piclist 2001\04\26\061106a >
Thread: Pin impedance
picon face BY : Peter L. Peres email (remove spam text)

What's your limiting resistor when discharging ? It should be fairly large
(current under 1mA). The curve is not linear with time but you already
know that. The intersect point with the threshold voltage will not change
linearly with R, because of the shape of the curve. The RC timeconstant
values are only valid for 10%-90% standard step measurements. If you want
a really linear input wire a NPN as a Miller integrator with your
(smaller) cap and use that with your present circuit. I don't think that
$0.01 for a BC547B will bankrupt your project. ;-) You can use RBPU for
pullup. I'd recommend the double slope approach using a Schmitt Trigger
input pin...


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