Today’s class went much better than last Friday’s.

I took the advice one of the students gave me last we and started with a “do now” question.  (She had actually suggested an “exit ticket”, but I don’t have the time management skills to leave a block of time at the end of class.)  The question I asked was a design task that was easy if you knew what you were doing, but subtly harder than the standard sort of text-book question, because it was a design question, not an analysis question:

You have sensor whose resistance varies from 1kΩ to 4kΩ with the property it measures.  Design a circuit whose output voltage varies from 1v (at 1kΩ) to 2v (at 4kΩ).

I gave the students 10 minutes to work on this at the beginning of class.  A good question to prompt discussion (according to the peer instruction blogs and websites) should be answerable by 30–80% of the students.  More than that and the question was too easy to be useful, and less than that and the question is too hard for peer discussion to be worthwhile.  It turned out that no one had gotten it after 10 minutes (too hard to use as a peer instruction question), so we used it as the basis for a class discussion.

Almost everyone realized that the desired circuit was a voltage source and a voltage divider (not too surprising, since that’s the only circuit they’ve used so far).  The majority also realized that the variable resistor had to be on the lower leg, between the output and ground, and a couple of the students could articulate why.  I suggested the common heuristic of trying extreme values (0 and ∞) for the variable resistor, to see whether the output voltage would go up or down as the resistance changed.

The students were then able to set up the simultaneous equations to solve for the input voltage and the fixed resistance.  The hole in everyone’s thinking when working on the problem initially is that they had not considered the voltage of the source as a design parameter to solve for, though one student had asked about it. This was the blind spot I was expecting, so I was able to use it as a teachable moment.  After we had the equations set up using mainly student input, I gave the students another minute or two to solve them, and about half the class was able to solve them correctly in the time provided. (I suspect that everyone could have if given enough time, but I didn’t want to take any more time in class—those who didn’t solve it in class could practice their algebra at home if they needed to.)  One student had made an algebra or arithmetic mistake, and gotten a source voltage smaller than one of the desired output voltages.  This was also a good mistake to get, since we could use it to talk about sanity checks on results.

I think that the 20 or so minutes of class was well spent, as we uncovered several important misconceptions, and raised awareness of all unspecified variables as potential design parameters, reasoning using extreme values, and the usefulness of sanity checks.

After that, we spent some time discussing different temperature sensors.  From the students, I got thermistor, infrared thermometer, mercury thermometer+camera, and enzyme + other sensor (pH, conductivity, color, …). I added RTD, silicon band-gap, and thermocouple to the mix.  We talked a little about the advantages and disadvantages of each. At the end, I also threw in bimetallic strips and tilt switches for one-bit digitization of temperature.  I wonder how many students will look at the thermostats in their apartments and try to figure out what sensor they include.

For the remainder of the class, we talked about gnuplot commands, particularly the “plot” command.

After class, several of us went over to the lab, where my son met us and helped the students install the DataLoger, python, pyserial, the Arduino environment, and gnuplot.  While he was doing that, I borrowed an Uno R3 Arduino board and made sure that all the computers in the labs had the drivers installed for it.  We had 2 installation failures: on one Windows laptop, my son was unable to get the serial ports to work and one Mac laptop couldn’t install gnuplot.

The problem with the gnuplot installation on that Mac was not solvable by the techniques in the comments for Installing gnuplot—a nightmare, because all the methods there assume that you can install the command-line tools “make” and “gcc”.  The Mac had 10.6.8 installed, but the student had never bothered to install the development tools and had lost the original CD-ROM with the Xcode tools on it.  The Apple Developer site does not provide the command-line tools for anything older than 10.7.3.  The only workaround we could find was to download the 4.1GByte complete Xcode suite for OS 10.6.8, which I was not willing to wait around for. (Other students with 10.6.8 had no trouble installing gnuplot, because they already had the command-line tools, though they’d never used them.)

I did not look at the problem on the Windows machine (the student had to leave for class before I became available), but I don’t know that I could have done anything—my son knows more about Windows than I do, so if he was stuck, I probably would have been also.

Next year I’m going to want to do an install session before the first lab.  (Or, if we go to 2 labs a week, as the first lab.)

On Wednesday, I’ll start with another “do now” question, though I’m not sure what it’ll be on, since I’ve not yet gotten to the material for this week’s lab: how a microphone works. I’ll do a tiny bit of gnuplot (just the “fit” command) and try to get through how a microphone works and an idealized i-vs-v plot for the FET output of the microphone.

Today’s class went much better than last Friday’s.

I took the advice one of the students gave me last we and started with a “do now” question.  (She had actually suggested an “exit ticket”, but I don’t have the time management skills to leave a block of time at the end of class.)  The question I asked was a design task that was easy if you knew what you were doing, but subtly harder than the standard sort of text-book question, because it was a design question, not an analysis question:

You have sensor whose resistance varies from 1kΩ to 4kΩ with the property it measures.  Design a circuit whose output voltage varies from 1v (at 1kΩ) to 2v (at 4kΩ).

I gave the students 10 minutes to work on this at the beginning of class.  A good question to prompt discussion (according to the peer instruction blogs and websites) should be answerable by 30–80% of the students.  More than that and the question was too easy to be useful, and less than that and the question is too hard for peer discussion to be worthwhile.  It turned out that no one had gotten it after 10 minutes (too hard to use as a peer instruction question), so we used it as the basis for a class discussion.

Almost everyone realized that the desired circuit was a voltage source and a voltage divider (not too surprising, since that’s the only circuit they’ve used so far).  The majority also realized that the variable resistor had to be on the lower leg, between the output and ground, and a couple of the students could articulate why.  I suggested the common heuristic of trying extreme values (0 and ∞) for the variable resistor, to see whether the output voltage would go up or down as the resistance changed.

The students were then able to set up the simultaneous equations to solve for the input voltage and the fixed resistance.  The hole in everyone’s thinking when working on the problem initially is that they had not considered the voltage of the source as a design parameter to solve for, though one student had asked about it. This was the blind spot I was expecting, so I was able to use it as a teachable moment.  After we had the equations set up using mainly student input, I gave the students another minute or two to solve them, and about half the class was able to solve them correctly in the time provided. (I suspect that everyone could have if given enough time, but I didn’t want to take any more time in class—those who didn’t solve it in class could practice their algebra at home if they needed to.)  One student had made an algebra or arithmetic mistake, and gotten a source voltage smaller than one of the desired output voltages.  This was also a good mistake to get, since we could use it to talk about sanity checks on results.

I think that the 20 or so minutes of class was well spent, as we uncovered several important misconceptions, and raised awareness of all unspecified variables as potential design parameters, reasoning using extreme values, and the usefulness of sanity checks.

After that, we spent some time discussing different temperature sensors.  From the students, I got thermistor, infrared thermometer, mercury thermometer+camera, and enzyme + other sensor (pH, conductivity, color, …). I added RTD, silicon band-gap, and thermocouple to the mix.  We talked a little about the advantages and disadvantages of each. At the end, I also threw in bimetallic strips and tilt switches for one-bit digitization of temperature.  I wonder how many students will look at the thermostats in their apartments and try to figure out what sensor they include.

For the remainder of the class, we talked about gnuplot commands, particularly the “plot” command.

After class, several of us went over to the lab, where my son met us and helped the students install the DataLoger, python, pyserial, the Arduino environment, and gnuplot.  While he was doing that, I borrowed an Uno R3 Arduino board and made sure that all the computers in the labs had the drivers installed for it.  We had 2 installation failures: on one Windows laptop, my son was unable to get the serial ports to work and one Mac laptop couldn’t install gnuplot.

The problem with the gnuplot installation on that Mac was not solvable by the techniques in the comments for Installing gnuplot—a nightmare, because all the methods there assume that you can install the command-line tools “make” and “gcc”.  The Mac had 10.6.8 installed, but the student had never bothered to install the development tools and had lost the original CD-ROM with the Xcode tools on it.  The Apple Developer site does not provide the command-line tools for anything older than 10.7.3.  The only workaround we could find was to download the 4.1GByte complete Xcode suite for OS 10.6.8, which I was not willing to wait around for. (Other students with 10.6.8 had no trouble installing gnuplot, because they already had the command-line tools, though they’d never used them.)

I did not look at the problem on the Windows machine (the student had to leave for class before I became available), but I don’t know that I could have done anything—my son knows more about Windows than I do, so if he was stuck, I probably would have been also.

Next year I’m going to want to do an install session before the first lab.  (Or, if we go to 2 labs a week, as the first lab.)

On Wednesday, I’ll start with another “do now” question, though I’m not sure what it’ll be on, since I’ve not yet gotten to the material for this week’s lab: how a microphone works. I’ll do a tiny bit of gnuplot (just the “fit” command) and try to get through how a microphone works and an idealized i-vs-v plot for the FET output of the microphone.

In Temperature lab, part2, I carefully measured the resistance vs. temperature curve for the Vishay BC Components NTCLE413E2103F520L thermistor, finding that either my thermometer was badly miscalibrated, or the manufacturer’s data sheet was misleading.  I got a resistance of 9.7kΩ (not 10kΩ±1%) at 25° C and a B-value of 3174°K, not 3435°K±1%.  A big part of the discrepancy is that I calibrated over a different temperature range, and the measured values deviated most from the manufacturer’s spec at low temperatures.

Actually, the manufacturer’s data sheet is not as bad as all that.  They give specs for the ratio of the resistance at various temperatures to the resistance at 25° C, and their numbers do not fall along a simple curve.  The report the B-value as a 2-point fit for T=25° C and T=85° C, but they also report resistance values every 5° C from –40° C to 105° C.  One can compute the B-value for any pair:

But even if I use their calibration data down to 0° C, I don’t get as small a B-value as I get from my measurements. My calibration curve does not fit their spec, even if I look at the table, rather than the simple B-value model (though the table is closer to my measurements, it doesn’t match).

How big a difference would if make if I used their curve rather than mine?  At R=6685Ω, we get the same temperature either way (96.446° F).  At the resistance they claim for 0° C (27348Ω), their B-value model would give 33.90° F and mine would give 29.37° F.  At the resistance I measured for 0° C (25.5kΩ), their model would give 36.67° F, while mine gives 32.32° F (which is closer than the measurement error I had on temperatures).  So it looks like relying blindly on their B-value model introduces an error of over 4° F at low temperature, but in a digital thermometer for human body temperature, one would get nearly the same result with either calibration.

Voltage divider

The second part of the lab was to use the calibration curve to design a circuit to convert the resistance variation into a voltage variation that is nearly linear with temperature, at least over a small range.  The simplest circuit to convert a resistance to a voltage is a voltage divider, which just requires a voltage source and another resistor.

A simple voltage divider for converting the thermistor resistance to a voltage. Because I want the output voltage to increase with temperature, but the resistance R(T) decreases with temperature, I put the thermistor on the top branch and fixed resistance on the lower branch.
Circuit drawn with the Circuit Lab editor.

The formula for the output voltage is simple: , where we are approximating for temperature T in Kelvin. We’d like the voltage to be as linear as possible, which means we want the second derivative of Vout with respect to T to be zero. Obviously, we can’t do that for all values of T, but we might be able to do it for a particular value of T, and to have a small second derivative in that neighborhood.

We can take the derivatives by hand (or use a tool like Maple or Mathematica):


The second derivative is 0 if , that is if .

We can use this formula to set the value for any temperature, for example, if we want linearity around 98.6° F (310.15° K), we can set the series resistor to 4323.64Ω.

Expected voltage curve using a voltage divider with series resistor optimized for 98.6° F. Note that the curve is fairly linear from about 70° F to about 130° F (where the red non-linear curve and the green linear approximation at 98.6° F are quite close.

Of course, commercially available resistors don’t come in values like 4323.64Ω, so in doing a design one has to pick an available resistor, using the EIA table of standard resistor values.  A 4.7kΩ resistor looks pretty close, which would be color code yellow-violet-red.  Too bad that I don’t have one.  I do have a 5.1kΩ resistor, which would be optimal for linearizing at 90.06° F.  We should, of course, ask students to do the resistor selection for a different temperature than the one we use in an example, and they should be using their own B-value.

Testing the voltage divider

I set up the voltage divider on a breadboard.  Because I knew my wall wart had a huge ripple, I added a low-pass RC filter consisting of a 100Ω resistor and a 470µF electrolytic capacitor.  This slightly complicates the analysis of the voltage output, since the Vdd voltage is itself dependent on a voltage divider.

Low-pass filter to clean up the output of the wall wart. With the low-pass filter in place, we can model the power supply as a 5.166V source and 100Ω series resistor.

Here is what the low-pass filter looks like on the breadboard. The red and black wires from the bottom come from the connector to the wall wart. The 100Ω resistor runs vertically up the center, and the electrolytic capacitor connects across to the ground. The red clip lead is from the multimeter and is set up for measuring the voltage at the output of the filter.

Here is the full breadboard, showing the RC filter, the series resistor, the leads to the thermistor (the red and yellow lines with the crimp-on connectors), and the clip leads to the multimeter (the red and black alligator clips). I’ve found it very handy to have a number of double-sided header pins to make easy connection points on the breadboard for clip leads.

Here is the setup used for testing. The thermistor and thermometer are in the ceramic-cup water bath, with the thermometer held to keep the bulb in contact with the thermistor. You can see that the least-significant digit of the LCD display is a bit hard to read—applying some pressure to the display often makes it readable. Clip leads are essential to doing this experiment—holding the multimeter probes to test points would be a major hassle.

I only made 21 measurements of voltage, since I was not going to be fitting a model to the data, but just using the model I fit from the resistance measurements.

Plot of measured and theoretical voltage vs. temperature for the thermistor. The non-linear theoretical curve seems to be a pretty good fit (though it was not fit on this data, but on the previous series of resistance measurements).

The range of the voltages with the series resistor (from about 0.85v at freezing to 4.15v at boiling) is fairly reasonable for direct conversion to digital for the Arduino 10-bit ADC. In the middle of the range, the slope is about 0.0233 V/°F, which would give a resolution of about 0.2°F for Arduino readings.  (Of course, with the Arduino, we would not need the low-pass filter, and Vdd would be a well-regulated 5v, so this calibration curve would have to be redone.)  Even if we use the computational power of the Arduino to correct the non-linearity of the voltage curve, it is still useful to select the series resistor for the temperature we are most interested in, since that point gets the largest slope and hence the highest temperature resolution for fixed-size steps in voltage quantization.

Series-parallel

A lot of thermistor circuits on the web have a resistor in parallel with the thermistor, as well as the one in series. I wondered what the effect of this extra resistor was.

Circuit with resistor in parallel with the thermistor, as well as in series.

I tried analyzing this circuit also, using the same brute-force approach of computing the second derivative of the voltage and setting it to zero.  I got a result that surprised me initially:  the second derivative is zero if , where is the resistance one would get for putting the series and parallel resistors in parallel. This is exactly the same condition as before, but with replacing the series resistor .

After I thought about this for a while, I realized that I should have been able to get there directly. If you think of the circuit as connecting the thermistor to a voltage divider consisting of R2 and R1, then you can replace the ground and that voltage divider by the Thévenin equivalent, which would be a voltage source with voltage and a series resistor consisting of  R1 and R2 in parallel: .  The only reason to put in a parallel resistor would be to restrict the voltage range (which might be useful if the output were to be amplified, but is not useful if the output is going directly into an analog-to-digital converter whose full-scale range is 0 to Vdd).

The thermistors that I ordered (see More musings on circuits course: temperature lab and Buying parts for circuits course) arrived today, about 2.1 days after I ordered them.  So I’ll try playing with them today, and see whether I can do the lab I’m thinking of for the students.

My son is still working on the Arduino data logger.  The Arduino code has been done for a while, but he’s been working on the Python front end.  He’s decided to do two front ends: a minimal one with no GUI and a fancy one using PyGUI.  He’s run into some problems (like PyGUI not supporting autoscrolling in TextEditor components), but everything seems to be solvable at this point.  He had a GUI interface working, but decided he needed to refactor his code to have a proper API for the datalogger, so that the user interface and the datalogger core code were as independent as possible with a documented interface between them—he’s teaching himself a lot about software engineering for this project, which is the first one for which he has had an external client.

The action plan for testing out a temperature measurement lab that I posted in More musings on circuits course: temperature lab was

• Get some thermistors and some thermometer probe sheaths and see if I can make adequate temporary waterproofing for pennies per student.  I’ll probably have to solder on wires to lengthen the leads.
• Try calibrating thermistors using a multimeter, cups of hot and cold water, and an accurate thermometer.
• Try reading the thermistor using a voltage divider and the Arduino ADC.  Plot the temperature and Arduino reading over a wide temperature range (say, as a cup of boiling water cools).
• Try linearizing the thermistor readings  using a parallel resistor and voltage divider.
• Try designing an amplifier to read the thermistor with much lower current through it (and so less self-heating).

I’ve got three types of thermistors (all 10kΩ nominal resistance at 25° C), none of which are intended for immersion:

• Vishay BC Components NTCLE100E3103JB0 a very cheap (23.5¢ each in quantities of 10) with B-value 3977°K).  There is high variation in the resistance (±5%), but low variation in the B-value (±0.75%).  These are glass-bead thermistors with 2cm uninsulated leads, so will need waterproofing. I bought 10 of these, but am hoping that I don’t need them.
• Vishay BC Components NTCLE413E2103F520L (34.9¢ each in quantities of 10) has 4cm leads and is epoxy coated, but with the warning “Not intended for fluid immersed applications or continuous contact with water.”  It has B-value 3435°K, both resistance and B-value ±1%.  I plan to find out today if it is waterproof enough for the relatively short duration of the labs. I bought 10 of these also, and have the highest hopes for these being the ones we use in the lab.
• Murata Electronics North America NXFT15XH103FA2B100 (66¢ each in quantities of 10) with B-value 3431°K, both resistance and B-value ±1%. The 9.5cm leads make these likely to be the easiest to use with thermometer probe covers, but they are more expensive.  I bought 4 @ 87¢, so as to keep the cost per thermistor type below $3.50. Note: the specs give different B-values depending which pair of temperatures used—I’ll have to look to see if they have specs for higher-order models of the resistance as a function of temperature.

The setup at 110°F, showing 4.60kΩ as the resistance.

I’ll try the epoxy-coated ones with the 4cm leads first. The first step is calibrating one with an ohmmeter. I used clip leads to connect a cheap multimeter to the thermistor, then dunked the thermistor in a glass of warm water with a thermometer (note: we’ll need to get some thermometers for this lab). The thermometer I used is a pasteurizing thermometer that I’ve had for years—it is, unfortunately, calibrated in Fahrenheit, not Celsius. Of course, we ideally want temperature in Kelvin.

I put a table of the measurements on a separate page, to avoid cluttering up the post with a table of 28 measurements.

I don’t expect many of the students will have the patience to make 28 measurements, but if we provide some different hot water sources (water boiled in a teakettle and ice water in a thermos), they should be able to make 10 measurements across a wide range of temperatures. I stopped at 28 measurements, because the alligator clip broke the wire and I didn’t feel like stripping more of the insulation and reconnecting.

I tried to fit the Steinhart-Hart equation to the data, where R is the resistance in Ω and T is the temperature in degrees Kelvin. But gnuplot got as good a fit using just A and B as using the third order fit, with providing an excellent fit.

Fitting the first two terms of the Steinhart-Hart model for thermistor behavior.

Simple Arrhenius fit for the data. When I fit the equation , which is the equation most often used in thermistor specs, I get A=0.24Ω and B=3157°K, which is not that close to the spec (10kΩ @ 25°C, B=3435±35°K, so A=0.0992Ω). Note that the spec curve fits the data quite well for warm temperatures, but deviates badly for cold ones.

There could be systematic problems with either my temperature readings or my resistance readings. The multimeter is more suspect than the thermometer, but the readings would have to be off by 500Ω to get that sort of error in the B-value. The numbers look particularly bad around 110°F. I think I need to get some new batteries for my other other cheap multimeter (which I think is a bit better) and see if it gives more reasonable results.

Wait a minute! I have an old Fluke 8060A multimeter (between 26 and 30 years old) that has a bit of a wonky LCD display, and a blown fuse on the 10A input, but is otherwise still probably my most accurate meter. If its batteries aren’t dead, I may be able to use it to get better measurements.

I tried measuring some known resistors, to see if the meters are way off. I tried an 11.8kΩ ±1% resistor: Fluke meter says 11.77Ω and the suspect multimeter says 11.67kΩ, while for a 732Ω±1% resistor, the Fluke meter says 731.1Ω, while the suspect meter says 718Ω. So the suspect meter appears to be reading 1–2% low, while the Fluke meter is within the accuracy of the resistors. So I boiled up some more water and tried another series of readings (the 63 readings are in a table on a separate page to avoid clutter here.)

Note that there are three runs of data: cooling down from boiling, cooling down from hot tap water, and cooling with ice from room temperature. I found it easiest to do this work on the counter in the bathroom, where I could easily adjust the temperature by pouring out some water and adding more hot or cold water. I was also very careful to make sure that the bulb of the thermometer was in contact with the thermistor while making the measurement, so that they were as nearly the same temperature as I could make them.

Let’s see what sort of fit we can get with this data.

The second data set, with 63 data points, is much cleaner than the first data set, and is well fit by a simple 2-parameter model (leaving out the third term of the Steinhart-Hart equation. Because it is hard to read the thermometer to better than 0.5°F, the fit is really quite good.

The data once again does not match the specified B-value of 3435°K±1%, but B=3174°K, though the curves are quite close for warmer temperatures.

The data sheet reports the 3435°K B-value as a B25/85 measurement, that is, it is based on values at 25°C (77°F) and 85°C (185°F).  If we just fit over that range, we can get the B-value up to 3236°K, which is still a long way short of the specified B-value.

Since I now trust that the resistance measurements are fairly good, the large error in the B-value has to be either from the thermometer or the thermistor. Because I don’t have a more accurate thermometer, and I’m not inclined to get one, I’m a bit stuck at this point in determining whether the thermistor meets the spec or not.

Doing a large number of measurements like this took far too long for the first part of a lab.  A lot of the time was spent waiting for the temperature of the water bath to change (or trying to make change by adding water or ice). We’ll probably have to ask for just 5 or 6 values, perhaps providing them with 5 or 6 water baths in thermoses that they can run their thermistors through one after another, to get the measurements rapidly. We will need some decent thermometers, capable of reading to ±0.2°C, if they aren’t too expensive.

Tomorrow I’ll try doing the electronics part of the lab, adding series and parallel resistors and measuring the voltage with the Arduino.

I’m going to need some parts to play with for the circuits course.  While I probably could get the parts I need at work from the Baskin Engineering Lab Support (BELS) staff, it would probably involve a bit of hassle, as there isn’t even a course number for the course yet, and so they’d have a difficult time figuring out which department to charge for the 30¢ and 60¢ parts—the cost in staff time (and my time) would be ridiculous.  So I decided to buy my own parts with my own money from Digikey.  My experience with them in the past is that in-stock parts generally get delivered by US mail within 2 days of ordering (it helps that they are not far away).

One experience we don’t give the students (at least, not until the senior design project) is trying to figure out what parts to buy.  It can be an overwhelming task—DigiKey has in stock 10,917 chips that come up in response to a search for op amps. We can reduce that to 1,249 if we restrict ourselves to through-hole rather than surface-mount parts. Adding a request for rail-to-rail outputs reduces that number to 314.  Sorting by price and looking through those under $1 shows many from Microchip Technology, each with slightly different specs.  I don’t see any way that a student in a beginning circuits class could make sense of most of the specs.

Thermistors are almost as bad, as there are a lot of specs for them also, and the price range is huge.  You have to know that you want NTC (negative thermal coefficient) devices, which gets you down to 1,369 thermistor types.  Eliminating surface mount parts reduces the number to 479, ranging in price from 23.5¢ each to $20 each.

I’ve decided to play with three different thermistors:

• Vishay BC Components NTCLE100E3103JB0 a very cheap (23.5¢ each in quantities of 10) 10kΩ thermistor with B-value 3977K).  There is high variation in the resistance (±5%), but low variation in the B-value (±0.75%).  These are glass-bead thermistors with short leads and will need some sort of waterproofing for the labs.
• The epoxy-coated Vishay BC Components NTCLE413E2103F520L has 50mm leads and is epoxy coated, but with the warning “Not intended for fluid immersed applications or continuous contact with water.”  It is a 10kΩ thermistor with B-value 3435K, both ±1%, and costs 34.9¢ each in quantities of 10. It may be waterproof enough for the relatively short duration of the labs, and the 2″ leads may make it easier to use with disposable thermometer covers from the drugstore.
• Murata Electronics North America NXFT15XH103FA2B100 a 10kΩ thermistor with B-value 3431K, with low resistance variation (±1%) and moderate B-value variation (±1%).  Note: the specs give different B-values depending which pair of temperatures used—I’ll have to look to see if they have specs for higher-order models of the resistance as a function of temperature. Although these thermistors cost more (66¢ each in quantities of 10), they have 100mm insulated, flexible leads, which should be long enough that we use these in a coffee-cup water bath, though they come with the warning not to use them in wet or humid locations, nor “Places with salt water, oils, chemical liquids or organic solvents”.  The long flexible leads may make this one the easiest to use with disposable thermometer covers.

It looks like I’d have to go to $2–$4 per part for thermistor probes with a brass, copper, or plastic sheath, and even then the manufacturers don’t say that they are waterproof.

I also decided to get myself some op-amp chips to play with, since we will certainly be assigning some op-amp labs. Because I don’t have a bench power supply, I want to use a single power supply, like a 5v wall wart (or the 5v supply for the Arduino).  I also want a DIP package, so that I can use the op amp on a breadboard.  I looked for cheap op amps on Digikey, and the best choice I found was a Microchip Technology MCP6002-I/P (33¢ each in quantities of 10) for a dual op amp with rail-to-rail output.  It is a bit slow (1MHz gain-bandwidth product, 0.6V/µsec slew rate), but has a low input bias current (1pA) and will run on a single power supply anywhere from 1.8V to 6V, so should be easy to use with batteries or the Arduino power supply.

I will have to be careful not to blow up the chips with my function generator, though, as I believe it has a 10V peak-to-peak output.  Maybe I won’t have to worry about it—I just tried to turn on my function generator to check the output voltage, and it won’t turn on.  The fuse looks ok, but I don’t even get a power-on light, much less any signal at the output.  I don’t know whether I want to try debugging it or not, given that I don’t even have a manual for it, much less a schematic.  It worked fine the last time I turned it on, so I’ve no idea what the problem is.  I suppose I should have expected it, buying cheap, old equipment on e-bay, but I didn’t expect the function generator to fail after I had checked that it was working.  It will be harder for me to develop a lab that uses a function generator, though, if I don’t have a functional one to test the lab with.  I wonder whether it is better to try to fix the one I have or get another one.

The students will have Agilent 33120A Function/Arbitrary Waveform Generators to work with, which are very nice instruments, but out of my price range ($1300 refurbished on e-bay, $2200 MSRP).  I could get a cheap Victor VC2002 for about $130, which is about as good a price as the used stuff on e-bay.

I’ve decided to do a lot of my musing about the course design on my blog, so that others could contribute to the course design (or at least participate vicariously).  I think that having an open log of our thinking on the course design will be useful to grad students or new faculty who are having to do their own course designs, to show how us old farts do it (whether they wish to copy our methods or avoid using them).

The relevant posts so far are

Changing teaching plans
More on electronics course design
Yet another project idea
Another way to think about course design

I’ve got my son working on Arduino software to act as a data logger, so that students can have an easy-to-use tool that requires no programming, but which is easily modified if they want to do their own programming.  He has Arduino code for alpha testing already, and I think he’ll have a user interface ready for beta testing by the end of the week, but he’ll only have tested it on a Mac—we’ll need to get him access to a Windows machine to do testing there, because there are some operating system dependencies in talking to USB devices like the Arduino from a Python program. He’s been consulting with me on desired specs for the data logger, but doing all the coding himself. It is good practice for him both in interrupt handling and in user-interface design. He’s also having to think about portability for the first time, as the data logger has to run on Windows and Mac OS X (I think that Linux users should have no trouble with the version that works on Mac OS X, but we probably won’t be testing it). He’ll have to write user documentation and installation instructions also. Some of the packages he likes to use (like PyGUI) are enough of a pain to install that he’ll probably provide a stripped-down interface without a GUI for those who want to do minimal installation (Arduino software, Python, and PySerial are only essentials).

The first lab I’ve been thinking about is for temperature measurements. For the small temperature ranges normally needed in biosensor applications, the sensitivity, low thermal mass, and low cost of thermistors make them probably the best approach, and I think that they are what are used in the cheap digital oral thermometers sold in drug stores.  I wonder what techniques the manufacturers use to get medically acceptable calibration for those devices.

I’ve been thinking about the thermistor lab—it seems like we could do that in stages: first just reading resistance with a meter, then adding a series resistor to make a voltage output, then adding a parallel resistor to try to linearize the exponential curve a bit, finally adding larger series resistors and an amplifier to avoid self-heating currents through the thermistor and to allow nearly the full range of the ADC to be used.  This series of lab exercises could be spread out over the quarter, as students learn more about circuits.

For them to calibrate the thermistor, we could use hot and cold water baths, if the thermistor was in a waterproof package. From what I can see, that raises the price of a thermistor with leads from about 25¢ (for something like the NTCLE100E3333JB0 by Vishay BC Components) to $1.55 (for USP10982 from US Sensor) or $3 (for USP10973RA from US Sensor).  [Prices from DigiKey 10-unit price.]

I think that having the students do their own waterproofing is probably not a good idea.  Potting components gets to be a mess, and adding a large blob around the thermistor will slow its response time a lot.  I wonder whether using 5¢ disposable thermometer probe covers would work, or whether they tear too easily.  I probably need to look at some at the drug store, to see whether there is thicker one on the market that the cheap thermistors would fit into.

If waterproofing a cheap thermistor turns out to be too difficult, we need to think about whether to use the more expensive parts or work out some cheap, measurable temperature sources that are not wet.  We could make something like is used in PCR machines, with a couple of blocks of aluminum and a Peltier device, but I don’t think that the price is worth it—better to use the sort-of waterproof probes, a cup of water, and a thermometer.

I’ve noticed that for some applications, people are choosing voltage-output temperature sensors that rely on the thermal
coefficient of a transistor, rather than on a thermistor, like the MCP9700-E/TO from Microchip Technology (25 cents).  They have a fairly linear 10mv/degree C output, but their absolute accuracy is even worse than thermistors. These may be a better choice than thermistors in many applications, but would not provide the same teaching opportunities for a circuits class.

Using a thin-film platinum resistor temperature sensor (RTD) like the US Sensor PPG102C1RD would allow more accurate temperature measurement without calibration. With calibration, RTDs can be the most precise electronic temperature sensors, though I don’t know if the high precision is available in thin-film resistors, or only in the more expensive wire-wound ones. I suspect that repeatability from part to part is higher in the wire-wound RTDs, but that the thermal coefficients are the same, so that calibrating at one temperature should give about equal accuracy either way.

The naturally linear response of RTDs (100Ω at 0°C and 138.5Ω at 100°C in a very straight line) does not lend itself to as much circuit design as thermistors. On second thought, converting the 3850 ppm/°C resistance change into a voltage range readable by the Arduino ADC is not a bad circuit exercise, particularly if self-heating is to be avoided, though it is not as difficult as flattening the highly nonlinear response of a thermistor.  The biggest problem with RTDs is their price: at over $10 for a bare sensor they may be too expensive for class use.

Another possible temperature sensor is a thermocouple, which generates a voltage based on the temperature difference of two electrically connected junctions of dissimilar metals. One article in the engineering toolbox claims that thermocouples are cheap and RTDs expensive, and I think that is true if you are looking for high-temperature devices (like thermocouples for detecting pilot lights in furnaces), but not so true if you are looking for careful measurement in corrosive wet materials (like most biosensing applications). See Choosing and using a temperature sensor for more info comparing RTDs and thermocouples. Thermocouples have relatively low precision and sensitivity, and they measure only the difference in temperature between two points, and so are probably not very interesting for biosensing.

Action plan for testing out a temperature measurement lab:

• Get some thermistors and some thermometer probe sheaths and see if I can make adequate temporary waterproofing for pennies per student.  I’ll probably have to solder on wires to lengthen the leads.
• Try calibrating thermistors using a multimeter, cups of hot and cold water, and an accurate thermometer.
• Try reading the thermistor using a voltage divider and the Arduino ADC.  Plot the temperature and Arduino reading over a wide temperature range (say, as a cup of boiling water cools).
• Try linearizing the thermistor readings  using a parallel resistor and voltage divider.
• Try designing an amplifier to read the thermistor with much lower current through it (and so less self-heating).