Gas station without pumps

2016 November 27

Cyber Monday sale 2016

Filed under: Circuits course — gasstationwithoutpumps @ 12:01
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My Black Friday sale was a moderate success (12 copies of my book sold for total royalties of about $25).  The sale continues through Cyber Monday (end date 29 Nov 2016), with the same coupon reducing the price to $2.56:

https://leanpub.com/applied_electronics_for_bioengineers/c/Black-Friday-2016

I’m not going to get rich off of this textbook, but I’m hoping that a few other teachers of electronics will pick it up and use it for a course.

2016 November 22

Black Friday sale 2016

Filed under: Circuits course — gasstationwithoutpumps @ 20:29
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Although I personally celebrate Buy Nothing Day this Friday, I have bowed to the marketing wisdom of the LeanPub publishers and am having a sale on my book this weekend.  From 2016 Nov 25 until 2016 Nov 29, the coupon link

https://leanpub.com/applied_electronics_for_bioengineers/c/Black-Friday-2016

will reduce the minimum price of my book from $3.99 to $2.56, approximately a 36% discount!  (At other times, it will just offer the usual pricing.)

Once you’ve bought the book, you can download any future versions that I publish through LeanPub (and I expect to have at least one more version before classes start in January).

Note: I’ll be sending coupons for free copies to the students registered for the course some time in December, so they don’t need to spend even the $2.56 of the Black Friday discount.

2016 November 19

Electronics course over-enrolled

Filed under: Circuits course — gasstationwithoutpumps @ 22:35
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Course registration has been going on for next quarter, and my Applied Electronics for Bioengineers course has more students wanting (or needing) to take it than we had planned for.  The largest previous class was last spring, with 45 students, so I had planned for a modest increase to 66 students this year (3 lab sections of 22 students each).  That filled up before the first round of registration was over, so I squeezed in 6 more students making room for 72 students (3 lab sections of 24 students each).  Those slots are now full and there are 12 students on the waiting list.  I can’t increase the lab sections any more (24 students in the lab is already crowded as there are only 12 benches).

I can’t add another lab section for several reasons:

  • The lab space is shared with another class that has 86 students enrolled (with a capacity of 120) and it took a while to negotiate even the 6 weekly slots for my 3 sections.
  • My lecture hall only seats 75.  Somewhat surprisingly, there are a couple of larger halls available in the MWF 2:40-3:45 time slot, though only one is a reasonable walking distance from my office (particularly given the amount of stuff I often carry to lectures for demos).  (If I’d been looking for a T Th slot, it would have been impossible at most times, as all larger halls are already scheduled.)
  • I’m already scheduled for classes, meetings, labs, or office hours 25 hours a week, not counting my Academic Senate committee or my undergrad directors’ meetings (which haven’t been scheduled yet).  I’ll probably be in the instructional lab TTh 1pm–7:30pm, making my lab hours be 13 hours a week.  (Students will have 190 minutes a week of lab, but there are three back-to-back sections, plus I need to set up and clean up.)
  • I don’t have a TA, so I’ll be doing all my own grading (8 problem sets and 5 large design reports for this course, and 2-3 design reports for the freshman design seminar). I will be able to hire undergraduate group tutors to help me answer questions in the lab—having two people in the lab to ask questions of makes a big difference.  I think I’ve got three tutors (one per section) lined up, but the official hiring doesn’t happen until some time in December.

The problem comes from a combination of factors:

  • requiring this electronics course of all bioengineering students (to reduce the pressure on the EE circuits course)
  • growing enrollment in bioengineering
  • growing enrollment at UCSC generally, without corresponding growth in instructional resources (instructional labs, classrooms, TAships, … )
  • my deliberately not  taking up a TAship for this course in previous years, to free up scarce teaching resources for more junior colleagues (though I fear that some of these resources have been squandered on courses taught by other senior faculty who didn’t really need TAs for their classes that were small or had only modest grading loads).

It looks like next year we’ll have to plan around 100 students:  the 84 students that seem to be the steady-state demand, plus 12 left over from this year, plus a tiny amount (probably too little) for growth.  That means that I’ll need 5 lab sections of 20 next year (or, with squeezing, 4 sections of 24).  I don’t think that I’ll be able to do all the grading myself for that large a class, particularly not with supervising that many labs.  So I’ll probably need a TA, who will have to come from another department, as none of the grad students in our department have electronics expertise. I might be able to get by with undergraduate group tutors and graders (though I’ll need enough hours from them that this might run into union limits—I believe the TA union considers it improper to have more than 20 hours a week of undergrad assistance).

 

2016 October 11

Lego as LED holder revisited

Filed under: Circuits course — gasstationwithoutpumps @ 18:01
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In Lego as LED holder, I wrote

I think that the Lego bricks work about as well as the old trans-illumination wooden blocks that I’ve been using for a few years, and they are certainly much easier to make, requiring only drilling two ⅛” holes.

I’ll want to play around with different illumination, also.  Lots of cis-illumination pulse monitor kits seem to use green LEDs, for example.  Do those work better?

Since then, I’ve played around with a couple of different approaches for using Lego.  The first, and most obvious, move was to use separate bricks for the LED and the phototransistor. If the phototransistor brick is clicked onto the top of the LED brick, then there is the ABS of the brick top between them, and very little light leakage.  I tried both 1×1 and 1×2 bricks, which cost about the same at about 3¢ each on the used market.

Because I was too lazy to wire leads onto all my different LEDs, I also tried just sticking the LED in the breadboard and resting my finger on the bricks—it isn’t very sturdy, but for quick testing it is not bad:

The LED can support the bricks, if you don't press too hard.

The LED can support the bricks, if you don’t press too hard.

I tried using several different LEDs. I got good results with 700nm and 607nm peak LEDs, but nothing but DC drift with green (565nm) LEDs. I would have tried a yellow LED, but I only had ones in 5mm packages, which is too big even for the axle holes, so the poor results there may have been due to mechanical, rather than optical difficulties (some signal was visible).

Here are the results with a 700nm red LED:

There is a lot of DC drift, but the underlying 3–4mV signal is clear.

There is a lot of DC drift, but the underlying 3–4mV signal is clear.

So, I have (at least) three choices for how to do cis illumination with Lego bricks, all of which I like better than using wooden blocks:

I can drill off-center holes in Lego Technic bricks, or centered holes in 1×1 or 1×2 bricks.

I can drill off-center holes in Lego Technic bricks, or centered holes in 1×1 or 1×2 bricks.

Drilling the bricks is very easy, and it would be even easier, if I made a jig that aligned the brick, rather than having to fiddle with the drill-press vice. I might even have the students drill their own bricks. I did not put Lego bricks on the parts list, but if I have to buy $5 worth of Lego bricks for this Winter’s class, it is no big deal.

2016 October 9

Lego as LED holder

Filed under: Circuits course,Uncategorized — gasstationwithoutpumps @ 15:00
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In Pulse monitor using log amplifier, I talked about the problems I was having with using wooden blocks for holding an LED and phototransistor side-by-side, because the wood was too transparent, and the rather clumsy test I made using electrical tape:

By cutting between the two 3mm holes I could put black electrical tape to block the short-circuiting light path.

By cutting between the two 3mm holes I could put black electrical tape to block the short-circuiting light path.

I suggested in that post that I would buy a chunk of black ABS plastic for $10 and try making the finger cradles out of that. I realized later that I already have some black ABS plastic, in the form of Lego bricks. If I could use them, that would save me a lot of trouble, and provide a more easily duplicated block for others to use.

I ended up trying a black 1×2 Technic brick (which would cost about 1¢–3¢ each on Bricklink), drilling two ⅛” (~3mm) holes in the faces on either side of the axle hole. The axle hole in the Technic brick provides a light barrier between the two optoelectronic components:

View of the Lego Technic brick from the bottom, showing the light barrier between the two optoelectronic parts. The bottom needs to be covered (with another brick or electrical tape), and the optoelectronic components need to be taped in place.

View of the Lego Technic brick from the bottom, showing the light barrier between the two optoelectronic parts.
The bottom needs to be covered (with another brick or electrical tape), and the optoelectronic components need to be taped in place.

View of the drilled Lego Technic brick, showing the optoelectronic components.

View of the drilled Lego Technic brick, showing the optoelectronic components.

I had hoped to be able to insert the 3mm LED and phototransistor from the bottom of the brick, but there was not enough clearance to do so easily, so I inserted them from the opposite face of the brick.

I tried recording the light levels with the front face taped over with black electrical tape, and with a finger covering it. The difference in voltage was large, indicating that the light through the finger was much more than the light leakage around the axle-hole light barrier. I was using an LTR-4206 phototransistor and a 1N914 diode followed by a unity-gain buffer.

I got around 394mV with the finger and 275–284mV with the holes taped. The variation on any given recording run with the holes taped was only about 0.3mV, but different runs, with different amounts of sunlight falling on the brick gave different levels. The minimum difference between the finger and the taped block is about 110mV, which translates to a 19.8dB difference in light (or a factor of 9.8).

But I was not able to get a pulse measurement with cis IR illumination.  I could get a signal of about 1mV peak-to-peak with ambient (shaded sunlight) illumination, which corresponds to about 0.18dB, or 2% fluctuation in finger opacity, but that depended critically on the pressure on my fingertip—if it wasn’t just right, I got no visible pulse signal, just noise.  I could get a bit more consistent results by putting the Lego block between my index and middle fingers and using a rubber band to clamp the fingers together.  If the squeeze was just tight enough to throb, then I got fairly clean results, and I could get them fairly consistently, but I’m not sure whether others will be able to get similarly consistent results.

I also tried taping up the block with no IR illumination, to measure the dark current.  I got 67mV, which should correspond to about 10nA, which is pretty good, since the spec for the LTR-4206 phototransistor give the max dark current as 100nA.

Bottom line: I think that the Lego bricks work about as well as the old trans-illumination wooden blocks that I’ve been using for a few years, and they are certainly much easier to make, requiring only drilling two ⅛” holes.

I’ll want to play around with different illumination, also.  Lots of cis-illumination pulse monitor kits seem to use green LEDs, for example.  Do those work better?

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