At 09:02 PM 2/28/2011, you wrote:
>On Mon, 28 Feb 2011 13:07 -0800, "alan smith" wrote:
> > I have a sensor, that when its activated, it goes from a 15ohm to open
> > circuit.  This is a battery powered device, so low current is critical.
> > It will bounce back and forth, as its a motion detector of sorts.
> >
> > The approach I was going to take (and build up to test later) is to put=
 a
> > 330K resistor in series being fed by the box 3.3V power, and connect th=
e
> > A/D of the PIC between the sensor and the resistor.  Normally it will
> > pull some small amount of current and thus a voltage drop to measure, a=
nd
> > on a open circuit if course it should read zero.
> >
> > Any other ideas on measuring for an open circuit with very low power
> > consumption?
>
>Hopefully you have an extra pin to use.
>
>If the sensor is normally low resistance then I would use an output pin
>to energize your 330K resistor and only switch it on for a few
>microseconds right before polling the sensor, then turn it off. No need
>to use an A/D unless your sensor has a variable output.
>
>Cheers,
>
>Bob

Assuming at least 20pF capacitance you might want to go a bit longer
than a few microseconds.. say 3 * RC or > 20usec. Longer if there's a
lot of wire to the sensor or if the sensor has substantial capacitance.

Even at 100usec, and checking every 10msec, you'd have an average current
draw of maximum 100nA (sensor low resistance), which is not very significan=
t
for battery life (self discharge current of, say, a CR2032 is several
times that).


>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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