SIM 300/900 (GSM Module)


SIM 300 is a GSM modem with a simple serial interface. SIM 300 modem can accept any GSM network operator SIM card and act just like a mobile phone with its own unique phone number. With this module one can send/receive sms, connect to internet via GPRS and receive calls. The modem can either be connected to PC serial port directly or to any microcontroller. When purchasing purchase the entire board. As it comes with RS232 to TTL converter and ethernet port. Also do check the module by calling a few times when in the shop.

You can purchase this module online. Some of the sites are listed below:



Fig.1 SIM300 Module

There are two LEDs on the board. One is power LED and the other is the network LED. When you insert your SIM card into the slot and power ON the device the power LED will be turned ON. After few seconds the network LED will start blinking after an interval of 3 seconds. If this happens it means signal is proper but if it is blinking faster it means that there is no network. If your mobile phone has network then this module should have network at the same location(provided the antenna is connected.) Make a call and it should ring. Do it a couple of times before purchasing from a store.

AT commands

These are the Haye’s command set also called AT commands. AT stands for attention. These commands are used to control the modem. Using these commands the modem can be operated. There are different commands for sending/reading sms etc. For further information about the history you can read the Wikipedia article.

The AT command set can be downloaded here.


Above table lists few of the commands. The most basic command is AT and the response is OK. If you get OK then it means that everything is working fine.


Now to test out the commands or for direct interfacing with the PC or laptop you can use USB to RS232 adapter.


Fig. 2 USB to RS232 Adapter

You will need to install prolific drivers. These will be included in a small CD that accompanies the adapter or you can download here.

Once you do that there will be a COM port available now. Some of the basic commands are explained in the following video.