SIM900A Extension Module GSM/GPRS 900/1800MHz Board Antenna Kit

Does anyone know more about the SIM900A Extension Module GSM/GPRS 900/1800MHz Board Antenna Kit? I bought this from ebay and heard it might need to be flashed to be able to use outside China/Hong Kong/Philippines. Here’s my thoughts about the pin configuration. The top 2 Vcc lines are for the Serial Output ports. They really don’t need to be powered unless I’m using those ports. Not really sure what purpose they serve. I can still use the GSM module by powering VccMCU with 5V. Check out my board and please let me know your thoughts. :astonished: :wink:

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I bought this board, too. Did you find a manual for this board?

OK, I got this module running. :D I connected the module with a USB-Serial-Converter (TTL-Level!) to my PC.

USB-Serial          SIM900A-Module
     Tx      --->     SIMR
     Rx      --->     SIMT
     GND     --->     GND

I connected the Pins VCC4,VCC5 and VCC_MCU together to my power supply with a Voltage of 4.1V. And GND of the module to the GND of my power supply.

Then I powered up the module and got a connection. If you send AT (CR+LF) yout get the answer "AT OK". It doesn't matter which baud rate you have selected because the module has auto baudrate detection.

My Module was locked and I had to flash the SIM900-Firmware on my SIM900A. The original firmware version on my module was: 1137B03SIM900A64_ST_ENHANCE

I used the appliction "SIM900 Series download Tools Develop 1.9" and flashed the firmware version 1137B01SIM900M64_ST_ENHANCE

Here is a good article for flashing new firmware:

Here you can find different firmwares for the SIM900:

I'm not sure if there is a voltage regulator on this module!? How do you now that the module can handle a supply voltage of 5V on VCC4 and VCC5 ? (The range of the power supply for the SIM900 is 3.2V to 4.8V as specified in the SIM900 data sheet) I have no manual/datasheet for this module as mentioned earlier...

Okay, I asked my seller for more information about this module. Here is the answer:

the pin following: GND -GND SIMR---SIM900A RXD, TTL level, can not be directly connected to 232 level SIMT---SIM900A TXD, TTL level, can not be directly connected to 232 level RST---SIM900A reset, active low VCC_MCU---when SIM900A module communicate with 5V TTL level, this pin is connected to DC 5V; when communicating with 3.3V TTL, this pin is connected to DC 3.3V. VCC5 ---- DC 5V 1A input. VCC4 ------ DC3.5--4.5V input,suit for lithium battery. Note: Two-way power (VCC4, VCC5) connector(you can choose one):

When I connect 5V to VCC5 and VCC_MCU everything works fine. (VCC4 is open now)

I'm using this library:

When you use this library, you have to connect the module like this: SIMR --> Arduino Pin 3 SIMT --> Arduino Pin 2

Thanks for the help. I wonder if the reset pin is the same as NRESET mentioned in the article for flashing. I'll have to look at the schematics of the SIM900A modem.

Can you provide a link to the item on eBay?

Hi all,

I've also bought this board (but mini version v3.4.1) and have a question about usb to serial (ttl).

My USBSerial is cheap china one with 5v/3.3V/Tx/Rx/GND. After connecting 3 pins Tx->SIMR RX->SIMT and GND to one GND pin, I cannot communicate with module itself.

Any hints on that?

Tnx in advance.

look at the comments section of this.

Hi all, I've written up 2 Instructables that may be of interest to you involving my Sim900A Mini module v3.4...


A titbit Ive learned since then is that the Arduino IDE library "GSM" used GSMSoftwareserial for its serial comms and you cant use Softwareserial at the same time as the 2 are incompatible serial interfaces. Im investigating how to use the UNO hardware serial port now.

If you look at you will see that Australia ( and luckily for me ) all African countries are supported on the SIM900A.

On the mini 3.8.2 there are level converters on the back of the board, and a choice of 5v or 3v for the Rx and Tx lines, and the board runs on 5v.