[OpenTRV-dev] OT: GSM remote control in car

[Kevin Wood]

I would check how you're deriving the supply voltage for the module. I
think the SIM900 module itself only needs 3.3 / 3.8 V but the current is
up to 2A peak when transmitting. Mine includes a regulator on the shield
that drops this from a nominal 5V supply input.

I'd be very surprised if your battery voltage is dropping low enough that
an LDO regulator wouldn't be able to supply the module from 12V, but I
suspect you might be regulating down to 5V, then down to 3.8, etc. and
losing a bit of headroom with each step, which might cause you a problem.

Dropping 12V to 3.8 at 2A might be a tall order for the regulator without
better heatsinking, but I wouldn't mind betting the average current is
quite low, given that the module won't work in every GSM timeslot, so you
might well get away with it.

Also, the accessory circuits on some cars are disconnected when the key is
in the "III" (cranking) position, so it might be simply that you are
losing the supply completely.

BTW: Sorry for my absence on here of late. I'll update you with what I've
been doing when I get a chance as it might be interesting.

Kevin



> its a sim900 based module and datasheet says 2A at max...
>
> how do i calc how big the cap needs to be for every X sec i want the
> cap to power it? it not its quite extreme but better be safe than
> sorry
>
> the module runs at ~4.8V, but need to check the regulator on the
> mega2560 based board the module mounts in to
>
> 2015-03-03 17:10 GMT+01:00 Adrian Godwin <EMAIL ADDRESS HIDDEN>:
>> Probably a cap and diode will fix it. Measure the current the GSM module
>> takes (while transmitting) to be dure. If not, use a tricklecharged
>> battery
>> When you turn the starter, the battery voltage can fall very low and may
>> well drop below what the module power supply needs.
>>
>>
>> On Tue, Mar 3, 2015 at 3:56 PM, Bo Herrmannsen
>> <EMAIL ADDRESS HIDDEN>
>> wrote:
>>>
>>> Sorry for the Off Topic
>>>
>>>
>>> But i have on the shelf a gsm remote control i build last year... it
>>> works fine but as soon i start the engine the GSM module goes
>>> disconnects from the cell network
>>>
>>> i could just power that off and turn it on should it loose connection
>>>
>>> but i wondered if a fix could also be a big cap and a diode ? or maybe
>>> a small lithium pack and a small charge circuit?
>>>
>>> /bo
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