Hi,

A few points:

(1) It's not a very good web page that you used. You'd be better off learning from a book (or a better website)

(2) You should just try it, and use a multimeter to see what happens. Instead of the GSM module, you could use (say) a 3V motor as your "load", and instead of an Arduino, use (say) a 9V battery as your "source". Decide on resistor values to give you approximately 3V, measure with your multimeter to confirm the output voltage, then connect your 3V motor to the output of the potential divider, and measure again what voltage you're getting. That will teach you a lot, and then you'll probably want to replace the motor with something else (you could use say another resistor to be your "load" instead of the motor. Again, measure things with a multimeter. Try to explain what's happening, using Ohm's Law (this will be described on Wikipedia, or any electronics book).

(3) Are you sure you can still get a 2G SIM card for your SIM800A board? And that it will work specifically in the town/city where you are? And that it will work for your particular need? It's an (almost) dead technology. You might want to consider at least a 4G/LTE module.

(4) Worth double-checking the documentation in case there is any other option to power the module from a different voltage level (such as 5V or 3.3V or 12V or whatever). Or buy a board that does allow powering from a more preferred voltage.

(5) If you're dead set on using the board that you have, then either use a bench power supply set to the actual voltage required (and measure it with a voltmeter before and after connecting the module - don't assume the bench power supply is providing what it says it is), or you'll need to obtain an adjustable DC-DC converter board, and again set it to the required voltage, and again confirm with a multimeter. This point (5) will work, whereas what you're trying won't work (see points (1) and (2) to know why the method you're using won't work.

Hi Aswinth,

Voltage dividers are not really useful if you want to deliver power to a circuit ( You can compute current by V/(R1+R2)). You can use LDOs or DCDC power supply.
https://simcom.ee/documents/SIM800A/SIM800A_Hardware%20Design_V1.02.pdf page number 19. You need a good 2A current source for powering the module.

You can use LM317 as well.

Welcome. Asking a question around here will knock the newness off you. Be prepared for the response to be turned into a complete redesign. There are some brilliant minds that visit here. They make suggestions us mortals would never consider.

I recommend that don't use the Arduino power supply as a source to power other circuits. Simple things like driving an LED can be accommodated but using it to power another module is frowned upon. 

That leaves you with needing a power source. Typically this would be regulated to ensure the voltage is constant. The voltage you are suggesting 4.2 is an odd beast in electronics. Odd in the sense it is not common. I did a quick search on the module and found one reference that it will accommodate 5-18VDC input. That to me suggests the board has a built in regulator to achieve the voltages the board needs.

With today regulator components simple regulated power supplies are not that complex to create. You need to know output voltage and current requirements to spec the device. I use an LM7805 for 5V only because I'm old and it feels familiar.

You may also consider buck/booster modules to generate the voltage. They come completely self contained. Give them an input voltage (i.e. 12VDC wall wart from the thirft store) and you can adjust the output to your hearts content.

Here is a link to some reputable calculators. www.digikey.ca/.../online-conversion-calculators one stop shop for many common electronic calculations.

Hi, as someone pointed out already, a voltage divider is not going to work well for powering up anything. It's a passive arrangement that can only work if the load is constant and known ahead of time so you can factor it in in the equation. It is not meant for variable loads or for any large amount of current. In the case on a GSM module the current consumption swings wildly between the standby state and active communication.
I've seen some SIM modules use a diode to drop the voltage to 4.2V but I wouldn't recommend that either. If the module can't be powered by 5V directly I would recommend a DC buck converter.
Now, even if you had the means to lower the voltage down to 4.2V (buck converter or a LDO voltage regulator)  I wouldn't recommend pulling power directly from the Arduino either, as the Arduino power regulator can only handle I think up to 1A (and that is assuming that the Arduino itself is getting power from a sufficiently powerful supply and not, for example, a USB port, which can normally supply 0.5A max). Tthe GSM module can draw up to 2A according to the chip's documentation so take that into account.

All voltage sources (batteries, power supplies, etc) have internal resistance, so the output voltage is diminished by the voltage "drop" across the resistance. This voltage drop is equalvto the resistance multiplied by the current your load draws. 

Fresh batteries and well-designed power supplies have very low internal resistance, so their output voltage drops tend to be very low. However, resistive voltage divider networks generally have much higher output resistance, so their voltage drops can be too high to allow them to be used as practical power supply alternatives. [You *can* make voltage dividers with output resistance as low as that of a battery or power supply, but this involves using low resistor values that draw a LOT of current - typically too much for the Arduino's 5V pin 

The SIM800A's maximum operating current is less than the 5V pins maximum rating, and since the SIM800A can use an input voltage of 4.4V you can put a 1N4001 (or any other 1N400x) diode between the Arduino's 5V pin and the SIM800A's input vtage pin unless your Arduino is being powered by a relatively high voltage source. 

https://youtu.be/sZxCpPYpkyY

ROFL - I will be gentle 

I use a ton of these adjustable LM2596 buck power supplies, link provided as an example. 

I usually use them on projects with a battery, 12V or 7V , to provide a regulated voltage output of my choice. 

You can hang it off the 5V and adjust it to whatever you want within its spec. There are flavors of the same module with adjustable BUCK-BOOST and also a current limit pot. I use the BUCK with current limit to drive hi-power multi-amp LEDs.  

https://www.amazon.com/LM2596-Converter-3-0-40V-1-5-35V-Supply/dp/B08NV3JCBC/ref=asc_df_B08NV3JCBC?mcid=f521f6434d7d3e24a6fe6ce8ac3cb5cd&hvocijid=11348621047901591685-B08NV3JCBC-&hvexpln=73&tag=hyprod-20&linkCode=df0&hvadid=721245378154&hvpos=&hvnetw=g&hvrand=11348621047901591685&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9008684&hvtargid=pla-2281435177178&th=1

Used and abused them as a inexpensive power source.

I have at least a dozen on hand just for impromptu projects. I've also used them as a dc motor driver, like an infinitely adjustable gearbox. 

Thank you, this is exactly what I needed. I used a 12V 2A adapter and built a regulator. i dint use MIC29302 but LM317 instead

Thanks