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2018 July 9

Analog Discovery breadboard adapter

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 11:16
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As I mentioned in Analog Discovery Impedance Analyzer, I recently bought two new attachments for my Analog Discovery 2.  I reviewed the Impedance Analyzer in the earlier post, so in this one I’ll review the breadboard breakout.

The breadboard breakout provides a simple way to attach the Analog Discovery 2 to a breadboard, without using the female headers that come with the device.

Here is the breadboard adapter, plugged into the end of a breadboard.

The Analog Discovery 2 can plug into the breadboard vertically, which is compact, but requires disassembly to put the test setup back in its box for carrying.  Here it is shown plugged into the last 15 rows of the breadboard, but I had to move it in two rows to keep the weight of the AD2 from tipping the breadboard.

I tried doing a little work with the breadboard adapter and found it to be a mixed blessing. I used it for testing a circuit where I needed both oscilloscope channels, one power supply, and one waveform generator, which would normally use 7 of the 30 wires on the AD2.  Some of the wires (the power, ground, and oscilloscope 1- and 2- wires) could be quite short, as they connected to the power busses on the breadboard, but the other wires had to be fairly long, as they had to skip past all the trigger and logic-analyzer inputs that I wasn’t using.  I could have plugged the adapter into the breadboard the other way around, but then the AD2 itself would interfere with convenient wiring.  It would have been nice to have the most frequently used connections at the tip of the adapter, instead of the base of the adapter.

For a fixed setup, where the oscilloscope channels are always looking at the same signals, the breadboard adapter is more convenient that the standard flywire connections, which have a tendency to slip off the double-ended male headers that I use for connecting them to the breadboard.  The female headers of the flywires are not designed for many cycles of attaching and detaching, and end up getting too loose after a while.

But for debugging, when the oscilloscope channels have to be moved rapidly from node to node, the breadboard adapter is less convenient than having the separate flywires—unless much longer wires are used (with the attendant problems of extra inductance and capacitive pickup of 60Hz interference). Losing 17 rows of the breadboard to the adapter is also a problem, as it leaves only 47 rows of a standard 64-row breadboard, or 15 rows of half-length breadboard for building the test circuit.

I think that I will use the adapter for lecture demos, where I have fixed wiring to carry around, as I can spend less time setting up the demo just before class, at the cost of slightly more time the night before. My standard lecture setup will use a full-length breadboard with the adapter in one end and a Teensy LC in the other end (for PteroDAQ demos) using up 31 of the 64 rows, leaving me with the equivalent of about a half-length breadboard in the center for the circuitry being demonstrated.

I don’t know yet whether I’ll find the adapter useful for regular debugging—probably not much.

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2017 May 16

New problem in class-D lab

Filed under: Circuits course — gasstationwithoutpumps @ 22:37
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Today was the last day for the class-D power amplifier lab, and the students had a problem that we’d never encountered before—the breadboards kept squirting the nFETs out of the breadboard (sometimes the landed several inches away).

We were using the same nFETs as last year,NTD4858N, which comes in a TO-251-3 Stub Leads (which they call IPak) package.  The problem is that this year’s breadboards have their contact springs deeper than in previous years, so the stub leads barely reach them.  I don’t know whether the breadboards were ordered from the supplier I had found (http://www.meerkatsystems.net/html/10000023.html) or whether they substituted one from one of their favorite suppliers.  It would be good to know, as this year’s breadboards seem to be inferior to previous year’s.

Next year, I think I’ll specify the nFET to be PSMN022-30PL,127, which comes in a TO-220 package and sits more firmly in the breadboard.

Tomorrow I’m giving a quiz in class—something I try to avoid doing, but so many students have not been showing up for class nor turning in the required pre-lab homework that I was compelled to assess them some other way.  My guess is that the grade distribution will be similar to the distribution for the sum of the homework so far (out of 50 possible points):

 1.0  1
 1.5  2
 2.5  1
 3.0  1
 3.5  3
 4.5  1
 5.0  1
 5.5  1
 6.0  1
 7.0  3
 7.5  2
 8.5  5
 9.0  1
 9.5  4
10.0  4
11.0  1
11.5  2
12.0  3
12.5  4
13.0  1
14.0  1
14.5  1
15.0  1
16.0  3
16.5  2
17.0  2
19.0  2
19.5  1
20.0  1
20.5  1
21.5  2
24.0  1
25.5  1
26.0  1
27.0  1
28.0  1
28.5  1
30.5  1
31.5  1
36.5  1

I further conjecture that there will be a very high correlation of scores (so I won’t really learn all that much about the students). But I’m prepared to be surprised—I made the quiz deliberately fairly easy, so it is possible that students who have struggled with the design problems of the homework may be able to do the quiz.

2014 April 3

Thermistor lab second half took too long

Filed under: Circuits course — gasstationwithoutpumps @ 22:31
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I thought that splitting last year’s thermistor lab into two parts would mean that it would fit comfortably into 2 3-hour periods, but I still spent 5.5 hours in the lab today with the students, even though they had done most of the measuring on Tuesday.  What went wrong? and how can I prevent problems like this in future (both later this quarter and next year)?

The students got the rest of their parts today (except for the thermometers, which will come too late for the students to use). I checked out the digital thermometers that we’d been loaned before class, and found that 4 of the 10 were clearly wrong, reading about 2°C too high.  That was in addition to the thermometer I found on Tuesday that was about 20°C too low. Tomorrow, I’ll have to take some time to talk about the differences between precision, repeatability, and accuracy.

I started by having students finish the homework assignment together (Design a circuit to convert a 1kΩ–3.3kΩ variable resistance sensor to a 1v–2v voltage output, with 1v for the 1kΩ resistance and 2v for the 3.3kΩ resistance. Use standard resistor values that you have in your kit), and wire it up using the power supplies and a multimeter. I also used this as a time to teach them about how to use the power supplies (letting them warm up, setting voltage and current before turning on the output, turning on and off the output without touching the power switch). I also told them about the resistor color code and two ways to use resistors in a breadboard: using 0.4″ spacing and having the resistor horizontal near the breadboard, or the “flying resistor” with 0.1″ spacing and a vertical resistor. I suggested that they might find the flying resistor easier to work with in some of the later breadboarding, and suggested that they get in the habit of always having the band for the most significant digit at the top of the resistor.

One of the students had done something very clever before lab—he’d taken the 64 tapes of 20 resistors each, sorted them by size and then used transparent tape along one edge to make a booklet of resistor sheets.  The half hour or so of time that took him will probably save him many times that much during the quarter—I had him show his work to everyone and suggested that they take the time to do likewise.  He also found, while doing the sorting, that two of the resistor tapes were mislabeled (he already knew the resistor color code, so could see at a glance that the 47kΩ and 100kΩ resistors had their labels swapped.  I think that this student is already thinking like an engineer—he may not have the math skills of a professional engineer, but he has the attention to detail and forethought that characterizes the really competent engineers. I hope that this early promise continues throughout the quarter.

One big problem with the lab time was that almost no one had actually done the pre-lab design problem to optimize the resistance to pair with the thermistor, so they spent an hour or two struggling to set up the equations and do the calculus during lab. Everyone did eventually get a reasonable value for their design resistor by setting up the equations and doing the calculus (with the aid of Wolfram Alpha), but sitting at a bench in the lab doing math is clearly not an optimal use of lab time. What can I do to prevent that delay? Demanding that they show up to lab with the solutions did not work last year or this, so I need a different technique.

Tomorrow I’ll talk to the students about time management (writing up the report for Tuesday’s lab either Tuesday or Wednesday, so that only the new stuff on Thursday needs to be added, for example) and doing design exercises before coming to lab.

I’m thinking that next year I may split the thermistor lab into 3 parts: part one is what we did on Tuesday minus the soldering; part two would be just setting up and testing the DAQ boards (installing software, soldering, and doing some simple tests) and wiring up the simple voltage-divider exercise; and part three would be wiring up the thermistor and resistor to the DAQ board, checking linearity, and plotting temperature as a function of time. I could then require that the design report due on Friday be mainly about the data collected on Tuesday and the calculations for the design of the temperature-to-voltage converter. By forcing them to turn in the design report with the calculations long before they come to lab, there is a better chance that they’ll do the design work outside lab time.

That would rather mess up the weekly pairing—I wonder whether to think of that as a bug or a feature. If I forced repartnering before the temperature recording got done, students would be coming in with two different design reports and having to reconcile them. That might cause them to actually read each other’s reports, which could be a good thing. I’ll have to see what ripple effect this rescheduling would have on the rest of the quarter.

Finishing up the soldering that was started on Tuesday did not take up much time—within about half an hour everyone had a board that had good-looking solder joints on all 64 connections.

Installing PteroDAQ on the KL25Z boards also took more time than I’d like, mainly because of the necessity of first installing MBED.org’s firmware fix, then installing PteroDAQ, then switching to the other USB port to run PteroDAQ. Downloading from the bitbucket site confused a lot of the students (I find the site confusing that way also—there should be an obvious big button that downloads everything without “cloning” a repository or doing anything mysterious. I ended up passing around my flash drive so that people wouldn’t have to deal with bitbucket. Almost everyone managed to get PteroDAQ working eventually, but it took more time than I expected.(One student may be having hardware problems with the KL25Z board or the USB cable—there was no 3.3v power measured.  Another student had a bad USB cable—the board worked ok with other cables.) I wonder if there is a way to make the bitbucket repository more user-friendly for someone who just wants to install and run the software—it seems to be set up only for developers, not users.

Setting up their circuits on the breadboards took much longer than I expected.  Some of the problems were anticipated (like the students having a hard time finding the appropriate resistors, or clamping the insulation in the binding posts of the power supplies and so not getting any voltage), but a lot of the delay here was a mystery to me—students didn’t seem to be making progress for quite a while, but I couldn’t tell what was delaying them. I’ll have a hard time fixing the delay problem here if I don’t know its cause.

So far the students have learned to use the multimeter as an ohmmeter and as a voltmeter, and next week they’ll learn to use it as an ammeter.  We’ll also get a little use from the function generator and oscilloscope next week. I think I’ll also start introducing AC voltage tomorrow, instead of waiting until next Monday, unless the students have a lot of questions about the writeups they need to do (I expect that there will be some gnuplot questions).

 

 

 

 

 

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