SIM800L GSM/GPRS Part II – Seeking Stability

My first SIM800L dev board was a flop when I tried to use it in an actual project. As mentioned at the end of my last post, the reliability of that SIM800L dev board was horrible. The SIM800L chip would reset regularly – as in roughly every 5 texts or less.

After pondering the problem, I decided I wanted a SIM800L dev board that doesn’t have an onboard voltage regulator. I want control over the power myself even if I am going to have to provide 4.2V rather than 5.

I found this dev board on amazon which fits the bill. No power regulation but it does still give me access to the RST signal:

Further, one of the SIM800L ‘hints’ I stumbled across while troubleshooting my power issue is to use a DC-DC converter rather than a voltage regulator. The comment was it can respond to the current surge faster. I found 7such a module that provides 3A which significantly more than the 2A voltage regulator I was using so I decided to order one of those from Amazon as well:

Clean Power

First, I setup and tested the DC-DC converter. This unit take 4.5V to 24V in and produces 0.8V to 20V. It is a buck converter, so the input voltage needs to be higher than the output. I’m converting 9V to 4.2V so no problem there. There is a tiny trimpot on the converter that is used to adjust the voltage. Before ever connecting this to the cellular chip, I made sure it was producing the proper voltage.

Once the DC-DC converter was functioning, I then setup all of the filtering as specified by the SIM800L datasheet:


Ca is 100uF and Cb is 1uF. The zener diode helps protect the SIM800L chip from over voltage.

Soldering Headers to SIM800L Dev Board

This dev board will not sit on a breadboard properly. But if you only want VCC, RST, TX, RX, and GND that is no problem. All of those signals are on the same side of the board and the pitch is .1″ so a header fits fine on that side.

Connecting SIM800L to PC via FTDI

Before I plug these new components into my project, I want to make sure everything works properly. To do that, I connect the SIM800L dev board to my PC via an FTDI converter. In this case, I want to use a 3.3V FTDI converter. The FTDI converter will only have ground, TX, and RX connected.

Here is the schematic:


not the best picture, but here is the actual setup:



I inserted this power supply and cellular chip into my ailing project and no power issues! When the thermal printer on the 9V power supply kicks in, there is little noise on the 4.2V supply. The project is still undergoing stress testing, but so far the 4.2 power supply and the SIM800L dev card have not had any issues.



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13 Responses to SIM800L GSM/GPRS Part II – Seeking Stability

  1. Pingback: SIM800L GSM/GPRS Cellular Eval Board First Look | Big Dan the Blogging Man

  2. Kevin Szabo says:

    Hi Dan. I am an occasional reader of your blog. I am pleased you have cleared your power problems, but as an experienced EE I want to let you know that I expect your power decoupling is marginal at the moment. The problem is the distances from decoupling caps to the load, and inherent inductance of the leads you are using.

    A lot has been written on decoupling because bad power is the source of so many intermittent failures. I pulled up MT-101 from analog devices (a tutorial on decoupling) but there are many others by manufactures such as Linear Technology and Texas Instruments.

    Click to access MT-101.pdf

    A series in EDN:


    • Dan TheMan says:

      Hi Kevin,

      Thanks for the links, I appreciate it. They are on my task list to read this coming week. I put the final project on strip board and the caps are much closer to the SIM800L sub-assembly. I’ve run several hundred texts thru it w/o problem, so hopefully the problem is corrected for this project. But I definitely want to understand the underlying issue better so I don’t have future problems.


  3. FTP says:

    Thank you for this will try this out. I am not an expert but just trying out something. I have been having troubles with powering SIM800L this module via mobile battery (3.7v) the module is able to detect the SIM and all but for some reason its unable to send out messages. Its probably reseting itself i see blink approx for 5 sec

    • Tejas Arlimatti says:

      I’m having the exact same problem. It says it has detected a SIM but doesn’t show connectivity when I type AT+COPS? And, ofcourse, it resets itself very regularly (about 5 secs) once (i can tell, because it stops blinking then, and the UART doesn’t work), and its back to square one. SIM, but no connectivity. Can you help with this? Were you able to solve your problem?

  4. FTP says:

    Thank you for detailed post, I will try out this as I am facing some strange problem that the this module is able to detect some of the AT commands do work ok but some of the SMS related AT commands error out.With SIM inserted the light blinks only for about 3-5sec and stop for a while and does it again. I am guessing that the module is getting restarted due to power problems as i am trying to power this module with my 3.7v Li mobile phone battery.

    Hoping that above circuit would help me.

    • Dan TheMan says:

      Hmmm, I wouldn’t expect one to have a problem using a lithium battery as they can provide a lot of power quick. But I don’t know that for a fact.

      If you have a bench power supply, you might try using that instead to see if the problem goes away. At least then you’ll know for a fact it is the battery.

  5. Phil says:

    Hi, the original dev board you had that takes 5 volt directly is using a couple of diodes to drop a bit of voltage off to bring it into range, these can be seen on the board (the larger black components with positive indicator stripe). This is a crude way of obtaining the correct voltage, and the voltage isn’t regulated and will vary depending on the load. This is likely causing the instability.

    You could simply short out those diodes then feed in the correct voltage from the same DC-DC regulator you are using now, I’ve not seen the board yet (you first one you purchased) but it looks like it already has some filtering to it, and this filtering is going to be better than anything else mounted off the board as it is closer to the VCC inputs. Once you start adding a few cm’s of cable filtering starts to get cancelled out by the cable inductance.

    Regards, Phil

  6. clovis yutaka says:

    hi do you know if this buck converter can take 5V (or 5.3V) to supply SIM800L?
    setting the output to 3.9V or something.

  7. sachin says:

    sir_g I m going to buy 2 battry of 2800mAh of 3.7v and thinking to connect in parallel circuit so pllz tell me that would it be better power supply for sim800l for start up and stand last for a day. And without using buck converter

  8. Steve says:

    I found using the red SIM800L board from my bench PSU (2.5A) I could get replies to AT (OK).

    Any command requiring connection to the cellular network resulted in “error”.

    The solution was a 100uF and 22uF cap near the SIM800L.

    I suspect the inductance of the test leads was limiting the rise time of the current pulses, despite the fact the PSU could obviously source the current for lower frequencies.

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