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

Tue Mar 3 16:57:35 GMT 2015

its a small board... of course will have to check if there are an
regulator on it... but someone just yelled dinner so will have to wait
after that

2015-03-03 17:55 GMT+01:00 Kevin Wood <EMAIL ADDRESS HIDDEN>:
>> and then i looked at the schematic and figured that voltage is first
>> lowered to 5V and then down to 3.3V...
>
> Are you sure?
>
> It looks to me like the GSM module is powered from the output of U1
> directly OR: is what's shown as the GSM module on that schematic actually
> a GSM shield with a further stage of regulation?
>
> Kevin
>
>
>
>
>> me allready getting a bit dizy :-D
>>
>> but i know that a healthy battery should be able to keep voltage at
>> 9V...  but once the engine is running that will jump to 15V and slowly
>> lower as the battery is charged up
>>
>> and then i looked at the schematic and figured that voltage is first
>> lowered to 5V and then down to 3.3V...
>>
>> maybe a small 7.2V lipo pack is the most easy way arround it... but of
>> course not the cheapest
>>
>> 2015-03-03 17:39 GMT+01:00 Adrian Godwin <EMAIL ADDRESS HIDDEN>:
>>> The fundamental calculation is C = V/(it)
>>>
>>> i.e. the capacitance is the voltage change you want to see divided by
>>> the
>>> product of current and time. As Kevin says, applying this can be
>>> complicated
>>> if there are multiple regulation stages. The current may also be
>>> dependent
>>> on the voltage. If you can provide the holdup at 12V while regulating
>>> that
>>> down to 3v3 with a switching regulator, you will get good value, as the
>>> current will be smaller than the module uses (due to the constant power
>>> effect of the regulator) and a large voltage drop (about 9V) will be
>>> allowed.
>>>
>>>
>>>
>>> On Tue, Mar 3, 2015 at 4:30 PM, Bo Herrmannsen
>>> <EMAIL ADDRESS HIDDEN>
>>> wrote:
>>>>
>>>> the remote control is direct to battery :-D
>>>>
>>>> or else... yes... it would loose power at engine start...
>>>>
>>>> it for a car preheater that has its own fuse box etc and not tied in
>>>> to the cars electrical other than at the battery terminals
>>>>
>>>> 2015-03-03 17:27 GMT+01:00 Kevin Wood <EMAIL ADDRESS HIDDEN>:
>>>> > 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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