Versatile Arduino/Netduino GSM shield
I needed a way to send SMS messages from a Netduino board, so I first tried the SM5100B shield. It worked somehow but it was not very stable and I did not like the hardware design. It also lacked some features. So I decided to build my own. Here is what I did (if you already used Arduino/Netduino the nomenclature below will be familiar, if not read a little bit about Arduino or Netduino, its a really nice microcontroller platform):
I wired both serial ports (so you can open a data connection and still send AT commands through the other line), A2 and A3 are used for ignition and hard reset of the gsm module and D12 and D13 are used for the ring indicator (signals events) and power indicator. These two lines are optional so I added an smd jumper pad which is connected and can be cut easily. the same is for the second serial interface.
I also added a battery holder for backing up the RTC in the module, so you do not lose the time stored in the module. Also very handy.
The SIM card holder is a push-lock and is on the bottom of the PCB facing to the side with the usb and power connector, so when a housing is used you can cut a slot into the housing and change the sim-card without opening it.
The USB port can be used to configure the GSM board, but this is optional. All settings can be made through the RS232 interface.