Rick Hughes coughed up some electrons that declared:

It would be handy to be able to have a detector that can give an audible
signal when it starts to rain.

I often have sports kit outside drying (Scuba, boat etc) and can't rely
on it staying dry long enough to dry kit.


I have found a link to one project
http://www.techlib.com/electronics/raindetectors.htm

Anybody anything better .... the key would seem to be the sensor ... large
enough surface are to catch first few drops of rain, stable enough not to
corrode in a couple of weeks.

Stainless steel would last forever, more or less. When I was a kid, I made a
water level detector for a caravan water tank (Aquaroll) with 5 bits of
stainless rod bent around the pipe-straightener that dipped in the
container. One common and 4 sensor points. Connections were done with
crimped stainless wire, sleeved and run out of the container, then soldered
into a DIN plug. Worked very well, never corroded.

Anyway I digress.

Perhaps get something like (anything will do as long as it's got stainless
rod):

http://cgi.ebay.co.uk/NEW-PATON-CALV...em?refid=store

Cut short strips, solder wires on the ends (Bakers Fluid and silver solder
to tin the stainless worked for me, but that's dated information, may be
other ways).

Lay in a pattern like this:


----------------
================
----------------
================
----------------
================

Connect the --------- together, connect the ============ together, but
separate from the ----------

Epoxy down on a bit of plastic (outside of a waterproof box if you want keep
it self contained, with the electronics inside). Space rods so that a drop
of rain can couple two rods.

If you don't fancy trying to solder the rod, what I did was hacksaw a 1/8"
slot in the end, then use a heavy vice to crimp a bit of wire in the slot.
If you're using copper wire, a bit of solid core from some mains twin+earth
might work, but cover the crimp with epoxy.

Well, that's my theory. Sure there'll be many more equally good ideas. The
above will cost you a few pounds depending if you get lucky at the pound
shop for the stainless and whether you need to buy appropriate solder and
flux.

Cheers

Tim