Tenth day of freshman design seminar

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.