Electron wrote:

> Even the
> original digital odometer may have one, in fact the "peak current of 400u=
A vs
> average current of 9uA" I measured may well be due to a cap in parallel t=
o the
> reed input which gets charged by a high value pull-up resistor (math sugg=
ests
> 330kohm), the pulse closes the reed only for few degrees of wheel rotatio=
n,
> so in the open state the capacitor is charged (albeit relatively slowly) =
by
> the resistor, the input at the microcontroller raises (slowly, but as it'=
s a
> schmitt trigger this doesn't cause problems) and the capacitor opposes a =
low
> impedance to eventual high frequency transients. When the reed closes, th=
e
> capacitor discharge (with relatively high current) and the digital pin go=
es
> low very quickly.

I suggest being careful with the discharge current. I have replaced 100s=20
of reed relays which have been cold welded in the closed position by a=20
small excess current flow (they could be cleared by vibration). I think=20
you need to work out the maximum frequency of the reed opening and=20
closing (i.e. using max speed and wheel diameter) then choose a=20
capacitor with an additional resistor in series with the reed to limit=20
the current flow on closure. This resistor will also form a low-pass=20
filter in conjunction with the capacitor which should help with your=20
noise problem.

George Smith

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.