Do I have to connect RX and TX of SIM900A with any pins (say TX, RX of Arduino) for network purpose? Shouldn't the network status LED blink slower after connecting the power sources with SIM900A?

  • Your question is somewhat unclear. You will need to connect the SIM900A to the Arduino, yes, if you wish the Arduino to communicate with it. Are you referring to the network status LED of the SIM900A? Assuming that the SIM900A was previously unpowered, then, upon applying power, how can the LED blink slower? Surely, if the SIM900A is unpowered then the LED won't blink at all? I must be missing something. See TUTORIAL TO INTERFACE GSM SIM900A WITH ARDUINO Jul 17 '18 at 15:31
  • I mean I have powered the device by using 12V 2A adapter. After connecting the power the Network LED will blink fast for a few moment. Then after establishing network connection it will blink slowly. Am I right?
    – opu 웃
    Jul 17 '18 at 16:02
  • Yeah, that is correct. See my answer below. You might want to edit and reword your question using the explanation in your comment :-) Jul 17 '18 at 16:04
  • Do I need to connect the TX and RX of GSM anywhere only for slowly blinking the Network LED i.e to confirm that the network has established?
    – opu 웃
    Jul 17 '18 at 16:05

To answer your questions in turn:

  • It is not necessary to connect to the Arduino solely for the purpose of making a GSM network connection.

  • When the SIM900A is powered on, the Status LED will blink once every second.

  • When the GSM connection has been established the Status LED of the SIM900A will blink every 3 seconds.

Follow this guide, from Tutorial to interface GSM SIM900A with Arduino:

Initial startup of SIM900

  1. Insert your SIM card to GSM module and lock it.
  2. Power up the SIM900A by connecting it to Arduino's 5V and GND pins
  3. Connect the Antenna
  4. Now wait for some time (say 1 minute) and make note of the blinking rate of ‘status LED’ or ‘network LED’ (D6). The GSM module will take some time to establish connection with mobile network
  5. Once the connection is established successfully, the status/network LED will blink continuously every 3 seconds. You may try making a call to the mobile number of the sim card inside GSM module. If you hear a ring back, the GSM module has successfully established network connection.

Connection to Arduino

The SIM900A has the following pins:

  • 3VR;
  • 3VT;
  • 5VR;
  • 5VT;
  • VCC, and;
  • GND.

For serial communication between the Arduino and SIM900A module. you have to connect:

  • SIM900A 5VT to Arduino D9 and;
  • SIM900A 5VR to Arduino D10

Basic Communication

  1. To change SMS sending mode: AT+CMGF=1

  2. To read SMS in text mode: AT+CNMI=2,2,0,0,0

  3. To make a call: ATD+60XXXXXXXXX (replace X with number you want to call, change +60 to your country code)

  4. To disconnect/hangup call: ATH

  5. To redial: ATDL

  6. To receive a phone call: ATA


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