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

[Kevin Wood]

> 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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