I have it connected like in the image with the grounds merged. But the sim module doesn't seem to like the setup and powers down, my guess is there is something wrong with the current on the grounds. At the end my plan is to use a pro mini at 3.3v powered also from the battery so think I should not have this problem.
But while I am developing the project I would like to use the UNO, if somebody has any insights it would be helpful.
Also I test commands on the sim module with a ttl converter with pretty much the same connections, with merged grounds the same way and don't have this issue.
Hey, sorry for the delay, hadn't time to work with the module until this week.
Nick_Pyner:
What you propose looks reasonable but the SIM800 is not famous for being frugal with current. Is your battery up for the job?
Yes, the battery works ok, I tried connecting the module and a pro mini 3.3v and it lasts about 3 days, the battery being 1000mAh.
ricky101:
This layout works for me, the USB/TLL being my 5V UNO
Hey, thanks for your suggestion, finally got it working with the configuration you mention, but had to include another voltage divider in the Rx connection from the UNO, without it I was getting still getting and OVERVOLTAGE error from the sim module.
