Now that I have a power resistor and heatsink, and have verified that my power supply is capable of delivering 50W, I can try making a thermal control system for an incubator box as I hope to get the freshman design class to do.

Before building a complete control system and tuning a proportional, PI, or PID controller, I decided to check each of the components:

• 1.8Ω resistor and heatsink (already characterized in still air in the previous post). Initially I was going to use the 8.2Ω resistor, but it heated so slowly once bolted to the heatsink that I wasn’t sure that students would have the patience to wait for it—they might conclude that things weren’t working.
• NTD4858N-35G nFET for PWM control of the heater.
• fan.  I bought a SanAce 40 109P0412P3H013 fan with PWM control and tachometer feedback, and I wanted to be sure that I could control the fan speed and read the tachometer.
• thermistor. I had some NTCLE100E3103JB0 thermistors around that I had never used.  They’re not ideal for measuring temperature of resistors (they only go up to 125°C), but they should be find for measuring air temperature around 35°C, which is what the incubator will mainly be used at.
• Arduino board (actually a SparkFun RedBoard, which is plug-compatible with the Uno R3, but has has a more reliable USB interface and is slightly cheaper.

I started out hooking up the nFET and the 1.8Ω resistor and making sure that the nFET did not get too hot.  It seems to be ok.  When I was using the 8.2Ω resistor, I measured the voltage drop across the resistor and the across the nFET, getting a 57.6mV drop from drain to source, with a current of about 9.024V/8.21ohm = 1.099A.  That’s about a 52mΩ on-resistance, and I was expecting more like 7mΩ–10mΩ.  My gate voltage was around 5V (bigger than the 4.5V of the data sheet), which should have given me lower on-resistance.  The only things I can think of are that I had more wiring resistance than I realized (quite likely, but not likely enough to add over 40mΩ), and that I was measuring around 1A, not around 10A, so perhaps there is a small-voltage effect that I don’t know about.

I should probably test the voltage drop again with 1.8Ω resistor, and see whether the on-resistance is still so high.  Better probe placement may get me more accurate voltage measurements also.

The fan runs fine at 9.212V at about 6850 RPM.  Setting the PWM input line of the fan to 0 drops the speed to about 710RPM, and setting the PWM duty cycle to a half sets the speed at about 4120RPM.  The fan is a bit noisy for such a tiny fan at the highest speed setting, but reasonably quiet at lower speeds.  I suspect that bolting the fan to a piece of masonite as a baffle would reduce the fan noise, as I think quite a bit of it was from vibration between the case of the fan and the metal plate it was sitting on.

The tachometer on the fan provides an open-collector output that I read with an interrupt input on the Arduino (pin 2, interrupt 0). I recorded the time between interrupts and converted it an RPM measurement.  The tachometer worked fine when I was just using the fan, or when the resistor was either completely off or completely on, but when I tried using PWM on the resistor, the tachometer readings became nonsense.

I looked at the tachometer signal with my oscilloscope and saw that the PWM transitions for the resistor resulted in huge spikes in the tachometer output that triggered extraneous interrupts.  I suspected noise coupled through the power supply. Adding a 10µF bypass capacitor to the 9V power supply to the fan reduced the problem considerably, and a 470µF aluminum polymer electrolytic cleaned up the power supply even more.  The 10µF alone was enough to eliminate the extraneous spikes in the middle.

I think that I should try adding some gate resistance to the nFET to slow down the rise and fall of the PWM signal a little, to reduce the inductive spikes and make the bypass capacitors more effective.

I noticed that I was still getting some readings that were half the duration that I was expecting.  These could have been caused by ringing at the other transition of the tachometer pulse, so I tried eliminating the ringing by adding some capacitance to the line and changing the pullup resistor.  These attempts were not very successful, so I decided that hysteresis was needed.  I put a Schmitt trigger between the open-collector output and the Arduino interrupt input, and the signal got a lot cleaner.  There were occasional double pulses at one edge, though, but I found that adding a 1nF to 10nF capacitor in parallel with the pullup resistor for the open collector output smoothed out the high frequency noise enough to get clean, single transitions out of the Schmitt trigger.

I hooked up the thermistor in a voltage divider with 5.1kΩ on the other leg (which maximizes the dV/dT sensitivity at 40.1°C). I used the parameters on the data sheet to plot a calibration curve for the thermistor:

Calibration and sensitivity curves for the thermistor, based on the data sheet and a 5.1kΩ pulldown resistor.

The maximum sensitivity of the thermistor circuit is around 33.3 degrees C (~10.4 Arduino LSB/°C).  That’s not a very high sensitivity, particularly given the noise of the ADC.  Note that maximizing the slope at 40.1 °C is not the same thing and having the maximum of the slope at 40.1°C.  If the maximum of the slope was at 40.1°C, the slope there would be less than it is in this plot.

My son wonders why I’m using the Arduino board for this project, rather than the FRDM-KL25Z board that I use for the circuits class or the Teensy 3.1 ARM development board. The ARM processors have more power, more memory, and much better analog-to-digital converters—and the KL25Z board is cheaper.  If I were doing this project for myself, I would certainly prefer the KL25Z board. But it is a little harder to get a beginner started on that board—just getting the first program onto the board is a pain if you don’t have a Windows machine (due to the broken bootloader the P&E Micro wrote).  There are instructions now for replacing the firmware from a Linux system, but I’ve not checked yet whether these instructions work from a Mac.  Even once you get working firmware onto the boards, the development environments are not beginner-friendly.  Well, that is certainly true of the MBED environment or bare-metal ARM environment for the KL25Z boards, but the Teensy 3.1 board supposedly can be programmed from a plugin for the Arduino IDE, which might be simple enough for beginners.  This is something for me to look into more.

Of course, one reason I’m using the Arduino Uno or Sparkfun RedBoard is that they are 5V processors, and most of the power nFETs I’ve looked at need 4.5V on the gate to turn on fully.  There are power nFETs now with lower gate voltages, but most of them are only available as surface-mount devices.  I don’t want to have to add an extra transistor or buffer chip as a level changer for the PWM circuit.

The problem is that these students will be brand new to programming, brand new to electronics, and brand new to engineering—and the course is only a 2-unit course, not a full 5-unit course, so the total time students are expected to spend on the course is only 60 hours. I want them to be able to design stuff quickly, without spending all their time learning to use tools or trying to find workarounds for limitations of the devices they are using. It already bothers me that they’ll probably need to use a Schmitt trigger to clean up the tachometer input, but at least hysteresis was a topic I was planning to cover! (The need for bypass capacitors bothers me less—they are so ubiquitous in electronics that I’ll have to cover them no matter what.)

It’s after midnight now, so I’m going to call it a day.  Here is my to-do list on this project:

• Check the V_DS voltage at 4A on the nFET. Is the on-resistance still much too high?
• Try adding a 1kΩ gate resistance to slow down the transitions on the PWM, to see if that reduces the inductive spikes and the noise-coupling through the 9V power supply.
• Write a simple control loop for the fan speed, so that the fan speed can be held constant even when the power-supply voltage changes.  This may be an opportunity to try the P/PI/PID tuning, since the control loop should be fairly fast.
• Write a simple control loop for controlling the temperature at the thermistor, by adjusting the PWM for the resistor.  This might get messy, as the fan speed probably affects the rate of transfer from the resistor to the thermistor (the thermistor is in the air stream blown over the resistor, not touching the resistor).
• Put the whole thing into a styrofoam box, to see whether extra venting is needed to allow things to cool down, and to see how tightly temperature can be controlled.
• Design and build baffling for the fan to get better airflow in the box.
• Figure out how to get students to come up with workable designs, when they are starting from knowing nothing. I don’t want to give them my designs, but I want to help them find doable subproblems.  Some of the subproblems they come up with may be well beyond the skills that they can pick up in the time frame of the course.

I’ve been following Garth’s CS Education Blog for a while, and his post Teaching programming is not getting easier resonated with me, particularly the lines

In programming memorization is a trivial part of the skill set needed to succeed. The primary skills needed are problem solving, strategizing, devolving problems into sub-tasks, interpreting, and general full bore head scratching. Those are an absolute bugger to teach, especially to kids that are not all that interested in learning those difficult skills.

I am not primarily teaching programming to beginners—my bioinformatics course has three programming courses as prerequisites and my applied circuits course does not require students to do any programming—but the same issues come up in all my courses.

Even after three previous programming courses, a lot of students have not had much practice at breaking a problem into subproblems or intelligent (rather than random) debugging. The block diagrams and “systems thinking” for the applied circuits class are precisely analogous to the decomposition of a software problem into modules, classes, or subroutines. Even in the senior thesis writing course that I taught for the last couple of years, a lot of the feedback is on getting students to structure their writing—to look at the thesis as having parts that communicate different information or with different audiences and getting the interfaces between the parts to work.

Almost all engineering requires similar problem-solving skills of decomposition into subproblems, designing and debugging parts independently, and debugging the interactions between parts. Teaching these skills in any context is difficult, and many teachers end up teaching special-purpose tricks that solve one type of problem but that does not help the student learn to solve novel problems.

Garth recognizes the problem in his own teaching:

I used to be a math teacher and math has somewhat the same thinking requirements and the same issues.  The big difference is the kids would have 10 home work problems a night, 8 of which were very easy to do so they would do those and ignore the hard ones.  The result would be an 80%.  With math there are a lot of problems with incremental steps of difficulty for almost any new concept.  Those students that can do the 70 or 80% in math survive just fine.  In math I usually have several different teaching strategies for a concept.  I have multiple “gimmicks” for devolving problems to make them easier to solve.  I have other math teachers to ask for new approaches and a whole lot of cool stuff on the internet to use as resources.   Programming on the other hand has diddly.

He learned some strategies for teaching math: assigning large numbers of small problems of gradually increasing difficulty, giving up on teaching most of the students to reach mastery, and providing gimmicks for students to memorize for the common problem types. These approaches have not worked for him teaching computer science—why not?

One difference is that the computer is not very forgiving of students who get things almost right—one punctuation mark wrong and the computer does the wrong thing or rejects the student’s attempt. Getting 70–80% of the way is not enough—students have to get the details right and not just the general picture.

Another difference is the one he notes: there are a lot more math teachers than CS teachers, particularly in K–12, so there is a lot more pedagogical content knowledge (knowledge of how to teach a subject) available in math. He notes that many CS teachers rely on a rather simple pedagogy:

After watching a number (3) of programming teachers teach it seems the teaching strategy is pretty consistent: show and tell and hope.

I wonder how much of the math pedagogy is really effective, though. A lot seems to be of the memorize-this-trick form, which gets students through their standardized tests, but does not develop transferable skills in problem decomposition or debugging.

The content in math and physics courses is also much more stable than in CS courses—what is taught at the high-school level has not changed much in the last century.  The main differences have been a loss of some tools (slide rules and trig tables) in favor of tools that are easier to use and teach with (calculators). Having a stable subject to teach allows teachers and textbook writers to experiment with how to teach, rather than what to teach. Although a lot of pedagogical experimentation fails (and the field of education is not very good at separating the successes from the failures), there are a lot of techniques available to choose from.

CS, however, keeps changing what is considered essential for a first course.  Fortran, LISP, Pascal, C, C++, Java, Python, Perl, Scratch, Processing, Alice, and other languages have all been proposed as “first” languages, and the programming language is often chosen for social rather than pedagogic reasons.

What topics are taught and in what order are often driven by the choice of language.  For example, LISP makes it easier to talk about recursion early, but makes it difficult to talk about strict type checking. Java and C++ force spending a lot of time on explaining data types and data type declaration. Python allows easy handling of sets and associative maps (“dict” in Python), but makes talking about information hiding and data abstraction somewhat more difficult. Scratch allows early discussion of race conditions in parallel programs, but not of complex data structures or program syntax.

CS teachers disagree about what order is most appropriate to present the topics in—not just the week-by-week order, but even what belongs in the first year and what in the second or third year. I think that in many cases the order doesn’t matter all that much—there are several different ways to get to a similar endpoint, and different students will respond well to different approaches. In the new bioengineering curricula that I’ve proposed, different concentrations have different programming requirements, with the bioelectronics track requiring bottom-up programming that teaches low-level interfacing to microprocessors in C, and the biomolecular track starting with bioinformatics-like programming tasks in Python.

Some of the teaching practices at colleges have not been helpful for developing desired skills.  For example, automated grading programs, which look just at I/O behavior of programs, are becoming more popular in huge college CS classes (and especially in MOOCs). But with automated grading students get almost no feedback on the decomposition into subproblems and clarity of documentation—those skills that are most needed for advancing in the field or transferring the learning to other domains.

I did end up teaching a tiny amount of programming in my freshman design seminar this past quarter: Arduino programming in C for gathering information from a thermistor or phototransistor and using it for simple on-off control (Twelfth day of freshman design seminar and Sixteenth day: Arduino demo).  I did not spend enough time on the programming, and a lot of it was “show and tell and hope”, so I suspect that only one or two of the students can do any programming independently now, but several who were not interested in programming became more interested in learning, which is all I expected of the course. The 2-unit course is only about 1% of their undergraduate education, so I can’t expect to make huge changes in their competence.

Next year I will spend more time on programming and on physical prototyping in the freshman design course, as those are areas that the students identified as having the most effect on them. So I may get to the point in the freshman seminar where I’ll also be facing the challenges of teaching CS concepts to beginners, rather than just piquing their interest.

One thing I think I will do in the freshman design seminar next year is to make the students actually wire up a thermistor or phototransistor to an Arduino board early in the quarter.  Having both a hardware and a software component to a design should help students learn problem decomposition and debugging, as there are obvious hardware/software boundaries—it can’t all be just one mushy “thing” in their heads.

The thermistor is particularly attractive as it requires several changes of representation—from temperature to resistance, to voltage, to ADC reading, back to  a numerical representation of temperature. Since the course is intended for bioengineers, the notion of sensors and different representations of what they sense is an important concept to build on, and temperature is a concept that they are familiar enough with that they can easily check whether what they are doing is working. Note that the data representation here is not just a software concept—the main constraint on the design is the analog-to-digital interface on the Arduino, which only measures voltage between 0 and 5V.

Having a thermistor lab early also works as part of a build-a-physical-prototype theme. I’m not going to use lab fees and handing out a lab kit, either, but make them find and order their own parts. One of the problems this year was students not realizing that getting parts requires a lot of lead time—having them experience that early in the quarter will make them more diligent about getting parts in time later in the quarter. To reduce shipping costs, I may have everyone look for parts separately, but then have them pool the orders, if they can agree on which parts they want to get.

In the coming quarter, in the Applied Circuits course, I’ll be trying to work more deliberately on both systems thinking and information representation, getting the students into being explicit about both earlier in the quarter.  (The first lab is a thermistor lab, which I am in the process of rewriting the lab handout for, which may be why it was the example I thought of using for the freshman design course.) I’ve heard discouraging reports about how little transfer there is of problem-solving skills between different subject domains, but I’m hopeful that having students encounter the same problem-solving concepts in several different domains will help them make the transfer.

I’ve been following Garth’s CS Education Blog for a while, and his post Teaching programming is not getting easier resonated with me, particularly the lines

In programming memorization is a trivial part of the skill set needed to succeed. The primary skills needed are problem solving, strategizing, devolving problems into sub-tasks, interpreting, and general full bore head scratching. Those are an absolute bugger to teach, especially to kids that are not all that interested in learning those difficult skills.

I am not primarily teaching programming to beginners—my bioinformatics course has three programming courses as prerequisites and my applied circuits course does not require students to do any programming—but the same issues come up in all my courses.

Even after three previous programming courses, a lot of students have not had much practice at breaking a problem into subproblems or intelligent (rather than random) debugging. The block diagrams and “systems thinking” for the applied circuits class are precisely analogous to the decomposition of a software problem into modules, classes, or subroutines. Even in the senior thesis writing course that I taught for the last couple of years, a lot of the feedback is on getting students to structure their writing—to look at the thesis as having parts that communicate different information or with different audiences and getting the interfaces between the parts to work.

Almost all engineering requires similar problem-solving skills of decomposition into subproblems, designing and debugging parts independently, and debugging the interactions between parts. Teaching these skills in any context is difficult, and many teachers end up teaching special-purpose tricks that solve one type of problem but that does not help the student learn to solve novel problems.

Garth recognizes the problem in his own teaching:

I used to be a math teacher and math has somewhat the same thinking requirements and the same issues.  The big difference is the kids would have 10 home work problems a night, 8 of which were very easy to do so they would do those and ignore the hard ones.  The result would be an 80%.  With math there are a lot of problems with incremental steps of difficulty for almost any new concept.  Those students that can do the 70 or 80% in math survive just fine.  In math I usually have several different teaching strategies for a concept.  I have multiple “gimmicks” for devolving problems to make them easier to solve.  I have other math teachers to ask for new approaches and a whole lot of cool stuff on the internet to use as resources.   Programming on the other hand has diddly.

He learned some strategies for teaching math: assigning large numbers of small problems of gradually increasing difficulty, giving up on teaching most of the students to reach mastery, and providing gimmicks for students to memorize for the common problem types. These approaches have not worked for him teaching computer science—why not?

One difference is that the computer is not very forgiving of students who get things almost right—one punctuation mark wrong and the computer does the wrong thing or rejects the student’s attempt. Getting 70–80% of the way is not enough—students have to get the details right and not just the general picture.

Another difference is the one he notes: there are a lot more math teachers than CS teachers, particularly in K–12, so there is a lot more pedagogical content knowledge (knowledge of how to teach a subject) available in math. He notes that many CS teachers rely on a rather simple pedagogy:

After watching a number (3) of programming teachers teach it seems the teaching strategy is pretty consistent: show and tell and hope.

I wonder how much of the math pedagogy is really effective, though. A lot seems to be of the memorize-this-trick form, which gets students through their standardized tests, but does not develop transferable skills in problem decomposition or debugging.

The content in math and physics courses is also much more stable than in CS courses—what is taught at the high-school level has not changed much in the last century.  The main differences have been a loss of some tools (slide rules and trig tables) in favor of tools that are easier to use and teach with (calculators). Having a stable subject to teach allows teachers and textbook writers to experiment with how to teach, rather than what to teach. Although a lot of pedagogical experimentation fails (and the field of education is not very good at separating the successes from the failures), there are a lot of techniques available to choose from.

CS, however, keeps changing what is considered essential for a first course.  Fortran, LISP, Pascal, C, C++, Java, Python, Perl, Scratch, Processing, Alice, and other languages have all been proposed as “first” languages, and the programming language is often chosen for social rather than pedagogic reasons.

What topics are taught and in what order are often driven by the choice of language.  For example, LISP makes it easier to talk about recursion early, but makes it difficult to talk about strict type checking. Java and C++ force spending a lot of time on explaining data types and data type declaration. Python allows easy handling of sets and associative maps (“dict” in Python), but makes talking about information hiding and data abstraction somewhat more difficult. Scratch allows early discussion of race conditions in parallel programs, but not of complex data structures or program syntax.

CS teachers disagree about what order is most appropriate to present the topics in—not just the week-by-week order, but even what belongs in the first year and what in the second or third year. I think that in many cases the order doesn’t matter all that much—there are several different ways to get to a similar endpoint, and different students will respond well to different approaches. In the new bioengineering curricula that I’ve proposed, different concentrations have different programming requirements, with the bioelectronics track requiring bottom-up programming that teaches low-level interfacing to microprocessors in C, and the biomolecular track starting with bioinformatics-like programming tasks in Python.

Some of the teaching practices at colleges have not been helpful for developing desired skills.  For example, automated grading programs, which look just at I/O behavior of programs, are becoming more popular in huge college CS classes (and especially in MOOCs). But with automated grading students get almost no feedback on the decomposition into subproblems and clarity of documentation—those skills that are most needed for advancing in the field or transferring the learning to other domains.

I did end up teaching a tiny amount of programming in my freshman design seminar this past quarter: Arduino programming in C for gathering information from a thermistor or phototransistor and using it for simple on-off control (Twelfth day of freshman design seminar and Sixteenth day: Arduino demo).  I did not spend enough time on the programming, and a lot of it was “show and tell and hope”, so I suspect that only one or two of the students can do any programming independently now, but several who were not interested in programming became more interested in learning, which is all I expected of the course. The 2-unit course is only about 1% of their undergraduate education, so I can’t expect to make huge changes in their competence.

Next year I will spend more time on programming and on physical prototyping in the freshman design course, as those are areas that the students identified as having the most effect on them. So I may get to the point in the freshman seminar where I’ll also be facing the challenges of teaching CS concepts to beginners, rather than just piquing their interest.

One thing I think I will do in the freshman design seminar next year is to make the students actually wire up a thermistor or phototransistor to an Arduino board early in the quarter.  Having both a hardware and a software component to a design should help students learn problem decomposition and debugging, as there are obvious hardware/software boundaries—it can’t all be just one mushy “thing” in their heads.

The thermistor is particularly attractive as it requires several changes of representation—from temperature to resistance, to voltage, to ADC reading, back to  a numerical representation of temperature. Since the course is intended for bioengineers, the notion of sensors and different representations of what they sense is an important concept to build on, and temperature is a concept that they are familiar enough with that they can easily check whether what they are doing is working. Note that the data representation here is not just a software concept—the main constraint on the design is the analog-to-digital interface on the Arduino, which only measures voltage between 0 and 5V.

Having a thermistor lab early also works as part of a build-a-physical-prototype theme. I’m not going to use lab fees and handing out a lab kit, either, but make them find and order their own parts. One of the problems this year was students not realizing that getting parts requires a lot of lead time—having them experience that early in the quarter will make them more diligent about getting parts in time later in the quarter. To reduce shipping costs, I may have everyone look for parts separately, but then have them pool the orders, if they can agree on which parts they want to get.

In the coming quarter, in the Applied Circuits course, I’ll be trying to work more deliberately on both systems thinking and information representation, getting the students into being explicit about both earlier in the quarter.  (The first lab is a thermistor lab, which I am in the process of rewriting the lab handout for, which may be why it was the example I thought of using for the freshman design course.) I’ve heard discouraging reports about how little transfer there is of problem-solving skills between different subject domains, but I’m hopeful that having students encounter the same problem-solving concepts in several different domains will help them make the transfer.

Today’s class in the freshman design seminar went well. I started by returning the drafts of the design reports and giving some generic feedback. I realized on reading the reports that I had not given a good explanation of what I meant by describing the components of the system—two of the groups had given me long parts lists on the first page of their reports, something that would only really be appropriate in an appendix. I explained that what I wanted was what the main blocks in the block diagram were, and that they should use the block diagram to organize their report, writing a page for each block. I also suggested that they use the block diagram to partition the project among the group members, with each group member working on a different component, then getting back together to reconcile any discrepancies. Note that this is much more like real engineering group work than the usual K–12 group project, which is usually done most efficiently by turning the whole project over to the most competent member of the group.

After the feedback on design reports, I offered the students a chance to get a demo of building an Arduino program with sensing and motor control. This was a completely extemporaneous demo—I had gathered a number of possibly useful components, but had not tested anything ahead of time nor even figured out what order to do the demo in.  I asked the students if they wanted me to start with sensing or control—they asked for the motor control first.

I started by pulling a motor out of box of motors I had gotten when the elementary school my wife works at cleaned out their closets.  I told the students that I had no idea what the spec of the motor were, but since it came from an elementary school, it probably ran on 3v batteries.  I tested the motor by hooking it up first to the 3.3v, then to the 5v power on my Arduino Uno.  It spun just fine on 3.3v, but squealed a bit on 5v, so we decided to run it on 3.3v.

I then pulled out the Sainsmart 4-relay board that I had bought some time ago but never used.  I explained how a relay worked, what single-pole double-throw meant, and normally open (NO) and normally closed (NC) contacts. I used the board unpowered with the NC contacts to spin the motor, then moved the wire over to the NO contacts to turn the motor off.  I then hooked up power to the board and tried connecting input IN1 to power to activate the relay.  Nothing happened. I then tried connecting IN1 to ground, and the relay clicked and the motor spun.  The inputs to the Sainsmart board are active low, which I explained to the students (though I did not use the terminology “active low”—perhaps I should have).  I did make a point of establishing that the relay provides very good isolation between the control logic and the circuitry being controlled—you can hook up AC power from the walls to the relay contacts without interfering with the logic circuitry.

Having established that the relay worked, the next step was to get the class (as a group) to write an Arduino program to control the motor using the relay. With me taking notes on the whiteboard, they quickly came up with the pinMode command for the setup, the digitalWrite and delay for the loop, and with only a tiny bit of prompting with a second digitalWrite and delay to turn the motor back off.  They even realized the need to have different delays for the on and off, so we could tell whether we had the polarity right on the control.  Here is the program we came up with:

#define RELAY_PIN (3)

void setup()
{   pinMode(RELAY_PIN, OUTPUT);
}

void loop()
{
  digitalWrite(RELAY_PIN,LOW); // turn motor ON via relay (or off via transistor)
  delay(1000);  // on for 1 second
  digitalWrite(RELAY_PIN,HIGH); // turn motor OFF via relay (or on via transistor)
  delay(3000); // off for 3 seconds
}

I typed the code in and downloaded it to the Arduino Uno, and it worked as expected.  (It would be nice if the Arduino IDE would allow me to increase the font size, like almost every other program I use, so that students could have read the projection of what I was typing better.)

I then offered the students a choice of going on to sensing or looking at pulse-width modulation for proportional control.  They wanted PWM. I explained why PWM is not really doable with relays (the relays are too slow, and chattering them would wear them out after a while.  I did not have the specs on the relay handy, but I just looked up the specs for the SRD-05VDC-SL-C relays on the board: They have a mechanical life of 10,000,000 cycles, but an electrical life of only 100,000 cycles.  The relay takes about 7msec to make a contact and about 3msec to break a contact, so they can’t be operated much faster than about 60 times a second, which could wear them out in as little as half an hour.

So instead of a relay, I suggested an nFET (Field-Effect Transistor). I gave them a circuit with one side of the motor connected to 3.3V, the other to the drain of an nFET, with the source connected to ground.  I explained that the voltage between the gate and the source (V_GS) controlled whether the transistor was on or off, and that putting 5v on the gate would turn it on fairly well. I then got out an AOI518 nFET and stuck it in my breadboard, explaining the orientation to allow using the other holes to connect to the source, gate, and drain.

I mentioned that different FETs have the order of the pins different, so one has to look up the pinout on data sheet. I pulled up the AOI518 data sheet, which has on the first page “RDS(ON) (at VGS = 4.5V) < 11.9mΩ”. I explained that if we were putting a whole amp through the FET (we’re not doing anywhere near that much current), the voltage drop would be 11.9mV, so the power dissipated in the transistor would be only 11.9mW, not enough to get it warm. I mentioned that more current would result in more power being dissipated (I²R), and that the FETs could get quite warm. I passed around my other breadboard which has six melted holes from FETs getting quite hot when I was trying to debug the class-D amplifier design. The students were surprised that the FETs still worked after getting that hot (I must admit that I was also).

I hooked up the AOI518 nFET using double-headed male header pins and female jumper cables, and the motor alternated on for 3 seconds, off for one second. We now had the transistor controlling the motor, so it was time to switch to PWM. I went to the Arduino reference page and looked around for PWM, finding it on analogWrite(). I clicked that link and we looked at the page, seeing that analog Write was like digitalWrite, except that we could put in a value from 0 to 255 that controlled what fraction of the time the pin was high.

I edited the code, changing the first digitalWrite() to analogWrite(nFET_GATE_PIN, 255), and commenting out the rest of the loop. We downloaded that, and it turned the motor on, as expected. I then tried writing 128, which still turned the motor on, but perhaps not as strongly (hard to tell with no load). Writing 50 resulted in the motor not starting. Writing 100 let the motor run if I started it by hand, but wouldn’t start the motor from a dead stop. I used this opportunity to point out that controlling the motor was not linear—1/5th didn’t run at 1/5th speed, but wouldn’t run the motor at all.

Next we switched over to doing sensors (with only 10 minutes left in the class). I got out the pressure sensor and instrumentation amp from the circuits course and hooked it up. The screwdriver I had packed in the box had too large a blade for the 0.1″ screw terminals, but luckily the tiny screwdriver on my Swiss Army knife (tucked away in the corkscrew) was small enough. After hooking up the pressure sensor to A0, I downloaded the Arduino Data Logger to the Uno, and started it from a terminal window. I set the triggering to every 100msec (which probably should be the default for the data logger), the input to A0, and convert to volts. I then demoed the pressure sensor by blowing into or sucking on the plastic tube hooked up to the sensor. With the low-gain output from the amplifier, the output swung about 0.5 v either way from the 2.5v center. Moving the A0 wire over to the high-gain output of the amplifier gave a more visible signal. I also turned off the “convert to volts” to show the students the values actually read by the Arduino (511 and 512, the middle of the range from 0 to 1023).

Because the class was over at that point, I offered to stay for another 10 minutes to show them how to use the pressure sensor to control the motor. One or two students had other classes to run to, but most stayed. I then wrote a program that would normally have the motor off, but would turn it full on if I got the pressure reading up to 512+255 and would turn it on partway (using PWM) between 512 and 512+255. I made several typos when entering the program (including messing up the braces and putting in an extraneous semicolon), but on the third compilation it downloaded successfully and controlled the motor as expected.

One student asked why the motor was off when I wasn’t blowing into the tube, so I explained about 512 being the pressure reading when nothing was happening (neither blowing into the tube nor sucking on it). I changed the zero point for the motor to a pressure reading of 300, so that the motor was normally most of the way on, but could be turned off by sucking on the tube. Here is the program we ended up with

#define nFET_GATE_PIN (3)

void setup()
{   pinMode(nFET_GATE_PIN, OUTPUT);
    pinMode(A0, INPUT);
}

void loop()
{ int pressure;
  pressure=analogRead(A0);
  if (pressure < 300)
  {    digitalWrite(nFET_GATE_PIN,LOW);  // turn motor off
  }
  else
  {   if (pressure>300+255)
      { digitalWrite(nFET_GATE_PIN,HIGH);  // turn motor on full
      }
      else
      {    analogWrite(nFET_GATE_PIN,pressure-300); // turn motor partway on
      }
  }
}

Note: this code is not an example of brilliant programming style. I can see several things that I would have done differently if I had had time to think about the code, but for this blog it is more useful to show the actual artifact that was developed in the demo, even if it makes me cringe a little.

Overall, I thought that the demo went well, despite being completely extemporaneous. Running over by 10 minutes might have been avoidable, but only by omitting something useful (like the feedback on the design reports). The demo itself lasted about 70 minutes, making the whole class run 80 minutes instead of 70. I think I compressed the demo about as much as was feasible for the level the students were at.

Based on how the students developed the first motor-control program quickly in class, I think that some of them are beginning to get some of the main ideas of programming: explicit instructions and sequential ordering. Because we were out of time by the point I got to using conditionals, I did not get a chance to probe their understanding there.

Today’s class in the freshman design seminar went well. I started by returning the drafts of the design reports and giving some generic feedback. I realized on reading the reports that I had not given a good explanation of what I meant by describing the components of the system—two of the groups had given me long parts lists on the first page of their reports, something that would only really be appropriate in an appendix. I explained that what I wanted was what the main blocks in the block diagram were, and that they should use the block diagram to organize their report, writing a page for each block. I also suggested that they use the block diagram to partition the project among the group members, with each group member working on a different component, then getting back together to reconcile any discrepancies. Note that this is much more like real engineering group work than the usual K–12 group project, which is usually done most efficiently by turning the whole project over to the most competent member of the group.

After the feedback on design reports, I offered the students a chance to get a demo of building an Arduino program with sensing and motor control. This was a completely extemporaneous demo—I had gathered a number of possibly useful components, but had not tested anything ahead of time nor even figured out what order to do the demo in.  I asked the students if they wanted me to start with sensing or control—they asked for the motor control first.

I started by pulling a motor out of box of motors I had gotten when the elementary school my wife works at cleaned out their closets.  I told the students that I had no idea what the spec of the motor were, but since it came from an elementary school, it probably ran on 3v batteries.  I tested the motor by hooking it up first to the 3.3v, then to the 5v power on my Arduino Uno.  It spun just fine on 3.3v, but squealed a bit on 5v, so we decided to run it on 3.3v.

I then pulled out the Sainsmart 4-relay board that I had bought some time ago but never used.  I explained how a relay worked, what single-pole double-throw meant, and normally open (NO) and normally closed (NC) contacts. I used the board unpowered with the NC contacts to spin the motor, then moved the wire over to the NO contacts to turn the motor off.  I then hooked up power to the board and tried connecting input IN1 to power to activate the relay.  Nothing happened. I then tried connecting IN1 to ground, and the relay clicked and the motor spun.  The inputs to the Sainsmart board are active low, which I explained to the students (though I did not use the terminology “active low”—perhaps I should have).  I did make a point of establishing that the relay provides very good isolation between the control logic and the circuitry being controlled—you can hook up AC power from the walls to the relay contacts without interfering with the logic circuitry.

Having established that the relay worked, the next step was to get the class (as a group) to write an Arduino program to control the motor using the relay. With me taking notes on the whiteboard, they quickly came up with the pinMode command for the setup, the digitalWrite and delay for the loop, and with only a tiny bit of prompting with a second digitalWrite and delay to turn the motor back off.  They even realized the need to have different delays for the on and off, so we could tell whether we had the polarity right on the control.  Here is the program we came up with:

#define RELAY_PIN (3)

void setup()
{   pinMode(RELAY_PIN, OUTPUT);
}

void loop()
{
  digitalWrite(RELAY_PIN,LOW); // turn motor ON via relay (or off via transistor)
  delay(1000);  // on for 1 second
  digitalWrite(RELAY_PIN,HIGH); // turn motor OFF via relay (or on via transistor)
  delay(3000); // off for 3 seconds
}

I typed the code in and downloaded it to the Arduino Uno, and it worked as expected.  (It would be nice if the Arduino IDE would allow me to increase the font size, like almost every other program I use, so that students could have read the projection of what I was typing better.)

I then offered the students a choice of going on to sensing or looking at pulse-width modulation for proportional control.  They wanted PWM. I explained why PWM is not really doable with relays (the relays are too slow, and chattering them would wear them out after a while.  I did not have the specs on the relay handy, but I just looked up the specs for the SRD-05VDC-SL-C relays on the board: They have a mechanical life of 10,000,000 cycles, but an electrical life of only 100,000 cycles.  The relay takes about 7msec to make a contact and about 3msec to break a contact, so they can’t be operated much faster than about 60 times a second, which could wear them out in as little as half an hour.

So instead of a relay, I suggested an nFET (Field-Effect Transistor). I gave them a circuit with one side of the motor connected to 3.3V, the other to the drain of an nFET, with the source connected to ground.  I explained that the voltage between the gate and the source (V_GS) controlled whether the transistor was on or off, and that putting 5v on the gate would turn it on fairly well. I then got out an AOI518 nFET and stuck it in my breadboard, explaining the orientation to allow using the other holes to connect to the source, gate, and drain.

I mentioned that different FETs have the order of the pins different, so one has to look up the pinout on data sheet. I pulled up the AOI518 data sheet, which has on the first page “RDS(ON) (at VGS = 4.5V) < 11.9mΩ”. I explained that if we were putting a whole amp through the FET (we’re not doing anywhere near that much current), the voltage drop would be 11.9mV, so the power dissipated in the transistor would be only 11.9mW, not enough to get it warm. I mentioned that more current would result in more power being dissipated (I²R), and that the FETs could get quite warm. I passed around my other breadboard which has six melted holes from FETs getting quite hot when I was trying to debug the class-D amplifier design. The students were surprised that the FETs still worked after getting that hot (I must admit that I was also).

I hooked up the AOI518 nFET using double-headed male header pins and female jumper cables, and the motor alternated on for 3 seconds, off for one second. We now had the transistor controlling the motor, so it was time to switch to PWM. I went to the Arduino reference page and looked around for PWM, finding it on analogWrite(). I clicked that link and we looked at the page, seeing that analog Write was like digitalWrite, except that we could put in a value from 0 to 255 that controlled what fraction of the time the pin was high.

I edited the code, changing the first digitalWrite() to analogWrite(nFET_GATE_PIN, 255), and commenting out the rest of the loop. We downloaded that, and it turned the motor on, as expected. I then tried writing 128, which still turned the motor on, but perhaps not as strongly (hard to tell with no load). Writing 50 resulted in the motor not starting. Writing 100 let the motor run if I started it by hand, but wouldn’t start the motor from a dead stop. I used this opportunity to point out that controlling the motor was not linear—1/5th didn’t run at 1/5th speed, but wouldn’t run the motor at all.

Next we switched over to doing sensors (with only 10 minutes left in the class). I got out the pressure sensor and instrumentation amp from the circuits course and hooked it up. The screwdriver I had packed in the box had too large a blade for the 0.1″ screw terminals, but luckily the tiny screwdriver on my Swiss Army knife (tucked away in the corkscrew) was small enough. After hooking up the pressure sensor to A0, I downloaded the Arduino Data Logger to the Uno, and started it from a terminal window. I set the triggering to every 100msec (which probably should be the default for the data logger), the input to A0, and convert to volts. I then demoed the pressure sensor by blowing into or sucking on the plastic tube hooked up to the sensor. With the low-gain output from the amplifier, the output swung about 0.5 v either way from the 2.5v center. Moving the A0 wire over to the high-gain output of the amplifier gave a more visible signal. I also turned off the “convert to volts” to show the students the values actually read by the Arduino (511 and 512, the middle of the range from 0 to 1023).

Because the class was over at that point, I offered to stay for another 10 minutes to show them how to use the pressure sensor to control the motor. One or two students had other classes to run to, but most stayed. I then wrote a program that would normally have the motor off, but would turn it full on if I got the pressure reading up to 512+255 and would turn it on partway (using PWM) between 512 and 512+255. I made several typos when entering the program (including messing up the braces and putting in an extraneous semicolon), but on the third compilation it downloaded successfully and controlled the motor as expected.

One student asked why the motor was off when I wasn’t blowing into the tube, so I explained about 512 being the pressure reading when nothing was happening (neither blowing into the tube nor sucking on it). I changed the zero point for the motor to a pressure reading of 300, so that the motor was normally most of the way on, but could be turned off by sucking on the tube. Here is the program we ended up with

#define nFET_GATE_PIN (3)

void setup()
{   pinMode(nFET_GATE_PIN, OUTPUT);
    pinMode(A0, INPUT);
}

void loop()
{ int pressure;
  pressure=analogRead(A0);
  if (pressure < 300)
  {    digitalWrite(nFET_GATE_PIN,LOW);  // turn motor off
  }
  else
  {   if (pressure>300+255)
      { digitalWrite(nFET_GATE_PIN,HIGH);  // turn motor on full
      }
      else
      {    analogWrite(nFET_GATE_PIN,pressure-300); // turn motor partway on
      }
  }
}

Note: this code is not an example of brilliant programming style. I can see several things that I would have done differently if I had had time to think about the code, but for this blog it is more useful to show the actual artifact that was developed in the demo, even if it makes me cringe a little.

Overall, I thought that the demo went well, despite being completely extemporaneous. Running over by 10 minutes might have been avoidable, but only by omitting something useful (like the feedback on the design reports). The demo itself lasted about 70 minutes, making the whole class run 80 minutes instead of 70. I think I compressed the demo about as much as was feasible for the level the students were at.

Based on how the students developed the first motor-control program quickly in class, I think that some of them are beginning to get some of the main ideas of programming: explicit instructions and sequential ordering. Because we were out of time by the point I got to using conditionals, I did not get a chance to probe their understanding there.

My counts of which days were which in the freshman design seminar were all messed, so three of my blog posts were misnamed:

Today we started by having the students turn in their Arduino programming homework, then start writing the program as a group. I told them that I particularly wanted those who had trouble with the assignment to provide input—I’m trying to get them to realize that questions and confusion are normal, and that the right action to take in college is to ask questions, rather than to hide ignorance.

This particular assignment was expected to be hard for them—I had not done the scaffolding for it that I had originally planned, but threw them into it with very little preparation. I told them that, but also that I was trying to get them used to looking things up and figuring them out, rather than waiting to be told exactly what to do. Again, I’m trying to get them out of the “regurgitate what the teacher said” mode that K–12 education has trained them into. If I accomplish nothing else this quarter, I hope to increase their willingness to ask questions (of their teachers and of the things they read).

We did get the program written, with some digressions into the difference between “==” and “=” in C++ and the convention in C and C++ that 0 is false and any other value is true. I also managed to work in the importance of good variable names to tell people what things meant, though this particular program doesn’t need any variables.  The students now have the notions of serial execution, conditional expressions, if-statements, digital I/O, serial communication (we had a digression into baud rate), and the Arduino setup/loop structure, which may be enough for their projects—they may also need analogRead(), which I should be sure to demo on Monday.

I then typed in the program we had created together and demoed it with Arduino. I had deliberately left in a bug that I had spotted (no space between the printing of the different fields), and the class spotted it and came up with a reasonable correction when the first output came out.

We only had about 10 minutes left, so I gave them feedback on their project proposals:

• Type homework for college classes!  Two of the groups had turned in hastily scribbled notes.
• Give explicit specifications for the project.  How big an incubator? How precisely does the temperature need to be controlled? How fast does temperature have to change for PCR? What temperatures are needed and how precisely? How many tubes need to be run through the PCR machine at once?  How much acceleration does the centrifuge need to produce? How precisely does the speed need to be controlled?
• Provide a block diagram giving all the components of the system (power supply, motor, fan, rotor, temperature sensor, … ) and lines showing the connections.  I talked about the importance of specifying the interfaces between components so that people could work simultaneously on different parts, and the need to renegotiate interfaces if the initial specification of them caused problems.

We talked a bit about prototyping—I want them to build something and learn enough to make an improved design, even if they don’t get a fully functional prototype.

For Monday, I assigned them the task of fleshing out the specifications for their project and producing a block diagram, filling in as many details as they could.  The group tutor is going to try to find time to meet with each group for an hour before then, to help them with flesh out their designs.

My counts of which days were which in the freshman design seminar were all messed, so three of my blog posts were misnamed:

Today we started by having the students turn in their Arduino programming homework, then start writing the program as a group. I told them that I particularly wanted those who had trouble with the assignment to provide input—I’m trying to get them to realize that questions and confusion are normal, and that the right action to take in college is to ask questions, rather than to hide ignorance.

This particular assignment was expected to be hard for them—I had not done the scaffolding for it that I had originally planned, but threw them into it with very little preparation. I told them that, but also that I was trying to get them used to looking things up and figuring them out, rather than waiting to be told exactly what to do. Again, I’m trying to get them out of the “regurgitate what the teacher said” mode that K–12 education has trained them into. If I accomplish nothing else this quarter, I hope to increase their willingness to ask questions (of their teachers and of the things they read).

We did get the program written, with some digressions into the difference between “==” and “=” in C++ and the convention in C and C++ that 0 is false and any other value is true. I also managed to work in the importance of good variable names to tell people what things meant, though this particular program doesn’t need any variables.  The students now have the notions of serial execution, conditional expressions, if-statements, digital I/O, serial communication (we had a digression into baud rate), and the Arduino setup/loop structure, which may be enough for their projects—they may also need analogRead(), which I should be sure to demo on Monday.

I then typed in the program we had created together and demoed it with Arduino. I had deliberately left in a bug that I had spotted (no space between the printing of the different fields), and the class spotted it and came up with a reasonable correction when the first output came out.

We only had about 10 minutes left, so I gave them feedback on their project proposals:

• Type homework for college classes!  Two of the groups had turned in hastily scribbled notes.
• Give explicit specifications for the project.  How big an incubator? How precisely does the temperature need to be controlled? How fast does temperature have to change for PCR? What temperatures are needed and how precisely? How many tubes need to be run through the PCR machine at once?  How much acceleration does the centrifuge need to produce? How precisely does the speed need to be controlled?
• Provide a block diagram giving all the components of the system (power supply, motor, fan, rotor, temperature sensor, … ) and lines showing the connections.  I talked about the importance of specifying the interfaces between components so that people could work simultaneously on different parts, and the need to renegotiate interfaces if the initial specification of them caused problems.

We talked a bit about prototyping—I want them to build something and learn enough to make an improved design, even if they don’t get a fully functional prototype.

For Monday, I assigned them the task of fleshing out the specifications for their project and producing a block diagram, filling in as many details as they could.  The group tutor is going to try to find time to meet with each group for an hour before then, to help them with flesh out their designs.

Yesterday’s class was originally planned to be an Arduino demo of analogRead() and Serial.print(), using either a pressure sensor or a phototransistor, but the class ended up going in a totally different direction.

At the beginning of class, I passed around a cheap vernier caliper and showed students how to measure the disposable cuvettes to 0.05mm precision. This was part of my continuing effort to expose students to simple tools that they should have seen in high school, but for the most part have not.

I started the class by collecting the proposals for design projects.  Since these were group efforts, there were only three proposals: a centrifuge, an incubator, and a PCR thermal cycler.  I’ve not read the proposals yet (I’m still under the weather, and ended up falling asleep last night right after dinner, and only waking up this morning barely in time to get to the department research seminar at noon), but I did read the titles in class, and started talking about what computer aspects there might be for each project.  The centrifuge is mainly a mechanical design, but a non-contact sensor (probably optical) for measuring the rotor speed would be useful—one might even want to include a motor speed control, but that depends on what they use for a motor. The other two designs both depend on regulating a temperature.  Important parameters include how tightly controlled the temperature has to be and how fast you have to move from one temperature to another.  An incubator generally needs to have a fairly fixed temperature, and only needs to respond to slow heat loss, except when the incubator is opened.  A PCR machine has to switch rapidly between three temperatures,  generally around 95° C, 50° C, and 75° C.

So we ended up talking about thermal control.  First I described the basic idea of having a “set point” and of simple on-off heater control, with high threshold and a low threshold.  We talked about temperature sensors, including the old-fashioned bimetal-strip thermostats, thermistors, thermocouples, and RTD sensors. I talked a little about how to choose among the different types (thermocouples for high temperature, RTD for high precision, thermistors for low cost and ease of interfacing).  I also mentioned semiconductor temperature sensors, which are used in a lot of integrated circuits (like CPUs and GPUs on their laptops), but are not very good for general temperature measurement.

We focused on thermistors as the simplest to use with the Arduino, and I showed them a data sheet for the NTCLE413E2103F520L thermistor we use in the Applied Circuits lab.  This lead to a discussion of how the thermistor resistance varies with temperature, and I showed them the Wikipedia page on thermistors, with its discussion of the Steinhart-Hart equation and the “B” parameterization of the formula.  I also explained what the B_25/85 specification on the data sheet meant (measuring resistance at only 25° C and 85° C, and solving for B).

I then tried to get the class to come up with a circuit to convert resistance variation into voltage variation, but they were stumped. So I showed them a voltage divider and had them work out as a class what the output voltage was using just Ohms law. They were then able to see that replacing one resistor with a thermistor would allow them to see a voltage variation as a function of temperature. I told them about the first homework in the Applied Circuits course (figuring out the optimal value for the fixed resistor, to get the maximum change in voltage with temperature at a particular operating temperature), but did not assign the problem. Not everyone in the class has had calculus yet, and the problem really does require being able to differentiate and use the chain rule.

I ran out of time before doing anything with the Arduino! I did assign them an Arduino homework, though:

For Wednesday 2014 Feb 19 (no class Monday Feb 17), write an Arduino program that will report over the USB cable once every two seconds the status of pins 8, 9, and 10, whether that pin is high or low. The report should be viewable on the Arduino Serial Monitor. It should look something like

 8:HIGH 9: LOW 10: LOW
 8: LOW 9:HIGH 10: LOW
 8: LOW 9:HIGH 10: LOW
 8: LOW 9: LOW 10: LOW

As a matter of common programming style, there should be a “block comment” at the beginning of every program telling what the program does (from a user’s standpoint, not how it works from a programmer’s standpoint), who wrote it, and when it was written. You may work on the programs in pairs (not larger groups), but the names of everyone who worked on

Turn in a printout of your program. This program is simple enough that I don’t need evidence of it working—for other class you may be asked to turn in the source code electronically, so that the graders can test the program, or provide input-output pairs that show evidence that the program is working correctly.

For those who find this program too easy, you can challenge yourself to do more ambitious programs:

• Read analog inputs from A0 through A5 and report the values.
• Accept characters from the USB serial line that change which pins you examine. (For example, getting the character ’8′ with Serial.read() might turn on looking at pin 8, and getting ‘*’ might turn it off looking at pin 8.)
• Write a little control program that turns on the on-board LED (pin 13) when some combination of conditions is true and off when the conditions are false.

At the end of class, I had everyone use my wire strippers to cut and strip a few small pieces of wire, so that they could do the hardware portion of the Arduino programming.

Yesterday’s class was originally planned to be an Arduino demo of analogRead() and Serial.print(), using either a pressure sensor or a phototransistor, but the class ended up going in a totally different direction.

At the beginning of class, I passed around a cheap vernier caliper and showed students how to measure the disposable cuvettes to 0.05mm precision. This was part of my continuing effort to expose students to simple tools that they should have seen in high school, but for the most part have not.

I started the class by collecting the proposals for design projects.  Since these were group efforts, there were only three proposals: a centrifuge, an incubator, and a PCR thermal cycler.  I’ve not read the proposals yet (I’m still under the weather, and ended up falling asleep last night right after dinner, and only waking up this morning barely in time to get to the department research seminar at noon), but I did read the titles in class, and started talking about what computer aspects there might be for each project.  The centrifuge is mainly a mechanical design, but a non-contact sensor (probably optical) for measuring the rotor speed would be useful—one might even want to include a motor speed control, but that depends on what they use for a motor. The other two designs both depend on regulating a temperature.  Important parameters include how tightly controlled the temperature has to be and how fast you have to move from one temperature to another.  An incubator generally needs to have a fairly fixed temperature, and only needs to respond to slow heat loss, except when the incubator is opened.  A PCR machine has to switch rapidly between three temperatures,  generally around 95° C, 50° C, and 75° C.

So we ended up talking about thermal control.  First I described the basic idea of having a “set point” and of simple on-off heater control, with high threshold and a low threshold.  We talked about temperature sensors, including the old-fashioned bimetal-strip thermostats, thermistors, thermocouples, and RTD sensors. I talked a little about how to choose among the different types (thermocouples for high temperature, RTD for high precision, thermistors for low cost and ease of interfacing).  I also mentioned semiconductor temperature sensors, which are used in a lot of integrated circuits (like CPUs and GPUs on their laptops), but are not very good for general temperature measurement.

We focused on thermistors as the simplest to use with the Arduino, and I showed them a data sheet for the NTCLE413E2103F520L thermistor we use in the Applied Circuits lab.  This lead to a discussion of how the thermistor resistance varies with temperature, and I showed them the Wikipedia page on thermistors, with its discussion of the Steinhart-Hart equation and the “B” parameterization of the formula.  I also explained what the B_25/85 specification on the data sheet meant (measuring resistance at only 25° C and 85° C, and solving for B).

I then tried to get the class to come up with a circuit to convert resistance variation into voltage variation, but they were stumped. So I showed them a voltage divider and had them work out as a class what the output voltage was using just Ohms law. They were then able to see that replacing one resistor with a thermistor would allow them to see a voltage variation as a function of temperature. I told them about the first homework in the Applied Circuits course (figuring out the optimal value for the fixed resistor, to get the maximum change in voltage with temperature at a particular operating temperature), but did not assign the problem. Not everyone in the class has had calculus yet, and the problem really does require being able to differentiate and use the chain rule.

I ran out of time before doing anything with the Arduino! I did assign them an Arduino homework, though:

For Wednesday 2014 Feb 19 (no class Monday Feb 17), write an Arduino program that will report over the USB cable once every two seconds the status of pins 8, 9, and 10, whether that pin is high or low. The report should be viewable on the Arduino Serial Monitor. It should look something like

 8:HIGH 9: LOW 10: LOW
 8: LOW 9:HIGH 10: LOW
 8: LOW 9:HIGH 10: LOW
 8: LOW 9: LOW 10: LOW

As a matter of common programming style, there should be a “block comment” at the beginning of every program telling what the program does (from a user’s standpoint, not how it works from a programmer’s standpoint), who wrote it, and when it was written. You may work on the programs in pairs (not larger groups), but the names of everyone who worked on

Turn in a printout of your program. This program is simple enough that I don’t need evidence of it working—for other class you may be asked to turn in the source code electronically, so that the graders can test the program, or provide input-output pairs that show evidence that the program is working correctly.

For those who find this program too easy, you can challenge yourself to do more ambitious programs:

• Read analog inputs from A0 through A5 and report the values.
• Accept characters from the USB serial line that change which pins you examine. (For example, getting the character ’8′ with Serial.read() might turn on looking at pin 8, and getting ‘*’ might turn it off looking at pin 8.)
• Write a little control program that turns on the on-board LED (pin 13) when some combination of conditions is true and off when the conditions are false.

At the end of class, I had everyone use my wire strippers to cut and strip a few small pieces of wire, so that they could do the hardware portion of the Arduino programming.

The astute reader of this blog may notice that there was no “eighth day of freshman design seminar” post.  I was sick last Wednesday and unable to attend class, so I had the group tutor (a senior in bioengineering) take the class and have them discuss possible projects to take on.  I asked them to turn in proposals yesterday, but forgot to collect them—I’ll collect them tomorrow.  We’re about halfway through the course, so it is time for students to start on their projects.

I returned two homeworks yesterday: the colorimeter design and the RGB LED resistor sizing.

The colorimeter designs were not very good, lacking necessary details, but were somewhat better than previous spectrometer attempts. I think I’ll try reversing the order of those assignments in future, as the colorimeter is a simpler device. The biggest problem with the designs is that most of them were pieced together from web pages, with no citations.  Two of them were blatantly copied from Science Buddies, which has a decent design, but the students did not cite the source. I yelled “Cite your sources!” at the class, and explained that I could have flunked several of them out for plagiarism, and that in an upper-division course I would have. I hope they get the message, so that they don’t fail out later on. I decided not to prosecute academic integrity cases in this 2-unit, optional course, though I am making the science-buddy copyists redo the assignment.

I then explained to students the mistake I had made in the photodiode explanations (see Lying to my students) and corrected the understanding of the “open-circuit voltage” spec from the photodiode datasheets. I think that the students are a little more comfortable about finding things on datasheets now—I hope that lasts for them.

We then went over one of the RGB LED datasheets and did the resistor sizing for it.  About a third of the class had done a decent job on that assignment, and I cleared up the common mistakes:

• If a battery is used in a schematic, both ends need to be connected.  Other options are to use +5v and Gnd port symbols, or a +5V DC voltage source symbol.
• The LED diode must be forward biased (with a large current flow), and the triangular shape of the diode symbol shows which way conventional current flows.
• The voltage needed for determining the resistance is the voltage across the resistor, not the voltage across the diode, so it is 5v–V_F, not V_F.

I think I managed to get these points across—I relied fairly heavily on asking the students to do each step, so I’m pretty sure that at least half the class can now size a resistor for an LED.

Finally we could get to some new material. I wanted to show them how to program an Arduino, so we built up the standard blinking-LED first example for an Arduino.  To make it a little more interesting, I started with a true statement—I did not know whether the LED on pin 13 was hooked up with the anode or the cathode connected to pin 13.  We looked at the two possible circuits and how they would behave differently when the pin was high and when it was low.  I then explained “void setup()”, “void loop()”, and “pinMode(13,OUTPUT);”.  I had the students come up with the body of loop, feeding them the important constructs (digitalWrite and delay) only once they had expressed the action they wanted.  We ended up with a loop that help pin 13 high for a second and low for ¼ second.  After I typed in the program we had written, I showed them how to select the appropriate board type and download it to the Arduino.  The light blinked, and the students were able to figure out from the pattern of on and off that the LED was connected between pin 13 and GND (with a series resistor), with the anode towards pin 13.

I ran out of time and material at about the same time (a first for this quarter), and assigned the students to read about Arduino programming from the Arduino reference website, with particular attention to “if”, “while”, “pinMode”, “digitalWrite”, “digitalRead”, “analogRead”, and the timer functions.  I expect to go over some analogRead stuff in class tomorrow, and assign a small programming assignment over the weekend, probably using “Serial”.