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

[Bo Herrmannsen]

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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