Gas station without pumps

2015 May 21

Pulse monitor lab part 2 and power-amp lecture

Filed under: Circuits course — gasstationwithoutpumps @ 22:18
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Wednesday’s lecture covered a reasonable amount.  I went over PWM again, stressing the key concept: that the relevant value is the average value of the output. I showed the derivation of the average value from the high value, low value, and duty cycle.  I introduced nMOS (and, to a lesser extent, pMOS) field-effect transistors, and showed how they worked as switches, which is the only way we’ll use them.  I introduced both nMOS low-side switches and cMOS output stages, and talked about shoot-through current.

I did not get to comparators, open-collector outputs, and how to create PWM signals from a comparator and a triangle-wave generator, which is the meat of tomorrow’s lecture.

I spent over 9 hours in the lab again today, because I’ve promised the students that I’ll be in the lab 5 p.m.–7 p.m Tuesdays and Thursdays for the rest of the quarter, but the students were done with this week’s lab by 6 p.m.  I think all but one or two of the groups got working pulse monitors.  Debugging was difficult today.  The most common problems were

  • Wires in the wrong holes on the breadboards—usually an off-by-one error.
  • Phototransistor connected up backwards. Students kept thinking in terms of “long lead” and “short lead” and somewhat arbitrarily assigned a positive and negative meaning to them, without going back to the datasheet to see which was the collector and which the emitter.  This was difficult for me to debug, because the phototransistors had wires soldered on and were covered with electrical tape, so I could not see the original wire length nor the flat on the package, and had to trust the students’ claims about which wire was which.  In one case the problem was only debugged after I connected my LED and phototransistor (where I knew the color coding I’d used) to the student’s circuit and saw that the circuit was working.
  • Centering the high-pass filter and second stage of the amplifier at 0V, even though we were using a single-sided power supply, so they needed to make a virtual ground.
  • Swapping the wiring of the LED and the phototransistor.
  • Insufficient gain in the second stage.
  • Difficulty getting appropriate amounts of pressure on the finger to get a good pulse fluctuation.

The students having the most trouble had not prepared a schematic to work from or had very sloppy un-color-coded wiring, or both.  I’ll have to remind students that taking a half hour at the beginning to set things up carefully can save hours of debugging—too many want to dive in and wire up stuff without a clear idea what they are going to do, just hoping that it will somehow get fixed by the group tutor or me debugging their work. I refuse to do debugging for a group without a schematic, and in some cases I gave students some properly colored wire and asked them to rewire the circuit so that it could be debugged.

One student was unable to get a pulse reading from his finger on anyone’s circuit (including mine).  I don’t know what the difficulty was—he attributed it to too much caffeine today, so that he could not hold his finger steady in the block.  I suspect that he might have been pressing down too hard and squeezing the blood out of his finger.  One student even managed to get a pulse  reading through fingernail polish (she was wearing a red polish, which apparently was transparent enough to still work).

I had set up my transimpedance amplifier (which has adjustable gain from 66MΩ on up) with an oscilloscope for students to see what a good signal looked like.  There is a lot of 60Hz noise capacitively coupled in, but I was getting a 0.5V pulse signal with only about 200mV of interference.  I probably want to lower the corner frequency for the low-pass filter, to reduce the 60Hz signal further (one group that had miscomputed their corner frequency had very low interference, but their signal was also attenuated too much—I think that a reasonable compromise position can be found).  Even groups with very high 60Hz interference were able to get clean signals by sampling at a multiple of 1/60th of a second (20Hz or 30Hz, for example)—the aliasing eliminates the 60Hz signal, turning it into a DC offset.  The only ones who ran into trouble with 60Hz interference were ones who had the gain set so high that the 60Hz interference caused clipping, obscuring the pulse waveform added to the interference.

I expected this lab to be a little easier for students than it turned out to be, but I think that several students who had been having trouble with some of the concepts (like the virtual ground) were now getting the idea.  I hope so, as most of what they did in this lab will be directly applicable to the EKG lab in the last week of classes.

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