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

[Kevin Wood]

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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