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Latest solar trough configuration:

A few details:

1. Those are just simple, band-sawn parabolic arches. The mirror is attached with velcro. Works awesome. Ly.
2. I suspended the mirror from the pipe thinking that would keep the focal length constant while allowing simples changes of elevation. Why do I always go straight for the complicatedest solution in the universe?

And data from same:

The highest temp on there is almost 140°C. That's over 280°F. According to my rough calculations, the four steepest upward slopes indicate powers in the range of 20-25 watts. (Some of those dropouts are me messing around with the setup, some are cloud cover. Also, I later discovered that true solar south is like 15 or 20° east of where I've been pointing.)

Two big changes from the last run, other than the already-mentioned one of shortening the excess pipe.

1. Painted the pipe flat black.
2. Used a laser pointer to adjust the focus. I mounted a frosted glass square (I happen to have a bunch I bought for just such a use as this) at what I thought would be the focal length. Then I stood back and aimed the laser pointer in a roughly perpendicular way and looked where the point fell. This really needs a System to keep it perpendicular, but anyway I was able to determine that my focus was off by over an inch.

I'm thinking the Mark III will be the last iteration. Pointing in the right direction, and simplifying the pointing a little, may let me add another 10-20 degrees to the peak but that will be about as far as I can go with these simple materials and using an open-air design.

Belated Mirror Squeezer Results

I did one "live" run of data collection from the parabolic mirror squeezer, but after I did it I realized there was a major problem. The pipe was much longer than the mirror width, so it overhung. Too late, I realized that this meant there was unheated oil in there, making the temperature measurement invalid. And I don't even know what direction the error was worse in, since the overhang could also act as cooling fins. Anyway, here's the graph of the results:

It looks very similar to the original run in a hotbox:

One major difference: Check the x-axis. The parabola run is at least 4x steeper.

Anyway, if I get some sun this weekend I should be able to re-run with more accurate results.

Things That Don't Work vs Things That Do

I've never been completely happy with my temperature logger. It's a bit fragile in the sense that if anything at all goes wrong, and there's no way to tell that at the time, I lose the entire run. I lost a run Memorial Day weekend.

Instead of storing the measurements on the Arduino, I'd like to instantly beam them onto my computer far inside the safety of the house. That way I can track things realtime as well as be assured that I have them. Coincidentally, for my birthday, I got both another Arduino¹ and Making Things Talk.

The book describes a great number of schemes to make microcontrollers talk to each other and to computers. You can use wireless networking, bluetooth, XBee, etc (I have only the vaguest notion what some of these are). Naturally the easiest protocols require the most expensive hardware. I only need one way, slow communication, so I got a simple RF module.

Claim:

It works just like a serial port! Just connect the transmitter to the TX pin and the receiver to the RX pin! It Just Works(tm)!

Reality:

No.

Perhaps my unit was faulty. I found many tutorials and guides across the internets and while results varies, I can't ever really say it worked. I did see data appear for a short time, but mainly what I saw was noise. Or nothing at all, which is even less explicable.

Last night I had a brainwave. Or brainstorm. Something happened to my brain and it resulted in an idea. Why not use a wireless laptop as the go between? The kids have these OLPC dealies. The laptop has a USB port and does WiFi. I have a WiFi router (specifically purchased, used, to work with these laptops). About 30 minutes and 10 lines of Python later, I was reading values from /dev/ttyUSB0 and sending them out over a socket to my desktop to another 15 minutes and 20 lines of Python.

The guts of the entire scheme are already there. But with so much success so fast, I'd like to add features. For instance, instant graphing of values on both ends. A protocol so that the laptop knows if there's been an error and can tell me, out in the field. When I have more than one sensor, I'll need a way to indicate which sensor had what value. It'll be like a complete Science Sensing Station!

¹If you are at all interested in robots, sensors, controlling stuff with computers, electronics or just plain messing around, I highly recommend the Arduino. That SparkFun item is all you need, assuming you have a USB port (and possibly a cable). Well....you may also need some external electronics, depending on what you want to do. LEDs, resistors, motors, etc.

The Heated Mouse

Not only do I work in the basement of my building and not only are there lots of computers (and thus lots of AC) but one of the AC vents is right above my desk. Oh and of course I sit still all day. So I get cold hands. More specifically, I get a cold mouse hand.

The obvious¹ solution to this problem is a heated mouse. Google can tell you this isn't a new idea but google can't tell you where to actually buy one. The only place that sold them is now dead and in any case seems to have been a scam. People didn't get their shipments or they did but the contents either failed to work as heaters or failed to work as mice.

But this can't be complicated. USB is a powered connection, meaning that a mouse can get power from the computer. If it can draw enough, it can run a tiny heater. As a matter of fact, the USB spec says that a device can draw up to 500mA at 5V, which is 2.5 watts. That's not a lot of heat, but if you were holding it literally in the palm of your hand I thought it might be enough to ward off pneumonia.

USB optical mouse and resistance wire from hair dryer.

Entire inside of optical mouse, USB cable unplugged from board. Red/black are power, don't mess with the other wires (data).

(I really should have photographed this entire build, but I was feeling my way through and didn't want to waste time with a bunch of dead ends.)

The first thing was to get a USB optical mouse. USB for the power, optical to have plenty of space inside. I had one lying around. Then I needed a way to turn electricity to heat. Resistance wire out of a trashed hair dryer turned out to be perfect. Around 3" of wire (coiled length), coated with liquid insulation, provides ~20 ohms of resistance. That draws about 250mA, providing around 125mW of heat. (I stayed well below the spec just in case, plus the mouse itself needs 100mA, according to the label.)

The rest was just a bunch of boring details. How to keep the heater wire inside the mouse. How to attach the power to the coil. How to get the mouse back together. Thinking about safety. Like, am I going to burn down the building? I don't think so...

Does it work? I've got it plugged in right now and it's working great. Works fine as a mouse (didn't even have to reboot, even though I switched from PS/2 to USB) and as a heater (I switched because my hand was cold and now I'm comfy).

I'd say the only problem is that the heat is applied more towards the palm when it needs to be more towards the fingers. It works as-is, but maybe I could uncoil the resistance wire and spread it out a little more around the inside of the mouse to hit all the areas.

¹To a dork.

Acrylic Mirror Failures Learning Opportunities

The acrylic mirror samples arrived. I got 3 mirrors about the size of a credit card, but thicker, like a piece of glass.

OBJECTIVE: Create a "trough mirror" that focuses onto a line.

KEY FACT #1: Acrylic mirror softens and bends at around 230-250°F.

KEY FACT #2: A parabola is a mathematically perfect focusing shape, but (a small section of) a circle is plenty fine for my needs.

FIRST ATTEMPT:

I got a big, glass jar with smooth sides. I put it lying down inside the oven. I balanced the mirror on top, shiny side pointing down (and with the protective plastic sheet still on it). I had to tape the mirror in place.

Starting at around 215°F, I slowly heated the oven up until the mirror ends started drooping. After probably an hour of watching it slooooooowly bend, I just reached in with an oven mitt and press-formed it to the glass.

RESULT: Meh. The shape is great and the focusing is accurate but the mirror got all foggy. It looks more like polished metal now. Also the spots where the tape was touching are distorted.

SECOND ATTEMPT:

My theory was that I should have the mirror pointing up, which might make the cloudiness go away and would make using tape unnecessary.

This time I used a regular soup pan tipped on its side. I put the mirror inside so it could form to the inside curve of the pan. Left the protective plastic on. Set the oven to 250°F (that's what I had worked up to from the first attempt) and waited. And waited. And waited.

Once again, I eventually just reached in and press formed it. When I removed the sheet....still cloudy.

THIRD ATTEMPT:

I watched the video again. Oh, I'm supposed to remove the protective plastic. Replay the second attempt, but this time remove the thing first.

After waiting the requisite Long Time, I could see the mirror was already foggy even before I press forming it. Aha! Not the plastic or the form!

WHAT'S PROBABLY GOING ON:

I have small samples, so gravity isn't enough to bend them until they are very soft from being in the oven for 45 minutes or more. In that amount of time, some chemical degradation (or something) is causing the cloudiness. If I rig up some way to put a weight on top of the sample, maybe I could speed that up. Or I could just reach in there earlier and do it by hand.

But now I'm out of samples. I can bend and rebend the cloudy ones just to test out some weighting system and/or get my timing right. But I'll only be able to check if the cloudiness disappears if I buy more mirror. Which I can do, but I hate the shipping charges. Ah well.

The Temperature of What?

So I had all these posts about temperature logging and one post with the actual logged temperatures...but the temperature of what?

A solar hot box!

You probably already know what this is, but just in case you don't: It's basically a tiny greenhouse. Or like a car left in the sun at noon in August. Only it's even hotter, since it is insulated, painted black and pointed right at the sun.

Inside the box I put a jar with 250 ml of cooking oil and poked a hole in the lid for a temperature probe. That's what these temps are.

Why did I choose cooking oil? Because I didn't want evaporation to be a problem. For one thing, it would fog up the inside of the glass. For another, it would cap my max temperature at 100°C (not that that turned out to be a problem in this case). And lastly, it would change the amount of water in the bottle and I needed that to be a constant because I did some calculations with it.

Knowing the amount of oil and the temperature change (plus looking up the specific heat of vegetable oil), I can calculate the rate at which energy is entering the oil. For the above graph, I got 2.5 watts for the steepest part of the curve. However, I see that the site I just linked to has the specific heat of veg oil as 1.67 kJ/kg K and I was using 2.5. So maybe the power is really more like 3.7 watts.

Knowing the area of the collector I can also calculate the amount of power falling into the box. That's about 75 watts. So the end-to-end efficiency was only about 3-5%. Not that great.

Imagine if you put a cup of water on the table and then turn the furnace thermostat up to 90°. How much energy are you going to waste before the water gets hot? This illustrates the 3 main problems:

1. Air passively surrounding a container of liquid isn't going to heat it very fast.
2. There's a lot of volume of air being heated uselessly.
3. During all this time, heat is escaping the cracks, windows, chimney, etc. In the case of the hot box, the glass front gets very hot and is radiating a lot of the energy right back out.

If a hot box is like an oven, the next version will be like a microwave. Don't heat up the air, just beam energy right into the substance.

Temperature Logging Part III: The Exciting Conclusion

Some pics of the final state: You might notice an extra button. I decided to write to EEPROM rather than an array after all. The reason was an unreliable power situation. A 9V battery lasts for a few hours and I didn't want to lose hours of data at the last second. So we have a switch for power on/off, a button for "send data" and a button for "start collecting data". The last one is I can control when the EEPROM starts getting overwritten the next time I turn it on.

That relatively minor change forced a paradigm shift in what processing was done where. Originally, I was just reading the voltage ratio from the pin and sending it to the PC that did the work to convert to degrees. The pin reports a value in 10 bits (i.e. 0-1023) but the EEPROM stores only bytes (i.e. 0-255). I could have renormalized to the smaller range, but only at the loss of a lot of precision. Instead, I now calculate the temp right on the Arduino. Surprisingly (to me, anyway), you can use the regular math library functions like log(). So then I end up with a temperature that practically speaking won't get above 255 or below 0, and even if it did I could add or subtract a constant to recenter it to my working range. I will describe what temperature I was logging in another post, but for now here's the pretty graph.

Temperature Logging Part II

OK, so I had the temperature (sorry if this reads like a novel--I'm writing this one after the fact), but I already had that with a kitchen thermometer. The whole point of this is data collection. From the docs and examples, I think most people are using the Arduino attached to a computer, which makes logging a simple matter of reading the serial port and writing to the hard drive. But the temperatures I'm going to be logging will be located far away from a USB port.

My initial plan was to write the values to the "EEPROM" (no, me neither), but:

• It only supports 512 bytes. At one sample every two minutes, that's only 4 hours.
• It only supports a limited number of writes. Large, but limited.
• I don't really need the ability to save the values through a power cycle.

Instead I went with an array in RAM. I just have to be sure to get the data before I turn it off or the battery runs out.

And how do I get the data off? I have a button that tells the thing to dump the data to the serial port. This is really a simple feature, but I learned a lot while doing it. That's because, even as an electronics n00b, my analog electronics sk1llz leave my digital ones in the dust.

Here's the schematic for a digital switch. Notice that it doesn't really work like an analog switch. That's because you aren't sending electricity around a digital circuit like with a simple flashlight, you are sending voltage around the circuit. Or so this one experience indicated to me. D2 is the digital port. When S is open, as shown, D2 is HIGH (i.e. 5V) with respect to ground. If I close S, current flows (which is why R exists--to limit the current, which means R should be as high as feasibly possible, at least so I infer), but more to the point D2 and ground are electrically connected, meaning it is now LOW (0V).

Question: I think I could have had D2 at LOW by default and switched to HIGH. Would that save power? Ideally, a voltage doesn't have a current, but I don't know how the voltage comparison happens internally.

Anyway, this circuit and the previous one can be put in parallel. The thermistor one reads temps until the buffer array is full and the switch one waits for someone to press it and when someone does, it spews data.

All that remained was to make it small and robust enough to survive an afternoon outside, with light breezes, sunshine, etc. So I soldered the components onto a board (seriosly, this project is like 1/4 of all the soldering I've ever done--I'm a total, total N))B) and slapped it into a tupperware container.

Temperature Logging Part I

There have been many times when I've wanted to log temperatures. No useful reasons, I just like collecting data. The problem is that temperature logging devices are like $50 and up and of course they only do that one thing. After the orgy of temperature data collection is done, it just sits on the shelf.

But I'd been hearing about this Arduino dealie, which is an open-source microcontroller that only costs about $35. I figured that I could put a thermistor on there and make my own logger, which would not only be cheaper but I'd also have a versatile device for other projects. I've never used a microcontroller before, plus electronics above the level of a flashlight circuit confuses me, so it's been an adventure. Fortunately the Arduino is supereasy.

For that matter, the electronics of this were prettyeasy. The hardest part was doing the math to convert a thermistor reading into a temperature one. I harvested a thermistor from a broken (except for the thermistor) electronic thermometer. (It was easy to identify, since it was sticking way up away from the rest of the circuit board and and had "therm" written next to it.)

A thermistor varies its resistance based on temperature. That means that if you apply a voltage, you'll get a varying amount of current out. But the Arduino input port measures voltage level, which means you need to turn a varying resistance into a varying voltage instead. That's easy with a voltage divider.

"Th" is the thermistor, and A1 is the steak sauce Arduino analog input port. The total voltage drop across both resistors is a constant 5V, but how much drops across each resistor depends on the ratio of their values. So when the thermistor changes, the voltage at A1 also changes.

Now you just need a way to convert a voltage reading to a temperature reading. I assumed it would be a linear relationship, but it isn't, it's exponential. When you buy a thermistor, apparently they give you two numbers. A baseline resistance (at 25°C,) and a number B. Apparently you are supposed to know this equation:

R_T = R_Tze^{B(1/T - 1/Tz)}

Oh sure, THAT equation! R_T is the resistance of the thermistor, which you can figure out based on the read voltage and known values of the circuit. R_Tz is the resistance at 25°C (or whatever baseline), which is Tz. T is the temperature you want and B is the magic number.

I didn't get my thermistor from any fancy-shmancy store so I didn't have the magic number. Instead, I used the thermistor to measure hot and cold water (calibrated by a kitchen thermometer). Plugging in those readings I was able to solve for B. Just using math got me within 2 or 3 degrees (over the range 0-100°C--I haven't tested outside that range). Then a little empirical fudging got me right on the money.