Building A Rain Alarm?

[BenMR]

Hey all,

I've been asked to build a rain alarm for my mum. She's partially sighted and whilst she does cope very well, sometimes she doesn't quite see the rain in time to bring her washing in which usually results in it having to be partially dried again.

I know absolutely nothing about the weather, but I've come up with 2 ways of doing this that I wanted to get a more informed opinion on:

1: Purchase some form of rain sensor, find a way to connect it to a computer and write a program that is run when the sensor is activated.

Prose:

Should be very accurate since it would be placed in our garden

Prices for sensors seem to be around £26, making it much cheaper than buying a complete station

Size is a bit of an issue in the planned location and the sensors seem quite small

Cons:

It may require a bit more hardware hacking than I've done before; most of the sensors trigger a relay and I have no idea about cheap ways of monitoring relays over USB

The program will need to be able to distinguish between a single drop of rain and more worrying amounts. This *shouldn't* be that much of a problem to code, but it will require a large amount of real life testing before it works well

A small amount of drilling will need to be done to root wires through the window frame since wireless sensors are noticeably more expensive

2: Find a source for weather a few miles away and have the program look at this information to determine if the alarm should be triggered or not.

Prose:

No investment

The program could be run on my server thereby negating the need for a computer to be on 24/7

It should be able to sound the alarm just before it starts raining, the idea being that mum will be able to rescue the washing before it starts

Cons:

Potentially inaccurate

My questions:

How reliable would the second method be - e.g. if I found a station near to me and the station reported that it was raining, how sure could I be that the rain will travel in my direction? Is there any vaguely accurate mathematical way of determining this? Now that I've written it down it sounds quite stupid, but this method would be much easier to implement.

Has anyone done anything like this before? Does anyone have any tips on monitoring relays over USB or anything else to do with the project?

I plan to open source all of the code once it’s done in the hope that other people will be able to benefit from my work, but as I said above, I wouldn't mind a bit of knowledgeable input before I start buying things.

Any pointers at all would be absolutely amazing.

Cheers,

Ben.

[Jane Louise]

Hi Ben ,welcome to Netweather.I'm sure someone will be along soon to answer your question.

[Terminal Moraine]

Well, Ben, I know absolutely nothing about the technical side of what you're doing but I do know a bit about the weather.

In my opinion it would be almost impossible to devise some sort of electronic early warning system for rainfall, the problem being that rain can approach from any direction ( although some directions are more favoured than others )

Rain generally results from either convective showers or frontal zones and, while the latter tend to be rather more predictable with regard to timing of arrival, there are frequent occasions when they slow down, speed up or stall.

Even avid radar watchers can be fooled by any of the above and relying on remote data would require a very dense network of stations encompassing 360 degrees of your site so that the speed of progress of any rainfall, from any direction, could be determined.

Rainfall from showers is even more difficult to predict purely from remote sensing. Not only would you need the dense station network outlined above but you have the added complication of showers spontaneously developing close to home under suitable conditions and 'slipping under the radar' so to speak.

I would think your best chance of an early warning system is to use a rainfall detector which triggers an alarm once a pre-set number of raindrops per second has been reached. I have no idea of the technical problems involved in accomplishing this, and it may result in a few occasions where your mum gets in the washing unnecessarily, or it gets slightly wet before she can get it in but it seems the best way to go.

Edited October 16, 2011 by Terminal Moraine

[suffolkboy_]

A good few years ago now, I remember making a rain sensor for a design project, not to measure the amount of rainfall, but just when it was raining. Very simple to make with the right materials. I will try and best describe it with no pictures.

It was basically a normal circuit, with a battery and buzzer, however, half way along the circuit it had a "switch" which was only "on" when there was moisture on a board. The board was simply a piece of plastic, almost like a cicuit board, with wires running very close in parallel, like this:

--------------------------------------------------------------------------

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--------------------------------------------------------------------------

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This board was placed half way on the circuit, therefore, as the wires were not touching, there was not a complete circuit. As soon as a single drop of rain fell on the board, it would look like this (the O is a drop of rain):

--------------------------------------------------------------------------

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----------------O--------------------------------------------------------

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--------------------------------------------------------------------------

The drop of rain falling between the wires (not shown very well above) would complete the circuit and thus make the buzzer sound.

The wires do need to be very close, I would suggest finding someone who can make cicuit boards ( a geeky friend maybe)

It worked, although it was quite a long time ago.

Hope that helps.