piclist 2002\03\23\120737a >
Thread: sensor interface
face picon face BY : Olin Lathrop email (remove spam text)

> I like the idea of a fixed current source through the sensor, I'll have to
> look more into this. One of the original problems I had was keep the
> through the sensor low enough.
> >Since
> >you have software that can apply corrections, this could be as simple as
> >200 ohm resistor to are regulated 10V supply, for example.
> This will be an automotive project, so I'd like to just use my existing
> power supply, and not have to add another.

If this is automotive, then you must have +12V available.  If you want your
A/D range to be 0-5V, then the current source will need some headroom above
that.  You could filter the 12V supply a little (you will need to do this in
any case because automotive 12V supplies are VERY dirty), and then run a
current source from there to your sensor.  It could be as simple as a
transitor, resistor, and zener, but you aren't going to get 8 bits over
temperature and voltage variation with that.  By the time you create a
current source that is solid to 8 bits, you will end up with a lot of parts.
I think a fixed voltage regulator around 10V with a resistor to your sensor
is going to be easier.  This is definitely a a win if you have more than one
sensor.  You make the nice clean 10V source once, then each new sensor just
needs its own resistor.

I just thought of another idea.  I really like this one.  Should have
thought of it before, duh!  Start by filtering the +12V supply (all
solutions will start with this) with L-R-C.  Now you've got a clean (spikes
removed) but fluctuating voltage.  Assume it will be from 10 to 16 volts.
From there just run the appropriate resistor to each sensor so that the
sensor voltage comes out to 4V or so with max supply and max sensor
resisance.  Now use a simple resistor divider from the filtered 12V to make
the A/D reference voltage for the PIC.  Make sure this is guaranteed to stay
under 5V with max filtered supply voltage.  The fluctuations in the filtered
supply cancel, so there is no need for a regulated voltage or current

Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspam_OUTspamspamembedinc.com, http://www.embedinc.com

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See also: www.piclist.com/techref/io/sensors.htm?key=sensor
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