I'm trying to build https://www.instructables.com/Mailbox-Phone-Alert/ and got myself the GSM900 board and the Uno for this. However I am confused about some things and I am not sure if I actually have an hardware problem with one (or both) components.
My understanding is that I would need a power supply that gives me 2A at 5V. Obviously a normal USB port on my computer doesn't give this. So I have a separated 2A supply connected to the GMS900 board.
But when I unplugged the USB connection from the Uno, it also powers down. If I connect the power adapter directly to the Uno it powers up, but the GSM900 doesn't (no matter which position the power switch is). Right at the beginning both powered up no matter where I connected the power to.
I did some experiments with using 4x AA batteries (6V) and later 8x AA (12V) to run the whole project from batteries. I connected those packs to both available power jacks. To see if it would be possible to power the setup by batteries... The specs that I have seen where saying that 12V should be fine for either of the boards.
For the current setup that seems to work:
Uno connected to USB.
GSM900 placed on top of the Uno and connected to the power brick.