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

Tue Mar 3 17:13:27 GMT 2015

Darn! I feel hungry now. ;-)

> 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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