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'Rain detector = Cow tipping again! [ROT]'
1999\04\17@011151 by paulb

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Clint Sharp wrote:

> if the altitude is lower than the average for any significant length
> of time the cow is either lying down, thus signalling impending rain,
> or dead, in which case you need to go outside and find a new cow,
> therefore enabling you to see if it's raining.

 Cow tipping again!

 Sort of.  I'm not sufficiently far into the "country" to know that
cows lie down in the rain, but last time I was driving in the rain (it
was indeed, driving rain) I thought I saw some standing.  Must look
closer next time.

Peter Grey wrote:

>> Let be a little serious for a moment.

 Now *that's* asking!

> With conductivity it is amazing how long a drop of water will stay on
> 2 electrodes.

 That would depend on how much voltage you applied, wouldn't it?  AC
would remove the drop by dielectric heating even if it is a poor
conductor.

Wagner Lipnharski wrote:

> Just make a resistors bridge, 4 x 560 ohms, three resistors packed in
> epoxy and protected from rain, just one exposed to the environment
> (rain).  All resistors assembled close to each other.  Measuring the
> delta voltage between the two middle points of the bridge would tell
> you if the exposed resistors is dissipating temperature faster, it
> means it is exposed to water.  Ambient temperature also changes the
> voltage, but the other resistors also receive it through the epoxy.

and

> If you want to increase the sensitivity, just put resistor (x) exposed
> too.

 Well, you would certainly want to do that, wouldn't you?  I think the
balance would want to be closer actually - just shield rather than
encapsulate the "control" resistors/ thermistors, and shade all from the
sun.

> The other good point is that the resistor would dry itself in few
> minutes.

 Yes, that really is the clue to this one.

Sean Breheny then wrote:

> Use an IR led and a lens to spread the beam out so it is an inch wide.
...
> Position the emitter/ lens system opposite the detector/ lens system,
> with an inch between them.  Have a large funnel to collect rain and
> channel it between the two lenses.

 This is a bit more like it, but I'd go for a longer path, no funnel
and the transmitter opposite a mirror, with a mirror/ receiver pair at
right angles.  This would detect scatter (a small quantity above zero)
rather than loss (a small quantity less than a large one).  These are
two options on smoke detection too.  The whole thing should be in a
large black short vertical pipe for protection and to minimize daylight.

 Finally, Wagner Lipnharski and William Chops Westfield have suggested
audio detection looking for percussive sounds on a diaphragm (tilted to
allow drainage; the sounds will change a bit when the surface is loaded
with drops).  Must be well clear of trees!

 Those last three really are clever!
--
 Cheers,
       Paul B.

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