering how much trouble people seem to have had I was surprised how quickly it worked!

The only two things I ran into was:

  1. The first SIM card I used didn't want to talk to the GE865. Probably because it is for a 3G modem and not a phone. Anyway, the second (normal) SIM card worked perfectly.
  2. Using the Arduino IDE's serial monitor as terminal interface didn't really work. But using "screen /dev/tty.usbserial-A9007ND2 9600" from the Terminal prompt worked flawlessly.

I had zero problems with the power supply. Even though I'm not using the recommended low ESR capacitor. Maybe because I'm in a densly cell-phone-covered area. Or maybe because the battery is capable of delivering up to 2A by itself.

And everything worked: sending and receiving SMS'es, retrieving a web page using GPRS. Nice! Now I just need to connect it to a DTMF decoder and a Speakjet :-)

Here's a picture of the breadboard setup:

(click to view large version)

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11. July 2010 22:10 by Jens Willy Johannsen
Categories: Telit GE865

Continuing work on the simple-simple setup, I needed to connect the GE865 to the breadboard. And preferably in a non-permanent way.

I decided to go with 0.1" MOLEX connectors and I chose to use three connectors (only two of which is connected in the simple setup).

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Keep it simple

8. July 2010 15:49 by Jens Willy Johannsen
Categories: Arduino | Telit GE865

– at least to begin with.

I have received the stuff (from Lawicel in Sweden since they had everything in stock) so let's fire it up...
Having read about how many problems can arise with these modules, I have decided to try out a simple-simple test first.
That means connecting the GE865 to a 1000 mAh LiPo battery (without the low ESR cap which I haven't received yet), connecting a manual push button on the on-off pin and a LED to the status LED pin (and a SIM card, obviously).
And then use the Arduino's serial monitor to send and receive commands.
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“ET phone home…”

4. July 2010 15:33 by Jens Willy Johannsen
Categories: Arduino | Telit GE865
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
  1. 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.

  2. 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...
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