Archive for July, 2010
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:
- 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.
- 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:
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).
– at least to begin with.
- 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).