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

[Adrian Godwin]

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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