I doubt this is possible at all, but it isn't possible to have a GSM shield 'sleeping' & then waking upon receiving a text message, And then send the command in the text to the arduino?
Only because i plan to have the GSM shield in a vehicle and i'm worried about the power consumption.
This specs on this model say that the average consumption is around ~500mA (with 2A bursts when TX/RX)
Also, between the sparkfun GSM shield: http://www.sparkfun.com/products/9607
And this one from eBay: http://www.anbere.com/10509N/SIM900/Schematic.pdf
Which would you recommend?
I also cannot understand from the datasheets of both shields, as to which pins it uses, only because my project uses pins 13,12,11 and 4 (i can change this of course, but i want to know what pins are free for me to use)
And in regards to the power, i'm going to be using a 12v DC-DC converter to power the GSM shield, and the arduino, and i also have another shield that i've designed to go on top with all the transistors and relays for my project.
The question is, with the sim900 shield, it wants 9v and around 2A, so can i use a 12v to 9V converter and connect it to the VIN pin on the arduino? Will the arduino accept 9V through VIN?
I think the IComsat module draws the power form the Arduino (at you link: "9~20V, compatible with Arduino"), also the Sparkfun module use the power provided from the Arduino. I can not see any power jack on both shields. On the Arduino page you can find - eg for the Uno - "Input Voltage (limits) 6-20V" http://arduino.cc/en/Main/ArduinoBoardUno . So I think the onboard Power Regulator will manage this.
It seems that VIN is the same as as the power on the power jack:
VIN (sometimes labelled "9V"). The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin. Note that different boards accept different input voltages ranges, please see the documentation for your board. Also note that the LilyPad has no VIN pin and accepts only a regulated input.
Sorry, I have to correct / modify my post above. I had read a warning about using the Arduino Power directly with the Sparkfun Shield:
The GSM shield can often require up to 2A of current in short bursts – especially when turned on, reset, or initiating a call. However your Arduino board can only supply up to just under 1A. It is highly recommended that you use an external 5V power supply capable of delivering 2A of current – from an AC adaptor, large battery with power regulator, etc. Otherwise there is a very strong probability of damaging your shield and Arduino. Ignore this at your own risk. When connecting this supply DO NOT use the DC socket on the Arduino. Instead, connect the 5V (positive) from the supply to the 5V pin on the GSM shield, and the negative to the GND pin.