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Sensing and Control for hobbyists and schools: Weather: Misc Non-1-Wire
This page is for non 1-Wire weather sensing. For weather work using 1-Wire, go to this page.
Dalsemi Discussion Archives.
Humidity / Temperature Sensor from Sensiron.
Measuring Snowfall. (Now a separate page of its own. Will open in new tab or window.)
Measuring Sunshine, Cloud Cover.
The discussion group at Dalsemi.
has looked at all sorts of things at one time or another.... and many of those discussions go into things beyond the narrow interests of those using 1-Wire (aka MicroLan) chips. In due course I'll try to write up some of the discussions here. You'll find the following in the archives: humidity, lightning, cloud cover, snow.
A great little non 1-Wire humidity / temperature device is available for Sensirion. It only costs about $15, and they might even send you a free sample if you ask nicely, and are in a position to be able to interface it to something. You connect power and ground wires, of course, plus only two more! One is an input to the device: a clock to control data transfer. The other is bidirectional, being used for serial data to and from the device to send in questions and send out answers. The fact that this device is the size of a couple of grains of rice just makes it that much cooler, don't you think? The user only sees digital data... all of the ADC work is done in the device. The sensor's part number comes from "Sensor, Humidity and Temperature", but perhaps SHT11 is still unfortunate!
Sunshine and cloudcover: Let's face it- the main question anyone wants answered by the weatherman is "Will it be a nice sunny day?" And yet, quantifying "sunny" is not as easy as you might think.
A simple light detector will give you some information... but beware: not all respond to light as the human eye does. A sensor which is fine for detecting a light beam, as in a break-the-beam burglar detector may not "see" colors we notice. Another may be more sensitive to ultraviolet light than we are, and consider a cloudy day as bright as a sunny day, because lots of UV gets through clouds, even though we don't see it.
A simple light cell might report the following as identical conditions: Dusk on a clear evening, noon when there's significant cloud cover. And how do you get a sensor to tell you the difference between a summer sky with a mixture of white and blue, and a boring winter day with no blue, just a thin grey equivalent to the average of the summer day's sky?
Getting the right solution for your needs will involve careful thought about those needs. Someone who is making a controller for electricity generating solar panels will probably want to "read" the sky differently from someone seeking to control the hvac in a greenhouse.
Links to hobbyist information always welcome! (See this to contact me.)
There was an interesting article about sensing lightning in the November 2004 issue of the British magazine Everyday With Practical Electronics ("EPE"). Instead of monitoring for strikes, the circuit looked at the potential difference between the ground (as in terra firma) and a point above the ground. The article described a nice unit you could build which displayed an indication of the voltage, and lit LEDs at various levels. The LEDs were supposed to advise you of the current likelihood of a strike. All well and good. What I found more interesting was the antenna and simple first part of the circuit which detected the voltage. That produced an output in a range of about 1.5 v - near 5v.... which could easily be passed on to a data logger, or to a 1-Wire chip for integration with a bigger 1-Wire system. (See my page about 1-Wire hardware for more details.) The EPE antenna was made as follows:
Start with a piece of quality shielded cable, and start from where you want the electronics to live. Lead the cable outdoors, bury several meters of the cable in the ground. This was recommended as a safety measure. PLEASE NOTE: STICKING WIRES IN THE AIR WHERE THERE MAY BE LIGHTNING WILL ALWAYS BE DANGEROUS. YOU, not me, not EPE, must TAKE RESPONSIBILITY for what YOU do. If you build anything as a result of this document, it is UP TO YOU to assess and manage the risks involved.
Sorry... where was I? Oh yes... the cable... after the buried section, you run the cable up a mast. I would guess that about 2 meters would suffice. It must not be too near trees, etc.
Strip away the shield from about 10cm of the end of the cable, leaving the insulation around the wire that was inside the shield. (The wire OF the shield is removed.)
To the end of the unshielded 10cm, attach, say, 7 stiff wires, each about 7cm long. Fan them out in a cone shape. Sharpening the tips may help.
That's the antenna! I doubt that the measurements are critical. You have, essentially, set up a very efficient lightning rod. Before now, I have made one out of an old rake, which I suspect would be as effective! (You'd still need the screened cable. The rake just replaces the fan of wires.)
Two critical bits:
While the fan of sharpened wires needs to be out in the weather, the length of unshielded wire should be OUT of the weather. A length of PVC pipe would be just the thing. The fan of wires must connect (electrically) to the inner conductor in the shielded cable. They should be insulated from everything else. This will be hard to achieve fully when it rains. Just THINK when you are mounting the mast, and do what you can to reduce paths along which voltage can "leak" between the fan of wires and terra firma. A drip collar or two on the PVC pipe might help, or might be more than is necessary. Consider the design of insulators you have seen for electrical transmission lines.
The other critical bit is as follows: The screen of the screened cable needs a good connection to terra firma. Again, we are in lightning conductor territory. The usual way is to drive a metal rod deep into the ground. (The drier your locale, the deeper it needs to be.) The outer insulation on the screened cable would then be carefully peeled away, to expose the screen, and that firmly attached to the rod in the ground. (Such rods are available from good hardware stores. You are NOT using it quite like it would be used in the case of a lightning rod. In that case, your fan of wires would be connected to the rod. You don't want to do that.)
With that antenna, or similar, and again: I offer thanks and credit to EPE, from which the ideas come, you should see a small potential difference (voltage) between the screen and the conductor in the center. The author of the article monitored that voltage, and USUALLY saw a relationship between what it was doing, and lightning activity in the area. In the article, he presents some graphs I found interesting of patterns he's noticed.
I said the voltage was small a moment ago. It is quite easily boosted to a "sensible" range using an op amp. The article's circuit used one gate from a TL074CN quad jfet... but I strongly suspect that virtually any would do. A couple of variable resistors and a fixed resistor complete the circuit.
If you are attracted to the advanced features in the EPE circuit, EPE have a PCB service. See the EPE website, where you can, I think, also order a copy of the magazine. As they sell back issues, I'm afraid I might be inviting a copyright infringement lawsuit if I were to offer to send you copies of the copy I have, sorry. Their circuit really does do much more than I've covered here, and they estimate to build the thing would only cost about $20.
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