Gas station without pumps

2014 January 27

Sixth day of freshman design seminar

Today I went into class with a long list of things to get done, but didn’t quite get to all of them:

  • Feedback on first homework.
  • Look at data sheets together.
  • Get class consensus on resistor values from homework due today.
  • Demo the Arduino Data Logger with the phototransistor and photodiode.
  • Discuss next homework (designing a colorimeter).
  • Start talking about Arduino programming.

The feedback on the homework went pretty much as planned.  I told them that the homework was not graded, but that I had both individual and general feedback on it.  Here is a summary of the general feedback:

  • College homework should be typed.  Professors expect it, even if they never say so.  The one exception is math homework, and I recommend to students that they learn LaTeX and typeset even their math.
  • Homework should always be stapled, not loose sheets, which get separated and lost.
  • Hand-drawn pictures are ok for this class (and many other classes), but I strongly recommend learning to use a drawing tool.  Adobe Illustrator is a popular one for those who have money, but Inkscape is an adequate tool for 2D diagrams and is free, though its user interface is rather clunky.  For more professional engineering drawings, I believe that AutoCAD has a free (or very low-cost) version for students. Sketchup and Blender are popular free tools for 3D modeling.  For schematic capture, I now use DigiKey’s SchemeIt, which I demoed briefly for the students (after having some trouble with the wireless connection in the room—I’ll have to check to see whether there is a live DHCP port by the projector cable in the room).
  • Most students added little to what we did in class. I pointed out that K–12 teachers mainly wanted them to spit back what they had been told, but that college professors were usually looking for added value—stuff from reading outside class or from original design.
  • I pointed out the importance of vocabulary (“diffraction” vs. “refraction”, “focus” vs. “collimate”) and of getting the right physical phenomena (Bragg’s Law for diffraction gratings, Snell’s Law and optical dispersion for prisms).  I told them to read the Wikipedia article on optical dispersion, so that they could understand the complexity of determining the wavelength-to-refraction-angle transformation, which is highly dependent on the material the prism is made of.
  • I also suggested that just dumping factoids (like the Bragg’s Law formula) on the paper without explaining the connection to the design didn’t really buy them much.
  • I pointed out the difficult design problem I had given them (300nm–700nm) with a diffraction grating would result in the second diffraction of 350nm at the same location as the first diffraction spot for 700nm—to handle both one would need two optical filters: one for the long wavelength, one for the short.  Even if we limit the range we’re interested in (say to 400nm–700nm), we’d still need a filter, since the sensor would still detect the 2nd-order 350nm spot, even though we weren’t interested in it.
  • I showed a couple of designs for a collimator (a lens and a slit, or a pair of slits on either end of a black tube) and explained why collimation was needed for a spectrometer (none of them had included a collimator).

The feedback took about the amount of time I expected, and I think I managed to communicate the problems without crushing anyone’s egos.  I was careful to tell them that I was not grading them on the homework, but providing feedback for them to do better later on things that would count—particularly that other faculty would often have these expectations of them without ever articulating them.  This freshman class is intended in part to help the students adapt to the college culture in a low-stakes environment.

Simple circuits for measuring light with an Arduino.

Simple circuits for measuring light with an Arduino. Update 2014 Feb 6: Q1 is intended to be an NPN phototransistor, not PNP as shown here!

We then looked at the WP3DP3BT phototransistor data sheet together.  First, I explained the mechanical drawing (dimensions in mm, the diameter sign , the two different ways that the case indicates which lead is which—both the flat and the shorter lead indicating the collector). This prompted a question about the naming of the collector and emitter (since it seemed strange to them that the collector went to the power lead and the emitter to the resistor), so I briefly explained that it was a NPN transistor, that the N’s stood for negative doping resulting in an excess of electrons as charge carriers, and that the emitter emitted the electrons and the collector collected them. I don’t know if that helped anyone.

I then asked the students what they needed help understanding for the numeric part of the data sheet. We ended up talking about 5 of the 7 parameters provided, covering a lot of different things (like that nA stood for nanoamps, not “not available”—a confusion I had not anticipated). I briefly went over milli-, micro-, nano- and explained that engineers preferred using those prefixes to expressing powers of 10, so that the prefer to express the dark current as 100nA, rather than 10-7A. Some scientific calculators provide engineering notation, in which only multiples of 3 are used as the power of 10, and the numbers are between 1 and 999.999999… .

I had to explain the difference between collector-to-emitter and emitter-to-collector voltages, and show the current vs. VCE curve with the two breakdowns. We talked a bit about the saturation voltage (0.8V with an irradiance of 20mW/cm2 and a current of 2mA). I’m not sure I understand that specification that well myself—it mainly tells me that we want to stay well below a 2mA current.

I asked the students for their resistance values from their homework, expecting some fairly random values that would reveal different misunderstandings. What I had not expected is that most of the class had nothing—not even a guess—at the resistance. I would have expected them to ask questions on the class e-mail list if they didn’t understand, but the notion of asking each other (or a faculty member) for help still seems completely foreign to them.

So we spent some time going over how to interpret the on-state collector current: 0.2 nA at an irradiance of 1mW/cm2 of 940nm light. I then had the look for more information that was given in the question, which no one had in front of them:

For Monday, 2014 Jan 27, as individuals (not groups), find a data sheet for the phototransistor WP3DP3BT. Also, select a cheap photodiode that is available in the same size and shape of package as the WP3DP3BT phototransistor and look up its data sheet. For the photodiode and the phototransistor, report the dark current, the voltage drop across the device (that would be collector-emitter saturation voltage for a phototransistor and the open-circuit voltage for a photodiode), and the sensitivity (current at 1mW/cm2 at λ=940nm, which is the wavelength where silicon photodiodes and phototransistors are most sensitive). Find a plot of the spectral sensitivity of a silicon photodiode or phototransistor (it need not be from the data sheets you found—all the silicon photodiodes and phototransistors have similar properties, unless the packaging they are in filters the light). We want to make a circuit so that the full-scale (5v) reading on the Arduino corresponds to an irradiance of 204.8μW/cm2 at 940nm, so that each of the 1024 steps corresponds to an increment of 0.2μW/cm2.

Eventually someone figured out that we wanted a 5v output to correspond to 204.8μW/cm2. I asked what current that irradiance produced. Note that this is a simple linear scaling of the 0.2 nA at an irradiance of 1mW/cm2. It took several minutes for them to do this on their calculators, and several tries before the class agreed on a value (luckily the right one). Now that they had a voltage and a current, I asked them for the resistance that was needed. One student quickly mentioned Ohm’s law, and they set about doing the division. It took them a couple of minutes to do this division on their calculators, and then most of them got it wrong (getting values in the µΩ range!).  Eventually they managed to converge to 122.1kΩ, after almost settling on 12.2kΩ, but what I had expected to be a 30–60-second computation for computing the resistance had taken 10–15 minutes.  The arithmetic and algebra skills of college freshmen are even lower than I had feared.

I showed them a chart of standard resistance values and helped them round to 120kΩ.  I showed them a 120kΩ resistor and measured it with a multimeter to make sure I had the right resistor.  I passed around an Arduino board and a breadboard and explained the point of ther breadboard. I hooked the resistor up in series with the phototransistor (on a pre-prepared breadboard) and used the Arduino data logger to show them the voltage changing as I covered and uncovered the phototransistor. (Next year I should probably reduce the sensitivity they are requested to match to 0.1µW/cm2 per step, as the classroom light was bright enough to move the voltage almost full scale.)

Class had been over officially by 10 minutes at this point (the first time I looked at my watch), so I gave each student a cuvette and asked them to look up what a colorimeter was and design one around the cuvette.

We still need to discuss the photodiode resistance value (I’ll see if anyone figures it out by Wednesday, when I’ve asked them to turn in the homework for real).  We have lab tours on Wednesday, though, so there won’t be time to discuss colorimeters before they design them.  I hope they have the sense to read about them on Wikipedia or the many web sites that give high school labs using them. The actual assignment was

By Mon 2014 Feb 3, design a colorimeter around the cuvette you picked up in class. Your design report should describe the function of the device, explain how it works, have a detailed drawing (with dimensions) of it, have a materials list of what is needed to build it, and give instructions for using it. If there are any computer components, an outline of the needed software should be included also.

1 Comment »

  1. […] Sixth day of freshman design seminar […]

    Pingback by Twelfth day of freshman design seminar | Gas station without pumps — 2014 February 19 @ 23:12 | Reply

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