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Topic: Programmically turn power on modem SIM808. Is my solution safe ? (Read 338 times) previous topic - next topic

Derektor

Hi.

I'm not too friendly with electricity, that's why I have question.

I use Arduino with SIM808 GPS/GPRS modem. Works very good. BUT. SIM808 have specjal button to power modem (along with button to power the shield).

I wanted to be able to power on/off modem programmically, and (mesuring the circuit on the SIM808 board) I found, that if I set to LOW digital pin on Arduino board, and I will touch pin to this power button on the SIM808 board, I acheive what I desire - modem is powering on/off.

My question is - is it safe from electric point of view ?

Please check out my schema and confirm, I can attach the cable to the pin of power button on SIM808 board to have my funcionality permanent.

Or - if needed- what resistors/capacitors/any thing else I should add to make boards (Arduino and SIM808) safe from short circuit or other demages.

Thanks in advance.


The code to set the pin is obvious:

--------------

void setup() {
   
   pinMode(3, OUTPUT);
   
   digitalWrite(3, LOW);

}

void loop(){
}

raymw

You really need to have a schematic/circuit diagram for the modem board. Unless they say that grounding an on/off power switch is the way to power it off, then it could be that you are grounding a power rail, and some protection circuitry is tripping it out, thinking that it is a short circuit - which it could be. I'm not familiar with the modem, but the way you have explained things, it does seem not quite right.

Derektor

Is this topic maybe an answer ?

https://forum.arduino.cc/index.php?topic=390856.msg2692330#msg2692330

One say, that grounding obeys the electric rules and schema (chipset is almost the same).

Does it ?

cattledog

Code: [Select]

pinMode(3, OUTPUT);
digitalWrite(3, LOW);


Setting a pinMode to OUTPUT and writing it LOW is a good method for connecting the pin to ground, but an Arduino pin can only safely sink 20ma of current.

Does the switch need to be pressed for some period of time for the on/off toggle to occur?

You will need a schematic or a measurement to determine the current to ground on the switch line of the module when the power switch pressed. My guess would be that the current is low and within the range of an Arduino pin, but you will need to test.


Derektor

> Does the switch need to be pressed for some period of time for the on/off toggle to occur?

Yes. About 20 seconds.

I can't find schematics for this shield (or I'm too blind to see them :( ).

I found info, that it is the way to start modem programically. Problem is, that this info is of SIM900 and I look for solution for SIM808.

Any help ?

cattledog

Quote
> Does the switch need to be pressed for some period of time for the on/off toggle to occur?
Yes. About 20 seconds.
That makes me think it is a low/no current end switch driving software or possibly some internal RC timer.

Quote
Any help ?
If you can not measure the current as previously suggested another possibility is to place a current limiting resister in series with the Arduino pin. 1K will limit the current to 5 ma. With resistance in the line to ground, does the modem turn on and off properly?

Moving up in complexity to active components, You may want to look at the use of a optocoupler, transistor or mosfet to switch the line to ground.

What is the application, and if the pin or Arduino fails in service, what are the consequences?



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