I recently purchased this GSM/GPS module.

I would like to power this module via a USB cable but can see that it has varying input voltages.

So bearing in mind that USB is 5V and that I'm an electronics newbie (I've only worked with evaluation boards that regulate the voltage for me), I'm not sure how to proceed.

On the site it recommends the following voltages:

  • GSM: 3,2 ~ 4,8 V
  • GPS: 2.8 ~ 4.3 V
  • VCHG: 5 V

I assume 'VCHG' has something to do with charging. I can't see any battery in the schematic below so I'm not sure what that's used for but I can also see that 'TEMP BATT' is pin 2. It also has an ON/OFF switch on pin 1, is this a red herring?

If VCHG won't help (I had hoped that would be some form of input that would regulate the voltage for me but I'm not sure), what's the easiest way to reduce the voltage down to the recommended amount? Should I aim for half way between the varying voltage values? Also, if I was to use GSM and GPS, do I only need to power pin 17 (GPS) as it appears that they connect after the R13 resistor.

The datasheet for the SIM928A can be found here.

enter image description here

  1. Find a data sheet for that thing and figure out what Vchg is all about.
  2. Find a data sheet for that thing and find out how much current it needs when operating. Most especially the maximum current it needs when the GSM module is transmitting.
  3. If it needs more than 100mA, then don't count on using it with USB power.
  4. Use a 3.3V regulator to drop the 5V from USB to a voltage that both the GPS and GSM pins will accept.

3.3V regulators can be had as linear that you can use with minimal parts on a PCB that you make yourself. You can also get 3.3V switching regulators that are more efficient but more complicated to build. You can get complete modules for both types of regulators.

In response to comment:

You will NOT be operating that thing off of USB power.

The datasheer you linked to in the comment includes suggested circuits for the voltage regulator, and mentions that the GSM module draws up to 2A when transmitting. It also recommends using 4.1V, which is comaptible with both the GPS and the GSM parts.

So, USB is right straight OUT.

USB is only supposed to supply up to 100mA until the USB host (in the PC) has negotiated with the device via data communication, at which point you can only draw upto 500mA. Since you won't be doing any data communication, you can only count on 100mA. Despite this, you will often get 500mA because some (many or most?) host manufacturers just ignore the specifications and supply full current to all devices. It just may not work on some USB ports.

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  • \$\begingroup\$ Thanks for your answer! Just to confirm, by datasheet do you mean this: soselectronic.com/a_info/resource/c/SIM928A.pdf \$\endgroup\$ – jskidd3 Apr 15 '16 at 12:39
  • \$\begingroup\$ Yeah, that thing. Edit your question and put that link in so it isn't buried down here in the comments. \$\endgroup\$ – JRE Apr 15 '16 at 12:46
  • \$\begingroup\$ What about if I power it through my Arduino Micro. It has a 3.3V pin out that I could perhaps use? I can see the recommended voltage is 4.1V so it probably won't be enough - but will it sort the amperage issue? I have another SIM808 EVB working fine off of my Arduino Micro 5V supply. \$\endgroup\$ – jskidd3 Apr 15 '16 at 12:48
  • \$\begingroup\$ The arduino micro has a datasheet, too. The info you need is also right on the Arduino site. See what it says about the 3.3V pin? That's a far cry from 2A. \$\endgroup\$ – JRE Apr 15 '16 at 12:54
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    \$\begingroup\$ @JRE - FYI the solution for this apparent contradiction (i.e. Arduino can't supply 2A, but the GSM module needs 2A), is that GSM modules only need 2A in bursts. With appropriate design and using large capacitors, then power sources with lower current limits can supply the necessary average current. Some earlier related discussions: electronics.stackexchange.com/questions/219235/… electronics.stackexchange.com/questions/17137/… HTH :-) \$\endgroup\$ – SamGibson Apr 15 '16 at 13:54

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