The cheapest GSM shield ever

I cannot understand how GSM shield can cost 50, 60, 100$, although a full cellphone with keyboard and screen costs just 30$.
I think the cheapest way to give Arduino some GSM capabilities is to directly connect it to a real, full cellphone.
This could be done by implementing FBUS protocol libraries for nokia phones (does it already exist such a library?), but for not-Nokia phones?
Could interfacing to the keyboard be a solution?
As far as I know, a simple diode can be used as a switch, if you connect it in a proper way.

Could this work?
Should I connect the "floating" pin just to GND?

Often Cell Phones are subsidized by the carriers.

So we could have "GSM-shields" subsidized :slight_smile: if this works. ?

first thoughts:

  • using the keyboard to send a message can be done, ( why not make a robot arm to press the keys? non disruptive to the phone :slight_smile:
  • but to read an incoming message is far more complex.

I did some experiments, and it looks like we do not even need diodes, it's just enough to directly connect keys to arduino and properly enable pins, although I didn't yet figure out what "properly" means... :roll_eyes:

Anyway, this method cannot be used at all to read messages!

But some years ago I experimente with Microchip PIC: connecting the PIC to the phone via audio cable allows a python program on the phone to send data to PIC (very small amount of data), without needing to implement the damn FBUS protocol (which that phone didn't even had).

The hardest part was to get phone identifying audio tones sent by PIC (it was too slow to perform FFT properly)... Connecting directly to the keyboard should close the circle.