On Fri, 31 Jan 2003 01:25:58 +0200, Peter L. Peres wrote:
My (probably wrong) calculations show that a 2mm wire in a 10mm i.d.
tube,
20 cm long, should give about 0.5 pF in air and 41 pF in water, with
the
capacitance changing linearly with water level. Neglecting the
influence
of the paint dielectric, which is supposed to be very thin. I used the
plate capacitor formula for this (not the one for cylindrical caps)
so the
numbers are slightly off.

I tried this method in my rain detector, from a circuit published in Circuit Cellar and Elektor sometime ago. Just couldn't get it to work, the 'capacitor was to be the frequency determining component of the astable. I also tried detecting a pulse duration difference between wet and dry with a timer working out the wet threshold.

In the end I've gone back to the astable resistive probe technique and causing an interrupt on RB0.

As a supplementary question, what precautions might work as far as protecting the  PIC from lightning? I've pondered with a opto isolator between the HC4093 output and the Pic pin plus or a MOV on the actual sensor board or some form of back to back Zener Diode arrangement. I don't mind if the opto or the gate disappear in smoke, but the Pic is just that wee bit more expensive.
Any ideas?

Colin


--
cdb, bodgy1@optusnet.com.au on 31/01/2003

I have always been a few Dendrites short of an Axon and believe me it shows.

Light travels faster than sound. That's why some people appear bright until they speak!

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