T&L Publications

I'm looking for a sensor to detect rain outside a window. What I'm thinking of should be reliable (no falses due to wind, bugs, temperature changes, etc.), have some sensitivity adjustability, and be relatively simple (self-built is okay). Also, it should not involve the use of vacuum tubes ( ) I'd like to get as many varied ideas on this as possible. Whew! Thank you all!

Well the simplest, is a piece of copper clad veroboard or a PCB that you are willing to etch into some parallel straight lines with the, connect a number of the lines together.<p>This becomes a resistive or capacitive sensor, depends how you drive it.<p>When the water bridges the two parallel lots of copper a short circuit is formed, or continuity if you prefer. Two trailing leads then go to your warning circuit. However, to stop nasties happening to your copper electrodes with the water and everything, you need to send an AC pulse to the sensor. A 555 timer or a couple of logic gates as an oscillator will do the trick.<p>An alternative sensor which will need lots of experimentation (in other words I could never get it to work satisfactorily) is to get some co-ax cable, shape it into a U shape. The two ends of the cable DO NOT come into contact with the rain, the co-ax exhibits capacitance between the screen and the core about 30pF in my experiments, water being a dialetric will change the capacitance of the cable which can be the C in the oscillator of above. The frequency will shift as the co-ax gets wet. For this to work the rain needs to build up so the co-ax sensor would need to be in the ground or in a pot or similar.<p>You could use a plain peizo sounder as a sensor and have a circuit that senses the vibrations, just need an op-amp to amplify the low voltage output. Again i could never get this to work reliably.<p>There is ultrasonics, or from Qantum they have a capacitive sense chip which can be used as a moisture detector.<p>I have a number of rain detector circuits that I built to go with my lawn sprinkler project, sadly N&V didn't want to publish them.<p>I can email you some photo's of probes and the circuit if you wish. I used the stainless steel dough hook from a kitchen mixer as the probe.<p>Colin

The 555 moisture sensor is the simplest and easiest to build. <p>I have the plans if you need them.

bodgy, this is a question, repeat question,<p>How does water on the outside of the coax change the dialetric constant of the insulation between center conductor and shield on the inside?<p>If coax was driven balanced (neither center conductor nor shield grounded, or reversed (center conductor grounded and shield driven); then I could see water on the outside increasing the capacitance to ground because of the increased conductive surface area.<p>And I can see that putting it in the ground would help, because rain water is nearly an insulator. Once its in the ground, salts and minerals are disolved, making it more conductive.<p>In any case, I think increasing the frequency would help a coax sensor.<p>??????????????????????????

Dale,<p>I can understand your puzzlement as basically I was typing bollocks - I just had coax on my brain this morning.<p>What I meant to say kiddies was insulated wire, two equal lengths, sealed at the furthest ends and the nearest ends go to the sensing circuit.<p>It works as rain water is not pure, and, as it is a poor insulator, as the cable gets covered the capacitance of the cable will change and hence the frequency will alter if used as the C in an oscillator circuit.<p>M. Vacher published some stuff about this effect in 1998.<p>Colin

TA

I think you just need a square of poster cardboard normally held up by a spring, but held down by a small piece of TP. If it rains the TP gets wet and weak and breaks, allowing the cardboard sign to pop up outside the window. Mark the words "IT'S RAINING OUTSIDE" on the cardboard. Just under that in small print have the words "If you don't believe me look for yourself". <p>No batteries required! Uses no vacuum tubes!<p>[ July 16, 2004: Message edited by: Bob Scott ]</p>

I assume you mean a non-contact sensor suitable for use turning on the windshield wipers. I think it has something to do with IR light and how light reflects differently off wet and dry glass.<p>see auto.howstuffworks.com/wiper4.htm and the associated PDF link on the same page.<p>TRW makes a sensor but it looks more complicated than required due to the serial comm link used to interface to the car's controller.<p>Looks like an IR emitter LED and an IR detector biased and connected to a comparitor (for sensitivity adjustment) is basically what it is.<p>Any kind of direct contact sensor you make would suffer from wind blowing the water off or dirt build up requiring constant maintenence.

Thanks for the replies!

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Bob Scott:
I think you just need a square of poster cardboard normally held up by a spring, but held down by a small piece of TP. If it rains the TP gets wet and weak and breaks, allowing the cardboard sign to pop up outside the window. Mark the words "IT'S RAINING OUTSIDE" on the cardboard. Just under that in small print have the words "If you don't believe me look for yourself".
Give a thumbs-up for para-science."The simpler the better"