Blog has some sensors I've not seen on this forum yet - http://www.designspark.com/blog/six-less-ordinary-sensors -
Other rare sensors?
Nice post Rob...
The one that measures a "nuclear event" is a bit like the oil pressure light on your car engine methinks: if it comes on, it's too late.
Lightning detectors are very common in Johannesburg which has really scarily dangerous thunderstorms. Golf courses have sirens to warn players to get indoors if a storm is imminent; and I remember from a few years back when I was a rowing official, Regatta Control always had one and we got all the crews off the water when a storm was getting close.
The lightning one is indeed fascinating, especially if the Arduino could also includes sound input to correlate the signals
=> calculate the distance. Direction might be more difficult at first sight...
The commercially packaged ones have a few leds for distance, kind of green / yellow/ red thing, I forget the distances... 2km and 5km iirc. Storms here are lethal.... and my house is right on one of Joburg's highest spots, about 1800m asl. (Just a little bit higher than in NL I think Rob?)
bHogan has a radiation monitor... http://brohogan.blogspot.com/
carl47's version of the lighting detector... lightning detector - Exhibition - Arduino Forum
Another interesting one from carl47... http://arduino.cc/forum/index.php/topic,53569.0.html
@JimboZA
Must be a magnificent view from your house!
I live in southern part of NL and that is max 10m above sea level (we live in flatland 
@CB
thanks!
Well not as sexy as the others posted here, but I recently bought one of these listed below, finally a proper light sensor, reading in proper units, with good dynamic range, at a very nice price. LDRs wired as voltage dividers are pretty lame to get working well, can have poor and limited range, and their low cost are their main claim to fame, these light sensors should point to a better solution.
http://www.ebay.com/itm/370724372512?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
Lefty
Yet I still can't find a high-speed humidity detector (plenty of low speed ones, but I need one that can detect distinct changes in atmospheric moisture level in less than a second)...
@cr0sh
There are formulas how the speed of sound depends on temperature and humidity,
See - http://www.rane.com/pdf/ranenotes/Enviromental_Effects_on_the_Speed_of_Sound.pdf -
If you take a calibrated ping sensor with a known distance and a good temp sensor you should be able to determine the humidity from that quite fast.
part of the math is here - Ultrasonic sensor to determine water level? - #12 by robtillaart - Sensors - Arduino Forum -
rewrite this formula: (SOS = speed of sound)
float sos = 331.45 * sqrt(1 + t/273) * (100 + RH * 0.11 * power(1.82, ((t-273)/10))); // t in Kelvin
<>
sos / (331.45 * sqrt(1 + t/273)) = 100 + RH * 0.11 * power(1.82, ((t-273)/10))
<>
sos / (331.45 * sqrt(1 + t/273)) - 100 = RH * 0.11 * power(1.82, ((t-273)/10))
<>
RH = (sos / (331.45 * sqrt(1 + t/273)) - 100) / ( 0.11 * power(1.82, (t - 273)/10 ));
if temperature stays constant this becomes
RH = SOS / someMagicNumber;
so speeding up the math will change this to a lookup table
RH = SOS / someMagicNumbers[t];
SOS can be detemined by the time it takes to cross the given distance.
JimboZA:
The one that measures a "nuclear event" is a bit like the oil pressure light on your car engine methinks: if it comes on, it's too late.
That one sounds like fun, but testing it would be a real bear.
That one sounds like fun, but testing it would be a real bear.
Reminds me of a colleague telling me a story about when he was in the Royal Air Force- one of the techs was asked to test a box of fuses. So he hooked them each up to a power supply and a rheostat and made a note of the current at which they popped. Thereby "testing" them....
robtillaart:
@cr0sh
There are formulas how the speed of sound depends on temperature and humidity,
Well - that's definitely very interesting, but I am not sure whether it could work in the application I am thinking of (the paper seemed to indicate long distance measuring, but I didn't read it in detail).
First - realize that this project of mine is no where close to being implemented or worked on, yet - it's all blue sky right now. Basically, I have an old Friendly Robotics mower that I eventually want to get working again. I plan on replacing the border sensing wire system with a hall-effect switch system and permanent magnets placed around perimeter areas (at least, I think this will work). I will probably also replace the entire on-board controller with something custom (and likely Arduino-based, but it may use something else).
What I want to implement, though, is something more intelligent than a random "bump-n-mow" system - something that knows where there is grass to be mowed, and where there isn't (either because there isn't any grass, or because the grass has already been mowed there). I've thought of numerous possible means, some simple, some more complex, some robust, some less so - one or more methods might be necessary to implement. One of the methods I've thought about was to measure the humidity level.
Basically - the idea (which I haven't experimented with yet) is that freshly mowed grass could be distinguished from un-mowed grass by humidity level. It may or may not be a workable idea; it may need to be something used in combination with other methods to make that determination. Ultimately, the mower robot could know when and where there is unmowed grass, and turn on/off the blade motors as needed as it mows, rather than running them continuously (and thus, it should have a longer run-time).
Just thoughts right now, like I said - but I am saving that information you provided in case it might help when I get to it...thanks!
You maybe could use a strong light masked to shine out in front of the mower at just over cut-grass height, detectors on the front of the mower could be set to pick up reflected light from "tall" grass close in front.
You could use a laser and rotating mirror even, might be easier to pick up in daylight.
Can the Hall detect a buried wire with current flowing through it? You could lay a path through the yard and use the Hall to make a line-follower mower.
something that knows where there is grass to be mowed,
colour sensor?