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

Tue Mar 3 16:49:46 GMT 2015

how?

its tied direct to battery...

2015-03-03 17:46 GMT+01:00 Adrian Godwin <EMAIL ADDRESS HIDDEN>:
> It's very likely that it's the accessory power problem.
>
>
> On Tue, Mar 3, 2015 at 4:45 PM, Bo Herrmannsen <EMAIL ADDRESS HIDDEN>
> wrote:
>>
>> 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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>> >> >>>
>> >> >>>
>> >> >>>
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