SIM900A and Arduino Uno Serial communication problem

Hi there!
I'm making a project that makes use of SIM900A and arduino. But I am having trouble because when I test the sim900, it's output is:

Here is the code and connections
Arduino >> Sim900a
Pin 10 >> 5VR
pin 9 >> 5VT

Also, I've used a 4.5V battery for the power supply of the sim900a

#include <SoftwareSerial.h>
SoftwareSerial SIM900A(9, 10);
// Connect the SIM900A TX to Arduino pin 9 RX.
// Connect the SIM900A RX to Arduino pin 10 TX.
char c = ' ';
void setup()
  // start th serial communication with the host computer
  while (!Serial);
  Serial.println("Arduino with SIM900A is ready");
  // start communication with the SIM900A in 9600
  Serial.println("SIM900A started at 9600");
  Serial.println("Setup Complete! SIM900A is Ready!");
void loop()
  // Keep reading from SIM900 and send to Arduino Serial Monitor
  if (SIM900A.available())
  { c =;
  // Keep reading from Arduino Serial Monitor and send to SIM900A
  if (Serial.available())
  { c =;

What seems to be the problem?

The problem I see is the missing wiring. Which Arduino?

It's Arduino UNO sir

Here is the connection

It looks like there is a mismatch on serial parameters - baud rate, parity etc.

Can you elaborate sir?

You tried but a picture never replaces a wiring. In a wiring there are pin designations, power supply. Soo often powering is the issue.

Yes, check up that baudrates mtches!

A common reason for seeing unreadable garbage when doing serial comms is that the sender and the receiver are configured differently. For example perhaps the SIM900 is sending at 4800 baud but the receiving Uno has 9600.

What are the defaults for the SIM900 and is there a method to reset it to those defaults?

The default baud rate for sim900a is 9600bps and yes, there is a method to reset those defaults but to set it the sim900a must response first to the arduino

A simple first test would be to try some of the other standard baud rates.

Do you know what it is supposed to say?

1 Like

I'll try from 9600 to 115200. This is the sim900a range. The response is supposed to be "OK" when I send "AT" from the arduino.

I've got a response at 57600 baud rate

Thank you for the assistance

Software Serial will not be reliable at that rate. I'd reset the sim 900a to 9600.

Yes that's what I did

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