Solution can be found HERE

I am trying to interface Simcom SIM900A GSM (SIM900a Mini DEV Board v3.9.2) and I tryed interfacing it with Arduino UNO. I have connected as:


SIM900A - Arduino
VCC     - VDC (+5V)
GND     - GND
5VR     - Connect to Arduino TX (Pin 1)
5VT     - Connect to Arduino RX (Pin 0)

Power Supply (External):

5V 2A

Problem When I upload sketch of:

Example > GSM > TestModem:

to Arduino, I got this output from Serial Monitor:

Starting modem test...ERROR, no modem answer.

Checking IMEI...Modem's IMEI: 0

Reseting Modem... Modem is functioning properly: IMEI: 0

Other info:

  • By supplying 5V, 2A the SIM900A Power LED ON continuesly and Signal LED blink with certain period of time which mean I am okay with signal.

  • I connected SIM900A and Arduino with Male to Male + Female to Female as I don't have Male to Female wires. I tested the wires with Multimeter and it's prefectly working.

  • I tested uploading other sketches of sending sms but not successful. Note that, I have load + free sms on SIM.

  • Power supply is actually Adapter (no transformer but circuit) with output of 5.1 Volts

My previous post can be checked HERE


Yesterday just for a minute I noted: Both LEDs (Power + Signal) of SIM900A were blinking at same time. But then by restarting it gets Okay again.

Please help. I am likely to die becuase of tension.

  • 1
    Some points: 1. You can't use pins 0/1 for both the GSM and your debugging at the same time. You will have to use SoftwareSerial (ugh) on another pair of pins. 2. The GSM library is for the Arduino GSM Shield, not the SIM900A.
    – Majenko
    Sep 21 '17 at 9:02
  • Hello @Majenko, Thank You very much for your comments. With due respect if you allow to let me clarify before flaging my post as duplicate, despite the title being same we have different content, also by following the instructions in mentioned post I applied that but nothing happened * (actually they say you have to use different, mine is already in place) * I know rules of SO and I will never spam it with questions by bombording it with same content. I never ask, but I research before very thoroughly. Thank You.
    – fWd82
    Sep 21 '17 at 17:05
  • @Majenko Thank You. 1: As you said: I can't use 0/1 I am using SoftwareSerial mySerial(9, 10); Still nothing happening (I mean not sending msg). 2: If I can't use GSM Library what should I use to make this SIM900A Mini work? What I want to accomplish is Send Text Message. Please post Answer, so that help universe solve this problem. (As many people are facing same problem and I found no solution)
    – fWd82
    Sep 21 '17 at 17:10
  • 4
    You already asked this question at SIM900a Mini Modem, IMEI 0, Help with TX RX pins and have now indicated that the answer is found there. You should not have re-posted your already asked question. Sep 22 '17 at 16:16
  • 1
    Re-posting to get extra attention is pretty much exactly what is prohibited. Sep 22 '17 at 17:47


Detailed Solution can be found Here


My Pins connections were wrong, it should not be: 0 and 1 it should be 2 & 3

Also SoftwareSerial.h is not working with SIM900A


All connections should be like this:

SIM900A - Arduino
VCC     - VDC (+5V)
GND     - GND
5VR     - Connect to Arduino TX (Pin 3)
5VT     - Connect to Arduino RX (Pin 2)

Details Here

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