The SIM800L is a Quad-Band GSM/GPRS module with the core signals broken out into standard 2.54mm headers. This small module accepts a micro-SIM card to be inserted at the SIM-holder at the bottom side allowing any microcontroller/microprocessor with a UART port to communicate over the GSM network of choice. A helix antenna is provided and shall be soldered into the pin labeled “NET”. There is also a UFL antenna connector if an external antenna is desired.
This module gives the MCU/MPU access to SMS, GPRS, or data call services and therefore opening several possibilities for monitoring and control including IOT applications, among others.
Powering the Module. The module requires an input power of 3.4V to 4.4V and draws a current up to 2A during peak communications. A 4.0V from an adjustable linear regulator like the LM338 / LM350 or from a buck converter such as LM2596. A large capacitor across the Vcc is also recommended (e.g. 220uF). These requirements tell us that this module should not be powered from the 5V pin of the Arduino. A single 3.7V Li Ion battery or a power supply adaptor of higher voltage and stepped down to 4v are possible power sources.
Using with an Arduino. Using this module with an Arduino/Saleng Uno is relatively easy. At a minimum, connect the power source to the Vcc and Gnd pins, solder the antenna to the NET pin (or use an external atenna via the UFL connector) and connect the Rx and Tx pins to the Arduino/Saleng Uno’s Hardware or Software serial port.
Since AVR-based Arduinos such as the Uno, Mega, Nano, among others, operate at 5V logic while the SIM800L is at a lower logic voltage, a simple two resistor voltage divider network should be installed at the modules RXD pin. The TXD pin of the module may be directly connected to the Rx pin of the Arduino as the voltage falls within the VIH and VIL of the AVR. Other solutions include proper level shifting IC’s or discrete circuits.
In the Saleng Uno, the same scheme as shown in the diagram below may be used. However, for quick tests or for a pure 3.3V system, the Saleng Uno may be switched to 3.3V operation using the jumpers on board to eliminate the need for a level-shifting circuit.
AT-Commands. The AT command set is fully documented in the manual provided by the manufacturer. According to the manufacturer’s document, the default is that auto-bauding is enabled. During factory tests however, there may be changes in the baud rate (See AT+IPR command) and you may want to test with all possible baud rates: 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 230400 and 460800. We would recommend you start with a 9600 baud rate. A factory default command using AT&F0 is also recommended for first time use.
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