On 01/03/2011 02:02, Bob Blick 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 the
>> 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, an=
d
>> 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
>
With a capacitor and lower resistor you can use one pin.
When you poll, output on pin for a short while, switch pin to input,=20
then allow time and then read pin.

Low power and one pin.
--=20
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