The pressure-sensor lab went fairly well today. We once again borrowed soldering irons from the RF lab (where my co-instructor teaches), but we did not borrow enough board holders, so a number of students had to solder with the boards just resting on the benchtop. This was not a major problem, but having more board holders would be good.
Many of the students had done an adequate job of doing the design ahead of time, and even those who had not mostly managed to finish within 4 hours (but the lab is supposed to be scheduled for 3 hours). Most groups managed to demonstrate working PC boards with instrumentation amplifiers amplifying the differential output of the strain-gauge pressure sensors, generally with gains in the range 100 to 300. I have one group coming in on Monday to finish the soldering—they took the more cautious approach of debugging on a breadboard first, and had to do a little redesign to make everything work. I think that they’ll complete with no problems on Monday, as they had good notes on what their breadboarded circuit was, and had come up with what looked like a feasible layout.
Soldering was much more routine this time than on the first soldering lab. Students were able to check their own work for cold-soldered joints, and only one group forgot to trim the extra wire length on the back of the board after soldering (they had a short when they put the board down, from two untrimmed leads touching).
Some students called me over to help them debug at one point, but I had to refuse, as they did not have readable schematics that I could help them debug from. After I told them why I couldn’t help them, they redrew their schematics, and I then helped them check the design against their soldered board. It seems that they had soldered everything up correctly according to their schematic, and the DC voltages were right at all the nodes, but the twisted-wire cable to the pressure sensor had the wires scrambled. (Luckily, they had followed my advice of using 4 different colors for the wires, though they had not followed the red=+5v, black=0v convention—several students are still using colors at random.) When they screwed the wires into the right terminals, their circuit worked. It seems that schematics were not the only thing that they were sloppy with!
Some of the students used PDF markup tools to add the layout of their extra wires and parts to the prototyping board PDF worksheet. That seems to have worked well for them, producing neater and more easily checked layouts that the pencil scrawls that I (and many of the students) used. I hope they tell me in their lab reports what tool they used, so that I can recommend it for the EKG lab and for the pressure-sensor lab next year. No one did the wire lists that I recommended, but there were relatively few wiring errors (and those were inherited from errors in the schematics, so wiring lists would not have helped). I think I’ll leave the wiring lists out of the assignment next year.
One thing that surprised me (another moment of culture shock), was that the seniors in bioengineering did not know what a peristaltic pump was. I was trying to connect what they were doing to something they already knew, only to find out that what I thought was familiar to them was novel. I demonstrated the basic principle for a couple of them by hooking two ends of the flexible tubing up to the two ports of their differential sensor, and pinching the tube between a pen and the benchtop. By pulling the tube through the squeezed area, I could get a large pressure difference between the ports. Since peristaltic pumps are standard lab equipment in many of the labs they are working in, I was surprised that they have never used one or even known of their existence. I’m now wondering whether I should do the demo demonstrating the principle in class on Monday for everyone.