Invisible Man wrote:
Phil Addison wrote:
On Sun, 31 Aug 2008 08:31:30 +0100, in uk.d-i-y "Donwill" popple
@diddle .dot wrote:

"Rick Hughes" wrote in message
...
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.
An optical sensor might be better, what do they fit to modern cars
these days? the auto windscreen wiper seemed to work OK on the
Peugeot 807 I used to have .

Yes, wife has a 206 and its auto wiper works well (provided you can
remember the switch flick-sequence to activate it!). And it's inside, so
doesn't get wet. It detects wetness on the other side of the glass.
Could it use a light beam totally reflected at the glass surface, but
passing through when the surface is wet - or the other way round? It has
a very fast response, generates single sweeps in light rain.

Phil

Our Mazda 3 Sport has an optical system. I assume it works on uneven
spread of light when raindrops fall on the outside of the windscreen
where the sensor is on the inside. Trouble is I often drive it in the
dark on unlit roads and if there are not a lot of oncoming headlights it
does not work well enough. My wife tends to use it in daylight and
reckons the system is brilliant.

Optical rain detection systems typically work by launching IR light into the
windscreen.

The light is confined to the windscreen through total internal reflection (TIR)

When there is water present on the windscreen it no longer undergoes TIR,
but is effectively coupled out of the glass, resulting in a drop in the
received signal.

A simple system, the clever parts are establishing the launch and receive
locations such that the system works reliably without lots of unnecessary
triggering.

On coming light should not really be a problem providing the operating
wavelength is well chosen and the receiver appropriately filtered. I would
also assume the light source is modulated at a few kHz, which is a standard
technique to reduce unwanted background levels.

cheers

David