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Do you really want to use a PIC? I would have thought a TL431 driving
a LED would be more than adequate. Re the mounting - can you re-use
the hole for the existing warning light? (Too hard to get to I'd
guess).

For the current- you could use the existing cable and measure the
voltage drop across it. Embed a temperature sensor somewhere
convenient along its length for temperature compensation if desired.
You can either do the compensation in software or use a "copper
resistor" (choke) with an opamp to get quite close compensation.

e.g the attached. R3 is the cable being measured, R1 sets the gain, L1
is used for its DC resistance. R2 (optional) is about the same DC
resistance as L1 to counter bias current induced offset. If L1 is the
same temperature as R3 and is made of the same material, then the
temperature induced resistance changes cancel and you get a voltage
output proportional to the current trough R3. All you need to do is
provide a voltage supply to the amplifier so that it operates in a
linear fashion, and possibly some level shift circuitry to the next
stage.  You may need a bit of capacitance here and there also to
filter the result and prevent high frequency instability.

I've used this circuit on telecom power supplies with busbars carrying
currents up to about 3000A  and it can work well if you take care with
the details. The choke I used had an inductance of 12mH and a DC
resistance of about 110ohms. I also used a chopper stabilised
amplifier, but the input was only about 3mV for full scale. With a
longer sense wire, a more standard low offset opamp would be fine.

RP

On 23 July 2012 12:55, Carl Denk <cdenk@windstream.net> wrote:
> Thanks, but preferring warning LED, note below, most space for a meter
> 1" x 1" approximately.
>
> Looking for comments on whether to monitor volts or amps, and if amps,
> how to sense, no room for shunt, possible hall effect or ??
>
> On 7/22/2012 7:55 PM, Nily wrote:
>> Follow this link and you will find a lot of meters.
>> http://www.ebay.com/sch/Business-Industrial-/12576/i.html?_kw=3Dbattery+=
indica
>> tor
>>
>> Cheers
>>
>> Rodolfo
>>
>>
>> -----Mensaje original-----
>> De: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]En nombre de
>> Carl Denk
>> Enviado el: Domingo, 22 de Julio de 2012 03:57 p.m.
>> Para: Microcontroller discussion list - Public.
>> Asunto: Re: [EE] [PIC] Automobile low battery warning
>>
>>
>> Thanks for the thought, but I'm trying to keep the interior clean of
>> add-ons, and make it permanent. On the Fords we have the cigarette
>> lighter power is on all the time. If I was going with a display, it
>> would have to be less than 1" square footprint to fit the available
>> space in a convenient eye sight location. Others drive the vehicle that
>> are not as technically orientated as me, so it has to be simple with no
>> action from the driver when starting or stopping a trip, and only react
>> to a warning light with possibly reading the manual (would make an
>> appropriate entry there) or calling home.
>>
>> Another approach would be to tie to the OBDII, request the voltage and
>> display it or set the warning when needed, but that's a lot more work,
>> and more money.
>>
>> On 7/22/2012 6:17 PM, Marcel Duchamp wrote:
>>> A friend bought a gadget at a national auto parts chain store (Kragen o=
r
>>> whatever), that is a cigarette lighter plug with a 2.5 digit LED 7
>>> segment display.  The lighter power is switched so it doesn't run all
>>> the time.  It might do what you want for reading battery system voltage=
..
>>>
>>>
>>> On 7/22/2012 11:10 AM, Carl Denk wrote:
>>>
>>>> I'm thinking of adding a low voltage warning LED.
>> --
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>> View/change your membership options at
>> http://mailman.mit.edu/mailman/listinfo/piclist
>>
>>
>> -----
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>>
> --
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