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Topic: SIM900 - LM2596 Step-down 5V>4V adjustable output voltage (Read 5756 times) previous topic - next topic


Nov 21, 2013, 09:10 pm Last Edit: Nov 23, 2013, 01:16 pm by karlok Reason: 1

for some IC I need an input voltage 4V. It has TTL 3V3 Levels so I have a 5V>3V3 regulator and powering this with some 5V constant voltage power supply. Then it will transform 5V to 3v3 to supply ATmega328P (on a breadboard) and this will communicate with the module using 3v3 TTL I/O.

SIM900A-module.JPG > http://www.ebay.com/itm/281194207422

LM2596 : How to adjust the output voltage? It says:
Input voltage: 4V-35V
Output voltage: 1.23V-30V

and is
Switching frequency: 150KHz
somehow concerning this ?


Thank you


Nov 22, 2013, 12:59 am Last Edit: Nov 22, 2013, 01:07 am by DVDdoug Reason: 1
On the bottom of page 12 of the datasheet it gives the formula:

Vout = 1.23 x  ( 1+ (r2/R1) )

It also referes to figure 20 on page 9.

It might be a good idea to print-out the datasheet. ;)   I usually do that for all but the simplest parts, and even for simple parts I usually print-out the "important" pages like the pinout diagram.


hello and thanks for the formula including the resistor values r1  and r2.
Unfortunately I do not know whether the ebay module linked
is there any possibility to adjust the output voltage because it also says "adjustable output voltage" ?


There is a potentiometer (the blue "thing") on the board that lets you adjust the output voltage. Take a good look at the datasheet of the LM2596, I don't think it's possible to get 4V from an LM2596 with an input voltage of 5V. You will need to find a regulator with a smaller drop out voltage.


Do you mean it   is not even possible because the voltages 5v (input)  and 4v (output) are too close together?
So my input voltage is 5v. Are there buck converters (step down) from 5v to 4v. I'd prefer some fixed ouput regulator, however think 4v is not that frequently used...


I've been using the SIM900 modems for some experiments. Be aware that the SIM900A ONLY works in these countries:
http://zapro.dk/div/pdf/20111031145224.pdf - So AVOID it unless you live on one of those regions!  :smiley-eek:

// Per.


Do you mean it   is not even possible because the voltages 5v (input)  and 4v (output) are too close together?

Fig 8 on the datasheet shows that the dropout voltage is less than 1V at 1A load current. So if the load is 1A or less, that regulator should be OK.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.


Thank you for the answers. I have read buyer reviews concerning the sim900a at alibaba in which it was said that the module had the European GSM frequencies supported - quad band - and is also said to work in Europe by the seller, some tests proved the contrary so it will not work in Europe  (it seems to be blocked by the european carriers).
I am now looking for alternative GSM modems working in Europe (900, 1800 MHz is what I need) and are compatible with either 5v or do not draw more than 1A whereby in the latter case I'd use the regulator.


Just buy the SIM900 (not A) -model, this works all over the world.

You won't find a raw GSM-module running off of 5V - They are all made to run from 3,3V - 4,2V, as the core-chipset inside is designed for use on a single cell Lithium-Ion battery.

You also won't find a module that does now draw those 2 Amp peaks - after all, it has to be able to send at 2 Watt output power, and this takes some power from the battery!

// Per.


Nov 23, 2013, 01:13 pm Last Edit: Nov 23, 2013, 01:15 pm by karlok Reason: 1
Now I am thinking about buying this module
(ebay id = 200972757437, "SIM900 GPRS/GMS Shield (fixed price)", seller:  www_pcduino_com (385))

It seems to be some kind of Arduino shield that is made to be put onto an Arduino board, and this is being supplied with 5V maybe through some regulator to the SIM900 IC.
It got a WIKI page:

Is there any idea to recommend a specific one to me?


The Shield you found, have a dedicated DC-in plug that feeds the Voltage regulator on board, generating the 4V supply.

// Per.


here (SIM900 GPRS/GMS Shield - LinkSprite Playgound) it is said:
Voltage :   Min 4.8,    Typical 5.0,    Max    5.2    VDC

Power select - select the power supply for GPRS shield(external power or 5v of arduino)

that seems to be optional which power source to take, so I don't need to power it externally
current: max 450    mA

here (Arduino Playground - ArduinoPinCurrentLimitations) it sais:
Absolute Maximum Ratings:
DC Current VCC and GND Pins...... 200.0 mA

Does that mean I need to power it externally?


as far as I can see, it is the same as this one :


which, at seedstudio, has been replaced by this one


which improves the power input part and the start-up .
It seems that, with the V1, you actually had to use the shield power jack , because, if you powered it with arduino, during start-up, the shield could cause an arduino reset .

But......the price is nearly 1/2 for the one you've found, I think you can try it, but I'm almost sure that you'll have to power it using its power jack .


thank you for your finding.
I was first thinking about using  this SIM900 module and an UBLOX GPS for creating a GPS tracker with sending the position via internet (GPRS) to my computer.
Now I would prefer the TK102B because this is more compact and cheaper than building your own one.
If anyone wants to create a GPS tracker, I'd take
- the SIM900 module, not SIM900A  (35 USD, new version 60 USD)
- Ublox NEO6MV2 (ca. 11 EUR / 15 USD)
- some rechargable battery
- some enclosing

Anyway you need a SIM card with either cheap SMS charge or cheap Internet access. For Germany: I'd recommend Netzclub.

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