SIM900 and SIM800L GSM project

I've just ordered these modules for a project I'm thinking of giving to some of my IoT students.

SIM900 GSM shield - development board for Arduino

SIM800L GSM GPRS module

I would appreciate hearing from anyone who has practical experience of using these devices.
I've watched quite a few YouTube videos about controlling the devices using AT codes - looks like a "fun" thing to do and fairly straightforward.

I plan to allow my students to send & receive text messages to their mobile phones before getting them to work out how to send data, like temperature readings to a remote database or via MQTT to Node-RED.


Hi Dave,

I had one of these a while back... - the things I noted were...
a) does draw some power - so make sure you have a good power supply.
b) will need a Sim of the correct size obviously - and credit to go with it... (hopefully they won't go walkies)
c) GPRS is slowly being switched off in UK - (2G) - so check you still have coverage on your chosen network...


Thanks for your input.
I've seen that setting-up the correct power supply is one of the most common problems.
Seems the SIM900 can consume >2A when in transmit mode !!!

I am waiting for SIM7020G, I want to see what can be achieve with NB-IOT, one provider have covered ZA.

Will be interesting.

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Just checked with 1pMobile about the switch-off of 2G and got this response...

The current plans for the phase out of the 2G network are not due to begin until 2033 at the earliest,
this is still not set in stone so it could potentially be later.

I think 11-years from now should see me out.

The two devices, shown above, have been shipped from China and are due mid-Sept.
More toys to play with.

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OK - as long as they don't piggy-back on vodafone or ee you should be ok... (maybe)

What's happening with 2G network closure - Hyphabit.

Certainly some cells around here have gone (one had a IT failure and they said the repair was to upgrade the kit and that no longer included 2G)

End of life for kit in 11 years, long time. Normaly telco kit gets swapped out between 10-15 years, but lately we have seen around 5 years. Sure it is not a typo and should be 2023?

It's supposed to be 2033 (probably in EU) - but in UK voda and ee have said starting 23 ... and indeed some are failing/not being repaired now. so your "mileage may vary"....

Just a quick update.

My two GSM modules arrived this morning.
Quickly lashed-up the GSM-900 Module and got it working - yippppppeeee.

As a temporary measure, I took the SIM card out of my mobile phone and used it in the GSM-900.
Bit fiddly as my SIM is a nano-size so I had to find its original plastic carriers.

Connected an Arduino UNO to the GSM-900 Module and used a simple sketch that acts a relay.
I tried various simple AT commands then tried sending an SMS message to my wife's phone.
Worked a treat.
Then tried sending a reply from wife's phone to the SIM-900 - again works fine.
The message gets stored on the SIM card, so I had to use an AT command to read it or "all" messages.



Another quick update.
Aided by one of my grandsons we tried doing a voice call from the SIM900 to his mobile phone.

I found a headset with pink and green coloured plugs that fitted exactly into the SIM900 Dev Board.
We were delighted as we could make a call and answer a call.
Dead easy with the command ATD+441234567890; << Your mobile phone number goes here
It was great fun talking to each other via this inexpensive Dev Board (UKP £ 6.50)

Next step is to try sending data (e.g. a temperature reading) to a remote PHP file that writes the data into a MySQL database.


I had an early version GSM board that only supported 2G and with which I had no success. Which provider are you using?

I've used the 1p Mobile provider as someone on the forum recommended/mentioned them.
They are a UK-based company that use EE as their carrier.
If you are not in the UK, I have no idea if you'll be able to use them.

I have a 'refer a friend £5 sign-up discount voucher'. Let me know if you interested in using it?

I've actually moved my wife's mobile and mine to use their 1-year Talker plan for £36. This gives unlimited texts and unlimited calls and 250MB of monthly data. Suits us fine as we are not huge mobile-data users.

I've just ordered one of their Classic PAYG plans for £10 as I'm fed-up with pulling the SIM card out of my mobile phone to experiment with the SIM900.

I managed to get the HTTP-protocol working yesterday to send a temp/humidity/pressure reading from an Arduino via the SIM900 to a PHP page on a remote server that updates a MySQL dB.


Hi David
I had the LILYGO® TTGO T-Call which has SIMM800 module. Apparently this is newer than the 900 version. Both are limited to 2G which a lot of the carriers have dropped. I never got my board working even after buying a "THREE" simm card which apparently does support 2G. Maybe the board was faulty.
Anyway I still have the "THREE" card if I decide to buy another board.

Sorry, my mistake. "Three" does NOT support 2G but I have tried with EE which does and still had no success. I'm currently looking at SIM7600 boards which support 5G. Unfortunately they are more expensive.

This was my very basic setup. Connected an Arduino UNO to the SIM900 Dev board and used this sketch.

#include <SoftwareSerial.h>
SoftwareSerial GPRS(7, 8); // RX, TX

unsigned char buffer[64];
int count = 0;

void setup()

void clearBufferArray()
  for (int i=0; i<count; i++)
    buffer[i] = NULL;

void loop()
    buffer[count++] =;
    if (count == 64) break;
  Serial.write(buffer, count);
  count = 0;

  if (Serial.available())
    byte b =;
    if (b == '*')

Started PUTTY (or some other terminal program) on the correct COM port for the Arduino.

Then tried some AT commands....
Should return OK

AT+CMGF=1 Check if SIM900 will support SMS mode

AT+CMGS="+44123456789" <<< Enter the mobile number of the destination
When you press enter you get a > prompt
Type in your message and terminate it with a * (star) character (to send the message).

If that works you can try sending an SMS from the destination phone to the Dev Board.
You should get an indication on the terminal that a message has been received.
You can see the messages with... AT+CMGL="ALL"

You need a decent 5V PSU (that can supply 2A) for the SIM900 Dev Board.
The red LED blinks fast whilst it is looking for a network.
Once it has connected to your network the flash rate changes to once every 4 seconds.

I bought my SIM900 from AliExpress for £6.72 + VAT and postage (arrived in the UK in 2-weeks).

Hope this helps.

Hi David

I had some success using these AT codes on a modem many years ago.

The dev board I currently have is LILYGO® TTGO T-Call which is an integrated ESP32 and Sim800

It does not lend itself to simple serial interaction but instead has to address 6 or 7 inputs to the module using a specific library.

Anyway it is 2G only and I would like a more future proof model, hence the SIM7600 which I have ordered.


If you order something today, it should at least support 4G. In my country (Sweden) the shutdown of 2/3G is expected by 2025

I got sent this link this afternoon - might be relevant to your set-up.

You sure you will not be better off with CAT/M or NB/IoT? You need to move a lot of data to use a 4G

With the CAT/M or NB/IoT you should get better range, be able to use battery power (Dependent on the amount of data you move)

SIM7020E fall in the NB/IoT category.


You can try here