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

Wed Mar 4 12:59:09 GMT 2015

the funny thing is that during car start the modem will not transmit...

but i have allready ordered a transorb and a cap... i asked the
italian guys that made this and they spec the input voltage to be max
16V.... so i picked a 15V transorb... the voltage will rarely be over
15V anyway...

if that does not help i will try as you have suggested.... a fast
meter is sadly not in my toolbox... but i can get a old fashioned
scope from one in the family... but i guess that is not good enough...

the caps and transorbs should be here tomorrow... will then fit them
and drive arround a few days to see if it can keep connection to the
cell network

2015-03-03 23:56 GMT+01:00 Hadyn van der Berg <EMAIL ADDRESS HIDDEN>:
> Hi
>
> just check the voltage input of MC34063A - it can tolerate up to 40V and
> thus you shouldn't need a tranzorb protection device. If you are getting 40V
> spikes on your car battery then there is a problem.  If you still want to
> add protection, you only need to make sure the tranzorb clamps to less than
> 40V in worst case.  go for 26V or something like that - it will ensure the
> transorb never conducts under normal crank or charging cases, but still
> provides protection. I couldn't see where else +V goes to , so you need to
> check the max operating voltage of whatever's connected there as well.
>
> The MC34063A isn't a good choice for powering the system since (at 100kHz
> switching frequency) it may not respond fast enough enough to handle the
> high peak currents demanded by the GSM pulses (up to 2A bursts , I remember
> times in usecs, but online says 1/8 timeslots * 4ms period).
>
>
> VCC for the GSM modem is powered directly from the MC34063 and its output is
> set for 4V which is probably right. Usually these GSM modules can be powered
> directly from a Li-ion cell (max 4.2V / 4.35V). The modem should continue to
> work down to around 3.5V so your battery voltage would really need to drop
> for it to start failing.
>
> You could try reducing C4 to speed up the regulators response to the fast
> transients generated by the GSM modem as it transmits.  As Kevin suggested,
> double C2 to help the regulator . Use ceramic caps since they have lower ESR
> than large aluminum electrolytics. Datasheet recommends 1000uF on the input
> (C2).
>
>
>
> I see there is already 3x 470uF  (likely those 3x black ceramics in the
> picture next to each other) +100uF  but they are on the main board and not
> electrically close to the modem. If the tracks are not thick (really thick)
> and the caps not close to the modem, they won't do much to help it. Indeed,
> with b-2-b connectors in between there is too much inductance for them to
> help for peaks.
>
> From the picture though , the modem board has its own local ceramic cap and
> the schematics suggest 2 of them.  Really , that should be good enough.
>
> Any chance you can measure the input voltage and / or GSM supply rail with a
> fast meter as you turn on the car?
>
> Hadyn
>
>
> On 3 March 2015 at 20:11, Kevin Wood <EMAIL ADDRESS HIDDEN> wrote:
>>
>>
>> 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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