Ugh, that title… Let me explain. I have a Davis Instruments 6250 Vantage Vue wireless weather station. It really is a lovely instrument: solar powered, wireless, with all the sensors integrated into a single sleek package. It is small, affordable, and accurate enough to be published on sites like wunderground.com (the Weather Underground).
Like most weather stations, the temperature sensor for this one is housed in a solar radiation shield. This shield consists of a stack of plastic plates designed such that the sun can never shine directly on the sensor, nor can precipitation reach it, but the breeze can freely blow through the plates and cool the whole thing to ambient temperatures. This results in a more accurate temperature reading.
However, I live in the South West desert, where it’s hot. If the breezes fail, the inside of the radiation shield can become baked and the temperature reading can be 8 – 12 degrees higher than the actual ambient temperature. That can turn an actual 108° hot day into an apparent 118° record breaker. Since, as a weather geek, I want to publish accurate weather data, it’s a problem.
The solution is a device called an aspirator. Really that’s just a small fan designed to pull outside air through the radiation shield (it never blows onto the temperature sensor itself) and keep it cooled to the outside ambient temperature. Some professional weather stations have an aspirator option, but Davis doesn't offer one for the Vantage Vue, so this Instructable describes how I built my own.
While the design I’m presenting is specific to the Vantage Vue, the idea of a solar powered aspirator fan is applicable to any weather station’s radiation shield, and this design is adaptable.
Step 1: The Vantage Vue Solar Radiation Shield
Here’s an illustration of the radiation shield and how it attaches to the weather station. As you can see, it’s just a stack of 5 plastic plates held on by two screws. The top 3 plates have hollow centers that house the temperature sensor, and the bottom two are solid. On the Vantage Vue the radiation shield is quite small – there is barely room enough for the sensor, much less an aspirator fan. I knew whatever I built would have to be tiny.
Step 2: Goals
I set myself several goals for the project.
- Solar powered. The whole weather station is solar powered and wirelessly linked, so I didn’t want to run a power line up to an aspirator fan.
- Minimally invasive. I wanted to make as few changes to the Vantage Vue as possible, preferably none.
- Removable/reversible. If the whole thing turned out to be a bad idea, I wanted to be able to remove the aspirator and still have a functional radiation shield for the weather station.
- Small, cheap, and reliable.
Step 3: The Aspirator Parts
- Micro-motor (x1), 1 – 3 volts. I used the tail motor from a broken (crashed it too many times) Syma S107G RC helicopter.
- Micro-fan blade (x1). Also from the crashed helicopter. If you google “Syma s107g tail motor” you will find that there are many sellers of both the motor and fan blade, for from $4 – 8.
- Coke can (x1). Source of sheet aluminum. I’m sure a Bud can would do just as well.
- Solar cells (x2) ~1 volt in full sun and unloaded. I used the solar cells from a toy solar race car. I’m sure the cells from a couple of cheap solar garden lights would work as well.
- #6 (as many as required) galvanized steel washers (approximately 0.38” O.D., 0.15” I.D.). Fiber or plastic washers would probably be better.
Step 4: Tools
- Sturdy scissors
- Soldering iron and solder
- Hand-held single hole paper punch (~1/4 inch hole)
- Drill and ¼ inch bit
- Blue painter’s tape
- Fast set epoxy glue
- Liquid electrical tape
- Fine point Sharpie
- Straight edge/ruler
- White outdoor spray paint
Step 5: Build
First test your motor with your solar cells. Connect the motor to each of the solar cells one at a time and make sure the motor runs in full sun (I found that a bright LED desk light shining directly on the solar cell generated almost as much power as the sun, greatly simplifying the testing). There are two solar cells in this project, but because they are positioned on opposite sides of the radiation shield, the motor will have to run on just a single cell. It should spin fast enough that you can feel it vibrate gently in your fingers.
Using the scissors, cut up your aluminum can. Cut off the top and bottom to make a cylinder, then slit that up the side to make a sheet. Flatten out the sheet (I used a rolling pin on the carpet for that). You could use something already flat, like pie pan; however, a Coke can is factory coated to resist a remarkably corrosive environment – Coke! I figured a little bit of sun and weather could hardly hurt it compared to that stuff.
On a sheet of card stock (not your aluminum sheet), layout a tee-shaped pattern similar to the one shown. I recommend dry-fitting all your parts, following my pattern, on the card stock to give yourself plenty of opportunity to mess up in a non-threatening paper environment. Go so far as to cut the paper out, punch the holes, and use blue tape to secure the solar cells, so you’re sure everything will fit correctly. When you’re done, the paper cutout becomes your template for the actual aluminum cutout.
Some of the illustration’s dimensions are exact, but others will vary depending on the size of your solar cells. The A and B holes must be positioned exactly. Hole A (yes, I know there is an unintentional pun here, but I’m not going there!) will hold your motor and is centered in the contraption. Most of the rest of the dimensions are laid out relative to the A hole (damn, I just went there).
The B holes fit over existing tabs in the radiation shield’s plates, so their positioning relative to hole A has to be precise. These firmly anchor the contraption inside the shield.
The C holes allow you to run the solar cell wires though the aluminum sheet to the back of the motor at hole A. Positioned correctly for your cells, no part of their wires should be exposed above the aluminum sheet.
The ~0.75 dimension is the width of a solar cell. Use whatever your actual solar cell dimension is.
The ~5.0 dimension has to be long enough to accommodate your solar cells and everything else, but not so long that the cells wires won’t reach the center where the motor is.
Once you've perfected your paper template, trace it out on the aluminum sheet – a fine point Sharpie works well for this. Then cut it out with your scissors. Dry fit everything again on the aluminum sheet to make sure it’s all good.
Spray paint the aluminum bit white. While this might not be technically required, it gives the whole thing a professional air; not like something cut out of a Coke can. Allow to fully dry overnight.
Following package directions, mix a generous amount of epoxy glue. You want to get every bit of the back of the solar cells covered with epoxy so there are no gaps that moisture can get into. The solar cells are going to be in the weather and if any moisture gets behind them, they are going to corrode and fail. So slather the back of one solar cell with epoxy. Run the cell’s wires though the C hole and stick the cell down on the aluminum. There should be enough epoxy that it squeezes out from all sides of the cell. Use blue tape to hold it in place while the epoxy cures. Repeat on the other side for the second cell.
Securing the motor in the central hole can be challenging. The motor is fundamentally a vertical cylinder, while the sheet aluminum is a very thin and flat horizontal surface. I found that these #6 steel washers just about exactly fit the diameter of my motor, while still being wider than the ¼ inch hole in the aluminum. Again I used the epoxy glue to secure the motor in the washer, with the washer as close to the top of the motor as possible. Then, when it was cured, I dropped the motor through hole A and glued down the washer to the aluminum. You should do something similarly clever to secure your motor.
Step 6: Wiring
Strip the solar cell wires, and connect them together in parallel (positive to positive, negative to negative). Don’t trim or solder yet, just twist them together. Strip the wires on the motor and twist them onto to solar cells’ connections. In my case the polarity was positive to positive, but it may be the opposite for you, so again, don’t solder yet.
Take the contraption outside (or use a bright LED desk light) to test. The motor should spin strongly when either solar cell is lit.
The photo shows the wiring completed, soldered, and coated with liquid electrical tape. Don't do that yet.
Step 7: The Fan/Propeller
I used the prop (fan) blade that came with my motor, so I would have no problem attaching it to the shaft. If that’s not the case for you, then you might have to widen the hole in the prop, or perhaps glue the shaft in.
A problem I did have was that the fan, at 1.2 inches, was too long for the space inside the radiation shield. I had to use the scissors to nip it down to 0.87 inches. Be sure that both sides of the fan are the same length, otherwise the misbalance will cause wear on the motor bearings. I also nipped off the corners.
When attaching the fan to the shaft, place the fan on a flat surface and push the motor down onto it. The motor shaft is quite thin and needle like; if you hold the fan in you finders and push it down, you are quite likely to stab your thumb and bleed… a lot. I'm just saying...
Once the fan is on the shaft, test it again. The prop should spin rapidly and generate a noticeable downward draft. The prop should not blow in the direction of the temperature sensor. Instead it blows down, away from the sensor, allowing ambient air to be drawn gently into the radiation shield above. If your prop is blowing the wrong way, reverse the polarity of the connection to the solar cells.
Now you can trim excess length from all wires, and twist and solder them together. Test again to make sure all is well. If everything is okay, paint the solder joints with the liquid electrical tape. Let dry and paint a second coat.
Step 8: Performance
I was rather impressed by the vigor of the breeze coming from my little contraption, so I decided to do some measurements. I have an optical tachometer, so I put a bit of reflective tape on one fan blade and measured the RPMs. I also measured current and voltage with a multimeter. Using an LED desk lamp to the illuminate one of the two solar cells, these are my results:
- Voltage: 0.89v
- Current: 40.1mA
- Speed: 10,880 RPM
I’m sorry, what? Over 10 thousand revolutions per minute?! High end computer disk drives spin at 10 thousand RPM. I would have been happy with a thousand RPM, but getting ten times that was startling. By moving the LED light away from the solar cell, I found the motor would slow and finally stall at about 400 RPM.
Step 9: Assembly
The contraption goes above the two solid plates, and below the first hollow plate. This puts it as low as possible within the temperature sensor’s space. Small as my contraption was, I found there was still not enough space for it, so I had to make the following modifications to the radiation shield.
I drilled a ¼" hole in the solid plate directly below the motor. The butt of the motor slides into this hole. This was the only permanent modification I made to the radiation shield, but it made me sad to do it. I consoled myself with the fact that the hole could be patched with a tiny bit of tape, with no harm done.
I found that when mounted to the weather station, the fan blade was actually contacting the temperature sensor, preventing it from spinning. I added a couple of the #6 washers between each of the hollow plates to move everything down just slightly. I think only about a tenth of an inch extra space was required.
Step 10: Does It Work?
Yes, yes it does. Above is a YouTube video (http://youtu.be/cc_JDsjsW4Y) and a photo showing the aspirator fan pulling the smoke from in incense stick down and through the solar shield assembly.
Step 11: Operation
Once assembled and mounted to the weather station, you should be able to hear the fan whirring merrily away inside the radiation shield. It’s a high pitched happy sound. If it’s a calm sunny day, temperature reported by your station will probably also drop by several degrees.
Because of the way the the Vantage Vue weather station is mounted to its pole, the morning sun will illuminate one solar cell, and the evening sun will illuminate the other. At noon the station itself will shade both solar cells, but the temperature sensor and radiation shield are also in shade at that time, so it shouldn't matter much.
The design presented here is actually about my third iteration of this contraption. I've found that the solar cells and motor are surprisingly robust, and both have operated for more than a couple of years.