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

[Bo Herrmannsen]

So far i lean towards adding a 22000uF 25V cap and a 15V transorber...
15V is 1V below what the remote control board should be spec'ed for



2015-03-03 17:52 GMT+01:00 Kevin Wood <EMAIL ADDRESS HIDDEN>:
> OK. In that case, there probably isn't much else you can do but use a
> capacitor / battery to hold up the supply.
>
> You already have a diode there, so effectively increase C2 to give it a
> bit more resilience. I'm still slightly surprised that you have a problem,
> because I wouldn't expect the terminal voltage to drop below 8V or so.
>
> I would also put a tranzorb rated at maybe 18-24 volts across the 12V
> supply input after the fuse, just to catch any spikes from the car's
> electrical system. It's a very dirty 12V source and might spike high
> enough to kill something.
>
> Kevin
>
>> 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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