Tag Archives: SIM900A



Hi all, recently  I was thinking about the AT command. What’s the AT command you might ask? well it’s an instruction sent to a cellular modem. for example: It’s an instruction which is sent from any suitable device (in this case and Arduino and a GSM module) to a phone/modem or GSM module. you can request something to be sent back to you or you can send something to another supporting device.

The abbreviation for ATtention is AT. each AT instruction starts with “AT” followed by the rest of the command. An easy example is:

AT+CMGF?AT=meaning that it’s an AT command, +=An extended command, CMGF= Select SMS message format, ?=Query character.

Ok enough of that lets get into the project.

The goal is to send a message from the Arduino+GSM module to a mobile cellphone. This can be quite useful.. since you can use this message as a “trigger”  for almost anything examples can be hooking up a relay to open your garage door or triggering a servo to open a lock.

you can even program the Arduino to receive a specific message and to execute a series of commands apon receiving this message. the possibilities are endless. so I suggest you have fun 🙂

So we will need a few items to get this project going.

* Arduino Uno.
* GSM module (I’m using the SIM900)
* 4 Male to Male jumpers.

You can ignore the relay for this project!

The first thing you want to do is download the Arduino IDE from this link.

After you have installed the IDE you’ll need to paste and modify the included code into your sketch (save the sketch)

Next you’ll want to burn the sketch onto your Arduino.

So now you need to wire up the Arduino and the GSM module.
Thankfully the GSM module already has a MAX232 chip embedded into the board so we don’t need to worry about RS232 communication.
take a look at the pictures below and wire up your board.

GND=purple goes to 5V GND on the Arduino.
RX=red goes to pin 8 on the Arduino.
TX= yellow goes to pin 7 on the Arduino.
Trigger=grey goes to pin 2 on the Ardino.

The trigger is triggered when pulled low to GND

power to the SIM900 board gets connected to +5V and GND on the Arduino.

Now all you need to do is pull pin 3 on the arduino to GND (this is the trigger, in this case it will send a SMS)

You can also list all the messages saved to the SIM card since the SIM card is basically a tiny computer in itself!

Well that’s it for now HERE is the code saved in a nice zip file for you guys.