Hello friends! Today, we will send an SMS with an Arduino UNO and a SIM900D GSM module using AT commands. We will start with the connections, check that the modem can communicate with the mobile network, and then let the Arduino submit one message. You will also learn how to tell the difference between a successful command and a message that has actually reached its recipient. The SIM900D handles the radio connection and SMS service. The Arduino sends instructions through a serial connection and reads the modem's replies. We do not need a separate GSM control library for this example; we will use the Arduino serial facilities and send the commands ourselves. This makes the exchange easier to follow when something goes wrong. This tutorial concerns a legacy 2G modem. Before building the cir ...