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:
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.