“With such a wide range of GSM modules available for the hobbyist, most of us ended buying one. I purchased a SIM800L module locally, and ended up playing with the different commands of the module. Using the Arduino Uno and the Arduino IDE, I was able to turn my ideas into reality. This did not come easy, with the SINGLE BIGGEST ISSUE being the limitation of only 2KB SRAM. After a lot of research on the internet and different forums, I was able to overcome this limitation. Different programming techniques, a much better understanding of the Arduino compiler, and using the SIM card and EEPROM for additional memory, saved this project. After some changes to the code, a stable prototype was build and tested over a period of a week. A drawback of the limited SRAM was that the unit could not be fitted with a display and user keys. This resulted in a complete rewrite of the code. With no user interface, the only option left to continue with the project, was to make use of SMS messages to configure the unit, as well as the users. This turned out to be an exciting project, and more futures were added as the development continued. My main goal was to stick with the Arduino Uno, or in this case, the ATMEGA328p, and not use any surface mount components. This will make is easier for the general public to copy and build the unit.”