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

[Kevin Wood]

I would go a little higher than 15V on the tranzorb. You want it to have a
reasonable margin above the highest possible voltage you'll see, and a car
system could get quite close to 15V. 18V would be reasonable, I'd say.

Kevin

> 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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