LED Status Indicators
There are three LEDs on the SIM900 GSM/GPRS shield which indicates connectivity or power status. By observing these LEDs you can get a visual feedback on whats going on with the shield.
PWR: This LED is connected to the shield’s power supply line. If this LED is on, the shield is receiving power.
Status: This LED indicates SIM900’s working status. If this LED is on, the chip is in working mode.
Netlight: This LED indicates the status of your cellular network. It’ll blink at various rates to show what state it’s in.
- off: The SIM900 chip is not running
- 64ms on, 800ms off: The SIM900 chip is running but not registered to the cellular network yet.
- 64ms on, 3 seconds off: The SIM900 chip is registered to the cellular network & can send/receive voice and SMS.
- 64ms on, 300ms off: The GPRS data connection you requested is active.
The operating voltage of SIM900 chip is from 3.4V to 4.4V. To keep supply voltage safe at 4.1V, the shield comes with a high current, high accuracy, low-dropout voltage regulator MIC29302WU from Micrel – capable of handling load currents up to 3A.
You can add an external power supply to the shield with the 5.5mm DC jack, to which you can connect any 5V-9V DC wall adapter you have. Next to the DC jack, is a Slide Switch to select the power source labeled EXTERN. To use external power source (2A min), move the slider as shown above.
The SIM900 GSM/GPRS shield uses UART protocol to communicate with an Arduino. The chip supports baud rate from 1200bps to 115200bps with Auto-Baud detection.
With the help of jumpers you can connect (RX,TX) of the shield to either Software Serial(D8,D7) or Hardware Serial(D1,D0) of the Arduino.