Hello, I just started playing with arduino buying an arduino mini pro and 2 modules (namely a SIM800L and a NEO6M V2 gps) and I think I may need some help. I'm not sure this is the right forum (or section) so feel free to tell me if this is the case.
First thing I did (after succesfully messing a bit with arduino and some leds) was trying to connect the SIM800L to my computer through an FTDI adapter. Hence I plugged the RXD and TXD to TX and RX of the FTDI, the DTR to the DTR and the ground and VCC to a DC DC buck converter that looks just like this, which I bought from china and should be capable of some amps. I regulated it to 3.3V just before connecting it.
The LED on the module started blinking and I was pretty happy until it stopped and started warming a bit too much - it's burned now.
I then realized I had to connect the ground of the FTDI to the one of the converter, I did it and plugged everything in the GPS - TX to RX, RX to TX, GND to both the GND of the FTDI and the one of the converter and VCC to the converter, this time raising the tension a bit to circa 4.1 volts.
It started drawing 60-70 mAmps, that made me again pretty happy, but suddently it burned and started drawing some amps as well. VCC and GND are now directly connected.
I just can't figure out what's the problem: the converter seems ok, I can safely turn on some LEDs and keep them on for a long time. Do someone have any idea? It would be great.
Welcome to the forum.
Do you have some programming experience ? That will be helpful when using Arduino, any computer language.
If you want something that works, then you could buy something from Adafruit and follow their tutorial.
When you start with Arduino, it is easier to power your project via the USB cable and use only small currents. As soon as you use 12V then the magic smoke might escape.
If you want cheap components, that might be counterfeit, that do not always work together, that have the wrong circuit, then you can buy things from AliExpress/Ebay/Amazon. I have blown things with a cheap DC/DC-converter that could not deal with a open output.
First of all, I prefer a Arduino board with a USB connector. I have used a Pro Mini in the past, but when I want to update the sketch after a year, then I can not find that usb-serial module.
Can you give links to the modules that you have ? Please give links to where you bought them. A link for every module: SIM800L, NEO6M V2, FTDI adapter, DC/DC-converter.
Please tell us about your project, give us a broader view.
Perhaps you should no longer use that DC/DC-converter anymore.
Can you draw the parts and the wiring on a piece of paper and show it to us ? Draw also the power wires, the GND wire is the most important wire.
Any time you have questions about a hardware system with more than about 2 connections, you need to post a diagram and/or photos.
Blah blah blah descriptions are hard to follow, and notorious for leaving out the actual problem.
Thanks so much for the answers. I'm posting here the (actually pretty simple) schematics, excuse me if they're not pretty at all.
This was the first one. No surprise if the serial didn't work since the ground was not connected, but I wouldn't expect it to burn!
This burned as well, I really can't explain how. The dc input was 4.1V c.ca here.
My components are just like @Koepel said: the DC DC converter comes from aliexpress and is some years old, I just bought the others from ebay: here is the sim module, here the gps and this is the FTDI (that is still working ok with the arduino).
The converter is identical to this. I was stepping down a 12V supply and it seems pretty stable.
I'm simply trying to build a gps tracker that activates when it receives an sms and it seemed to me a good idea to get started with arduino. I would therefore need the mini since I need the project to draw as little current as possible (I hope it will run a month on some 18650 batteries, but I'm not very sure that's possible).
I know the components are very cheap, but that's the only way I can afford them. They still seems pretty solid and since I'm not I think the problem should be on my side... 
Answering @Koepel's last question: Yes, I have some almost solid programming skills (I know quite a bit of C) and that side of the project doesn't actually worry me, but I won't be able to program anything except for LEDs if I keep burning things :).
Anyway, thank you so much for the answers despite the badly written question!
Don't use that DC/DC-converter anymore. You need to test it and be sure that it is okay before you use it again.
What is powering the DC/DC-converter ?
Can you get another power supply ?
Do you have a multimeter ?
These are good quality DC/DC-converters: https://www.pololu.com/category/131/step-down-voltage-regulators.
Pololu tells what chip is used, they provide a schematic, they have a forum, they give support.
Of course I have a multimeter, and it's a pretty good one (I think it's the only good thing I have :)).
How do you think I can test the converter? It's powered by a 12v Vodafone adapter I found at home, but everything seems pretty ok: it just powered the arduino (via the non-regulated pin) for several hours without any problem. I tryed just now to wire some resistor to it monitoring the tension and there are absolutely no relevant changes (always less than 0.04v) until at least 150 mA draw.
Do you think wiring it through a resistor or in series to a capacitor could somehow solve my problem? I don't really know but you know more than me for sure.
Thanks for the link by the way, I saw there is a seller in my city and I may go there if I will need some piece!
Start simple
What is its output voltage ?
I can regulate it, it's 3.3V right now
What voltage does the Arduino require ?
Note that there are 3.3V and 5V models available
It's a 3.3v arduino mini.
The arduino works just fine by the way, the problems are with other modules which should work with this voltage as well.
Is it an Arduino mini or an Arduino pro mini ?
They are not the same
Ok, it's a pro mini then, I didn't know they're different
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