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The Thermistor Board is Calibrated!!!

Final Calibration.

Final Calibration.

The Thermistor Circuit Board is Calibrated!  I was able to successfully calibrate it at 0 deg C and 70 deg C.   Now is the time to think about what went right, what went wrong and what is the next step, both for this blog and with this project.

What Went Right!

First, what went right.   All of the analysis paid off.  The six channels all calibrated and when I had all six changing temperatures, they all tracked together.  I feel confident this will meet the requirements I set for myself.   The calibration potentiometers.pots, were not so sensitive that when I touched a screwdriver to it the values changed  dramatically and, they were not so insensitive that I developed a sore wrist from cranking on them.   The most right thing I did was the initial calibration using resistors in place of the thermistors to get the settings close.  This made it easier to troubleshoot and adjust the pots without having to fiddle with the ice-water bath or the high temp box.

What I would like to improve upon:

The final version of the circuit.

The final version of the circuit.

The first thing would be to change R6 so it would be impossible to get a zero value there.    It may be that I simply have to put a 1K resistor in series with it and still use the 10K pot to be able to cope with all the tolerances.  Another computer run will quickly determine the best solution.   The second improvement will be to use a pc board design program and either make the board myself or buy one I have designed.   I have already downloaded a program I will use to design the pc board, KiCad. I have yet to use the program.  There was pretty much only two choices in the hobby market Eagle or KiCad.  I chose KiCad because it is open source and available for the big three operating systems.  I will attempt to design, mask, etch and drill some small projects before attempting this circuit again.   (Never have someone else fabricate if you have a chance of doing it yourself!  Laughing at myself.)

The next big thing I would want to improve is the “environmental chamber”, AKA a cooler, a fan, light blub, and a hot pan of water.   The single 60 watt blub was right at the limit of being able to overcome the heat loss out of the cooler.   I was able to obtain 70 deg C. (158 deg F), but it was pushing the limits.  Temperature climbed slowly, but dropped quickly when the light was off.   The fan got noisy near that temperature, and I was probably pushing the limits of the wire insulation temperature rating for both the fan and the light bulb.   This is definitely one of those “Don’t try this at home” things.  I controlled the light bulb by having it plugged into a power strip and used the switch on the strip to turn the lamp off and on as needed.   I kept a fire extinguisher near by in case things got very ugly.  Should I do this again I will look into building a better box and a better method of heating and controlling the temperature.

Calibration involved taking the thermistors from the cold test area to the hot test area several times.  This was necessary because the two adjustments, R6 and R10 did have some effect on each other.  Cold was easy to obtain, it was simply ice water and that just means a whole lot of ice and and just enough water to equal the height of the ice in a container.  Hot was harder to obtain.

Where we are heading next:

There are two things I need to do next.  First, I need enclosures for all the components and especially the power supply.  The power supply was purchased as a kit when I bought the Arduino.  It was inexpensive but needs an enclosure and at that time I will install a switch, a fuse and binding posts or terminals.   This is an opportunity to try to learn a little more about metal working.   I have done some sheet metal work with duct work, but this will be much more precise.   As with everything, nothing to it but to do it.  (Yeah sure.)   The thermistor board and Arduino will also require enclosures, although I may see if I can put both of those in one common enclosure.

The next issue after that will be coming up with a way to have relatively long distance communications to the Arduino.  I want the communications so that the circuit board and Arduino can be used to measure temperatures outside the house, and communicate with the computer inside the house.   WIFI may be an answer, but wired communications such as RS485 may also be an answer.

Along with the communications issue will be some programming, both in the Arduino and in the computer.   I will have to write a ring buffer routine in the Arduino to store several A/D readings should the computer go down for some reason.   I am still thinking on that one.  The computer program will also take care of converting the A/D raw values into actual temperature.  When I do that part I will take care of both degrees C and degrees F because that is a simple conversion.

So… in general there is a lot to look forward to in the near future.   This weekend I will finish the crane to the point to do the final test on the boom.   I have decided to test it to destruction.   I will have my steel toe shoes on!

In theory we are just about done with Capacitors and Inductors.  I have a few more op-amp circuits to describe and then we will probably move into an area I call mech-elect.  That is electrical devices that cause motion or take motion and make electricity.  I hope to work though it like I have done with the past theory.  That is somewhat more technical than most hobby theory but not into complete math overload of engineering theory.   There is some time before we go there, so if any of you have any opinions of where to go, please e-mail me.

Meanwhile, I think I will pop the top on a beer and do a minor celebration.


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