SIM900 Relay Control Not Working Without Serial Port Connected

[adfischer]

I am trying to control a set of lights using an Arduino UNO, SIM900 board, and a relay board. The Sketch works beautifully as long as the Uno is connected to a PC and the serial port is active - either through the USB or direct to pins 0,1 however if I do not have the serial port active it does not work. I need the sketch to be able to process messages without having a PC connected. Any advice would be greatly appreciated. Attached is my sketch:

ChristmasLights.txt

[Cribstone]

Can you tell me which GPRS shield you're using?

[Cribstone]

I"m guessing the problem occurs when the SMS is translated back into the AT command set. Check out this thread about a similar issue and see if the fix would make sense in your case- http://www.edaboard.com/thread284929.html
I would also test that the SIM900 is actually receiving messages. This tutorial seems to have a good method- http://www.theengineeringprojects.com/2015/02/receive-sms-commands-using-sim900-arduino.html

My own experience with SIM900 modules was an exercise in frustration, and I ultimately dropped it for the "official" Arduino GSM Shield based on the assumption that it would enjoy stable library support in the IDE. Even that proved to be somewhat wishful thinking on my part as serial comms with the official shield are also very complex. The only other GSM hardware that looks promising is Adafruit's Fona platform since one can always expect excellent documentation and support from them.

Another possibility I would look into is the use of the MQTT protocol as with the Cheerlights project. While I'm not aware of any libraries that support SIM900 with MQTT, I know the Fona has support built-in.