Gas station without pumps

2015 March 18

Freshman design projects moderately successful

I just finished grading this year’s freshman design projects. I think that the projects were more successful this year than last year, in part because I kept the students focussed on electronics and programming (for which they had lab access and which I could help them debug), and in part because the projects were somewhat less ambitious.

There were two groups doing EKGs and 4 groups doing blood pressure meters.  Both EKG groups managed to demonstrate their projects working, as did one of the blood-pressure groups.  (I’m being fairly generous here about what “working” means—they had to get their electronics to work, capture the data, and plot the waveforms, but further interpretation or software was not required.)  The other three blood pressure groups did not manage to demonstrate their projects, but one of them managed to plot waveforms for the pressure measurements (without getting their high-pass filter and amplifier working for the pulse measurements).

Some things I learned for next year:

  • Tell the students what op amp to get.  A number of students picked op amps that turned out to be rather old-fashioned ones with very low input impedance (as low as 2MΩ), rather limited output ranges, and external nulling circuits. The cheap MCP6002 or MCP6004 chips would have worked better at lower cost.  In fact, I gave one group that seemed to have a good schematic (but couldn’t get their circuit to work) an MCP6002 chip, which they wired in place of the op amp they had been using, and their circuit worked immediately.  I would have done the same for other groups, but the others with poorly chosen op amps were about a week behind and did not have circuits that were that close to being functional.
  • Warn students sooner not to use FedEx.  My son’s and my experience with FedEx this year has been that they are ludicrously slow. At least one group was burned by a ridiculously long delivery time, having ordered with FedEx delivery just hours before I warned the class about them.  (The US Post Office is faster and cheaper for lightweight electronics orders from Digi-Key.)
  • Students who never ask questions in class probably don’t understand much that is going on—all the groups that successfully demonstrated their projects had at least one active participant in class.
  • Students who fail to turn in their progress report are almost certainly not going to complete the project on time—I need to be more assertive in getting them moving and demanding that they show me their schematics.  Almost everyone had errors in their schematics on their first design (and one of the successful groups went through 4 incorrect designs before getting to one that worked).  Students that are afraid to show me incorrect or incomplete work don’t get the feedback they need to correct the problems—I need to normalize errors more and insist on seeing stuff, even if it is wrong.
  • The MXP5050DP pressure sensors are very easy for students to use, though a bit pricey at $16 each.  The built-in amplifier makes doing pressure measurements with an Arduino fairly trivial (hook up the three wires of the sensor to A0, +5V, and GND).  They were a good choice for the freshman design seminar, though I’ll continue to use MPX2053DP sensors without an integrated amplifier for the applied circuits class—that assignment is intended to get students to design with an instrumentation amp and to understand a bit about strain gauges.
  • Get the students to plot stuff earlier in the quarter. One group tried installing gnuplot on a Mac in the lab in the last few hours, which did not go well for them.  They did eventually find a plotting program that they could install and run, but then did not have time to run the data they collected through the filtering program I’d written for the class.  Their signals were pretty clean, though, and the plots they produced were good even with just the RC high-pass filter in their amplifier, without digital filtering.
  • The students seemed (for the most part) pretty excited about the projects—even those whose projects didn’t quite work seem to have gotten a lot out of the lab times.  I should look in a couple of years to see how many have stuck with engineering majors (I suspect that some might switch to computer science or computer engineering, rather than sticking with bioengineering, but that’s ok).

2013 October 8

First blogging swag

Filed under: Data acquisition — gasstationwithoutpumps @ 20:51
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One hears about bloggers making money off their blogs from advertising (I get nothing from the ads that puts on my blog—it just pays for the free blogging service).  One also hears about bloggers getting sent products or books to review on their sites, as a form of cheap advertising.  I’ve reviewed a number of things on my blog, but they’ve always been things I’ve bought retail, not promotional items.

Yesterday I got my first free swag.  Bitscope liked my use of their Pocket Analyzer USB oscilloscope in my circuits blog posts, and are supposedly integrating some of my suggestions into their next software release.  They offered me a chance to use and review a pre-production version of a new product: a front-end for the Pocket Analyzer that gives it the ability to do true differential inputs and AC coupling, reducing some of the limitations of the DC-coupling and having the analog ground be the same as the USB ground.

I agreed to this on 2013 July 29, and they supposedly sent me one of the devices.  I never got it, though, thanks to the combination of the Australian and US post offices (perhaps it is still on a boat circling the Pacific Ocean). Had I gotten the device over the summer, I would have probably spent a week playing with it, reporting the results on my blog. Now that the Fall quarter is in full swing, I’ll have much less time, what with teaching classes, grading programs, supervising grad students, and helping write grant proposals. I’ll probably be only able to put in an hour or two a week on it (I’ve already spent far more than that today on this blog post, and I’ve still not done any meaningful testing).

Bitscope asked me last week when I was going to post something, and I told them that I was still waiting to get something.  So they sent me another one via Fed Ex, which came quite promptly:

Date/Time Activity Location
10/04/2013  –  Friday
1:26 am
Shipment information sent to FedEx
4:49 pm
Picked up
6:02 pm
Left FedEx origin facility
10/05/2013  –  Saturday
6:07 am
In transit
10/06/2013  –  Sunday
5:09 am
In transit
10:25 am
Arrived at FedEx location
1:29 pm
International shipment release – Import
3:27 pm
Departed FedEx location
5:26 pm
Arrived at FedEx location
10/07/2013  –  Monday
4:50 am
Departed FedEx location
7:49 am
At local FedEx facility
8:37 am
On FedEx vehicle for delivery
9:18 am

I found the routing a little strange (Los Angeles->Memphis->Oakland), but I can’t fault the delivery speed. Within the US, I’ve always had very good service from the US Post Office, with prompt, low-cost delivery, but deliveries across the Pacific can be a bit iffy. It generally takes 2–3 weeks to get something from China through the Post Office, longer from India, and my success rate from Australia is only 50% (small sample, though). It may be worth paying extra for more reliable shipping from Australia and India.

The probe PC board is about 6.2cm long and 2cm wide (8cm long with the connectors on the ends).

The probe PC board is about 6.2cm long and 2cm wide (8cm long with the connectors on the ends).


The back of the probe has a connector with 3 twisted pairs that go to the Pocket Analyzer, and two jumper positions for shorting plugs that change the gain of the preamplifiers in the differential probes. The label on the probe says 2× without the jumper and 20× with the jumper, but I was told by email 0.5× and 5×, so I’ll need to test the gain for myself. There is no documentation yet, since this is a pre-production prototype.  The jumpers for gain switches are a bit of a kluge, but I expect that sort of cost cutting on a USB oscilloscope.


The other end of the short (~14cm) cable from the probe plugs into the leftmost pins of the Pocket Analyzer, connecting to 5v, 3.3v, A, B, and 2 Gnd pins. There are two blank positions on the left end of the connector, corresponding to the space at the end of the connector on the Pocket Analyzer. I suspect that the intent is to use this as a key to reduce the probability of plugging the probe in wrong. It probably won’t help much, as there is nothing stopping anyone from plugging it in upside down at the wrong end of the connector—the Pocket Analyzer uses a symmetric connector that has no keying.


On the component side, we can see that there are are 4 main ICs (probably op amps or instrumentation amps, but I can’t read the labels through the heat-shrink tubing) and a number of discrete components. I’m assuming that the heat shrink tubing is a pre-production measure, and that they’ll have a classier-looking case once they go into production. (I’ve been recommending heat-shrink wrapping for the LEDs in the light gloves my son in designing, because I think that classier cases for the LEDs will be difficult for them to get made in tiny quantities, but I suspect that BitScope is looking at a larger production run than the first run for the light gloves).

Taking a closeup photo of the input end of the probe with my camera lets me see the labels on the ICs.

Taking a closeup photo of the input end of the probe with my camera lets me see the labels on the ICs. They seem to be MCP6292 E/SN chips (about 67¢ each from DigiKey in 100s).  Those are dual rail-to-rail op amps with a 10MHz gain-bandwidth product (7v/µs slew rate), with a 2.4v–6v single-sided power supply.
You can also see that the surface mount components have been hand-soldered, by someone who is worse at SMD soldering than me (that’s pretty bad). It looks to me like one of the components (a resistor?) has been shorted by a bad solder bridge. I assume they did some testing before sending me the probe, so I hope that this is an optical illusion, not a real short.

I’ve not heard back from the BitScope folks yet what voltage range is acceptable on the inputs, so I’ll have to be rather cautious at first—the op amps don’t want inputs more than 1v outside the op-amp power rails, and I don’t know what voltage dividers there are before the op amp.

I suspect that the bandwidth for the probe is around 5MHz in the low-gain setting, and 1MHz in the high-gain setting—I’ll probably have to take the setup into one of the student labs at the university to test this, though, as I don’t have much to use for comparison at home. It may be difficult to measure the bandwidth when the probe is plugged into the Pocket Analyzer, since the Pocket Analyzer itself has limited bandwidth. I can’t get it to sample faster than 5MHz, which gives an upper limit of 2.5MHz for the bandwidth. The online documentation for the BitScope claims 20M samples/sec is possible, but if I try to set that, the software claims that there is an access violation and does not change the setting.

I think that the noise floor of the differential probe will also be set by the rather high noise in the Pocket Analyzer. The high-gain setting of the differential probe will allow somewhat better signal-to-noise ratios, as I doubt that the differential probe is anywhere near as noisy as the Pocket Analyzer, which seems to have ±3LSB on an 8-bit DAC (about ±1% of full scale). I wonder whether BitScope will try to take advantage of some of the newer processors (like the Freescale Kinetis L family) that have higher resolution ADC with probably adequate sampling rates.

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