Building a Thermistor

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    Mike Richardson

    Does anyone have any plans for building a thermistor?
    I have tried several inexpensive “thermometers” but the reaction time is way to slow. On the flight line I have seen all sorts of fancy expensive units that plot graphs and do everything but “yell thermal”. The one that I did see and like at the last contest appeared to be a simple analog ohm meter (I think it was ohm) with a probe. As for what was hidden behind the curtain I’m not sure.
    I would appreciate any help on building a simple inexpensive unit.

    Thermals to all


    In my copy of Flying North there are several articles and circuits for both thermistors and anemometers developed by Jack North; they first appeared in Free Flight News from the 1970s onwards and the references are;

    Bubble Machines, Temperature Recorders or Both -: FFN, June 1978
    Anemometers for Thermal Detection – FFN, July 1983
    Anemometers Revisited – FFN, June 1988
    Recorders for Thermal Detection – FFN, November 1988
    Twenty Years of Thermal Recording – FFN, January, 1990

    All these articles are in Flying North, along with a mass of plans and articles covering Jack’s model flying life and his career in aerodynamics. Details can be found at:

    John Oian

    There are transistors that vary the output voltage according to the temperature, and are made so that the voltage output when read on a voltmeter will give a direct reading of the temperature. Below is a link to how one is set up and how it works and reads. The example reads as follows 1V = 100 degrees C, .5V = 50 degrees etc. There are some, I believe, that are calibrated to read degrees F. I don’t know how quickly they react to changes, but it might be worth looking into. The set-up looks pretty simple, just a transistor, resistor (apparently to be determined by the voltage of the power source), power source (battery) and volt meter. In the past I ordered a couple of the transistors that read directly in degrees F, but they are somewhere in the shop probably never to be found, along with the address of the source. I suspect that if you contacted an electronics parts supplier, for example, they could probably give you more information on what’s available, and maybe even what the reaction time might be.

    There’s even a test question at the bottom of the page to see if you really understand how the temperature conversion works.

    I hope this helps, John


    Thermistors may well have changed since Jack N orth’s work with them, but he did find that most commercial ones had too slow a response rate to be any good for our purposes. This meant that the glass capsule had to be carefully cracked open to expose the naked thermistor bead. Shrouding the bead from direct sun was of course necessary and the bead was mounted in a foam polystyrene housing to minimise this heating.

    John Oian

    The IC used in the above link is, I believe, a transistor with three legs and not a resistor (thermister) and claimed to be more accurate than a thermister. As far as response speed, I don’t know about that. It appears to come in at least three configurations. The usual looking plastic body transistor (shown), a sealed in metal unit, probably for liquids and likely not very quick to respond in air, and many surface mount versions. I don’t know what is inside the plastic body, or if it can be shaved to increase sensitivity without destroying it, “The sensor circuitry is sealed and not subject to oxidation, etc”. Maybe someone with some electronics know how can chime in on this.


    Roger Morrell

    Mike and John

    If you look on a f1B flight line you will see a number of people with a Dick Wood meter – this has graphic screen display that shows the temperature for about 4 and half minutes. I assume Dick still makes them and sells them . This is a micro controller based device that uses an analog thermistor. Dick solved the problems that you were wondering about with sensitivity and speed. i have used one for about 10 years and they work very well. Dick does not sell a kit.

    You will also see a much smaller device it has a center zero analog meter. This was designed by Vin Morgan in Australia. I believe Vin makes the plans available. This works differently from Dick’s device. It shows the current temperature related to an average ambient temperature. So if the current temp was at ambient the needle points vertically to zero , if the temp is higher than ambient to goes to the plus direction and less it goes to the negative. There is a knob to adjust sensitivity. A number of people such as Bob Piserchio have success with this device.

    I have made a microcontroller based version of Vin’s device and given a couple to friends. Mine uses an industrial temperature sensor that I think was made for automotive purposes, It is fast and accurate, it looks like a transistor but gives the actual temperature directly . I read the temperature and compute a rolling overage over 10 minutes and display the relationship between the current temperature and this rolling average on a screen. I do not sell this device nor do I make it available in kit form. I have plans to combine this approach with that used by Dick but this is not high on my priority list at the moment.

    there is a Johnny Cash song about a guy who steals all the parts for a Cadillac over a number of years and finally assembles it with the aid of a “ay-dept” kid. If you have access to such a kid who is good at STEM subjects at high school them you could build one using an Arduino microcontroller . this is a device made for educational purposes that has ways on connecting screens, IC like John mention and thermistors. essentially this is the way that Dick Wood’s device works. There is quite alot of information available on how to connect all kinds odd device , like thermistors to an Arduino. You do need to be able to the software for the Arduino, not hard for the “ay-dept” kid. The only problem is to find the kid who will help you with the project. I think that way back in Dick’s past he was a software developer but he did talk about working with his grandson on this or a similar project.

    If you look at the older devices such as the jack North as mentioned by Buster it is hard to find the parts as this is out dated technology and many of the parts are obsolete. Also the kind of temperature sensors currently available are much better than in Jack’s day. the data sheets for the sensor gives information about the sensitivity and response rate. Also modern machines like the Wood and Morgan devices can be mounted at eye level so you do not have to look down at the ground when using them so you can keep watching for those other important thermal indicators like other airplanes, birds, moving trees and streamer.



    I started by building a version of Martin Gregorie’s design based on a thermistor:

    It shows you when the temperature goes up and down but you need to keep adjusting it to keep the needle central as the temperature varies over the course of the day. A digital thermometer such as the Starline (is that still available?) is better, but when you go to the pole you need to wait to find out what the average temperature is so that you can accurately identify the changes you are looking for. I now use a remote sensor unit to measure temperature and wind which are displayed graphically on a PDA with selectable temperature, windspeed and time scales. It also computes the moving average temperature and windspeed so you can see immediately whether conditions are favourable or not. If you are into software development building things like this can provide great entertainment!

    Dean McGinnes

    I used a Starline unit for many years until it died. However, on a whim purchased a couple indoor/outdoor thermometers from Harbor Freight for very little…less than ten bucks each. I mounted them side by side with the Starline and discovered to my amazement that though they seldom agreed as to what the temp was, the response times were virtually identical!

    When the Starline croaked, I have used the Harbor Freight ever since. Not as good as a proper wind/temp with comparison to a moving average though.

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